fundamental aspects of sludge characterization

Fundamental aspects of sludge characterization

Herwijn, A.J.M.

DOI:10.6100/IR458461

Published: 01/01/1996

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Citation for published version (APA):Herwijn, A. J. M. (1996). Fundamental aspects of sludge characterization Eindhoven: Technische UniversiteitEindhoven DOI: 10.6100/IR458461

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Fundamental aspects of sludge characterization

A.J.M. Herwijn

\

FUNDAMENTAL ASPECTSOF SLUDGE CHARACTERIZATION

FUNDAMENTAL ASPECTS OF SLUDGE CHARACTERIZATION

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof. dr. J .H. van Lint, voor een commissie aangewezen door het College van Dekanen in het openbaar te verdedigen op

woensdag 17 april1996 om 16:00 uur

door

AREND JOHANNES MARIA HERWIJN

Geboren te Eindhoven

Dit proefschrift is goedgekeurd door de promotoren:

prof.dr.ir. P.J.A.M. Kerkhof

prof.dr. W.G.M. Agterof

en de copromotor:

dr.ir. W.J. Coumans

ter herinnering aan mijn vader,

aan mijn moeder

DANKWOORD

Vele personen hebben een belangrijke bijdrage geleverd aan de totstandkoming van het

proefschrift. Ik wil ten eerste mijn promotor,· Piet Kerkhof en copromotor Jan

Coumans bedanken voor het in mij gestelde vertrouwen en de aangeboden kans om het

verrichtte onderzoek vast te leggen in een proefschrift. De financiers van het onder

zoek en de leden van de begeleidingscommissie dank ik voor de ondersteuning.

Mijn naaste collega's Erik La Heij en Paul Janssen bedank ik voor de prettige en

uitstekende samenwerking. Paul Janssen bedank ik speciaal voor het uitvoeren van een

groot aantal experimenten en het vervaardigen van vele tekeningen die zijn opgenomen

in dit proefschrift.

Mijn waardering gaat ook uit naar de afstudeerders Paul Dohmen, Lotte Boon, Juul

Dzermans, Diederic van Dijke, Annemiek van derZandeen Moshe van Berlo, die alle

een essentiële bijdrage hebben geleverd aan het onderzoek.

De technici wil ik bedanken voor het ontwerpen en fabriceren van meetopstellingen.

Mijn kamergenoten, Gerben Mooiweer en Ton van der Zanden, bedank ik voor de

altijd prettige werksfeer.

De samenwerking met de vakgroep Colloïdchemie en Thermodynamica en speciaal

met de heer van Diemen heb ik altijd als zeer positief ervaren.

Mijn moeder en broer bedank ik tenslotte voor de morele ondersteuning in de

afgelopen jaren.

SUMMARY

Sewage sludge, which is a suspension with a dry solids content of 3 to 4 wt%,

originates from the purification of waste water. In the Netherlands the annual produc

tion of communal sewage sludge amounts to about 300 million kilograms of dry

solids. Before depositing the sludge, mechanica! dewatering is used. Chamber filter

presses, belt presses, and centrifuges are utilized to dewater sewage sludges. The

importance of mechanical dewatering will only increase. The deposit of sewage sludge

in agriculture and the preparation of black earth and compost will strongly decrease in

the near future to meet the requirements of the limitations of heavy roetal concentrati

ons and the allowed dosages. Moreover, one strives to reduce the deposition of

sewage sludge on tipping sites owing to insufficiently available space in the Nether

lands. The sewage sludge production will increase in the near future due to the growth

of the population, the higher degree of purification, and the introduetion of chemical

dephosphatation.

A better understanding of the mechanical dewatering process can lead to a better

preparation for these technological and social developments. A better insight into the

fundamental aspects of sewage sludge dewatering can be obtained by characterizing

sewage sludge. Characterization methods must be used which are of relevanee for a

better understanding of the mechanical dewatering behaviour.

In this thesis the determination of a great number of dewatering properties of sludge

and sludge cake determined with both existing and newly developed measuring

methods are discussed. Moreover, an attempt bas been made to interrelate the various

dewatering characteristics. Four different sludges originating from four different waste

water treatment plants have been studied.

Water is bound to sludge particles in different ways. Two methods were used to study

the solid-to-water bond strength: thermal analysis to determine isothermal drying

curves and the measuring of water vapour sorption isotherms at different temperatures.

The isotherms measured at different temperatures can be described very well with the

S-shaped temperature-dependent G.A.B. equation. With both methods it is possible to

determine the water bond enthalpy as a function of the sludge cake moisture content.

Both methods show that at decreasing moisture content the bond enthalpy is initially

zero ('free water') and differs significantly from zero at sample moisture contents

smaller than 0.3 to 0.6 kg water per kg dry solids ('bound water'). The sludge origin,

type of flocculant, and flocculant dosage do not appear to influence this critical

moisture content. On the basis of these measurements, one may conclude that the

maximum feasible dry solids content in a mechanica! dewatering process amounts to

about 65 to 75 wt%. Obviously, the amount of free water which has been entrapped

during filter cake formation dictates the attainable fina1 dry solids content in practice.

A great number of sludge dewatering characteristics at microscale and macroscale has

been determined. In order to determine these characteristics both existing and newly

developed measuring techniques have been used. Microproperties that have been

determined are composition (cluster properties like dry solids content, ash content, pH

and electrical conductivity), zeta potential, partiele size distribution, and rheological

properties. Macroproperties that have been determined are the specific cake resistance,

end dry solids content, porosity, permeability, capillary suction time, concentration

ferric ions and polyelectrolyte in the filtrate. A lot of dewatering characteristics of

sludge and sludge cake have been measured with a newly developed apparatus: the

filtration-expression cell. Micro- and macroproperties were determined as a function

of the flocculant dosage. The addition of flocculants improves the dewatering process.

Three types of flocculants per sludge type have been studied: the polyelectrolyte Röhm

KF975, the polyelectrolyte used at the sludge treatment plant, and ferric chloride in

combination with lime. It appears that the flocculant dosage has a large impact on the

dewaterability of sewage sludges. At the optimum flocculation condition (dosage and

mixing intensity) some characterization parameters show a maximum or a minimum:

minimum specific cake resistance, minimum vacuum suction time, minimum CST

value, minimum iron content in the filtrate, maximum dry solids content of the cake,

maximum permeability, maximum median floc diameter, and maximum degree of

thixotropy. At the optimum flocculation conditions, sewage sludges can ·be dewatered

at the highest rate and the highest dry solids content is reached.

The conventional Capillary Suction Time apparatus is often used in modem practice to

determine the sludge dewaterability. However, this apparatus has some disadvantages.

Filter paper, which is used as capillary medium, often differs in structure which

results in different porosities and permeabilities. These differences in filter paper lead

to a bad reproducibility of the measurements. Another disadvantage is the determinati

on of the position of the liquid front at only two times. Consequently, only little

information on the dynamic dewatering process can be obtained. A physical-mathema

tical model has been developed for the apparatus. With the model the position of the

liquid front can be calculated as a function of time. The influence of the various

process parameters on this relationship has been investigated. A modified CST

apparatus which registers the position of the liqoid front as a function of time has been

developed. Ceramics is osed as capillary medium. Ceramics is an isotropie materiaL

As a resolt the reprodocibility of the measorement was improved. The resolt of fitting

the model on the experimental data yields the specific cake resistance of the sludge

cake. Both flocculated and unflocculated slodges, can be studied. The specific cake

resistance is an intrinsic valoe, as opposed to the capillary soction time which on

among other things depends on the dry solids concentration of the suspension.

Before mechanical dewatering, sewage slodge is conditioned with flocculants.

Flocculants that are applied in practice are polyelectrolytes and ferric chloride in

combination with lime. Insight into the varioos destabilization mechanisms involved in

slodge flocculation may lead to a better fundamental onderstanding of the dewatering

process. An important parameter is the zeta potenrial which has been determined with

electroacoustophorese. The dominant floccolation mechanism indoeed by adding ferric

chloride to a slodge suspension is specific adsorption of mainly monovalent and

divalent positively charged hydrolysed ferric ions at active sites of the surface of

negatively charged slodge particles. Specific adsorption results in partiele charge

reversal from negative to positive. Typically, the charge reversal is sharp and

discontinoous. The discontinuity is attriboted to the presence of an undesired electroly

te in the suspension. Specific adsorption is energetically favoorable within a eertaio

pH range. Electroacoostophoresis seems not very suitable to study the mechanism of

flocculation indoeed by polyelectrolytes. The measuring probe cannot detect the

formed flocs. However, charge reversal was observed. The main floccolation mecha

nism indoeed by adding cationic polyelectrolytes is charge neotralization.

SAMENVATTING

Zuiveringsslib, bestaande uit een waterige suspensie met 3 à 4 gewichtsprocenten

drogestof, ontstaat bij de zuivering van afvalwater. De jaarlijkse produktie aan

communaal slib in Nederland bedraagt circa 300.000 ton op drogestofbasis. Alvorens

het slib af te zetten, vindt er mechanische ontwatering plaats. De mechanische

ontwatering gebeurt in Nederland met behulp van kamerfilterpersen, zeefbandpersen

en centrifuges. Het belang van de mechanische ontwatering zal naar verwachting

alleen maar toenemen. De afzet van slib in de landbouw en de mogelijkheden het slib

te verwerken tot compost of zwarte grond zal in de toekomst sterk verminderen ten

gevolge van de normen die er gesteld worden aan de toegestane verontreinigingen met

zware metalen enerzijds en aan de slibdosering anderzijds. Bovendien wordt vanwege

ruimtebeslag zoveel mogelijk gestreefd naar het reduceren van het te storten volume.

De produktie van zuiveringsslib zal onder andere door de groei van de bevolking, een

verdergaande zuiveringsgraad en de invoering van chemische defosfatering verder

toenemen. Door een beter begrip van het mechanisch ontwateringsproces van slib kan

op adequate wijze worden ingespeeld op deze maatschappelijke en technologische

ontwikkelingen. Meer inzicht in de fundamentele aspecten van het slibontwateringspro

ces kan worden verkregen door het zuiveringsslib te karakteriseren. Hierbij dienen

karakteriseringsmetboden te worden gebruikt die relevant worden geacht voor het

ontwateringsgedrag.

Dit proefschrift gaat in op het vastleggen van een groot aantal karakteriseringsparame

ters van slib en slibkoek die is bepaald met zowel bestaande als nieuw ontwikkelde

meetmethodieken. Tevens is getracht de verschillende parameters met elkaar in

verband te brengen. Er zijn vier verschillende slibben afkomstig van vier verschillende

rioolwaterzuiveringsinrichtingen onderzocht.

Water is op verschillende manieren gebonden aan slibdeeltjes. De slib-water binding is

onderzocht met twee methoden: thermische analyse voor de bepaling van isotherme

droogcurven en het meten van waterdamp-isothermen bij verschillende temperaturen.

De waterdamp-isothermen gemeten bij verschillende temperaturen blijken goed te

kunnen worden beschreven met de S-vorrnige temperatuurafhankelijke GAB vergelij

king. Met beide methoden is het mogelijk om de bindingsenthalpie als functie van het

vochtgehalte van een slibmonster te bepalen. Beide methoden tonen aan dat bij

afnemend vochtgehalte de bindingsenthalpie van slib/water aanvankelijk nul is ("vrij

water") om vervolgens sterk toe te nemen. bij vochtgehaltes lager dan ca. 0,3-0,6 kg

water per kg drogestof ("gebonden water"). De slibsoort, het flocculanttype en de

tlocculantdosering lijken geen invloed te hebben op dit kritieke vochtgehalte. Op grond

van deze metingen kan worden geconcludeerd dat in een mechanisch ontwateringspro

ces drogestofgehalten kunnen worden bereikt van 65 tot 75 gewichtsprocenten. In de

praktijk worden echter drogestofgehalten bereikt van 20 tot 30 gewichtsprocenten.

Blijkbaar is de hoeveelheid vrij water dat wordt ingesloten tijdens de filterkoekvor

ming ("interstitieel water") bepalend voor het te bereiken drogestofgehalte in de

praktijk.

Een groot aantal ontwateringskarakteristieken zowel op micro- als macroschaal is

bepaald van vier slibben. Voor het bepalen van deze karakteristieken zijn zowel

bestaande als nieuw ontwikkelde meetmethodieken gebruikt. Micro-eigenschappen die

zijn bepaald zijn samenstelling (clnstergrootheden zoals drogestofgehalte, gloeirest, pH

en electrische geleidbaarheid), zêta-potentiaal, deeltjesgrootteverdeling en reologische

eigenschaPpen. Ontwateringseigenschappen (macro-eigenschappen) die zijn gemeten

zijn de specifieke filtratieweerstand, einddrogestofgehalte, porositeit, permeabiliteit,

capillaire afzuigtijd (CST), concentratie ijzerionen en polyelectrolyt in het filtraat.

Vele ontwateringskarakteristieken van een slibkoek zijn bepaald met behulp van een

nieuw ontwikkeld meetapparaat: de filtratie-expressieceL De micro- en macroeigen

schappen zijn bepaald als functie van de dosering flocculant. Toevoeging van floccu

lanten heeft tot gevolg dat het ontwateringsproces sterk wordt verbeterd. Er zijn drie

typen flocculant per slibsoort getest: het polyelectrolyt Röhm KF975, het polyelectro

lyt dat wordt gebruikt bij de betreffende slibverwerkingsinstallatie en ijzerchloride in

combinatie met kalk. Het blijkt dat de flocculantdoseriitg een grote invloed heeft op de

ontwaterbaarbeid van zuiveringsslib. Er blijkt een optimale flocculatieconditie

(dosering en mengintensiteit) te bestaan waarbij een aantal parameters een maximum

of minimum vertoont: minimale specifieke filtratieweerstand, minimale afzuigtijd,

minimale CST waarde, minimale ijzerionenconcentratie in het filtraat, maximale

einddrogestofgehalte, maximale permeabiliteit, maximale mediaan vlokdiameter en

maximale thixotropie. Bij de optimale flocculatiecondities wordt het slib met de

hoogste snelheid ontwaterd en wordt het hoogste drogestofgehalte bereikt.

Het conventionele capillaire afzuigtijd apparaat wordt veel toegepast in de huidige

praktijk voor de bepaling van slibontwateringseigenschappen. Echter dit apparaat kent

enkele nadelen. Het filtreerpapier, dat gebruikt wordt als capillair medium, verschilt

vaak in structuur hetgeen resulteert in een verschillende porositeit eu permeabiliteit.

Dit verschil in structuureigenschappen leidt tot een slechte reproduceerbaarbeid van de

metingen. Een ander nadeel is dat de positie van het vloeistoffront slechts wordt

gemeten op twee verschillende tijdstippen, waardoor er slechts geringe informatie

wordt verkregen over het dynamische ontwateringsproces. Er is een fysisch-mathema

tisch model ontwikkeld voor het CST -apparaat. Met dit model kan de positie van het

vloeistoffront worden berekend als functie van de tijd. De invloed van de verschillende

procesparameters op deze relatie kan worden onderzocht. Een gemodificeerd eST

apparaat, waarin de positie van het vloeistoffront als functie van de tijd wordt

geregistreerd, is ontwikkeld. Als capillair medium is keramiek gebruikt. Het gebruikte

keramiek is een isotroop materiaal, hetgeen de reproduceerbaarbeid van de metingen

verbetert. Door het model toe te passen op het experimentele resultaat kan de

specifieke filtratieweerstand van de slibkoek worden berekend. Zowel geflocculeerde

als ongeflocculeerde monsters kunnen worden onderzocht De specifieke filtratieweer

stand is een intrinsieke ontwateringskarakteristiek, dit in tegenstelling tot de capillaire

afzuigtijd die afhangt van onder andere de drogestofconcentratie in de snspensie.

Vóór de mechanische ontwatering wordt het slib geconditioneerd met behulp van

flocculanten. Flocculanten die worden toegepast in de praktijk zijn ijzerchloride in

combinatie met kalk en polyelectrolyten. Inzicht in de verschillende mechanismen die

een rol spelen bij de flocculatie van zuiveringsslib leidt mede tot een beter fundamen

teel begrip van het slibontwateringsproces. Een belangrijke parameter hierbij is de

zêta-potentiaal, die bepaald is met behulp van electroakoestoforese. Het dominante

flocculatiemechanisme geïnduceerd door toevoeging van ijzerchloride aan een

slibsuspensie is specifieke adsorptie van voornamelijk monovalent en divalent positief

geladen ijzerhydroxydecomplexen aan actieve plaatsen op het oppervlak van de

negatief geladen slibdeeltjes. Specifieke adsorptie leidt tot ladingsomkeer van de

slibdeeltjes van negatief naar positief. Uit het verloop van de zêta-potentiaal als functie

van de pH van de slibsuspensie blijkt dat de ladingsomkeer scherp en discontinu is en

plaatsvindt bij een pH gelijk aan 6, het iso-electrisch punt van het systeem slib/ijzer

chloride. De discontinuïteit is te wijten aan de aanwezigheid van een ongewenst

electrolyt in de suspensie. Specifieke adsorptie is energetisch gunstig binnen een

bepaald pH-gebied. Electroakoestoforese lijkt niet erg geschikt voor de bestodering

van het flocculatiemechanisme geïnduceerd door polyelectrolyten. De meetsonde kan

de gevormde vlokken niet goed detecteren. Ladingsomkeer is echter wel waarge

nomen. Het belangrijkste flocculatiemechanisme geïnduceerd door toevoeging van

katione polyelectrolyten is ladingsneutralisatie.

CONTENTS

1 INTRODUCTION

2 WASTE WATER PROCESSING 7

2.1 Introduetion 7

2.2 The activated sludge process 8

2.3 Sludge stabilization 10

2.4 Thickening 12

2.5 Mechanica! dewatering 13

2.6 Description of various types of sewage s1udge 17

2.7 History and origin of studges investigated 19

2. 7.1 The Eindhoven waste water treatment plant 19

2.7.2 Waste water treatment plant 'Amsterdam-Oost' 20

2. 7.3 Oxidation ditch system 'Veghel-Uden' 21

2.7.4 The oxidation ditch system 'De Hooge en Lage Zwaluwe' 22

3 WATER BINDING IN SEW AGE SLUDGE 25

3.1 Introduetion 25

3.2 The presence of water in sewage sludge 26

3.3 Isothermal drying curves (TGA/DTA) 27

3.3.1 The TGA-DTA drying model 29

3.3 .2 Evaporation of pure water and the calibration of the DT A probe 31

3.3.3 Isothermal drying of a sludge cake 35

3.3.4 Experimental results 37

3.4 Water vapour sorption isotherms 41

3 .4. 1 Introduetion 41

3 .4. 2 Sorption roodels 43

3 .4. 3 Metbod of saturated salt solutions 46

3.4.4 Coulter Omnisorp 100 54

3.5 Conclusions 59

4 SLUDGE DEW A TERING CHARACTERISTICS 61

4.1 Introduetion 61

4.2 Plan of cbaracterization research 62

4.3 Composition 63

4.4 Piltration and expression 66

4.4.1 The filtration-expression cell 67

4.4.2 Modified Piltration Test 75

4.4.3 Conventional CST apparatus 78

4.4.4 The compression-permeability cell 80

4.5 Amount of iron in filtrate 84

4.6 Polyelectrolyte concentration in filtrate 86

4.7 Partiele size distribution 87

4.8 Rheological properties 95

4.9 Conelusions 100

5 MODIFIED CAPILLARY SUCTION TIME (CST) APPARATUS 103

5.1 Introduetion 103

5.2 A theoretical model descrihing the liquid flow in a CST apparatus 104

5.3 Parameter studies 107

5.4 Modified CST apparatus 110

5.5 Experimental results and discussion 113

5.6 Conelusions 119

6 FLOCCULATION BEHA VIOUR OF SEWAGE SLUDGE 121

6.1 Introduetion 121

6.2 The electrical double layer around a spherical sludge partiele 122

6.3 Colloidal stability in terms ofthe electrical double layer 128

6.4 Specific adsorption flocculation by metal coagulants 131

6.5 Polymerie adsorption flocculation 133

6.6 Experimental technique to determine the ESA signal and zeta

potential 137

6.7 Results and discussion 142

6.8 Model to describe hydrolyzable metal ion adsorption at the sludge

solid-water interface 154

6.9 Conelusions 159

7 CONCLUDING REMARKS AND PERSPECTIVES 161

NOTATION 165

LITERATURE 173

Appendix 1 : Process schemes and/or maps of the waste water and sludge 185

treatment plants

Appendix 2: Output ofMAPLE program 191

Appendix 3 : W orking scheme of sludge characterization 193

Appendix 4 : Shift of the absorption maximum of cobaltphtalocyanine due to 195

increasing added amounts of polyelectrolyte

Chapter 5 has been publisbed in Ind. Eng. Chem. Res., vol. 34, no. 4, pp 1310-1319,

1995.

1 INTRODUCTION

Sewage sludge dewatering is an important step in waste water treatment. Sewage

Slûdge is a rest product of the waste water treatment process, and consists of settleable

solids with the addition of the waste activated sludge generated in the biologica!

treatment stage.

In 1991, about 330 million kilograms of sewage sludge dry solids were produced in

the Netherlands. 25 percent of the total sludge solids removed was used as fertilizer in

agriculture, 20 percent in the preparation of compost and black earth, 51 percent for

landfills and 4 percent was incinerated [CBS, 1993]. In the near future, an increase of

the sludge production in the Netherlands is to be expected due to the increase in

population in the Netherlands, the completion of new waste water treatment plants,

and the introduetion of dephosphatation in 1995.

At the beginning of the seventies undewatered sewage sludge was nsed as a useful

fertilizer in agriculture. Due to the surplus of animal fertilizers and the more severe

legislation on the use of sewage sludge as fertilizer in agriculture at the end of the

seventies, mechanica! dewatering became more important. Nowadays, before its

ultimate disposal a large part of the total sewage sludge produced in the Netherlands is

dewatered. Different dewatering methods are used: mechanical dewatering by belt

presses, chamber filter presses and centrifuges (see section 2.5), and natural dewa

tering in lagoons, buffers and drying beds. The amounts of sewage sludge dewatered

mechanically and naturally in 1991 were 196 million kg and 11 million kg of dry

solids, respectively. The remaining part of the total sewage sludge produced (122

miltion kg of dry solids) was not dewatered at the waste water treatment plant [CBS,

1993].

Disposal of (dewatered) sewage sludge in agriculture and in the preparation of

compost and black earth have to meet the requirements of the Dutch Ministries of

Agriculture & Fisheries, and for Housing, Regionat Development, and the Environ

ment (V.R.O.M. 1). This implies on the one hand limitation of the heavy metal

concentrations in sewage sludge, and restrietion of the sludge dosage per hectare on

the other. The most occurring heavy metals which are present in sewage sludge are

copper, chromium, zinc, lead, cadmium, nickel, mercury and arsenic. The maximum

sewage sludge dosage on arabie land is equal to 2 tons of dry solids per hectare per

1 V.R.O.M. Volkshuisvesting Ruimtelijke Ordening en Milieubeheer

2 Cbapter 1

year. The maximum dosage on grassland is restricted to 1 ton of dry solids per hectare

per year. Disposal of sewage sludge in the North Sea is forbidden.

The amount of sludge disposed on tipping sites will be restricted before long. A dry

solids content of 40 wt% is required to dump sewage sludge on tips. Deposition of

sewage sludge on dumping sites bas to be restricted due to insufficiently available

space in the Netherlands, and the presence of heavy metals in sewage sludge.

The operaring costs of sewage treatment in 1991 amounted to 887 million guilders for

the plants and 229 million guilders for the transport systems [CBS, 1993]. Sewage

sludge handling is responsible for 20 to 50% of the total operaring costs. Sludge

disposal costs amount to 15% of the total running costs.

The current policy is to strive for 1) better controlling the mechanica! dewatering

process, 2) reducing volume and mass of sewage sludge and/or 3) more rapid

dewatering in smaller equipment, and 4) lowering annual costs of the total sludge

processing. Because of the increasing urbanization in the Netherlands, smaller waste

water treatment plants are required. As a consequence, new technologies (e.g. smaller

dewatering systems) have to be developed.

Sludge volume rednetion can be reached in different ways :

1. Rednetion of the production of sewage sludge [Eikelboom, 1993].

2. Impravement of the dewatering characteristics of existing dewatering techniques.

The study presented bere is focused on the second option. An important dewatering

parameter is the dry solids content of the sludge. The basic aim is to achleve higher

dry solids content of the dewatered sludges. Removal of water is accompanied with

volume reduction. In tigure 1.1 the sludge volume is given as a function of the dry

solids fraction for two different initial dry solids contents (2 and 5 wt%). The sludge

volume is inversely proportional to the dry solids content. An increase of the dry

solids fraction from 0.02 to 0.20 reduces the sludge volume by a factor of 10. Another

consequence of increasing the dry solids content of the dewatered sludge is the

rednetion of the energy needed for incineration. Above a dry solids content of 30 to

35 wt% the combustion process is self-sustaining and does not need additional fuel.

Incineration is used as post-treatment after mechanical dewatering. In 1991, only 4%

of the total sewage sludge produced (12 million kg of dry solids) was incinerated.

However, in the near future three big incineration plants for sewage sludge will be in

operation for about 35 % of the total sludge production in the Netherlands [Mars

kamp, 1993].

Introduetion 3

100

90

80

~ 70 ._,

~ 60

50

<I) 40 bi)

] 30 ... 20

10

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

dry solid.s fraction (-)

2% S%

Fig. 1.1 Sludge volume as a julletion of dry solids fraction for two initial dry solids contents

[Koot, 1974/.

The mecbanical dewatering of sludges by filter presses, belt presses and centrifuges

appears to be a unit operation wbicb is very poorly understood. The main reason for

tbis is the complexity of the sludge material. The cbemical composition of the sludge

material is caused by the biomass (bacteria, protozoa, fungi) and by organic and inor

ganic materials. External factors, sucb as the sewer system, weather conditions, and

climatological circumstances, also influence its composition.

Moreover, in a flitration process the sludge cake formed appears to be bigbly

deformable as welt. This means that when mechanical forces are exerted on the filter

cake a lot of water remains entrapped within the cake. lt is believed that tbis mecha

nism is to a high extent responsible for the attainable final dry solids content in a

mechanical dewatering process.

In the period 1979-1983 the 'Foundation for Applied Waste Water Research' (Dutcb

abbreviation: STOW A) has carried out a stndy on mechanica! dewatering of sewage

sludges. The study included a literature survey on the nature of the solid/water bond

strength in sewage, sludge properties, and dewatering results of mechanical dewatering

4 Cbapter 1

equipment. Moreover, studies on optimization of sewage sludge conditioning and the

operation of cbamber filter presses and belt presses were carried out. The results have

been presented in eight reports [STOW A, 1979 .. 1983]. The studies were principally

empirical by nature and much progress in fundamentally understanding the mechanica!

dewatering process could not be expected. A better fundamental understanding of the

sewage sludge dewatering process can help to improve the dewatering characteristics

(high dry solids contentand more rapid dewatering) of existing techniques.

The aim of the Dutch research program 'Future treatment technique for municipal

waste water (RWZI 2000)' is to develop new technologies for waste water treatment.

This research program is a co-operation between the 'Foundation for Applied Waste

Water Research' and the 'lnstitute of Inland Water Management and Waste Water

Treatment' (Dutch abbreviation: RIZA). Various research projects were initiated by

RWZI 2000 with respect to purification of waste water and sewage sludge processing,

such as fundamental aspects of sewage sludge dewatering, anaerobic prepurification,

magnetic dephospbatation, rednetion of sewage sludge production, and the Vertech wet

ox.idation process.

A research project entitled 'Fundamental aspects of sewage sludge dewatering' was

carried out at the Laboratory of Separation Technology of the Eindhoven University of

Technology in the period 1990-1994 [Herwijn et al., 1994]. Before the start of this

research project, the state of the art of mecbanical dewatering was investigated [van

Dijck et al., 1989]. Based on this preliminary study a proposal for a research project

'Fundamental aspects of sewage sludge dewatering' was written [Coumans and

Kerkhof, 1989] and accepted.

The global objectives of this research project were:

l. Characterization of sewage sludges. This implied determining sewage sJudge

properties which are expected to be of relevanee for a better understanding of

mecbanical dewatering processes. Characterization methods had to be developed

and tested on the applicability to diagnose and optimize the mecbanical dewatering

process. With suitable characterization methods the attainable dry solids content of

sewage sludge under certain process conditions can be determined.

The result of this study is presenled in this thesis.

3. To develop a physical-mathematical model for the description of the solid-liquid

separation of sewage sludge. The dewatering bebaviour of sewage sludge depends

not only on the sludge properties but also on the way in which the solid-liquid

separation process is performed. In practice the dewatering capacity of mecbanical

Introduetion 5

dewatering equipment is often smaller than the design capacity. Also, the dewa

tering results may fluctnate. Models which describe the operation of dewatering

equipment as a function of process and machine parameters can help to solve these

problems. A physical-mathematical model for the description of the dewatering

behaviour of sewage sludge is a basis for equipment models. The result of this

work was presented in another thesis [La Heij, 1994].

As a matter of course, both studies can not be considered as two different parts, but

are si:rongly interrelated.

Contents of this thesis

In this thesis various sewage sludge characteristics which are of relevanee for a better

understanding of the mechanica! dewatering process will be discussed.

In chapter 2 the various types of waste water processing and mechanica! dewatering

equipment will be dealt with. Emphasis is laid on four waste water treatment plants in

the Netherlands. Sewage sludges from these plants were characterized in this research.

In chapter 3 the solid/water bond strength in sewage sludge is discussed. The

solid/water bond enthalpy as a function of the water content is a fundamental charac

teristic which enables the prediction of the theoretically maximum dry solids content in

a certain dewatering process. The measuring methods used to deterrnine the bond

enthalpy are theemal analysis teclmiques and water vapour sorption isotherrns. In

chapter 4 various sludge dewatering characteristics and their interrelationships are

discussed. Macroproperties (e.g. specitic cake resistance, Capillary Suction Time,

permeability) as well as floc microproperties (e.g. composition, partiele size distribu

tion, rheological properties) are deterrnined in well-defined laboratory tests. Newly

developed characterization tests and tests used in previous studies are evaluated. The

sludge properties are investigated as a function of the flocculant dosage. In chapter 5 a

modified 'Capillary Suction Time (CST)' apparatus is discussed. The conventional

CST instrument provides an indication of the dewatering rate of sewage sludges. The

modified CST apparatus enables to predict the average specific cake resistance of

flocculated as well as non-flocculated sludges. In order to promote the dewatering

behaviour of sewage studges flocculants (iron chloride/time and polyelectrolytes) are

added. In chapter 6 the results of a preliminary study of the destabilization phenomena

occurring in sludge flocculation are presented. The zeta potential of the sludge

suspension is hereby a characteristic parameter. On the basis of the measurements a

model is developed to describe the hydrolysed metal ion adsorption at the sludge solid surface.

6 Chapter 1

Finally, in chapter 7 the main conclusions and perspectives for future research are

presented.

2 WASTE WATER PROCESSING

2.1 Introduetion

One of the major steps in waste water treatment is the removal of dissolved and

undissolved solids that otherwise might damage the effluent quality, and subsequently

the concentration of removed solids into a much smaller vohune for ease of handling

and disposal. The waste suspension generated in waste water treatment processes is

generally referred to as sewage sludge and exists in many forms and quantities. The

amount and quality of sludge depend on the origin of waste water, type of treatment

plant, and on the metbod of plant operation. Waste water entering the sewer system

originates from householcts (domestic sewage), andJor from industries (industrial

sewage). In the past, various methods were developed to remove organic matter from

sewage: sedimentation in tanks, the trickling filter and the activared sludge process.

The earliest metbod of waste water treatment was sedimentation in septic tanks.

Sedimentation of municipal waste water has a limited effectiveness, since only a part

of the waste organics is settleable. The sedimentation tank is a primary treatment in

which no deliberate attempt is made to remove oxygen-demanding materials.

The trickling filter, also called biological bed, was the first major breakthrough in

secondary waste water treatment. A secondary treatment plant was designed to remove

solids and to rednee biologica! oxygen demand (BOD). BOD is, by defmition, the

quantity of oxygen utilized during a certain time at a certain temperature by a mixed

population of micro-organisms in the aerobic oxidation process, and is a measure of

the biodegradable organic content of waste water. The oxygen demand at a tempera

ture of 20 oe during a period of 5 days is used as a standard, and abbreviated as

B0~0•

The trickling filter process occurring in a biologica! bed is based on slow movement

of waste water through a bed of rocks covered by biological mass, and results in rapid

rednetion of organic matter. Excess microbial growth is removed from the filter

effluent by a final clarifier. The waste sludge of the final clarifier is called filter

humus. In the Netherlands biologica! beds are still used for waste water purification

(total capacity in 1991: 1.638·106 inbabitant equivalents).

Another secondary treatment is the activated sludge process, which is the most

occurring waste water process in the Netherlands (total capacity in 1991: 1.7263·107

inbabitant equivalents [CBS, 1993]. The inbabitant equivalent is defined as the oxygen

8 Chapter 2

demand of sewage discharged by one persou in one day (see also section 2.7).

2.2 The activated sludge process

A major advancement in biological treatment took place when it was observed that

biological solids, developed in polluted water, flocculated organic solids. These

masses of micro-organisms (mainly bacteria), referred to as activated sludge, rapidly

metabolize pollutants from a solution and can subsequently be removed by gravity

settling.

In tigure 2.1 a typical process scheme of municipal waste water treatment is given.

The influent is stored in a buffer tank (1) to intercept high sewage flows and is

screened to separate large solids (2). The grit chamber (3) protects the mechanical

equipment and sludge pumps against abrasive wear. In the primary settling tank (4),

settleable solids are removed. The solids withdrawu from the bottorn of this tank by a

seraper are knowu as primary sludge (9). Primary settling produces a sludge of coarse

organic and inorganic solids. The organic part is actively decomposed by bacteria and

diffuses offensive odours.

influent--.-i2J:1/ i /

/ 2 1

7 Sludge processing

4

Fig. 2.1 Process scheme of a typical activated sludge processjor municipal waste water

treatment. l=bujfer tank, 2=screen, 3=grit chamber, 4=primary settling tank, 5=aeration

tank, 6=final settling tank, 7=sludge processing, 8=grit, 9=primary sludge, IO=secondary

sludge.

Waste water processing 9

Primary sludges thicken and dewater readily because of their fibrous and coarse

nature. Solids concentrations in primary sludge are in the 4 to 6 percent range.

The basic cbaracteristic of the whole system is the use of a mixed bacterial culture for

the conversion of pollutants. Conversion of organic materials to oxidized end products

(C02 and H20) takes place in the aeration tank (5). Bacteria metabolize dissolved

waste solids, produce new growth while taking in dissolved oxygen and releasing

carbon dioxide and water. The organic material serves not only as an energy source,

but also as a carbon souree for cell synthesis. In order to meet the required oxygen

demand, air is driven into the aeration tank by porous diffusers, by surface aerators,

or by some other means, such as brushes or aspirators.

The biomass produced in the aeration tank must be settled out in the secondary or

final settling tank (6), and partially recycled to the head of the aeration tank. The

excess of biomass mnst be wasted. This biological waste material is known as waste

activaled sludge or secondary sludge (10). Waste-activated sludge from the aeration

tank consists of flocculated microbial growths with entrained nonbiodegradable,

noncolloidal, and colloidal solids. It is relatively odour-free because of biological

oxidation, but the dispersed particles make it difficult to dewater. Secondary biological

sludge from aeration processes is less concentrated in solids than primary sludge.

So in activated sludge systems a part of the activated sludge is recycled and the other

part is withdrawn from the final settling tank. The primary and/or secondary sludge

must be thickened and/or mecbankally dewatered (7) before ulrimate disposal. The

purified efflnent is discbarged into surface waters.

Complete mixing aeration without primary sedimentation is popular for treatment of

small waste water flows. This type of waste water treatment, named oxidation ditch

[Pasveer, 1957], is often used to serve a town with a population of several thousands.

Typical ditierences between the normal activated sludge plant and the oxidation ditch

are:

The grit chamber and primary settling tank are missing in an oxidation ditch

system.

The greater size of the aeration basin in an oxidation ditch system. The purifica

tion system consists of a large ring-shaped aeration basin in wbich the liquor is

circulated and aerated by rotaring brushes. The sludge needs sufficient time to take

up oxygen for stabilization (see section 2.3).

The organic removal rate in an oxidation ditch is relatively small compared with

the regular activated sludge process. The organic removal rate is expressed in

10 Chapter 2

units of kg BOD per kg dry solids (ds) per day. Typical value of the organic

removal rate in an meidation ditch is 0.05 kg BOD/kg ds per day,whereas typical

values for an activated sludge plant are in the range of 0.1 to 0.2 kg BOD/kg ds

per day.

When a shortage of biological food is present in a system, bacteria liquidate

themselves and organic cells are oxidized. This process is called mineralization

and is typical for an oxidation ditch. In the case of an abondance of nutrients,

bacteria only metabolize a part of the dissolved and undissolved solids applied.

This is a typical characteristic for a high-loaded activated sludge plant. As a

consequence, the amount of surplus sludge produced is relatively higher in a

regular activated sludge system.

Elimination of primary settling dramatically affects the character of waste sludge. The

dry solids content of oxidation sludge is in the 0.5 to 2 percent range.

In tigure 2.2 a scheme of the successive steps in sludge processing is presented.

2.3 Sludge stabilization

Sewage sludge is a good growth substrate for bacteria and therefore putrefies very

rapidly, usually requiring stabilization prior to further use or disposal. The content of

organic matter must be reduced such that intensive putrefaction processes are no

longer able to proceed. Sludge stabilization may be of a biological, chemical or

thermal nature. In this section only biological sludge stabilization is dealt with.

Biological sludge stabilization can take place in the presence of oxygen (aerobic

stabilization) or in the absence of oxygen (anaerobic stabilization or digestion). More

than half (52 %) of the total sludge produced in the Netherlands in 1991 was

anaerobically digested, 37 % was aerobically stabilized, and the remainder was not

stabilized [CBS, 1993].

Allaerobic stabilization

In the conventional activated sludge plant anaerobic processes are generally used for

the purpose of sludge stabilization. The bacterial process consists of two successive

processes · that occur simultaneously in digesting sludge. In the fust stage large

components are broken down and converted into organic acids. This step is performed

by a variety of bacteria operating in an environment devoid of oxygen. In the second

stage, gasification is needed to convert the organic acids into 65 wt% methane, 35

purified water

Waste water processing

polluted

water

activated sludge process

primary

sludge

sludge stabilization

aerobic or anaerobic

thickening

mechanica! dewatering

incineration

secondary sludge

Fig. 2.2 Scheme of the consecutive steps in sludge processing.

11

12 Chapter 2

wt% carbondioxide (biogas), and trace amounts of nitrogen, hydrogen, and hydrogen

sulfide. Methane-forming bacteria are strictly anaerobic and are very sensitive to the

environmental conditions of pH and temperature. Digesters operate normally at 30 to

35 oe. The biogas produced in anaerobic digestion is an excellent fuel and is almost

always used to heat the digester tank, and/or buildings situated on the waste water

treatment plant.

Other advantages of the anaerobic stabilization process are:

Rednetion of the dry solids mass.

Increase of ash content due to conversion of organic matter. As a consequence

less odour probieros and a better dewaterability (less compressible cake) are to be

expected.

A disadvantage of anaerobic stabilization is the rednetion of the sludge heat content,

which might adversely affect a possible incineration process.

Aerobic stabilization

Aerobic stabilization may occur simultaneously in the aeration tank, or separately in

an additional tank. In small sewage works (e.g. the oxidation ditch system), simulta

neons aerobic sludge stabilization is applied. An important parameter in this stabiliz

ation process is the required sludge retention time needed for excessive mineralization.

The sludge retention time is strongly dependent on temperature. The organic removal

rate in such systems is relatively low.

In the separate aerobic sludge stabilization process, sludge undergoes aeration in a

separate tank, and the organic materials are decomposed by aerobic roetabolie pro

cesses. The organisms only experience an abundant nutrient supply during the initial

stages, and over a period 5 to 10 day period they reach the condition of stable,

underfed sludge.

2.4 Thickening

Thickening may be used as the first step in sludge processing for volrune reduction.

Different methods are utilized to thicken sewage sludges:

1. Thickening in separate tanks. Figure 2.3 illustrates a typical gravity thickener.

Sludge flow enters from behind an inlet well in the centre of the tank and is

directed downwards. The supernatant overflows a peripheral weir, while an

underflow of thickened sludge is drawn down from a bottorn sump in the tank.


Piekets attached to the collector arms stir through the sludge providing cavities for

the release of entrapped water.

2. Flotation thickeners. Waste sludge enters the bottorn of the flotation tank, where it

is merged with recirculated flow containing compressed air. Dissolved air flotation

is achieved by releasing fme air bubbles that attach to sludge particles and cause

them to float. The overflow, discharged by a mechanica! skimming device, is the

thickened sludge. Dry solids concentrations of between 3 and 7 percent are

reached.

3. Mechanica! thickening by centrifuges. Centrifuges used for sludge thickening

produce a solids concentration normally varying between 4 and 12 percent. In the

next section a description is given of the working principle of this dewatering

device.

4. Lagoons. Sedimentarlon in lagoons is a natura! dewatering process. Both the sun

and the wind influence the dewatering result. This metbod requires a large surface

area. Soil and ground water polintion are disadvantages of natura! dewatering.

Fig. 2.3 Schematic drawing qf a gravity thickener.

2.5 Mechanica! dewatering

In the Netherlands various methods are used to dewater sewage sludges. In table 2.1

14 Chapter 2

an overview is given of the distinct dewatering techniques used, and their typical

dewatering results achieved in 1991.

Table 2.1 Overview of dewatering results gained in 199l[CBS, 1993].

Dry solids Volume (1 03 m3) Dry solids fracnon

mass (106 kg) (%)

in out in out ===

Drying beds 4.4 127 17 3.5 31.6

Lagoons 6.7 222 69 4.0 20.9

Belt presses 112.9 3515 553 3.5 21.3

Filter presses i 65.1 1957 253 4.0 34.1

Centrifuges 17.9 544 104 3.5 21.0

Incineranon 10.4 64 4 99.9

Sewage sludge dewatering in filter presses and belt presses is performed by means of

filtranon and expression (see secnon 4.4). In filtranon and expression the water moves

relanve to the solids under the influence of a liquid pressure gradient. Solids are

deposited in tbe form of a cake on tbe up-stream side of a filter medium while tbe

clear liquid passes tbrough. A porous plate, filter paper, or textile fabric acts as a

filter medium. In order to improve tbe mechanica! dewatering behaviour of sewage

sludge, tlocculants are added to sewage sludge before dewatering.

The chamber filter press, a batchwise operaring filter, is an important dewatering

technique in tbe Netberlands. Figure 2.4 shows tbe basic layout of a chamber filter

press. The basic unit is constructed of a sequence of plates and frames mounted on

suitable supports, e.g. a pair of rails. The hollow frame is separated from the plate by

a filter clotb to create a series of clotbwalled ebarobers into which slurry can be forced

under pressure. The plates and frames are held togetber by hydraulic pressure, or by

means of a screw. Two cakes are formed simultaneously in each cbarober. The liquid

passes through tbe cloth, runs down the corrugated surface of tbe plates, and is

discharged at drain points. When the two cakes join, tbe ebarobers are full of cake and

the dewatering process is stopped. Subsequently tbe ebarobers are opened and tbe cake

is removed.

Slurry In let


Frame Plate

• ~w I I I 1 1 1

I ~~ i 1 l 1

I I I w ~--! ! !1

I I I I I l I t I I I i I i I I I I 1 1 1 1

I I

~'-."'-L-"'.J./-bw/%1

Flitrata outleta

Fig. 2.4 Sche1111ltic diagram of a chamher filter press.

15

In the course of the dewatering process the mechanica! pressure increases until a

maximum of about 15 bars is reached. Cake depths are commonly up to 50 mm. The

dewatering time takes about several hours. In order to promote the mechanica!

dewatering behaviour of sewage sludge, flocculants are added to sewage sludge before

dewatering. The type of flocculants usually used in chamber filter presses are iron

chloride in combination with time.

The belt press is a continuously operated machine and is nowadays the most applied

dewatering technique in the Netherlands (see table 2.1). The operating principle is

based on mechanically squeezing sewage sludge cake between two beits or filter

fabrics (see figure 2.5). The feeder delivers conditioned sludge to the porous lower

moving belt. The sludge successively passes the 'pre-dewatering zone', the 'pressing

zone', and the 'friction zone'. In the 'pre-dewatering zone', water is only removed by

gravitational force. The two moving beits apply pressure on the sludge cake in the

'pressing zone'. Further squeezing of the cake is applied in the 'friction zone', where

16 Chapter 2

shear stresses are exerted on the cake. The cake structure is disrupted and in this way

more dewatering is achieved. The cake is finally discharged and filtrate passing

through the lower filter belt is collected. The type of flocculants used to dewater

sewage studges in belt presses are organic polymers, also called polyelectrolytes

(p.e.). Compared to the chamber filter press, the dewatering results of belt presses are

worse (see table 2.1). This is due tothesmaller residence times of sludge particles in

belt presses (8 to 10 minutes), and the excess amount of dry solids (flocculants) added

prior to the dewatering in chamber filter presses.

pre dewaterlng zone

pressing zone

Fig. 2.5 Schematic diagram of the belt press.

friction zone

In the Netherlands centrifuges are getting more popular to dewater sewage sludges.

Centrifugal sedimentation is based on a density difference between solids and liquids.

The particles are subjected to centrifugal forces, which makes them to move radially

through the liquid either outwards or inwards, depending on whether they are heavier

or lighter than the liquid.

In practice, the scroll-type centrifuge (also called decanter centrifuge) is utilized to

dewater sewage sludges. lts operaring principle is shown in tigure 2.6. A charac

teristic feature of the scroll-type centrifuge is the horizontal conical bowl, containing a

screw conveyor that rotates in the same direction but at a slightly higher or lower

speed.

The sludge suspension enters through an axial stationary feed pipe at the centre of the

rotor and passes through the distributor into the rotaring bowl. On their way from the

entrance to the cylindrical end of the bowl the solids are separated from the carrier

liquid by the centrifugal force. Deposited solids are moved by the screw conveyor

towards the conical end of the bowl and are discharged. The supernatant is freely

discharged over a ring dam at the other end. Polyelectrolytes are used to flocculate

Waste water 17

sewage sludges to be dewatered in centrifuges. A major advantage of this type of

centrifuge is the operational flexibility. Bowl speed affects centrifugal forces on the

settling particles, and the difference between bowl speed and conveyor speed controls

the solids retention time.

Overflow

Cake

Fig. 2.6 Schematic diagram of the scrolt-type centrifuge.

2.6 Description of various types of sewage sludge

~ tinderflow (solids)

Feed

Depending on their point of origin and nature of treatment, different types of sewage

sludge may be distinguished. In the previous sections, several types of sludge have

already been mentioned. In this section, a general overview of sludge types occurriug

in the activated sludge process will be given.

1. Primary sludge.

Primary sludge is separated from the incoming sewage in the primary sedimen

tation stage. lts composition is made up of the settleable solids contained in the

municipal raw sewage. It excludes those materials collected on the sereens and the

grit separated out in the grit chamber. It undergoes thickening in the primary

settling tank to a solids concentration of 4 to 6 wt%. Primary sludge is of a

course, non-homogeneaus and variabie nature.

2. Activated sludge, (secondary sludge).

The activated sludge or sludge biomass present in the aeration tank is the natural

vehicle for the biological treatment process. Within its biomass the enzymatic

power is manifested which is required for the conversion of high-molecular sub

stances into the desired inorganic endproducts of treatment, or for incorporation

into the cell biomass. When sewage is aerated, the micro-organisms form colonies

(activated sludge flocs) and function as adsorptive surfaces to which biologically

18 Chapter 2

inert matcrials may be attached. The chemica! composition of activated sludge is

determined by the biomass itself (bacteria, protozoa, fungi), and by organic and

inorganic matcrials which are present either as deposits or inclusions. The

proportion of the individual constituents is dependent on the sludge loading (high

sludge loading=high organic fraction), the nature of the sewage, and the effi

ciency of the primary settling. In plants without prior settling (e.g. mddation

ditches) the activated sludge will contain high proportions of inorganic matter.

The combination of an aerated reaction chamber and the succeeding fiual settling

tank with parrial recycling ofthe separated biomass (recycled sludge) is the basis

of the activated sludge process. Separation and recycling of the sludge biomass

enable a high biomass concentration in the aeration tank. Sludge thickened and

separated from the final sedimentation stage is called secondary sludge, surplus

sludge, or waste activated sludge. Sludge flocs are required to settie rapidly with

no undesirable metabolic processes taking place, i.e. the flocs must be compact,

heavy, and inactive. The tenns •activated sludge', 'secondary sludge', 'surplus

sludge', 'waste activated sludge' refer to physically identical sludges.

Sludge withdrawn from the settling tank in the oxidation ditch system is called

oxidation ditch sludge.

4. Stabilized sludge.

Stabilized sludge refers to sludge which is treated such that putrefaction processes

do no longer proceed and offensive odours are no longer given off. During

biologica! sludge stabilization, the organic fraction is reduced by controlled

metabolic reaction processes until the desired degree of stabilization is achieved.

Two types of biologica! stabilization processes may be distinguished: aerobic and

anaerobic stabilization (see section 2.3). Sludge produced in the anaerobic

stabilization process (digestion) is called digested sludge.

5. Conditioned sludge.

Prior to mechanica! dewatering sewage sludge is conditioned by flocculation in

order to achleve a better dewatering result (see chapter 6). Flocculants used in

waste water treatment are inorganic salts and polyelectrolytes.

Types of sludge to be dewatered are primary sludge, secondary sludge (or a mixture

of both), aerobically stabilized sludge, digested sludge, and oxidation ditch sludge.


2. 7 History and origin of sludges investigated

In the scope of this study four different sludges originating from four waste water

treatment plants in the Netherlands were characterized:

1. A mixture of prirnary and secondary sludge originating from the activated sludge

plant in Eindhoven.

2. Digested sludge coming from the activated s1udge plant 'Amsterdam-Oost'.

3. Oxidation ditch sludge from the waste water treatment plant 'Veghel-Uden'.

4. Oxidation ditch sludge originating from the waste water treatment plant 'De

Hooge en Lage Zwaluwe'.

In the next sections some typical characteristics of the above-mentioned waste water

purification plants are given. First some definitions which are typical for waste water

processing will be introduced.

The capacity of a waste water treatment plant is expressed in inhabitant equivalents

(i.e.). One inhabitant equivalent is, by defmition, the biologica! oxygen demand of

sewage produced by one persou per day (unit: BOD/day). This oxygen demand is

equal to 54 g BOD~0 per day. Oxygen consumption of industrial sewage is also

expressed in inhabitant equivalents.

Sludge age is defmed as the ratio between the rate of sludge wastage or withdrawal

and the quantity of sludge in the aeration tank, and corresponds with the meao

retention time of sludge in the aeration tank (unit: days).

The organic removal rate is the amount of high-energy organic substrates (pollutants)

converted into low-energy mineral endproducts per kilogram dry sludge solids per

day, and is expressed in units of kg BOD/kg ds per day.

2.7.1 The Eindhoven waste water treatment plant

The map of this activated sludge plant is given in Appendix 1. The incoming sewage

originates from about 25 cities and villages situated in the 'Dommeldal'. The mixture

of prirnary and secondary sludge is stored in a buffer tank and transported to the

sludge treatment plant in Mierlo. A process scheme of this plant is also presented in

Appendix 1. The ratio between the amount of primary sludge and secondary sludge is

strongly dependent on the weather conditions. During heavy raio showers the supply

of prirnary sludge is high, due to sloicing of the sewer system.

20 Chapter 2

Characteristics

Waste water purification

Sewage composition: 60% domestic, 40% industrial

Design capacity: 750,000 i.e.

Influent flow: 5·107 m3/year

Organic removal rate: 0.20 kg BOD/kg ds per day

Sludge age: 6 days

Sewage sludge processing

Sludge flow: 5.6·105 m3/year

Meebankal dewatering equipment: 5 belt presses, 1 centrifuge, 2 chamber filter

presses

Dry solids content incoming sludge: 2.5 wt%

Dry solids content dewatered sludge: 22 wt% (belt presses), 24 wt% (centrifuge), 35

wt% (chamber filter presses)

Flocculant dose: 50-70 g/kg ds FeC13, 400-600 g/kg ds Ca(OH)2 (chamber filter

presses)

4-5 g p.e./kg ds (centrifuge and belt presses)

Flocculant type: Nalco 41162 from Nalco Company

Ultimate disposal: Dewatered sludge produced in the centrifuge and belt presses is

composted. Dewatered sludge from the chamber filter presses is

deposited on dumping grounds.

2.7.2 Waste water treatment plant 'Amsterdam-Oost'

The city of Amsterdam possesses four waste water treatment plants. Sewage origina

ting from the north and centre of the city is treated in the activated sludge plant 'Am

sterdam-Oost' (map of plant is given in Appendix 1). Half the total sewage produced

in Amsterdam is supplied to this plant. Since January 1993 treatment of sewage

sludges produced in the four plants bas been centralized in 'Amsterdam-Oost'. The

mixture of primary and secondary sludge is thickened in four gravity thickeners, and

is subsequently anaerobically stabilized in seven digesters, which operate at a. tempera

ture of 30 °C. The sludge retention time in the digesters is about 20 to 30 days. The


daily biogas production is about 25,000 m3• The biogas is bumt in gas engines to

generate electricity. The released heat is used to heat up both the digester tanks and

water for heating the buildings on the plantside.

Characteristics

Was te water purification

Sewage composition: exclusively dornestic sewage


Influent flow: 4.8·107 m3/year


Sludge age: 9 days


Sludge flow: 8.4·105 m3/year

Mechanica! dewatering equipment: 3 centrifuges (3·1,200 kg ds/hour), 4 chamber

filter presses (1,120 m3 sludge/day)

Dry solids content supplied sludge: 2.0-2.5 wt%

Dry solids content dewatered sludge: 25 wt% (centrifuges and chamber filter presses)

Flocculant dose: 5 g p.e./kg ds (chamber filter press), 7-8 g p.e./kg ds (centrifuge)

Flocculant type: Zetag 63 and Zetag 73 from Allled Colloids

illtimate disposal: After composting the material is dumped on tips.

2.7.3 Oxidation ditch system 'Veghel-Uden'

Sewage from the municipalities Heeswijk-Dinther, Uden, Volkel, Eerde, Zijtaart,

Vorstenbosch, Mariaheide, Erp, Keldonk, Boekel and Venhorst is supplied to the

oxidation ditch system 'Veghel-Uden' (map in Appendix 1). Differences with the

conventional oxidation ditch system are:

Sewage treatment includes a grit chamber.

Only a small part (2.5 %) of the sludge produced and thickened in two tanks is

further dried in lagoons. Sludge from lagoons (5 wt% dry solids) is incidentally

used as a fertilizer.

22 Cbapter 2

Cbaracteristics

Waste water puri:fication

Sewage composition: 35 % domestic, 65 % industial


Influent flow: 1.3·107 m3/year


S1udge age: 13 days


Sludge flow: l. 4·1 05 m3/year. A bout 10 % of the total sludge supplied consists of

digested sludge which was produced extemally.

Mechanica! dewatering equipment: 4 belt presses (4·350 kg dslhour)

Dry solids content supplied sludge: 3.5-4.0 wt%

Dry solids content dewatered sludge: 17 wt%

F1occulant use: 5-6 g p.e./kg ds

Flocculant type: Superfloc C496 from Cyanamid B.V.

Ultimate disposal: Dewatered sludge is composted and subsequently deposited on

dumping grounds.

2. 7.4 The oxidation ditch system 'De Hooge en Lage Zwaluwe'

The municipality 'De Hooge en Lage Zwaluwe' bas its own waste water purification

system. The process scheme of the oxidation ditch system is presented in Appendix 1.

Surplus sludge withdrawn from the fina1 settling tank is stored in buffer tanks. The

thickened sludge (dry solids content 3 wt%) is supplied to the waste water treatment

plant in Rijen (see Appendix 1) for further treatment. Sewage sludge originating from

the 'De Hooge and Lage Zwaluwe' plant is only a sma1l fraction of the total sewage

sludge supplied to the 'Rijen' plant. In 1992, a total amount of 122,215 m3 sewage

sludge bas been dewatered with two belt presses, 2, 765 m3 was coming from the 'De

Hooge en Lage Zwaluwe' plant, 43,640 m3 from the 'Rijen' plantand 75,810 m3 from

nine other waste water treatment plants.


Characteristics

Waste water purification

Sewage composition: 100 % dornestic sewage


Influent flow: 7 .1·105 m3/year


Sludge age: 20 days

Sewage sludge processing in 'Rijen'

23

Sludge flow: 1.2·105 m3/year; 2.8·103 m3/year from the 'de Hooge en Lage Zwa

luwe' plant

Mechanical dewatering equipment: 2 belt presses (2·30 m3 sludge/hour)

Dry solicts content supplied sludge: 3.5 wt%

Dry solicts content dewatered sludge: 20 wt%

Flocculant use: 6 g p.e./kg ds

Flocculant type: Zetag 87 from Allied Colloids

Ultimate disposal: Dewatered sludge is dumped on tips.

3 WATER BINDING IN SEWAGE SLUDGE

3.1 Introduetion

A clear onderstanding of the sludge solid-to-water bond strength is of importance to

get a fundamental insight into the dewatering behaviour of sewage sludges. Sewage

sludges typically have moisture contents of 95 to 99 wt%. The amount and type of

water present in sewage sludges can play an important role in defining their dewater

ing characteristics. Three models have been proposed to describe the types of water

present in sludges [Smollen, 1986; Vesilind, 1974; STOWA, 1981]. All three models

refer to water that is 'bound' in some fashion to sludge solids, whether the binding is

obtained chemically or physically. Water binding is expressed - with respect to pure

water - in lower values for vapour pressure, water activity, entropy and enthalpy of

water molecules.

Different techniques have been proposed for estimating the bound water content of

s1udges. 'Bound water' is defmed operationally by the measuring metbod used.

Smollen [1988 and 1990] reported on the use of the drying curve metbod for quanti

fying the bound water content of biologica! sludges. The hypothesis of the metbod is

that physically bound water evaporates from the sludge at a slower rate than free

water.

The dilatometric metbod is based on the hypothesis that bound water does not freeze

below the freezing point of pure and free water. The volume of freezable water could

be determined from the net expansion of the fluid level in the dilatometric unit. The

difference between this freezable ( or free) water and the total water was defined as the

bound water content. Dilatometric tests were used by Barber and Veenstra [1986] and

Robinson and Knocke [1992].

Katsiris and Kouzeli-Katsiri [1987] used differential thermal analysis (DTA) to

determine freezing curves and quantify the free and bound water fractions in waste

activated and digested sludge samples.

In this study two measuring methods are used: isothermal drying curves and water

vapour sorption isotherms. A defmition of 'bound water' used in this research study is

given in section 3.3.4. The sludge solid-water bond enthalpy can be obtained as a

function of the sample moisture content by the two methods mentioned. Know1edge

about the water bond enthalpy in a sewage sludge cake as a function of the moisture

26 Chapter 3

content enables the predierion of the theoretically maximum feasible dry solids content

in a certain dewatering process.

In this chapter a model is presented to describe the different types of water present in sewage sludge. Subsequeutly, the measuring teclmiques used and experiments carried

out to determine the solid-to-water bond strength are dealt with.

3.2 The presence of water in sewage sludge

Figure 3.1 schematically represents the way in which water may be present in a

sewage sludge suspension and a sludge cake.

a

ocs

• • r in particles

erstitial

Hydration layer Additive particles with

water

Fig. 3.1 Schematical representation of types of water present in sewage sludge. a) bulk water between jlocs; b) interstitial water inside floc; c) hydrated water to surface of jloc particles; d) incorporated water, e.g. intracellular water within floc particles [Kerkhof, 1991].

Water binding in sewage sludge 27

In a suspension or in a filter cake one can distinguish a bulk water and a floc pbase. A

three-dimensional floc network is formed by processes tbat aggregate the colloidal

sludge particles. Flocculants are used to promote the aggregation of the basic sludge

particles (see cbapter 6). The floc consists of a skeleton in which interstitial water is

present.

The basic sludge particles, like microbial cells, pieces of wood, etc., may contain

water inside (incorporated water). Moreover, additives like tlocculants and filter aids

may possess incorporated water. Water enclosed in organic cells is called intracellular

water. Further hydration layers may be present at the surface of floc particles, for

instanee bound to the ionogenic groups of the partiele surface. Hydrated water may

also surround the added flocculants (e.g. FeC13.6H20).

3.3 Isothermal drying curves (TGAIDT A)

In this study thermal analysis is used to determine isothermal drying curves of sewage

sludge cakes. Thermal analysis is a group of techuiques in which a physical property

of a substance and/or its reaction products are measured as a function of temperature

whilst the substance is subjected to a controlled temperature program. W ell-known

techuiques are thermogravimetrie analysis (TGA) and differential thermal analysis

(DTA).

In thermogravimetrie analysis (TGA) the change in sample mass (mass loss or gain) is

determined as a function of temperature and/or time. The resulting mass change

versus temperature curve (thermogram) generally provides information on the thermal

stability and the composition of the sample, the intermediate compounds, and the

residue. Thermogravimetry is universally applied to a large number of analytical

probierus in the fields of metallurgy, paint and ink science, ceramics, mineralogy,

food technology, inorganic and organic chemistry, polymer chemistry, biochemistry,

and others.

In differential thermal analysis (DTA) the temperature difference between a sample

and a reference material is measured as a function of the sample, inert material, or

furnace temperature when the sample is subjected to a controlled temperature pro

gram. Temperature differences between the sample and the reference material are due

to enthalpie transitions occurring in the sample material, such as those caused by

pbase changes, fusion, boiling, sublimation and vaporization, dehydration reactions,

dissociation and decomposition reactions, and other chemica! reactions. Differential

28 Cbapter 3

thermal analysis is generally used to determine the heat of enthalpie transition ( or

reaction), or the mass of the reactive sample.

The available thermal analysis equipment (SETARAM, TGA 92 with DTA probe)

provides the possibility to carry out thermogravimetry and differential thermal analysis

simultaneously. The equipment consists of: a cylindrically shaped furnace (4>=21.9

mm), a thermobalance, a temperatore programmer and controller, and a computer.

The filmace may operate with temperatores up to 1000 °C and is externally cooled by

water circulation. The filmace may employ a carrier gas, such as air, inert gases, or

reactive gases. In the experiments carried out nitrogen (inert gas) was used as carrier

gas to remove the vapours given off by the sample. The thermobalance permits

continuous weighing of a sample as a function of time and/or temperatore. The

controller contains an acqnisition and amplification card for the various signals and

transfers digitized signals to the computer. The typical measuring probe which is connected to the microbalance is presented in fignre 3.2. The measuring probe bas

been developed by Boersma [1955]. Two aluminium cups (4>=3.7 mm; height=3.9

mm) are positioned on the probe.

TO MICROBALANCE

HEFEREN CE

FURNACE WALL

Fig. 3.2 The typical measuring probe positioned in a cylindrically shaped jurnace (<!>=21.9

mm). The probeis connected toa microbalance.


The reference sample is an inert material in the temperature domain in wbich experi

ments are carried out; however, in tbis study an empty aluminium sample holder is

taken as reference. The sample material is a piece of sludge cake (weight 40 to 60

mg) obtained from a fUtration experiment (see secdon 4.4.1 ). Thermocouple A (Pt

Pt/Rh 10%) measures the temperature of the sludge cake sample. Either the sample or

furnace temperature is regulated by the temperature controller. In the experiments

carried out the sample temperature is kept constant.

During an isothermal drying experiment, the sludge cake sample will take up heat

needed to evaporate water. The temperature of the sample will then be lower than the

temperature of the reference material. The temperature difference is registered with

two thermocouples, Br and B, (Pt-Pt/Rh 10%), wbich are positioned just below the

sample bolders. The temperature difference is proportional to the heat flow to the

sample (see next section). Due to the position of the thermocouples (see tigure 3.2),

the measured temperature difference is not equal to the real temperature difference

between reference and sample. Because of non-quantitied heat resistances, the relation

between the temperature difference and the heat flow bas to be calibrated. A suitable

calibration procedure is based on the evaporation of pure water at constant temperat

ures.

3.3.1 The TGA-DTA drying model

In tbis section a model is presented that describes the isothermal drying of samples

with the 'Boersma' measuring probe (see tigure 3.2). The model enables the calcula

tion of the water bond enthalpy. The model is based on mass and energy balances for

both the reference and the sample cup. The following assumptions are made:

1. The aluminium cup possesses a high thermal conductivity; consequently both the

sample material and cup have the same temperature.

2. The convective heat transfer coefficients for sample material and cup are differ

ent. However, an average convective heat transfer coefficient is assumed for both.

3. The emissivity E for the reference cup and the sample cup are the same (taken to

be 0.2 for unpolished aluminium).

30 Chapter 3

Energy balance reference cup

where Ar Hr m.-Tgas Tr Twa11 ar E (J

= heat transferring surface area of reference cup [m2]

enthalpy of reference cup [J.kg-1]

mass of reference cup [kg] temperature bulk gas in furnace tube [KJ reference temperature [KJ furnace wall temperature [KJ convective heat transfer coefficient for reference cup [W .m·2 .K 1]

emissivity [-] Stefan-Boltzmann constant = 5. 8·10·8 W .m·2 .K4

Equation (3 .1) means tbat lhe sum of lhe convective and radiative heat flow equals lhe

rate of heat accumulation in lhe reference cup. By definition, the change in mass of

lhe reference is zero: d.m,./dt=O. It will be shown tbat during a drying experiment lhe

reference temperature appears to be virtnally constant: dT/dt""O.

Consequently dH/dt=Cp,/dT/dt""O. If it is assumed tbat lhe wall temperature equals

lhe gas temperature, equation (3.1) is rewritten as:

(3.2)

Thns:

(3.3)

Energy balance sample cup

where

d ~. dH 4 4 dt(m.HJ = H.-d.t + m, dt. = a,A.(Tg .. -T,) + EUA,(Twau-T.) (3.4)

A, A .. H, Miv jw m. T, a,

=

j~ssdllv

heat transferring surface area of sample [m2]

sample surface area for moisture transport [m2]

enthalpy of sample [J.kg-1]

enthalpy of evaporation of pure water [J.kg-1]

moisture flux 1hrough surface area [kg.m·2 .s'1]

mass of sample [kg] sample cup temperature [KJ convective heat transfer coefficient for sample [W.m·2.K1]


Equation (3.4) means that the rate of heat accumulation in the sample cup equals the

sum of the convective and radiative heat flow minus the heat flow needed to evaporate

water.

The pure water evaporation enthalpy Miv is a function of the water temperature T w (in

K):

where pure water evaporation enthalpy at 273.15 K and 1 bar = 2504·103 J.kg-l specific heat of water vapour = 1. 87·103 J.kg-1• °C 1

specific heat of water = 4.18·103 J.kg-1• °C1

Mass balance sample cup

(3.5)

(3.6)

Substitution of equations (3.3) and (3.6) in (3.4) yields the ruling equation for the

drying of samples with the combined TGA-DTA technique:

(3.7)

The first and secoud term on the right-hand side of equation (3.7) equals Q:

(3.8)

where 01eff is the 'effective heat transfer coefficient':

(3.9)

3.3.2 Evaporation of pure water and the calibration of the DTA probe

The result of an experiment in which pure water (initial mass 50 mg) was evaporated

at a constant sample temperature of 60 oe is presented in figure 3.3. Water mass IDw,

evaporation rate diDw/dt, and the signal S produced by the thermocouples Br and B, (in

p, V) are given as functions of time. The derivative diDw/dt is digitally calculated by the

computer and specifies the evaporation rate. The temperature difference (which is

32 Cbapter 3

proportional to S) and the evaporation rate are virtually constant during the experiment

(see figure 3.3). From equation (3.7) it follows that the evaporation of pure water with

the 'Boersma' probe is given by:

(3.10)

where HW = cp.w(T.- 273.15) = cp,w(). enthalpy of water [J.kg'1]

50 1.000

0 0.001

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

time (sec)

Fig. 3.3 Evaporation of pure water (initia/ mass 50 mg) at a constant sample temperature of

60 oe. Water mass 1nw (-), evaporation rate dmjdt (-·-·-), and the thermocouple signa/ S

(---) are registered as juncrions of time.

The water temperature is virtually constant during the experiment, thus

~/dt = Cp,w*dT.fdt = 0. The left-hand side of equation (3.10) equals zero, so

equation (3 .10) can be rewritten as:

(3.11)

The thermocouples Br and B. do not measure the real temperatures of the reference

cup and sample cup. Due to the position of these thermocouples, a measuring error is


introduced. It is assumed that both thermocouples are positioned in the temperatnre

boundary layer at a fractional distance f=(x/or) from the cup surface (see tigure 3.4).

i

i I Tgas

Fig. 3.4 Position of the thermocouple Bs in the temperature boundary la:yer (thickness br) at a

distance x from the sample cup suiface.

B· .. f.tT -T) = (T -T) \ gas s s,exp s (3.12)

B· r· (3.13)

Subtraction of equation (3.13) from equation (3.12) yields:

T-T = r s (Tr,exp (3.14)

The constant CroA ( = 11(1-f) > 1) is a correction factor wbich takes into account the

measuring error due to the position of the thermocouples. From equation (3.14) it can

be concluded that the position of the thermocouples B, and Br weakens the sensitivity

of the thermocouple signal. The measured temperatnre difference between the

reference cup and sample cup is calculated from:

34

s T -T =-r,exp s,exp ctc

Chapter 3

(3.15)

The constant ctc is a conversion factor which equals 6.625 p. V .K1• Substitntion of

equation (3.14) in (3.15) yields:

(3.16)

The sample temperatnre T, is maintained at 60 oe during the whole experiment. The

thermoconple signal S is virtnally constant in the experiment (see figure 3.3). As a

result the reference temperatnre T., which can be calculated from equation (3.16),

appears to be virtnally constant as well. Substitntion of equation (3 .16) in equation

(3.11) gives:

Q = aerrA-sCTGAS =- <imw(~-Cpw0,) ctc dt ·

The calibration factor Cr is defined as follows:

and thus from equation (3.17) it follows that:

c = Q f s

<imw 1 --(AU - C 0)·-dt ~-'v p,w ' S

(3.17)

(3.18)

(3.19)

The evaporation rate dffiw/dt is derived from the experiment and appears to be

virtnally constant. The sample temperatnre T., the thermoconple signa! S, and the

evaporation rate are k:nown as functions of time. Now, the calibration factor Cr (in

Wip. V) can be calculated as a function of time with equation (3.19). In figure 3.5 er is

depicted as a tunetion of the water mass.

The calibration factor appears to be virtnally constant during the experiment:

er 1.63 ± 0.05 mW/p.V. The resolution of the calibration factor is about 3%.

Consequently, the error in determining the heat of evaporation of water with the TGA

DT A technique is about 3%.


0.0018

·"'\ 0.0016 A .. r 0.0014

>' 0.0012

~ • 0.0010 I ~ 0.0008 4-o • u 0.0006 • • 0.0004 • •

0.0002 I 0.0000 r

0 10 20 30 40 50

mass mw (mg)

Fig. 3.5 The calibration factor c1 (in Wl~t V) as a fitnetion of the water rru2ss fnw.

3.3.3 Isothermal drying of a sludge cake

Sewage sludge cakes were driedat a constant sample temperature of 60 oe. The mass

of a sludge cake sample m. equals the sum of the mass of sludge dry solids ll\ts aud

the mass of water lllw:

where u = sample moisture content [kg w.(kg dsY1]

The derivative dm,/dt is given by:

dm,

dt du

mdsdt

The total heat accumulated in the sludge cake sample is given by:

where enthalpy of sludge solids [J. oe1]

enthalpy of pure water [J. 0 e 1]

(3.20)

(3.21)

(3.22)

36 Chapter 3

The derivative of m,;H, to time is represented as:

The enthalpy of the sJudge solids is given by:

Where Cp,ds = SpecifiC heat Of SlUdge SOlids [J.kg-t. oe-I]

0, = temperature [ 0 C]

The sample temperature remains constant during the experiment, thus

(3.23)

(3.24)

dHds/dt Cp.ds *dO,/dt = 0. As a result, the first term on the right-hand side of

equation (3.23) equals zero.

The enthalpy of water is given by:

where Cp,w = specific heat of pure water [J.kg-1• °C1

]

.Mlb = water bond enthalpy [J.kg-1]

(3.25)

The water bond enthalpy .Mlb is defined as the excess enthalpy to evaporale water out

of the sample. The derivative of llw to time is given as follows:

(3.26)

The drying process is isothermal, thus the first term on the right-hand side of equation

(3.26) is equal to zero. Substitution of equations (3.26) and (3.25) in equation (3.23)

yields:

d(m)I.)

dt

(3.27)

Substitution of equation (3.27) in the ruling equation for the drying of a sample with

the TGA-DTA technique (equation (3.7)) yields:

Water binding in sewage sludge

du [ iMHb m- -u--+ C 0 ds dt àu p.w s

Equation (3.28) cao be rewritten as:

Mlbl = Q + dm, LUi dt V

Q dm,

dt

37

(3.28)

(3.29)

The sample mass m., the evaporation rate dm./dt, and the heat flow to the sample Q

( =cr'S) are known at any given moment. The sample moisture content u is related to

the sample mass m. according to:

u(t) m,(t) -mds

(3.30)

Equation (3.29) provides the possibility to calculate the water bond enthalpy as a

tunetion of the sample moisture content.

3.3.4 Experimental results

The moisture-to-sludge solid bond strength was studied with four types of sludges

originating from four waste water treatment plants (see section 2.7). Sludge cake

samples obtained from a tiltration experiment carried out with the filtration-expression

cell (see section 4.4.1) were dried. Typical initial moisture contents varied between 4

and 7 kg w/kg ds. Different flocculants were used to condition the sludge sample prior

to filtration: FeC13/Ca(OH)2 , polyelectrolyte Röhm KF975, and the polymer applied in

practice. Three different dosages per flocculant type were used. The dosage Ca(OH)2

was maintained constant if the sample was flocculated with FeCl3/Ca(OH)2 •

In total 36 isothermal drying experiments were carried out. In all drying experiments

the sample temperature was kept at 60 oe. A drying experiment took about 2 to 3

hours, depending on the mass and initial moisture content of the sludge cake sample.

At lower drying temperatures the drying time increases and the sludge cake composi

tion may change due to biologica! activity. A uittogen flow of 1 liter/hour was

adjusted to remove the water vapour given off by the sample. The uittogen flow had

an upward direction.

38 Chapter 3

80011 1.000

7000

J ---.----:

-~-----:--80011 . /

[ '/"

i t·

5000 I: o.mo c:l I I

I ' 4000 I

I I

iS: 8000

I --- :1! liOOO --- ___",__--. --.--- O.Olll

~"' ] 111110

0

-111110 0.001

o.o 0.11 1.0 :Lil 2.(1 2.5 8.0

moistu:re content u [kg waterika ds)

Fig. 3.6 Result of the isothermal drying experiment (60 oe) with a Veghel sludge cake sample

flocculated with 3 g Röhm KF975/kg ds. The drying rate dm/dt (---), the heat flow Q and the bond enthalpy Mlb (-) arepresentedas functions of the sample moisture content u.

80011 1.000

7000

I 80011 -----:---

~ 11000

ê:t

' 4000

iS: 8000

1 liOOO

I ~..:.----

] 11100

(I

./ -11100

0.0

m.oistu:re content u [kg water/kg ds)

Fig. 3.7 Result of the isothermal drying experiment (60 oC) with a Veghel sludge cake sample

flocculated with 1 g Superflocfkg ds. The drying rate dm/dt (---), the heat flow Q(---),

and the bond enthalpy illlb (-) are presented as junctions of the sample moisture content u.


8000 :LOOO

7000'

i 1

.---~ 8000 /

~ /

i I 5000 I o.wo Cl

I

~ 4000 I/ i <l

>. 11000

~ I I ------- ~ 2000 I --- 0.010

lä -- .f' ] 1000

0

( -1000 0.1101

0.0 G.5 :LO L5 z.o 2.5 3.0

moisture content u [kg water/kg ds]

Fig. 3.8 Result of an isothennal drying experiment (60 oq with a Veghel sludge cake sample

jlocculated with 3 g Supeifloclkg ds. The drying rate dmjdt (---), the heat flow Q ,-.·-··J. and the bond enthalpy Mfb (-) arepresentedas functions of the sample moisture content u.

In figures 3.6, 3.7 and 3.8 the results of three drying experiment.<; are presented. In

the figures the drying rate dm./dt, the heat flow to the sample Q, and the water bond

enthalpy &Ib are depicted as functions of the sample moisture content. The drying

rate as a function of moisture content is also called the drying curve.

At the start of an experiment the evaporation rate and heat flow are relatively high. In

the first drying stage the evaporation rate remains virtually constant and free water is

transported. The drying rate during this period (the 'constant rate period') is determi

ned by the conditions in the continuons phase: temperature, humidity, and mass

transfer coefficient. The heat of evaporation in the sample equals the heat of evapora

tion of pure water. The type and added amount of polyelectrolyte do not cause a

marked difference between the drying rates of the constant rate periods. Halde [ 1979]

investigated the influence of an added concentration of Praestol 444K on the vacuum

drying rate of digested sludges. He concluded that the added concentration of poly

electrolyte had a minor influence on the sludge drying rate in the initial period.

40 Chapter 3

The figures show that at a sample moisture content of about 0.4 to 0.6 kg wlkg ds the

drying rate and heat flow start to decrease and the bond enthalpy starts to deviate

significantly from zero. Passing this critical moisture content the resistance to moisture

transfer inside the drying body starts to grow rapidly and mainly controts the mass

transfer. This second drying stage is called the 'falling rate period'. The movement of

water inside the sludge cake specimen may occur by various mechanisms, e.g.

capillary flow, liquid and vapour diffusion [Whitaker, 1977].

The critical moistnre content of 0.4 to 0.6 kg wlkg ds is found in every drying

experiment carried out. The 'bound water' content in a sludge cake was not affected

by the sludge and flocculant type and flocculant dosage. All graphs showing the bond

enthalpy as a function of sample moistnre content were similar. Moistnre having a

zero bond enthalpy is here by definition 'free water', and is mainly removed by liquid

flow in the filtration-expression process. Moisture having a bond enthalpy larger than

1 kJ/kg is called 'bound water', and cannot be removed in a mechanical dewatering

process at applied pressures smaller than 10 bar. 'Bound water' can only be released

in e.g. a thermal drying process. It is unlikely that a sharp physical boundary exists

that separates 'bonnd' water from 'free' water. The transition proceeds fairly continu

ously over the mentioned range in moistnre contents.

The condusion can be drawn that the maximum feasible dry solids content in a

mechanical dewatering process is about 65 to 75 wt%. The remaining parts of liquid

in the cake are bound to the sludge particles andlor represent incorporated water.

There seems to be different types of attachment for water bonding in sludges. Keey

[1972] distinguished three types of water binding, i.e. chemical (ionic and molecular), ·

physical (osmotic and adsorptive) and mechanical (capillary). Typical bond energies

for molecular, adsorptive, and capillary water are 5000, 3000 and 100 kJ/kmol,

respectively.

La Heij [1994] studied the effect of applied mechanical pressure on the equilibrium

dry solids content with the filtration-expression cell (see section 4.4.1). Dry solids

contents of 35 to 45 wt% were reached at low expression pressures (5 to 7 k:Pa) and at

optimal laboratory conditions. The considerable d:ifference in the theoretically

maximum dry solids content and the dry solids contents reached in laboratory

experiments indicates that most of the water remaining in the sludge cake is not

'bound' to particles. Clearly, other fractions of water have more impact on the final

cake solids concentration than the 'bound water' fraction. Probably, it is the water

present in the interstitial spaces (interstitial or interfloc water) that determines the final


cake dry solids content. In a mechanica! dewatering process the porous structure of the

cake material collapses and a lot of interstitial water remains entrapped within the cake

(immobilized water). The porous structure of the cake depends on the type of

flocculant used. The compressibility of sludge flocculated with polyelectrolyte is

higher than that of sludge flocculated with iron chloride/lime. The compressibility

influences the structure of the formed cake in terms of permeability [La Heij, 1994].

In mechanica! dewatering equipment the moisture content in sludge fllter cakes

amounts about 2 to 4 kg w/kg dry solids ( = 20 to 35 wt% dry solids). A simple

calculation learns that 90% of the water present in the fllter cake with a dry solids

content of 20 wt% is present as 'free water' (bond enthalpy equals 0) and the remai

ning 10% is present as 'bound water'.

The dry solids contents reached in practice are much smaller than the dry solids

contents reached at laboratory scale (35 to 45 wt%). This discrepancy indicates that

other factors, like bad flocculation conditions, floc break-up during transport, filling

problems of chambers in fllter presses, negatively affect the sludge dewatering process

in practice. More research is needed into the flocculation process and transport of

(flocculated) sludge in practice.

Higher mechanica! pressures are needed to remove a larger amount of interfloc water.

La Heij [1994] showed in laboratory experiments that by applying high mechanica!

pressures (6 to 10 MPa) in a hydraulic frame press a fllter cake solids content of 60

wt% could be reached. The remaining parts of water in these experiments were only

'bound' to the sludge particles!flocs or represented incorporated water.

3.4 Water vapour sorption isotherms

3.4.1 Introduetion

The secoud experimental technique to characterize the solid to water bond strength in

sewage sludge is measuring water vapour sorption isotherms. The water vapour

sorption isotherm of a substance is the constant temperature relation of the water

content in the substance and its thermadynamie water activity. The water activity <lw is

defined as a ratio of fugacities:

42 Chapter 3

(3.31)

where fw is the fugacity (or 'escaping tendency') of water in the mixtnre at equilibrium

and ~ is the fugacity of pure water at standani temperatnre and pressure. The fugacity

becomes equal to pressure as the system approaches ideal gas behaviour. Equation

(3.31) is then rewritten as:

(3.32)

where Pw is the partia1 vapour pressure of water in the system and p~ is the satnration

vapour pressure of pure water at the same temperatnre. The ratio PwiP~ is also called

the equilibrium relative humidity (RH) or relative vapour pressure of the system. For

a certain system, the water activity is a function of temperatnre and moistnre content,

and lies in the range 0 s aw s 1. In this context, bound water is defined as water ha ving

a water activity below the bu1k water activity (aw= 1).

Since sorption of water by a sludge cake is a spontaneous process, it is accompanied

by a decrease in the thermadynamie Gibbs free energy G. During sorption the entropy

S is decreased, caused by trapping vapour molecules at active sorption sites or in a

thin surface layer. The enthalpie change .:UI during the sorption process reads:

AH= ÄG + TÄS (3.33)

Thus the enthalpie change during this process must be negative (exothermic process).

At high water activities the molar heat evolved from sorption AH..,r from the vapour

phase equals the molar enthalpy of condensation ~nd ( = 44 kJ/mol at 20 °C).

Under these conditions water vapour sorption is comparable with condensation. The

enthalpy of sorption increases with decreasing water activity. The excess or net

enthalpy of sorption AHexc is defmed as:

(3.34)

and corresponds thermodynamically with the bond enthalpy (section 3.3.3). The excess

sorption enthalpy is indirectly obtained from sorption isotherms at different temper

atnres by applying the equation of Clausius-Clapeyron:


R (3.35)

where Llllw ( < 0) corresponds to the differential enthalpy of wetting, and R is the gas

constant. The contribution of the sorbent to the enthalpy effect may be neglected, so

Llllw ""' Llll.,xc · Gal [1967] gave an overview of available tecbniques for the deterrniuation of iso

therms for water vapour sorption. In the present study, water vapour sorption iso

therms were determined with two tecbniques: firstly the conventional tecbnique of

vacuum exsiccators with saturated salt solutions to control the water activity, and

second1y the Cou1ter Omnisorp 100, a fully-automatic measuring instrument.

In order todetermine the differential enthalpy of wetting, water vapour isotherms have

to be measured at different temperatures.

3.4.2 Sorption models

In literature about 80 equations to describe sorption isotberms are known [lglesias and

Chirife, 1982; v.d. Berg and Bruin, 1978], because the interactions between water

(sorbate) and dry substance (sorbent) are very complex. A sorption model which takes

into account all kinds of interactions will lead to a complex isotherm equation witb

many constants. However, a more practical approach is an isotherm equation contain

ing only a few parameters, which must have a physical meaning. It should be realized

that it will remain a simplification insofar as not all structural and interaction details

can be covered. In the present study the following two sorption models were studied.

B.E.T. model (0<1!,.,<0.35)

Brunauer, Emmet and Teller [1938] derived an isotherm equation for localized

multimolecu1ar adsorption onto independent sites, assuming the adsorbed molecules

beyoud the first one to have bulk liquid properties. The B.E. T. equation reads:

(3.36)

44 Chapter 3

where Cs is a constant depending on the interaction of the first adsorbed molecule

with the adsorption site and the temperature.

The temperature dependenee of Cs is given by:

[MI -MI l c "" c exp 1 cond

s s,o RT (3.37)

The adsorption enthalpy of water molecules in the first layer (MI1) is larger than the

condensation enthalpy of pore water (LUiconJ. The adsorption enthalpy in the second,

third, .... mlh layer equals the condensation enthalpy (LUI2 =LUI3 = ... =Mlm=MJ.,onJ·

G.A.B. model (0<~<0.9)

Andersou [1946] proposed that the molar sorption enthalpies of the secoud and follo

wing molecules on a sorption site are not the same for condensation of the bulk liquid.

The influence of the sorption site on the second, third,. .. and mlh layer is noticeable.

Anderson multiplied the water activity by a constant k, less than unity, to take this

inflnence into account.

The isotherm eqnation is then represented as:

(3.38)

where Cg is the Guggenheim constant. De Boer [1953] and Guggenheim [1966]

derived the same isotherm equation on a thermodynamic base. All three authors

worked independently. lsotherm eqnation (3.38) is referred to as the Guggenheim,

Andersou and de Boer (G.A.B.) model of adsorption.

The temperature dependenee of the Guggenheim constant is given by:

(3.39)

The factor k, which corrects for differences in properties of water molecules present

in the multilayer with respect to pure water, is also a function of the temperature:


[ (.6-H -.6-H ) l P k = ka exp m cond = ka exp(~) (3.40)

The G.A.B. model provides energetic infonnation on the averaged multilayer and

must be considered to be a more general model for localized homogeneons sorption

than the B.E.T. model.

In this stndy the temperature dependent G.A.B. model is preferred for experimental

isotherm analysis. The experimental data can be fitted with a S-shaped curve. The

G.A.B. equation represents a S-shaped curve. However, the G.A.B. model cannot be

used as a physical model for water sorption in sludge cakes. Multilayer adsorption

does not play an important role in sludge cakes. Water absorption in sludges particles

is the most important sorption mechanism in sludge cakes (see figure 3.1).

lnsulation

Sludge cake sample

Wheels

Thermostata

Water bath

Fig. 3.9 Schematic diagram of the whole expertmental set-up.

Metalplate

Glass vessel

Chapter 3

3.4.3 Metbod of saturated salt solutions

Equimnent and procedure

In this study a new experimental apparatus bas been developed to measure water

vapour sorption isotherms, based on the couventional technique of vacuum exsiccators

with saturated salt solutions [van Dijke, 1992]. The data for the water activity of these

solutions were taken from the tables of Greenspan [1977]. These tables relate the type

of salt solution and its temperature to the value of the water activity. Por keeping a

constant temperature during equilibration, the exsiccators were positioned in a

thermostated water bath(± 0.1 K).

A schematic diagram of the equipment is shown in tigure 3.9. Twelve glass vessels,

positioned in a revolving metal frame, were partially filled with twelve different

saturated salt solutions to control the water activity. Sludge cake samples having initial weights of about 1 gram were obtained from a tiltration experiment. The samples were

brought into small glass vessels which were positioned on small perspex tables. The

samples were pretreated with thymol, which is an effective fungicide for a relatively

long period (2-3 months) and does not influence the water vapour sorption behaviour

[v.d. Zande, 1993]. Prior to the start of an experiment the exsiccators were evacuated

to reduce the equilibration time. The samples will desorp water dependent on the

water activity. Regularly the samples were weighed. Attainment of equilibrium was

assumed if two subsequent weighings within 24 hours gave the same results. When

equilibrium was reached, the temperature of the water bath was changed and subse

quently the same samples were equilibrated again. At the end of the experiment the

equilibrium moisture content was determined by drying the sample at 105 oe. In this

way twelve points of each sorption isotherm were determined.

With the vacuum exsiccator method, water vapour desorption isotherms were measu

red of three different sludge cakes (Mierlo, Amsterdam and Veghel). The flocculant

used in the experimeuts was FeClsfCa(OH)2 • Each sludge cake sample was flocculated

with three different dosages of FeC13• The dosage Ca(OH)2 was kept constant for each

sludge type. In this way 9 different sludge cake samples were studied.

Equilibration time

Figures 3 .1 0 and 3 .11 show desorption isotherms measured at three temperatures of

two sludge cakes: Amsterdam sludge cake flocculated with 100 g FeC!lkg dry solicts

and 400 g Ca(OH)2/kg dry solicts and Veghel sludge cake flocculated with 50 g

0.32

0.28

0.24

~ ..!f 0.20

lit O.:IB

~ 0.22

~ '0.08

0.04.

0.0


0.~ 0.2 0.3

* * * 298..15

298.15

0.4 0.15 0.6

aW' (-)

0 0 0 313.15

- - 313..15

47

0.7 0.8 0.9 1.0

A A A 328..15

328.15

Fig. 3.10 Desorption isotherms of Amsterdam sludge cake flocculated with 100 g FeCf/kg ds

anti 400 g Ca(OH)/kg ds at three different temperatures. Symbols: experimental results.

Lines: model.

0.32

0.28

-0.24

.a .Jf 0.20

lit O.:IB

~ o.u ~ 0.08

0.04.

o.o o.~ 0.2 0.8

* * * 303 .. 15

803.15

0.4 0.15 0.8

aW' (-)

0 0 0 818.15

818..15

0.7 0.8 0.9 1.0

A A A 328..15

328.15

Fig. 3.11 Desorption isotherms of Veghel sludge cake flocculated with 50 g FeCf/kg ds anti

200 g Ca(OH)/kg ds at three different temperatures. Symbols: e.xperimental results. Lines:

model.

48 Chapter 3

FeC13/kg dry solids and 200 g Ca(OH)2/kg dry solids. The symbols show the experi

mental resnlts. One experiment took about 2-3 months. The graphs show isotherms

with only eight to ten data points. At high water activities (a_.> 0.8) the samples did

not equilibrate. Wolf et al. [1985] related the long equilibration time of food products

at high a_.-values to the problem of microbial deterioration of the samples.

GAB-fit

Upon increase of the isothermal temperature, water sorption equilibria shift towards a

lower moisture content at a given water activity. The temperature-dependent G.A.B.

equations (3.38 to 40) were fitted to the measured isotherms, using the sum of least

squares metbod for minimizing the absolute differences between measured and

calculated moisture contents. This was efficiently done by nsing the Statistica!

Analysis System package (SAS). The lines in the graph represent the temperature

dependent G.A.B. equation. The S-shaped G.A.B. equation describes the experimental

isotherms very well. The isotherms for these two samples are almast identical over the

entire range of water activities, especially at low water activities (a_. <0.3). It indicates

that the two sludge cakes, which originated from different waste water treatment

plants and were flocculated with different dosages of FeCl3 and lime, have about the

same water sorption isotherm. The small differences in sorption capacity may be

attributed to biologica! variations of the slndge cake samples. The same sorption

behaviour bas also been found for the other sludge cakes investigated [v.d. Zande,

1993].

Fit parameters of the temperature dependent G.A.B. model are given in table 3.1.

Van den Berg [1981] measured sorption isothermsof potato starch samples, which are

also organic products, at different temperatures. The sorption equilibria were analysed

in terms of the B.E.T. equation and G.A.B. equation. The typical monolayer value u1

for s1udge cake samples of about 0.1 kg wlkg ds was also found by van den Berg

[1981] for potato starch samples.


Table 3.1 Results of desorption analysis for sludge cake samples in termsof the temperature

dependent G.A.B. equation. Veghel 5 wt% FeCl3 means Veghel sludge flocculated with 50 g

FeC[/kg ds.

Sample cg.o(-) ko(-) Et E2 llt

(kJ/mol) (kJ/mol) (kg w/kg ds)

Veghel 5 wt% FeCI3 7.1E-10 0.081 64.52 5.525 0.089

Veghel 10 wt% FeC13 7.5E-7 0.080 46.97 5.628 0.087

Veghel 30 wt% FeCI3 2.2E-8 0.166 56.86 4.091 0.072

A'dam 5 wt% FeCI3 1.7E-8 0.015 56.07 9.45 0.096

A'dam 10 wt% FeCI3 1.2E-6 0.037 44.05 7.34 0.106

Mierlo 5 wt% FeCI3 3.4E-4 0.033 30.55 7.56 0.085

Mierlo 9 wt% FeCI3 3.2E-4 0.056 30.70 6.45 0.083

Mierlo 27 wt% FeCI3 l.lE-9 0.016 63.88 9~ 0.113

The GAB parameters Gg and k can now be calculated as functions of the temperature

with the equations (3.39) and (3.40), respectively. In figures 3.12 and 3.13, Cg apd k

are plotted as functions of the absolute temperature for the Amsterdam sludge cake

sample flocculated with 100 g FeClikg ds.

The Guggenheim constant Cg is a measure of the binding energy of tbe first adsorbed

layer. The binding energy of the first adsorbed layer decreases with increasing temper

ature. Van den Berg [1981] observed a shorter residence time of adsorbed molecules

in the fust layer when the temperature is increased. The sorption process becomes less

strongly localized. As a consequence, at constant water activity the sample moisture

content reduces with increasing temperature. This is confirmed by the experimental

results (see figures 3.10 and 3.11).

All parameters of the temperature-dependent G.A.B. equation are known, so it is

possible to calculate the sorption isotherm of a sludge cake at any given temperature,

which therefore needs not be measured.

Figure 3.14 shows desorption isotherms at eight temperatures for the Veghel sludge

cake sample flocculated with 100 g FeCl,lkg ds obtained by this analysis.

50 Chapter 3

350

300

260 -I 200 -ct

0 160

100

60

0 270 280 290 300 310 320 330 340 360

Tempersture (K)

Fig. 3.12 The Guggenheim constant Cg as a function of the absolute temperature for the

Amsterdam sludge cake sample jlocculated with 100 g FeClikg ds.

1.00

0.90

0.80

0.70

- 0.60 I - 0.60

...:.:::: 0.40

0.30

0.20

0.10

0.00 270 280 290 300 310 320 330 340 350

Tempersture (K)

Fig. 3.13 The G.A.B. parameterkas a function of the absolute temperature for the Amster

dam sludge cake sample flocculated with 100 g FeC[/kg ds.

0.8

- 0.7 ~ 0.6 ~ 0.5 ~ 0.4 ~ 0.3 ~ 0.2


/ /

51

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 aw (-)

Model -- 283.15 . 293.15 - - . 303.15 - - 313.15 - 323.15 - 333.15 - 343.15 ·------- 353.15

Fig. 3.14 Desorption isotherms of the Veghel sludge cake sample flocculated with 100 g

FeCl/kg ds at eight temperatures.

The temperature-dependent G.A.B. equation relates the sample moisture content u to

the water activity <lw and the absolute temperature T: u=u(Clw,T). The inverse function

<lw=<lw(u,T) is given by:

where

and

e 0·5(e u2 -2uu (e -2)+u 2e )05 +u(e -2)-u e g g 1 g 1 g g lg

El egO exp(-)

· RT

2uk(eg-1) (3.41)

(3.42)

52

Ez k =ko exp(-) RT

Entbalpy of wetting

Chapter 3

(3.43)

Application of the Clausius-Clapeyron equation (3.35) and the G.A.B. equation (3.41)

yields an analytical expression for the differential entbalpy of wetting Mlw. Unfortu

nately this expression, calculated by using the MAPLE"' package, is very large and

therefore presented in Appendix 2. Figures 3.15 and 3.16 show the differential

enthalpy of wetting, -Mlw, as a function of the sample moisture content u for two

sludge cake samples as derived from the procedure mentioned. The entbalpy of

wetting increases with decreasing sample moisture content and flatteus at very small

moisture contents. The shape of the bond entbalpy curve corresponds well with that

measured of other products containing organic cells, such as potatoes [Görling, 1955]

and wood [Pidgeon and Maass, 1930].

Figure 3 .17 presents the bond entbalpy as a function of the moisture content of the

Veghel sludge cake sample conditioned with 50 g FeC13/kg ds as obtained from

sorption isotherms and isothermal drying curves. Both curves are not identical. The

boud entbalpy obtained from drying curves does not flatten, but increases sharply

when the sample moisture content is reduced. At very small moisture contents the

evaporation rate dm,/dt reduces stronger than the heat flow Q. As a result, the ratio

Q.(dm,/dt)"1 used to calculate the bond entbalpy increases strongly with decreasing

moisture content.

At a eertaio bond entbalpy, the moisture content determined in an isothermal drying

experiment is larger than the moisture content obtained from the sorption isotherm.

During drying of the sludge cake, a moisture content profile is present in the cake.

The water content at the surface is smaller than the moisture content in the cake. In

the sorption experiment, the moisture content was measured in an equilibrium state.

As a result, the moisture concentradon profile in the sludge cake is flat. The average

moisture content of the sludge cake in the drying process is thus larger than the

average moisture content measured in the sorption experiment.

With the sorption analysis the bond entbalpy was only calculated over a small moisture

content range (u<0.3 kg w!kg ds), whereas the drying analysis yields valnes of the

bond entbalpy over the whole working range of sample moisture contents.


3000

2000

1000

0 ~----.---~----~-----.----,-----,-----,---~

o.oo 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

u (kg w/kg ds)

Fig. 3.15 Dif.ferential enthalpy of wetting as a junction of the sample moisture content.

Sample: Amsterdam sludge cake flocculated with 100 g FeCl/kg ds and 400 g Ca(OH)/kg ds.

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

u (kg w/kg ds)

Fig. 3.16 Dif.ferential enthalpy of wetting as a function of the sample moisture content.

Sample: Veghel sludge cakeflocculated with 50 g FeCl/kg ds and 200 g Ca(OH)/kg ds.

54

8000

~ 5000

~ 4000

~

f 3000

2000

i 1000 ] 2 0

-1000

0.0

Chapter 3

i

l I

. \. .I

. \ ~ \

o.t

\ \. '. ""-..

0.2 0.3

-~-.---·- ·,

0.4 0.5 0.6

moisture content (kg w/kg ds)

Fig. 3.17 Comparison of bond enthalpy as a function of water content for the Veghel sludge

cake sample flocculated with 50 g FeC[/kg ds as derived from isotherm analysis (-) and

drying analysis (- -).

The transition from 'free' to 'bound' water is difficult to determine in both graphs.

However, it seems that this transition predicted by the sorption analysis occurs at

somewhat smaller moisture contents. The condusion based on the drying analysis, that

only a small part of water (about 10%) present in a sludge filter cake is chemically or

physically bound to the sludge particles, is confirmed by the sorption analysis.

3.4.4 Coulter Omnisorp 100

The Coulter Omnisorp 100 instrument is an automated gas sorption analyser to

measure both adsorption and desorption isotherms. Adsorptive gases that are common

ly used are nitrogen, oxygen, hydrogen, argon, krypton, carbon dioxide, carbon

monoxide, and water vapour.


helium nitrogen

V5

Pil

furnace vacuum pump

outgassing section physisorption section

Fig. 3.18 Schematic diagram of the Omnisorp 100.

Figure 3 .18 represents a schematical diagram of the Omnisorp 100. The instrument

may be considered to be split into two sections: the ontgassing section and the

physisorption section. The ontgassing section is fitted with a furnace for heatiug

samples under vacuum up to a temperature of 450 °C. The physisorption section is

used for measuring the vapour pressure of the adsorbate, and comprises of a manifold

(black area in tigure 3.18), a sample port A, and a reference port B.

The Omnisorp 100 instrument may operate in two different modes: the contiuuous

volumetrie and the static volumetrie mode. In the contiuuous volumetrie mode, the gas

is continuously dosed to the sample at a very slow rate (typically at 0.3 ml/min) by

means of a mass flow controller. In the static flow method, the adsorbate is supplied

from the reference bulb into the manifold and subsequently to the sample. The

different valves (V) are operated by the software of the Omnisorp. To performa water

vapour sorption measurement, the static flow metbod is used. A small volume of

deionized water is added to the reference bulb. Reference bulb and sample bulb should

be kept at the analysis temperature. The prevailing pressure in the reference bulb is

the saturation pressure of water at the analysis temperature (23 torr at room tempera

ture).

56 Chapter 3

At the start of an adsorption measurement the sample bulb is evacuated. The vacuum

system comprises of a rotary vacuum pump and a diffusion pump. After evacuation a

gas dose of known volume and pressure is supplied to the sample and this dose is

equilibrated with the sample. Manifold or sample pressure and saturation pressure of

the adsorbate are measured by three capacitance-type pressure transducers fi.tted to the

manifold. PO is the reference pressure transducer, which continuously measures the

satnration pressure of the adsorbate. The low-range transducer P1 provides high

resolution pressure measurements within the range of 0 to 10 torr. P2 is a high-range

transducer, with a full-scale reading of 1000 torr. Equilibrium is reached when a

number of consecutive sample pressure readings are within the equilibration tolerance.

The number of consecutive pressure readings and the toleranee are speci:fied by the

operator. The total volume of gas present in the sample bulb is calculated from the

difference between dose pressure and equilibrated pressure. The 'dead volume', which

is defined as the volume of the sample bulb where no adsorption takes place, is

substracted from the calculated volume to determine the volume of gas adsorbed by

the sample. The determination of the dead space is performed with helium, which is a

non-adsorbing gas.

When the equilibration criterium bas been met, the same vapour dose is again supplied

to the sample and a new equilibrium sample pressure will adjust. In this way a new

data point of the adsorption isotherm (relation between relative vapour pressure and

volume of gas adsorbed) is determined. Afterwards the sample bulb is dosed again

with water vapour. Thus the whole adsorption isotherm is determined. The sample

measurement time is generally 24 to 72 hours, and thus this technique is less time

consurning than the vacuum exsiccator method. Water vapour adsorption can be

followed by vapour desorption. If a desorption experiment is required, first the

reference bulb is equilibrated with the sample before the run is started. After equili

bration a certain gas dose is removed from the sample bulb, the sample pressure is

suddenly decreased, and a new equilibrium will adjust, etc.

In the scope of this research, some preliminary experiments were carried out with

sludge cakes. Figure 3.19 presents the result of a desorption experiment. The sludge

cake used bad a mass of 0.03 g and originated from the Mierlo sludge treatment plant.

This experimental result is compared with the results obtained with the vacuum

exsiccator method. Both results agree. The amount of data points determined by the

Omnisorp (99) is much higher than the amount of data points obtained by the vacuum

exsiccator method (12).


0.22

1).20

ll.UI -~ 0.16

g: étlll.UI ~om ::som

liJM

11.82

0.00

0.0 0.1 OJI 0.8 O.<l D.5 OJI 11.7 0.8 0.9 10

aw(-)

LI!GBND -- GAB • • • Omm * * * &*

Fig. 3.19 Desorption isothenns of Mierlo sludge cake jlocculated with 45 g FeC[/kg ds and

200 g Ca(OH)/kg ds determined with the Omnisorp and the vacuum exsiccator method. lhe

line represems the G.A.B. equation fitted on the results of the Omnisorp experiment.

Consequently, the accuracy of the fitted parameters is much smaller in tenns of the

95% confidence interval (see table 3.2).

Table 3.2 Parameters of the G.A.B. equation fïtted on the Omnisorp experiment and the

results obtained by the vacuum exsiccator method.

Omnisorp

k (-) 0.752 0.605

k 95% confidence interval 0.745 <k<0.759 0.454<k<0.756

27.896 45.87

26.806 <cg< 28.986 Cg<79.37

0.081 0.095

0.0801 <u1 <0.0821 0.077 <u1 <0.1138

58 Chapter 3

Especially in the low activity range (aw<O.l) a lot of data points are available. This

low activity range is very interesting for tbe purpose of tbis stndy. For 3w < 0.1 the

bond enthalpy differs significantly from zero.

With the Omnisorp tbe water vapour isotherm is measured of only one sample, where

as with the vacuum exsiccator metbod the isotherm is determined by measuring at

twelve samples simultaneously. The sludge cake samples may differ in composition

due to biologica! variations.

It was only possible to determine the water vapour sorptilln isotherm at one temperat

ure: room temperature. The Coulter Omnisorp bas to be modified. The physisorption

section may be air-thermostated so that sorption experiments can be measured at

different temperatnres (working range: 25 to 70 °C). In the near future more experi

ments have to be carried out at different temperatnres to delermine the bond enthalpy

accurately and to aim at a better onderstanding of the working principle of the

apparatns.

Calibration of somtion apparatus

Sorption properties of biological or food produelS may differ from each other due to

biological variations of tbe substrate, differences in equipment design, and different

procedures in handling. A collaborative research program named 'COST 90 project'

was initiated in 1985 to study possible influences of the measuring metbod applied on

the adjustment of equilibrium conditions over a wide range of water activities [Wolf et

al., 1985]. Microcrystalline cellulose (MCC) was chosen as reference material,

because this material shows well-defined and stabie sorption properties. The sorption

isotherm of MCC was measured in 32 laboratories, which all took part in tbis study.

The mean isotherm of MCC was fitted with the G.A.B. model. The results of tbe

COST 90 project were summarized in recommendations for the measurement of

sorption isotherms [Wolf et al. , 1985].

To investigate how far the vacuum exsiccator metbod and tbe Omnisorp 100 meet the

requirements of the COST 90 project, experiments were carried out with MCC. The

results are shown in figure 3.20. Botb experimental isotherms show acceptable ag

reement with tbe mean sorption isotherm determined in the scope of tbe COST 90

project. The vacuum exsiccator and Omnisorp 100 adsorption run gave data which

show a slightly lower moisture content. This may be attributed to differences in the

structnre of MCC (particle size distribution, porosity) used.

8.18

0.17 0.18 o.u o.H

IL18

:i'0.:12

f~ JfiUW :::to.oe

o.olll

o.oc o.oa o.oz o.m


o.oo.,~~~~~~~~~~~~~~~~~~~~~~~~~T

0.0 0.1 0.8

--- OOSTilO

O.<l O.IS

aw

• • • OMNI80

0.7 0.9

***Salt

59

Fig. 3.20 Adsorption isothermsof microcrystalline cellulose at 25 oe_ (-) COST 90 project;

(···) Omnisorp 100; (***) Vacuum exsiccator method.

Moreover, the suppliers of MCC used in the COST 90 project and the MCC used in

our experiments are not the same.

3.5 Conclusions

In order to study tbe solid-to-water bond strengtb in sewage sludge, two different

methods were used: tbermal analysis to determine isotbermal drying curves, and the

measuring of water vapour desorption isotberms at different temperatures. Witb botb

metbods it is possible to determine the water bond enthalpy as a function of tbe sludge

cake moisture content. Botb metbods show tbat tbe bond entbalpy differs significantly

from zero at sample moisture contents smaller than 0.3 to 0.6 kg water/kg ds. The

sludge origin, type of flocculant and flocculant dosage do not influence tbis critical

moisture content.

Moisture having a bond entbalpy smaller than 1 kJ/kg is classified as 'free water' and

moisture ha ving a bond entbalpy larger than 1 kJ/kg is called 'bound water'. 'Bound

water' is not removable in a mechanical dewatering process. So the maximum feasible

60 Chapter 3

dry solids content in a mechanical dewatering process amOWlts to about 65 to 75 wt%.

However, in practice dry solids contents of about 20 to 30 wt% are reached. The

'bound' water fraction does not contribute significantly to the sludge moisture

retention capacity. Probably the interstitial or interfloc water which bas been

entrapped during filter cake formation mainly dictates the final cake solids concentra

tion.

The experimental isotherms measured at different temperatures can be described very

well with the S-shaped temperature-dependent G.A.B. equation. Sewage sludge cakes

from different waste water treatment plants show minor differences in sorption

capacity. Moreover, the different additives and their dosages influence the sorption

isotherm to a minor extent.

4 SLUDGE DEWATERING CHARACTERISTICS

4.1 Introduetion

Sewage sludges produced by municipal waste water treatment works exhibit wide

variatious in their physical, chemica! and biologica! properties due to differences in

the types of waste waters and the design and operation of waste water treatment

plants. Seasonal and weather conditious can often complicate matters even further. By

no meaus can sewage studges beregardedas well-defined systems with 'excellent' and

constant properties.

In academie research it is common practice to perform studies with well-defined

model systems. However, with respect to sewage sludges this is not an easy approach,

because the ruling properties are not sufficiently known. In the past, model sludge

studies were carried out sporadically. Haustveit et al. [1977] used an Arthrobacter

pure culture as model sludge. The pure culture was insulated from an activated sludge

sample. An important property of these bacteria was their ability to form aggregates.

Over the years many types of tests have been developed to measure the specific

properties of studges in relation to partienlar forms of treatment, such as activated

sludge processing, stabilization, thickening, dewatering, tipping, and incineration.

Vesilind [1985] gave an overview of tests on sludges. The tests were categorized as

being either physical, chemica! or biological.

It is the objective of this study to establish a set of sludge dewatering characteristics.

A set that may be cousidered to be a fingerprint of the sludge. Successively four

sewage studges originating from four different waste water treatment plants have been

characterized (see section 2.7). Eropbasis was placed on evaluating characterization

tests used in previous studies, and on developing and reviewing potenrial new

methods. The practical relevanee of characterization tests to monitor and control the

performance of the operational sludge treatment plant was studied. Up to now the

determination of sludge dewatering characteristics at sludge treatment plants is still

rather ambiguous.

Moreover, an attempt was made to establish (cor)relatious between floc micropropert

ies and filtration behaviour of studges (macroproperties) in well-defmed laboratory

tests. The sludge microproperties that were studied are composition, zeta potential,

partiele size distribution, and rheological properties. The dewatering behaviour of

studges was studied in terros of average specific cake resistance, cake solids concen-

62 Chapter 4

tration, porosity, permeability, vacuum snction time, eapillary Suction Time,

concentration ferric ions and polyelectrolyte in filtrate. The macro- and micropropert

ies were studied by using different types and dosages of flocculants. The flocculation

process appears to be a very critica! operation which affects the dewatering properties

of the sludge to a great extent.

4.2 Plan of characterization research

As pointed out in the previous section, four different sewage sludges were characteriz

ed. In Appendix 3 the woricing scheme of the sewage sludge characterization research

is presented. Sewage sludge samples were collected from the various treatment plants

at positions in the dewatering system just before the sludge is mixed with flocculants

and transported to the dewatering equipment. The sludge samples were storedat 5 oe.

La Heij and Janssen [1990] measured filtration properties of a sewage sludge sample

as a function of the storage time. Their condusion was that the major changes in

filtration characteristics (increase in specific cake resistance) occurred during the first

day, when the sewage sludge sample is collected at the sludge treatment plant. Eikurn

and Paulsmd [1974] measured the specitic cake resistance of primary and primary

chemical sludges as a function of the storage time at 10 oe. During 18 days of storage

the specific resistance increased to four to nine times its original value. Therefore the

characterization research on one sewage sludge sample must be carried out in a

relatively short time (one week).

The following characterization tests were daily applied to unconditioned sludge

samples to check possible changes in composition: A TP content and esT value.

The next characterization parameters were determined as functions of the flocculant

dosage: electrical conductivity, pH, zeta potential, concentration of ferric ions and

polyelectrolyte in filtrate, eST value, vacuum suction time (VST), dry solids content,

average specitic cake resistance, permeability, partiele size distribution, thixotropy,

and bond enthalpy. Three different flocculants were used per sludge type: ferric chlo

ride/lime, the polyelectrolyte Röhm KF975, and the type of cationic polyelectrolyte

used at the waste water treatment plant concemed. Flocculation was carried out

according to the conditions prescribed intheSTORA manual [1982, 1983].

The determination of the bond enthalpy has already been discussed in chapter 3. The

determination of the zeta potenrial will be discussed in chapter 6.

Sludge dewatering characteristics 63

In the next sections, methods to determine sludge dewatering characteristics are

discussed and some typical results are shown.

4.3 Composition

The dry solicts content of a sewage sludge sample or a filter cake is determined by

drying it in a furnace at 105 oe for 24 hours [NEN 6620]. The dry solids content is

expressed in termsof weight percentage of solids.

The ash content is related to the inorganic fraction of the sludge solids and is meas

ured by burning the dried sample at 600 oe for 30 minutes [NEN 6620]. The 'loss of

ignition' equals the organic fraction.

The pH and electrical conductivity are ordinary Iabaratory routines and do not need

any further introduetion here.

Dry solids and ash content, pH, and electrical conductivity of unconditioned sewage

sludge samples collected from different sludge treatment plants and used in the

characterization research are presented in table 4 .1.

Table 4.1 Overview of pH, electrical conductivity, dry solids and ash content of the investig

ated unflocculated sludges.

sludge type dry solids con- ash content pH electrical

tent (% of sludge solids) conductivity

(wt%) co·lm•l)

Mierlo 2.6 25 6.35 ; Veghel 3.2 33 6.50 0.21

Amsterdam 2.4 35 7.38 0.54

Lage Zwaluwe 0.9 36 6.90 0.25

Sewage studges from Mierlo, Veghel, and Amsterdam pass a thickener prior to

dewatering. Thickening accounts for relatively higher initia! dry solids contents.

The ash content of the Mierlo sludge sample is smaller compared to the other sludges.

Mierlo sludge is not stabilized, whereas at the other plants biological stabilization

processes occur. During sludge stabilization the organic fraction of the sludge solids is

reduced (see section 2.3).

64 Chapter 4

The pH of Amsterdam sludge is higher than the pH of the other sludges. Amsterdam

sludge bas been anaerobically digested. Aerobic bacteria are virtually not present in

the sludge sample. These bacteria are responsible for the following reaction:

Hydrogen carbonate is weakly acidic in nature and as a result the pH decreases.

The electrical conductivity of Amsterdam sludge is two to three times the conductivity

of the other sludges. Apparently a larger amount of ions or small charged particles are

present in the sludge sample.

Addition of lime to sludge increases the pH to about 12 (see figure 6.14). An increas

ing amount of added ferric chloride decreases the pH (see figure 6.10). Ferric ions are

acidic in a neutral pH medium (see secdon 6.4).

The increase of the ferric chloride dosage raises the electrical conductivity of the

sludge suspension. At relatively high ferric chloride dosages (250 to 300 glkg ds) the

electrical conductivity is about 1 to 2 0·1m·1 [van Berlo, 1993].

The type and added amount of polyelectrolyte do not influence the electrical conduc

tivity and pH of the sewage sludge sample [van Berlo, 1993].

ATP (Adenosine-5'-Triphosphate) is a specific indicator of cell viability because it

exists only in living cells. A TP acts as an energy buffer for living cells. The quantity

of ATP can thus be used as an indicator of the amount of living biomass in sewage

sludge samples [Patterson et al., 1970]. The technique to determine the A TP content is

based on the light-producing (luminescent) reaction with the Iuciferase enzyme derived

from fireflies. Since the Iuciferase enzyme will not penetrate bacterial cells, the ATP

must be extracted and presented in a solution for measurement. The total light output

(photons) is directly proportional to the amount of ATP present in a reaction mixture.

The light intensity is measured with a spectrophotometer.

Figure 4.1 presents measured ATP contentsas a function of the storage time (in days)

of two different sludges. The storage temperature was 5 oe. For both studges the A TP

content slightly increased with storage time.

Sludge dewatering charaderistics 65

25

20 -fl.) "CS bO 15 El -bO ::s. ._

10 ~ E-4 <

5

0

________________ .,.. _____________________________ .,.. -------------

1 2 3

day

Fig. 4.1 Measured ATP content of two sludge samples as a junction of the starage time (in

days). (•) Mierlo sludge, ( •) Amsterdam sludge. Starage temperature was 5 oe.

However, a decrease of the ATP content would be expected. During storage at 5 oe living cells die off. Hanstveit et al. [1977] measured ATP contents of activated sludge

samples as a function of the storage time under aerobic and anaerobic circumstances at

15 oe. In most cases the ATP content decreased during storage time.

It can be concluded that the use of this technique to measure the A TP content of

sewage sludge samples is questionable [Eikel boom, 1994]. Many technique factors like

temperature, ionic concentration of the sample, and presence of enzymatic inhibitors

in the extracted samples influence the light emission. Moreover, there are some objec

tions to this technique when it is applied to sewage sludge samples. The biochemica!

reactions occurring during storage of the samples are unknown. Possibly A TP is

produced or ruptured in these reactions.

66 Chapter 4

4.4 Filtration and expression

The dewatering of sewage studges in mechanical dewatering equipment can be

subdivided into two pbases: the tiltration and subsequently the expression pbase [Yeh,

1985; La Heij, 1994]. At the start of the solid-liquid separation process, a pressure is

exerted on the sludge slurry. Solid particles and/or flocs are retained on the filter

medium which acts as the separating agent. A cake is built up from the filter medium

and the clear liquid (filtrate) passes through. The cake thickness L increases in time.

Because of the frictional losses arising from the liquid flow through the cake, there

will be a hydraulic pressure gradient. In the tiltration phase the fluid pressure near the

filter medium approaches zero and the hydraulic pressure at the slurry-cake interface

remains constant (eqnal to the applied pressure). The drag on each partiele is commu

nicated to the next particle. Consequently the (so-called) solid compressive pressure

increases as the medium is approached. At every position in the cake the sum of the

hydraulic and compressive drag pressure equals the applied pressure. The tiltration

pbase appears to be described by a model in which non-linear elastic material

behaviour was assumed [La Heij, 1994].

The expression pbase starts when the slurry has disappeared and the cake is com

pressed. A material is said to be compressible if the volume of the material reduces

when pressure is exerted on it. Sewage sludges are highly compressible materials. As

a result, the average liquid content of a sewage sludge cake changes with changing

pressure. The cake thickness decreases in time in the expression phase. At the end of

this phase the liquid content and hydraulic pressure are uniform throughout the cake

and the liquid flow is zero. The expression phase can be described with a non-linear

visco-elastic model [La Heij, 1994].

In order to simulate the tiltration and/or expression pbase at laboratory scale and to

characterize the dewatering behaviour of sludges, three measuring devices were used:

the filtration-expression cell (FE-cell), the Modified Piltration Test (MFT), and the

Capillary Suction Time (CST) apparatns. The compression-permeability cell (CP-cell)

was used to determine dewatering properties of filter cakes under equilibrium

conditions.

Sludge dewatering charaderistics

r---·-------------- sludge sample

---~air escape ~~--- gas pressure

------~ perspex cylinder

--~piston

-----------~ sludge cake ----~filter medium

Jr---f-·-~ filtrate

D~:l::::::--~ balance

clasp

Fig. 4.2 Schematic drawing of the filtration-expression equipment.

4.4.1 The filtration-expression cell

67

A schematic drawing of the developed filtration-expression cell (FE-cell) is presented

in tigure 4.2. The cell consists of a perspex cylinder (inner diameter 70 mm, height

100 mm) with a porons roetal plate which is covered with a filter paper (Schleicher &

Schuell, 5893, ref.no. 300209). At the beginning of the experiment a gas pressure

(range 0 to 1500 kPa) is suddenly exerted on the sludge sample. A filter cake is built

up and the liquid is collected in a beaker glass positioned on a balance. The balance

continuously registers the weight of the filtrate, from which the filtrate volume can be

derived. If expression is also carried out, a closed piston moves downwards and the

cake is compressed.

With this experimental set up it is possible to carry out experiments under different

process conditions, such as gas (or expression) pressure, type of flocculant, flocculant

68 Chapter 4

dosage, amount of sewage sludge (or final cake thickness) and mixing energies during

flocculation. In this characterization research only tiltration experiments were carried

out with this device.

Expression experiments, which take a few hours, were carried out by La Heij [1994],

who used the experimental data to verify the modelling of the dewatering behaviour of

sewage sludges; he also studied the influence of pressure, cake thickness and slurry

concentration on the expression behaviour. Dewatering parameters that may be

determined from the expression phase are equilibrium dry solids content, expression

time and visco-elastic parameters. The FE-cell may also be used to measure hydraulic

pressure and porosity profiles [La Heij, 1994].

The liquid flow through a compressible porous medium has been extensively described

by La Heij [1994]. The constitutive equations, relating porosity e and solidosity e. to

the compressive pressure p., and the relation between permeability K and compressive

pressure p, are one of the basic concepts of the model. These equations can be

determined by carrying out experiments with the compression-permeability cell (see

section 4.4.4). Since the CP-cell experiments are time-consuming (only one experi

ment per day) this measuring device was not incorporated in the working scheme of

characterization.

The compressibility of sewage sludges is very small in the tiltration phase. The

tiltration phase can therefore be described by the integrated form of Darcy's law for

incompressible media, in which tiltration time t is related to filtrate volume V

[Svarovsky, 1990]:

where A = cross-sectional area of the filter medium [m2]

<;, = concentration of solids in the suspension [kg.m-3]

LW= applied pressure difference [Pa] R",= filter medium resistance [m-1

]

aav = average specific cake resistance [m.kg-1]

11 = viscosity of filtrate [Pa.s]

(4.1)

In deriving equation (4.1) it is assumed that the pressure difference across the cake LW

is constant. The filter medium resistance R", can be obtained by doing an experiment

with pure liquid (cv=O). Since filter medium blinding may be regarded as a minor


problem, Rro may be assumed to be constant in the dewatering period [La Heij, 1994].

The average specifïc cake resistance am a characteristic of the sludge cake, follows

from a regression of the experimental data with equation ( 4.1). This resistance was

used as a characterization parameter in this study. All tiltration experiments were

carried out with 100 ml sludge (exclusive flocculants), at a constant applied gas

pressure of 200 kPa. Flocculation was carried out according to the conditions

prescribed in the STORA manual [1982, 1983]. Mixing of flocculant with sludge was

carried out with a standard stirrer positioned in a 250 ml beaker. The distance from

the stirring rod to the bottorn was 15 mm. In the case of flocculation with iron chlo

ride/lime, fust sludge was mixed with iron chloride by stirring at a speed of 1000 rpm

for 15 s. The suspension pH decreased (see tigure 6.10). Afterwards lime was mixed

with the sludge sample with by stirring at a speed of 500 rpm for 60 s. Mixing of lime

with sludge led to an increase of the sludge suspension pH to 12 (see tigure 6.14).

Precipitation of insoluble ferric hydroxide (Fe(OH)3) occurred. In the case of condi

tioning with po1yelectrolyte, s1udge samples were mixed with a stirring speed of 1000

rpm. In the experiments an optimum stirring time, which gives the highest dry solids

content, was used. The optimum stirring time depends on the type of sludge, type of

polyelectrolyte, and the dosage used, and was determined for every combination of

sludge type-polyelectrolyte dosage with the Modified Piltration Test (see section

4.4.2). In the figures 4.3 and 4.4 results are shown of tiltration experiments with

Mierlo sludge flocculated with iron chloride/lime and polyelectrolyte (Röhm KF 975),

respectively.

From these figures it can be concluded that the average liquid rate for the sludge

conditioned with polyelectrolyte is much larger than the liquid rate for the sludge

conditioned with iron chloride/lime.

In the figures Darcy's equation (equation (4.1)) is also presented. Although incom

pressible cake tiltration was assumed, the model describes the experimental data very

wen.

In both experiments a sudden increase in filtrate volume can be observed. This can be

attributed to formation of cracks in the filter cake. The experimental data just before

the moment the cake cracked were used to fit with Darcy's equation.

The average specific cake resistance strongly depends on the amount of flocculant

used.

Fig. 4.3 Filtrate volume as a function of time according to experiment (• • •) and model (-).

Mierlo sludge jlocculated with 100 g FeCVkg ds and 200 g Ca(OH)zlkg ds.

110

100

~ 90

- 80

j 70

80

~ 50

J 40

30 20

lO

0

• • • • • • •

o ~ 2 s 4 s a 7 a 9 20 n u u H u m ~ time (s)

••• __., __ tel --- Mod!al

Fig. 4.4 Filtrate volume as a function of time according to experiment (•••) and model (-).

Mierlo sludge jlocculated with 9 g polyelectrolytelkg ds.


In tigure 4.5 the specifïc cake resistance is depicted as a function of the dosage of

ferric chloride. At low iron chloride dosages, the specific cake resistance is relatively

high due topoor flocculation conditions. Only a small part of the sludge solid surface

was occupied with ferric hydrolysis products. As a result polymerization of the

hydrolysis products did virtually not occur. With increasing flocculant dosage the

average specific cake resistance strongly decreased. Polymerization of the ferric

hydralysis produelS occurred, causing the formation of flocs. After a certain dosage

the specific cake resistance slightly increased, due to restabilization of the particles. At

excess flocculant dosages, partiele surfaces became saturated with specially adsorbed

iron hydrolysis products. The sludge particles had a high and positive charge and

electrostatic repulsion prevented further flocculation. A minimum average speci:fic

cake resistance and thus an optimum flocculant dosage (or small range) were found.

At tbis optimum dosage the sludge can be dewatered at the highest rate. An optimum

iron chloride dosage (or small range) was found for every sludge investigated in tbis

study [van Berlo, 1993]. Optimum ferric chloride dosages were found in the range of

90 to 120 g FeClikg ds and were independent of the sludge type (and thus slurry

concentration).

0 20 40 60 80 100 120 140 160 180 200

dosage of iron chloride (g/kg ds)

Fig. 4.5 Average specific cake resistance aav as a function of the dosage of FeCl3 (Mierlo

sludge). The lime concentration was kept constant at 200 glkg ds.

72 Chapter 4

-i -~

1013 ~----------------------------------------~

• tS 0 g • -lli'J • .... ! • • -! •

<.> <.> ;::: .... <.> 0 c:::a.. lli'J

2 4 6 8 10 12

dosage of Röhm K.F97S (g/kg ds)

Fig. 4.6 Average specific cake resistance cxav as a function of the dosage of polyelectrolyte

Röhm KF975 (Veghel sludge). Optimum mixing conditions.

The dosage of iron chloride used in the sludge treatment plant in Mierlo was about 40

to 60 g/kg ds.

The same trends were found for sewage sludges floccnlated with different types of

polyelectrolyte [van Berlo, 1993]. In fignre 4.6 the specific cake resistance is depicted

as a function of the dosage of polyelectrolyte Rölnn KF975. The curve can be

explained by the consecutive phenomena: poor floccnlation, charge neutralization,

bridging mechanism and restabilization (see section 6.7). An optimum polyelectrolyte

dosage was also found: 10 glkg ds. The optimum polyelectrolyte dosage depends,

among other things, on the type of polyelectrolyte (molecnlar weight, charge density)

used. In the characterization research Rölnn KF975 was nsed as a standard polyelectr

olyte for which optimum dosages were found in the range of 9 to 13 glkg ds.

A rnininnJm specific sludge cake resistance at a certain coagulant (FeCl3 and cationic

polyelectrolyte Zetag 63) dosage was also found by Katsiris and Kouzeli-Katsiri

[1987]. Lotito and Spinosa [1990] used a cationic polyelectrolyte in their study and

found a minimum specific cake resistance at a certain polyelectrolyte dosage.

Typical differences in experimental results between sludges floccnlated with ferric

chloride/lime and polyelectrolytes are:


• The average specific cake resistance cx.v of studges conditioned with FeC13/Ca(OH)2

at optimum flocculation conditions is five to ten times larger than cx.v for sludges

flocculated with polyelectrolyte (at optimum conditions). As a result the average

dewatering rate in the flitration phase is larger for sludges flocculated with polyelectr

olyte compared to sludge flocculated with ferric chloride/lime. The type of polyelec

trolyte (trademark) has a considerable influence on the average specific cake resis

tance. It was found that a dosage of 10 g of Röhm KF975/kg ds to Veghel sludge

yielded a specific cake resistance cx.v of 0.22·1012 mlkg, whereas the same dosage of

Superfloc C496 resulted in cx.v equal to 7 .3·1012 m/kg.

• The amount of added ferric chloride to obtain the optimum laboratory dosage is

about five to ten times larger than the amount of added polyelectrolyte.

In an additional research the effects of other variables like time dosage and mixing

intensity were studied [Janssen et al., 1994]. Expression experiments, which took 15

minutes, were conducted at 300 k:Pa. Increasing the lime dosage deercases the average

specific cake resistance cxav. Lime which acts as an ordinary filling material reduces

the compressibility of the cake. Consequently the three-dimensional sludge matrix will

get stronger, less pores will collaps and this way the dewatering rate will be prom

oted. However, a high lime dosage is disadvantageous because it introduces an excess

amount of dry solids (increase of volume and mass). This leads to an increase of

transport, dumping, and incineration costs. Moreover, the cake can be expressed less.

At a lime dosage of 200 g/kg ds, the dewateriug efficiency1 is at a maximum.

However, at this optimum dosage filter cakes will stick to the filter cloth in chamber

filter presses. For this reason, practical lime dosages are higher than the optimum

dosage. This problem needs more investigatiou.

The mixing energy largely influences the dewatering behaviour if polyelectrolyte is

used as conditioner. In a series of expression experiments, Mierlo sludge samples

were mixed with 4 g of polyelectrolyte Nalco 41/62 per kg dry solids with a stirring

speed of 1000 rpm. The stirring time was varied. The results of the series of experi

ments are shown in figure 4.7. The average specific cake resistance is depicted as a

function of the stirring time.

1 The dewatering efficiency is defined as the percentage of the initia! water mass removed in a filtration-expression process.

74 Chapter 4

stirring time (s)

Fig. 4. 7 aav as a function of the stirring time. Mierlo sludge flocculated with 4 g of polyelectr

olyte Nalco 41/62 per kg dry solids. Stirring speed was kept constant at 1000 rpm.

An optimum stirring time of 5 to 10 s was found. Mixing of polyelectrolyte Nalco

41/62 with sludge initially leads to large aggregates. A higher mixing energy accom

plishes floc breakup due to the high shear forces. A smaller floc/aggregate effective

diameter reduces the dewatering rate (see section 4. 7). Mixing energy does virtually

not influence the dewatering rate when iron chloride/lime is used as flocculant. Small

aggregates (20-200 /Lm) are formed which are more resistant against high shear forces

(see section 4.8).

The filtration-expression cell is an automated instrument to study the dynamic

dewatering behaviour of sewage sludges. The influence of different parameters

(flocculant type and dosage, pressure, slurry concentration) can be investigated. The

FE cellis not labour-intensive and can rather easily be used at sludge treatment plants,

for instanee to control and optimize the flocculation process. More research is

necessary to find out the added value of the FE-cell at waste water treatment plants.


4.4.2. Modified Filtration Test

The Modified Piltration Test (MFT) bas been developed by TNO (Heide aod Kampf,

1983). ln figure 4.8 a schematic drawing of the MFT equipment, consisting of three

parallel tubes, is given. This test is a modification of the Büchner fiJtration test. ln the

set up shown, three experiments cao be carried out simultaneously. The test provides

insight both into the dewatering rate aod the final dry solids content after dewatering

[NEN 6691].

The test comprises a vacuum expression of conditioned sludge, which is carried out

with a 7 cm diameter Büchner funnel at 50 k:Pa for 10 minutes. Cracking of the sludge

filter cake is prevented in the following way. After the tiltration phase a plastic foil is

laid loosely over the Büchner funnel. On the foil a 2 cm layer of water is brought,

causing the plastic foil to fit closely at the upper side of the sludge aod the inside of

the funnel. After 10 minutes of expression the test is end ed.

Fig. 4.8 Schematic diagram of the MFT equipment.

The weight of the filter cake before aod after drying at 105 oe is determined. From

this test the following dewatering characteristics cao be determined:

76 Chapter 4

• 'Vacuum suction time (VST)', which is defined as the time needed to collect 60 mi

of filtrate. The VST is a measure of the average filtradon rate.

• Final dry solids content (MFTds) of the sludge cake based on total solids (sludge

solids plus flocculant plus additives).

• Water content of the sludge cake (MFTid) expressed in kg water per kg initial dry

solids in the sludge sample. This moisture content is corrected for the dry solids from

additives and provides a better judgement of the effect of e.g. the nature and amount

of flocculant on the dewatering process. Another parameter, which does not take into

account the amount of dry solids from additives, is the dewatering efficiency.

Results of MFT experiments carried out with Mierlo sludge flocculated with different

dosages of iron chloride are presented in figures 4.9 and 4.10. The lime dosage was

kept constant. Both the vacuum suction time and the MFTid show a minimum at a

dosage of 100 to 120 g of FeC13/kg ds. At this optimum dosage the sludges are dewat

ered fustest and the highest dry solids content is attained. Both the average specific

cake resistance (figure 4.5) and vacuum suction time (figure 4.9) show the sametrend

as a function of the ferric chloride dosage. Optimum flocculant dosages (always to be

regarded as a small range of dosages) were found with the MFT for other sludge

flocculant combinations [Heide and Kampf, 1983, van Berlo, 1993]. The VST of

studges flocculated with iron chloride/lime is typically ten times larger than the VST

of sludges flocculated with polyelectrolyte (at optimum stirring conditions).

The MFT is an inexpensive and suitable procedure for the characterization of sludge

for dewatering with belt presses, but less appropriate for cbamber filter presses.

Dewatering times in belt presses are about 10 minutes. Unfortunately, the dry solids

content attained in a MFT bas no predictive meaning for cbamber filter presses. This

can be explained by the rather different conditions in a cbamber filter press, viz. long

expressiou times and high cake thickness. The cake thickness in a MFT experiment

equals 5 mm and in the cbamber filter press a few centimeters. The cake thickness

influences the dewatering time: the dewatering time increases with the square of the

cake thickness [La Heij, 1994]. A disadvantage of the use of the MFT is that the

metbod provides no clear insight into the dynamic expression behaviour. Moreover,

only pressure differences in a small range (0 to 100 k:Pa) can be exerted.

600

500

400

E-t 300 1':1.2

> 200

100

0

Sludge dewatering charaderistics 77

0 20 40 60 80 100 120 140 160 180 200


Fig. 4.9 Vacuum suction time as a function of the dosage of FeCl3 (Mierlo sludge). The lime

dosage was kept constant at 200 glkg ds.

20

15

10

5

0 0 20 40 60 80 100 120 140 160 180 200


Fig. 4.10 MFTid as a tunetion of the dosage of FeCl3 (Mierlo sludge). The lime dosage was

kept constant at 200 glkg ds.

78 Cbapter4

4.4.3 Conventional CST apparatus

The conventional CST apparatus is a simple automatic instrument for determining the

dewaterability of sewage sludges in a quick way [NEN 6690]. CST was presented for

the first time by Baskerville and Gale [1968] and is still used as a dewatering test in

sewage sludge treatment plants. The conventional CST apparatus ( tigure 4.11) consists

of a cylindrical tube (inner radius 10 mm) resting on a reetangolar piece of filter paper

(Whatman no. 17 or Schleicher & Schuell ref. no. 382455). The filter paper is

sandwiched between two reetangolar plates of perspex. When sludge, or any other

suspension, is poured into the stainless steel cylinder, liquid will be sucked into the

paper nnder the influence of the capillary suction pressure and the sludge head. The

capillary suction pressure of the filter paper used is about 10 to 15 kPa [Baskerville

and Gale, 1968, Leu, 1981.]. Thus, with this apparatus only dewatering in the low

pressure range is studied. The liquid front will move in a radial direction, forming

more or less an ellipse due to the grain of the paper. At two different positions (r=6

and 13 mm) two electrodes are fi.xed in the perspex plates. When the liquid front

arrives at the first electrode, an electrical signal will be given to a chronometer,

whereupon time measurement will start. When the liquid front arrives at the second

electrode, the time measurement will end. The time needed to move the liquid front

from the first to the second electrode is called the 'Capillary Suction Time', abbrevi

ated CST. From this CST the dewaterability of the suspension can be estimated; a

small CST implies a good dewaterability. The CST value is, however, oot an intrinsic

dewatering parameter. The CST value depends on the slurry concentration and paper

properties (see section 5.5).

The CST value was used as a parameter in this characterization research. The result of

a series of experiments is presented in tigure 4.12. The CST value is depicted as a

function of the ferric chloride dosage added to Mierlo sludge. AB expected, a mini

mum CST value was fonnd. A minimum CST value at a certain inorganic flocculant

was also fonnd for other sewage studges investigated [Baskerville and Gale, 1968, van

Berlo, 1993].

Measurements of specific cake resistance aav (figure 4.5), VST (figure 4.9), and CST

(figure 4.11) as functions of the dosage of FeC13 gave the same results. lnorder to

correlate the specific cake resistance aav with the CST parameter one needs to specify

the slurry concentration.

Sludge dewatering characteristics

1: cylinder 2: sludge 3: sludge cake 4: electrode 5: timer 8: filter paper

Fig. 4.11 Schematic drawing ofthe conventional CST apparatus.

79

Eikurn and Paulsrud [1974] showed a fairly good relationship between the specific

cake resistance and the CST value divided by the total suspended solids concentration.

From our experiments it appeared that the conventional CST apparatus is not suitable

for unflocculated sludges and sludges flocculated with polyelectrolyte. The reproduc

ibility of CST measurements carried out with unflocculated studges is bad [van Berlo,

1993]. The dewatering process is very slow. Different cake structures are formed due

to the large inhomogeneity of the unflocculated sludge sample, resulting in large

deviations in the CST value.

Large sludge flocs/aggregates are formed when studges are flocculated with polyelec

trolyte (see section 4.7). Due to the small inner radius of the CST tube, cake forma

tion bardly occurs. Since hardly any resistance against liquid flow occurs in the tube,

small CST values are measured. In the scope of the study presented here a modified

CST instrument has been developed (see cbapter 5). The instrument provides the

possibility to continuously measure the position of the liquid front as a tunetion of

time and to calculate a specific cake resistance from the experimental data.

80 Chapter 4

200

lSO

-en -E-t 100 ti')

u

so

0 0 20 40 60 80 100 120 140 160 180 200


Fig. 4.12 CST value as ajunetion of the dosage of FeC/3 (Mierlo sludge). The lime concentra

tion was kept constant. Initia! dry solids content is 2.6 glkg ds.

4.4.4 The compression-permeability cell

The compression-permeability cell (CP-cell) was developed by Ruth [1946] and is

used to determine both the permeability and porosity as functions of the compressive

pressure. A schematic drawing of this cell is given in tigure 4.13. The cell consists of

a perspex cylinder with a porous metal bottom plate covered with a filter paper

(Schleicher & Schuell, 5893, ref. no. 300209). The double plated piston, which is

positioned in the cell, bas an upper solid plate and a lower porous plate. A sludge

(cake) s~ple is situated between the bottomsupport plate and the lower porous plate.

A gas pressure is exerted on the upper closed piston and in this way the sludge will be

expressed. The filtrate is collected in a beaker glass positioned on a balance, which

registers the weight of the water obtained. When the equilibrium situation bas been

reached, the mechanical pressure equals the compressive pressure p.. After equilib

rium the mechanical pressure is increased and the cake will again equilibrate. This is

repeated a few times.


,. ... ______ ··---IE·····)o-displacement transducer

Ps ~='IME-- mechanica! pressure

closed ..... ~plate

···•• PL hydraulic pressure

E porous

........ plate ................ sludge cake

••·•••· .... botlom support plate

Fig. 4.13 Schematic diagram of the compression-penneability cell.

The cake porosity E or solidosity E,

followiug equation:

1-E) at any pressure can be calculated from the

where

u U1 + -- d - +-E [ 1 u']

(1-E) w d, dw

d,= density of dry solids [kg solids.m·3 solids] dw= density of pure water [kg water.m·3 water] u moisture content [kg water.(kg solids)"1

]

u' bound water content [kg bound water.(kg solidsY1]

(4.2)

Bound and/or iutracellu1ar water is not responsible for the free liquid flow iu the cake

and thus does not contribute to the filliug up of the porous space. The bound water

content u' is taken as 0.4 kg bound water/kg ds (see chapter 3). The average pure dry

solids density d, of Eindhoven/Mierlo sludge is 1280 to 1300 kg/m3• The cake

moisture content u at any pressure can be determiued from the experimental data:

82

where

Chapter 4

U = nt.v,oo + (~,oo ~) md.

ruw= mass of dry solids [kg] m..v . ." = mass of water in the cake at the end of the experiment [kg] ám..v = loss of water mass at any pressure [kg] ám..v,,., = totalloss of water in the experiment [kg]

After the experiment the cake is dried, so ruw and m..v."" are known.

(4.3)

Following this experimental procedure both equilibrium porosity € 00 and equilibrium

solidosity e,,,., as :functions of the compressive pressure p, are obtained [Tiller et al.,

1980, La Heij, 1994]:

(4.4)

and

e =e0

[1 + P,ltJ '·"'

8 Pa (4.5)

where 'A and fi are compressibility (or compactibility) coefficients. e0 and €80 are the

porosity and solidosity respectively, when the compressive pressure p, equals zero. p.

is a constant making the equations dimensionless. Equations (4.4) and (4.5) are called

constitutive relations.

The permeability of the porous sludge cake is measured by passing water through the

cake. The liquid flow Q1 through the cake is measured by using a balance which is

connected to a computer. The cake thickness L can be measured with a displacement

transducer (see tigure 4.13). When the cake thickness L and the hydraulic pressure

drop .6.P1 across the cake are known, the equilibrium permeability K." can be calcu

lated with the integrated Darcy's law:

(4.6)

where 1J is the viscosity of the filtrate.


The dependenee of the equilibrium permeability K"' on the compressive pressure p, is

represented as:

(4.7)

Some of the typical results presented here are a part of the research carried out by La

Heij [1994]. The CP-cell was not incorporated in the working scheme of the sludge

characterization. The main reason was the rather long time it takes to carry out a

single experiment, i.e. one day. Only one sludge type was studied: Eindhoven sludge.

The power law functions as given by the equations (4.4), (4.5) and (4.7) showed to he

the most appropriate equations to describe the experimental data. Both the permeabil

ity and porosity decrease with increasing compressive pressure p,. The solidosity E,

increases with increasing compressive pressure. Permeabilities and solidosities versus

compressive pressure were measured for Eindhoven sludge flocculated with different

dosages of ferric chloride. Both the compressibility coefficients ó and B did not change

with the dosage. lt indicates that the dosage of iron chloride has a minor influence on

the compressibility of the cake. However, the dosage of iron chloride has a major

influence on the absolute permeability of sludge cake. An optimum flocculant dosage

of 100 g/kg ds was found.

CP-cell experiments with Eindhoven sludge flocculated with polyelectrolyte showed

that the compressibility (o=2.3) is higher than the compressibility of sludges

flocculated with iron chloride/lime ( ó = 1. 6). The higher compressibility is causcd by

the large weak flocs which are formed during polymerie flocculation (see section 4.7);

a higher value of Kv and lower value of E,0 were found. The initia! permeability Kv is

high, which results in quick dewatering during the initia! stage of the filtradon phase.

The average permeability in the filtradon phase K,.v is inversely proportional to the

average specific cake resistance aav according to [Tiller, 1975]:

(4.8)

A higher initia! permeability for studges flocculatcd with polyelectrolytes corresponds

with a smaller average specific cake resistance (see section 4.4.1).

84 Chapter 4

The polyelectrolyte dosage hardly influences the compressibility and the equilibrium

solidosity. After expression times of about a few hours, the sludge cakes are com

pacted to the same structure.

The CP-cell provides information on the dewatering behaviour of sludge cakes under

static conditions. It is a valuable instrument to determine the compressibility of sewage

sludges and the optimum flocculant dosages (in terms of permeability). However, the

time needed to carry out a single experiment is about one day.

4.5 Amount of iron in the filtrate

Knowledge about the iron concentration in the flltrate is important, as it possibly

provides insight into the flocculation mechanism induced by ferric chloride/lime.

Besides, from an environmental point of view, the amount of iron in flltrate must be

as small as possible.

The amount of iron in the flltrate can be determined by measuring the light absorption

of an iron complex with a spectrophotometer. First all the Fe3+ was reduced to Fel+.

Subsequently the flltrate was treated with 2,2 '-bi pyridine which formed a red coloured

iron-2,2' -bipyridine complex.

0 ..... CIO .::: 2.00 0.15 -.;::: 0 -s 0.12 .6 1.50 g • .... 0.09 ·-'"0 1.00 -8 6 0.06 '"0

CIO ..... 0.50 0

0 • 0.03 toO CIO i:i • ~ ~ 0 0.00 0.00 u .... 0 50 100 150 200 250 300 0 0.

dosage of FeCl3 (g/kg ds)

Fig. 4.14 Percentage of added iron in the filtrate (•) and iron concentration in filtrate (A) as

a function of the dosage of ferric chloride. Mierlo sludge jlocculated with 200 g of time/kg ds.

Sludge dewateriug characteristics 85

Cl) .... CIS .... 100 0.50 ...... -~ 90 -Cl) • ---5 -80 0.40 0 !:= 6. El ·- 70 • • lilt • -g = 60 • • 0.30 0 .... -·-·- 6. ............

"'0 50 ' CIS Cl) ~:=

"'0 40 0.20 - = "'0 6 Cl)

CIS 30 6. ä ..... 0

0 u Cl) 20 0.10 g t>Q 6 CIS 10 6 .... d ·-Cl) 0 0.00 u .... 0 50 100 150 200 250 300 350 Cl) Q"

dosage of FeCl3 (g/kg ds)

Fig. 4.15 Percentage of added iron in the filtrate (•) and iron concentration in the filtrate ( t:.)

as a function of the dosage of ferric chloride added to Mierlo sludge. No addition of lime.

The light absorption was measured at 523 nm, which is the optimum wavelength for

Fe2 + absorption. The iron concentration in the filtrate follows from the Lambert-Beer's

law.

Examples of two series of measurements are presented in figures 4 .14 and 4 .15. Both

the percentage of added iron in the filtrate and the iron concentration in the filtrate are

depicted as a function of the iron chloride dosage added to Mierlo sludge.

In one series the time dosage was kept constant at 200 glk:g ds ( tigure 4 .14). The

lowest percentage of added iron in the filtrate corresponded with the smallest average

specific cake resistance. At the optimum dosage of 150 g of ferric chloride/kg ds, the

absolute iron concentratien in the filtrate was minimal (10-6 mol/I). The smallest iron

concentration in the filtrate was also found at optimum ftltration conditions for the

other sludges investigated [van Berlo, 1993].

The different flocculation mechanisrns are responsible for the results found. Ferric

hydrolysis products are formed which specifically adsorb at the surface of the sludge

particles. Li.kewise precipitation of insoluble ferric hydroxide occurs, which stays in

the filter cake rather than passing through the filter medium. At a relatively small

ferric chloride dosage (25 g/kg ds) the percentage of added iron in the filtrate is

realtively high. This is probably caused by colloidal and small particles in the f:tltrate

86 Chapter 4

which contain adsorbed iron and disturb the measurement. At high ferric chloride

dosages ( > 200 glkg ds) a slight increase of the percentage of added iron in the filtrate

has been measured. All active sites on the surface of the sludge particles/flocs are

likely occupied by ferric hydrolyis products. Moreover, there may oot be enough lime

to precipitate all the FeH. The excess amount of added iron dissolves in the filtrate.

Figure 4.15 shows the measured iron concentation in the filtrate resulting from

experiments with sludges that were only conditioned with ferric chloride. The amount

of iron in the filtrate is much higher without lime than with lime being added to the

sludge. This is due to the :tàct that Fe(OH)3 is precipitated when lime is added (an

increase of pH toabout 12).

The condusion cao be drawn that the determination of the iron concentradon in the

filtrate seems to provide useful information on the flocculation mechanisms and

optimum dewatering conditious when sewage sludges are conditioned with ferric

chloride/lime. Experiments with this characterization test cao be performed simply and

quickly.

4.6 Polyelectrolyte concentration in the filtrate

Several techniques for determining the polyelectrolyte concentradon in the filtrate

were investigated [van Berlo, 1993]. The most promising method is the formation of

an ionene/cobaltphthalocyanine complex [van Welzen, 1989]. It is well known that

cobaltphthalocyanine (CoPc(NaS03) 4) in a neutral aqueous solution exhibits two

absorption maxima in the visible region, duetopartlal dimerisation according to:

2M~D

The dimeric complex D shows a maximum at a wavelength of 628 om, whereas the

monomer M has its maximum at 662 om. The presence of increasing amounts of

ionenes causes a shift of the absorption maximum from 662 om to 628 om (see

Appendix 4). The shift is thought to be the result of the formation of polynuclear

CoPc(NaS03) 4 aggregates. Phthalocyanine absorbances were measured with visible

light spectroscopy.

A certain amount of ionene (positively charged polymer molecules) is needed to shift

the absorption maximum. This amount depends on the type (trademark) of polyelectro-


lyte used (molecular weight, charge density). The minimum amount of polyelectrolyte

per g CoPc(NaS03) 4 was determined for every polyelectrolyte used in this study:

0.74 g of Röhm KF975/g CoPc(NaS03) 4 ,

9.7 g of Zetag 63/g CoPc(NaS03) 4 ,

71 g of Nalco/g CoPc(NaS03) 4•

Filtrate samples were analysed with the technique described. A certain volume of

filtrate is needed to shift the absorption maximum from 662 nm to 628 nm and

corresponds with the predetermined amount of polyelectrolyte. The ratio between the

amount of polyelectrolyte and the filtrate volume equals the concentration of polyelect

rolyte in the filtrate. Unfortunately, it appeared that a quantitative determination of the

polyelectrolyte concentration in the filtrate is not possible. The shift of the absorption

maximum was not clear (high noise level), because of the disturbance of the measure

ment by colloidal particles. However, a trend could be observed that an increase of

the amount of polyelectrolyte added to sewage sludge increases the polyelectrolyte

concentration in the filtrate. The amount of colloidal particles in the filtrate may be

minimized by centrifugation of the filtrate samples prior to the absorption measure

ment. More investigation into this problem is needed.

4. 7 Partiele size distribution

The size of sludge aggregates (flocs) is of major interest in thickening and dewatering

operations. Karr and Keinath [1978] experimentally determined the significanee of

partiele size distribution of sewage sludges. Sludge solids were fractionated into the

following size ranges: settleable (> 100 p.m), supracolloidal (1 to 100 p.m), colloidal

(0.001 to 1 p.m) and dissolved ( <0.001 p.m). Sludge samples with high supracolloidal

solids concentrations had high specific cake resistances.

The microscopie appearance of different sludge samples under investigation are shown

in tigure 4.16. The appearance of unflocculated sludge (figure 4.16 a) is not clearly

defined, owing to a great number of dispersed particles in the liquid phase. Studges

flocculated with iron chloride and polyelectrolyte Röhm KF975 display flocs and a

few dispersed particles ean be seen in the liquid. The nature of the sludge flocs

produced by conditioning with ferric chloride is different compared to that of sludge

flocs obtained by flocculation with polyelectrolyte (see tigure 4.16b and 4.16c).

88

1000 J.tffi (a)

Chapter 4

1000 JLffi

(c)

1000 J.tffi

(b)

Fig. 4.16 Microscopie photographs of sludge: a) uriflocculated, b) flocculated with iron chlo

ride, and c) flocculated with polyelectrolyte Röhm KF 975.

The 'image analysing technique' was used in this research to examine the partiele size

distribution of a sewage sludge sample. The experimental equipment consists of an

optical microscope, a camera and a computer (figure 4.17). A picture of a sludge

sample was registered and digitalized and shown on the screen of the computer

system. The projected surface area of the particles/flocs was determined with the TIM"

package and subsequently mathematically converred to an 'effective diameter' of


spherical particles tbat have the same projected surface area as those of the measured

particleslflocs. Results were compiled to produce a partiele size distribution.

Different types of partiele size distributions can be defined: partiele size distribution

by number, length, surface or mass (volume). In this study the partiele size distribu

tion was based on the number of particles within a size interval.

camera

microscope

sludge sample

fl.~·. /. -.'. =

'image analysing'

Fig. 4.17 Schematic diagram of the image analysing equipment.

The distributions are given as either frequencies f(x) or undersize cumulative fre

quencies (expressed as fractions or percentages) F(x). The measured undersize

cumulative partiele size distribution is approximated by the Harris three-parameter

equation:

F(x) 1 _ [ 1 -(~)·f (4.9)

where Xo is the maximum diameter in the distribution and a1 and b1 are constants. The

frequency distribution is obtained by differentiation of equation (4.9).

90 Chapter 4

(4.10)

There is a great number of definitions for averaged partiele sizes. In this study, the

median diameter is defined as the partiele size for which half of the particles is larger

and half is smaller, i.e. the size which divides the area under the distribution fre

quency curve into two equal parts. Thus, at the median diameter F(x)=0.5.

The partiele size distribution of sewage sludge samples conditioned with different

types and dosages of flocculants were determined and evaluated according to the above

method. Figures 4.18 and 4.19 show the calculated partiele size distributions of

studges conditioned with different dosages of iron chloride and polyelectrolyte,

respectively. The calculated distributions were obtained by determining the three

parameters of the Harris equation. Sludge samples were flocculated at optimum

mixing conditions. It appeared that deviating the stirring and mixing conditions will

change the partiele size distribution, especially for sludges conditioned with polyelect

rolyte [van Berlo, 1993].

The effect of an increasing amount of flocculant is a shifting of the distribution curves

towards greater diameters. However, at sufficiently high dosages shift in size to larger

particles does virtually not occur any more (see tigure 4.20). This is attributed to

restabilization of the particles.

Typical values for the median diameter are:

- 5-10 pm for particles in unflocculated sludges,

- 8-100 pm for particles/flocs in sludges flocculated with ferric chloride,

- 500-2000 pm for particleslflocs in studges flocculated with polyelectrolyte.

In tigure 4.20, the median diameter is depicted as a function of the polyelectrolyte

dosage. Larger-sized particles (in terms of the median diameter) result in correspon

ding reductions in the sludge specific resistauce (see tigure 4.6). This empirical

relation was also found for other sludge-flocculant combinations [van Berlo, 1993;

Knocke and Wakeland, 1983]. Knocke and Wakeland [1983] examined the impact of

the floc size distribution on the flitration rates of metal hydroxide sludges. Any

treatment varlation (e.g. polymer addition or mixing intensity) that resulted in a shift

in partiele size distribution had a corresponding effect on the specitic resistauce of

metal hydroxide sludges.

Sludge dewateriug cbaracteristics 91

0.045

e 0.040

Sr 0.085 120 .......,

~ 0.030 40

t 0.025 - 80 0.020

ts 60 0.015 = :8 0.0:10

~ 0.005

0.000 ---- -..:-_::.:_-.::.:.::::. -::::...::-- - -

0 25 50 75 100 :125

diameter (J.tm)

Fig. 4.18a Partiele size distributions of Veghel sludge flocculated with different dosages of

iron chloride. The numbers in the graph represent the iron chloride dosage (in glkg ds).

1.0

--- 0.9 I

-o.a t ~7 0.8

~ o.5

j 0.4

0.3

0.2

0.1

0 25 50 75 1.00 1.25

diameter (}..t,m)

Fig. 4.18b Cumulative partiele size distributions (Harris equation) of Veghel sludge flocculat

ed with different dosages of ferric chloride. The numbers in the graph represent the iron

chloride dosage (in g!kg ds).

92

0.001.5

e ~ 0.0012 ........, til

~

i 0.0009

'êS 0.0006

@ û 0.0003

~ 0.0000

I

0

f I I,

I I

' I

Chapter 4

500 1000 1500 2000

diameter (,.Lm)

Fig. 4.19a Partiele size distriburions of Veghel sludge jlocculated with different dosages of

polyelectrolyte Röhm KF 975. 1he numbers in the graph represent the polyelectrolyte dosage

(in g!kg ds).

1.0

..-0.9 I

-o.s

j 0.7

! 0.6

(3o.5 j:: j 0.2

0.1 ..... --.--T""""""T""..,.......--.--..-.-...... --.--.,....,....,.......-.-T""""""T""..,-.-....-r..,.......-.-..-.-.,....-.-..-.-..,.......-.-.......... .,...

0 500 1000 1500 2000

diameter (J.Lm.)

Fig. 4.19b Cumulative partiele size distributions (Harris equation) of Veghel sludge flocculat

ed with different dosages of polyelectrolyte Röhm KF 975. 1he numbers in the graph represent

the polyelectrolyte dosage (in glkg ds).


800 • - • •

El 600 ::s. • - • ~ -Q.)

f1 400 ;a

f;1 ·-"Ct 200 ä • • 0 0 2 4 6 8 10 12

dosage of KF975 (g/kg ds)

Fig. 4.20 Median diameter versus the dosage of polyelectrolyte Röhm KF975.

CampbeU and Crescuolo [1982] investigated the impact of the sludge partiele size

distribution on the rheological behaviour. They determined partiele size distributions

using a Coulter Counter and found that a raise of the polymer dosage from 0 to 4 kg/t

dry solids increases the mean partiele size from 19 p.m to 14 7 p.m.

The specific surface <1v of a partiele is defined as the surface of the partiele divided by

its volume. Por a sphere, the specitic surface is inversely proportional to the partiele

diameter x:

6 (4.11) x

The specific surface distribution by number can be obtained by substitution of equation

(4.11) in equation (4.10):

f(a,.) (4.12)

In tigure 4.21, the frequency distributions of the specific surface of Veghel sludge

conditioned with different dosages of polyelectrolyte KF975, is given.

94

a ~ ......... til Q)

i ~

'ïS 8 t3 J)

0.0015

0.0012

0.0009

0.0006

0.0003

o.oooo 0.000

Chapter 4

'. f

. '

-2

0.005 0.010 0.015 0.020 0.025 0.030 0.035

specific surface (1/ /Lm)

Fig. 4.21 Specific suiface distributtons of Veghel sludge flocculated with different dosages of

polyelectrolyte Röhm KF975. The numbers in the graph represent the polyelectrolyte dosage

(in glkg ds).

Any shift in size to larger particles will rednee the specific surface area per unit

weight of slndge solids in the slndge matrix with a corresponding rednetion in surface

shear stresses encountered during dewatering. Conseqnently, the specifïc cake

resistance is smaller. Surface shear stress results from the difference in the partiele

and fluid velocity and is obtained by dividing the fluid drag force by the surface area

ofthe floc.

Theoretica! equations re lating the permeability K ( or specific cake resistance) of a

porons bed to the specitic surface area (or partiele diameter) like the Kozeny-Carman,

Happel-Breuner and Brinkman equations cannot be used for sludge filter cakes [van

Veldhuizen, 1991]. For iustance, the Kozeny-Carman equation is given by:


K (4.13)

where E is the porosity, 3v is the specitïc surface area, <4 is the median particle!floc

diameter, and kKc the so-called Kozeny constant, which is specitic for every porons

system. The Kozeny-Carman equation was derived for Poiseuille flow in an incom

pressible bed, whereas sludge is a highly compressible materiaL Moreover, the

specitic surface area of sludge particles!flocs is not constant during compression, but

will change. Because of this time-dependent complex structure and extremely compli

cated liquid flow, in this case empirica! relations should be used. The measured

overall permeability of a sludge cake is determined by liquid flow alongside the sludge

flocs as wellas through the flocs. Combination of equations (4.4) and (4.7) yields an

experimental relationship between permeability K"' and porosity E00 of a sewage sludge

cake, which takes into account all the sludge dewatering properties and conditions

mentioned:

(4.14)

Allthough it is difficult to obtain a representative and objective measurement of the

size of flocs/particles due to the irregular floc shapes, changes in measured partiele

size distribution due to different flocculant dosages can be determined and correlated

with the average specific cake resistance. One should also notice that the measured

particle!floc size distribution is a distribution of unstressed particles. Another disad

vantage is the labour-intensiveness of the measuring technique, which makes this

characterization testnotpractical for sludge treatment plants.

5.8 Rheological properties

The influence of chemica! conditioning on the rheological properties of sewage sludge

was exarnined by using a Searle type coaxial rheometer (Contraves Rheomat 115). A

schematic diagram of the rheometer is given in tigure 4.22.

A sewage sludge sample is introduced into the small gap between two cylinders. The

inner cylinder (Mooney-Ewart spindle) rotates and the outer cup, which holds the

sample, remains stationary. In an experiment the angular velocity w of the spindie is

adjust and read

Cbapter 4

motor

measurenutnt of torque

Fig. 4.22 Schematic diagram of the Searle type coaxial cylinder rheometer.

increased in 15 small time-steps (5 seconds) until tbe maximum speed is reached and

is subsequently decreased to zero, also in 15 steps.

The shear rate i' is related to the angu1ar velocity w according to:

. dw R.vw 'Y = r- = -- 13.36w

dr Rb-Rs (4.15)

where Rt, is tbe radius of tbe outer cup (2.425·10-2 m), R. the radius of the inner

spindie (2.25·10-2 m) and Rav = (Rt, + R.)/2. The maximum shear rate reached in an

experiment is 1008 s·1• The torque T needed to rnaintaio tbe angu1ar velocity is

measured and converted to a shear stress r:

T T T=-----

211T2l 2'11'R!l (4.16)

where lis the lengthof the iuner cylinder. Equations (4.15) and (4.16) are valid when

the gap widtb (Rt,-R.) is much smaller than the radius of tbe outer cylinder. The appar

ent viscosity rJ of tbe suspension is defined as:


T (4.17) .y

The result of an experiment is called a rheogram. The flow in a rheometer must be

laminar. Turbulence occurs in a Searle type rheometer above a critical Reynolds

number Recrit:

Re " ~R,(R" -R,)p > 41.3 J R" Re"' 11 Rb -R,

(4.18)

The turbulent flow consists of so-called Taylor vortices. Inequality (4.18) means that

Taylor vortices hold out above an apparent viscosity rJ > ')r/50 mPas.

A typical rheogram of a sewage sludge flocculated with iron chloride is presented in

tigure 4.23. From this rheogram the following information is obtained:

• The rheological behaviour of sewage sludge can be interpreted as pseudoplastic

flow, also called shear thinning, which is presented with the following equation:

(4.19)

where T0 is the yield stress and 'flv(.Y,t) the plastic viscosity, which depends on the

shear rate and time. The yield stress is the minimum stress which must be overcome

before true flow occurs. Below this yield stress, molecular bindings and interaction

between particles (London-van der Waals force, electrastatic repulsion) result in a

three-dimensional floc network. Above To the floc networkis ruptured and sludge flow

may occur. There is no strong and extensive floc network in sewage sludge, so the

yield stress To is very small. The plastic viscosity k decreases with increasing shear

rate, which may be the result of the rearrangement of the particles/flocs in the flow

direction.

At each shear rate an equilibrium exists between the aggregation of particleslflocs and

the degradation of flocs. The equilibrium floc size is determined by the various forces

acting on the aggregates: London-van der Waals attraction, electrostatic repulsion,

hydrodynamic forces, and thermodynamic forces like the Brownian motion. Both the

London-van der Waals force and electrostatic repulsion increase with increasing floc

diameter ( equations ( 6. 18) and ( 6.14)). The repulsive hydrodynamic force arises from

the distartion of the fluid flow due to the presence of particles, and increases with

increasing shear rate. The equilibrium size also depends on the flow history.

98 Chapter 4

13

12

11

~ 10 9

1:- 8

I 7 6

l 5 4

3

z 1

0

0

shear rate j (1/s)

Fig. 4.23 Shear stress as a function of shear rate for sludge flocculated with 90 g iron

chloride/kg ds; (-) Ascending curve, (---) Deseending curve.

Differences in equilibrium median floc diameter due to different constant shear rates

could, however, not be observed with the image analysing technique. CampbeU and

Crescuolo [1982] measured only a slight increase in mean partiele diameter from 50 to

60 p.m after the particles had been sheared to 113 s·1•

The plastic viscosity 'lip can be determined as a function of time in a stationary shear

experiment. It appeared that for flocculated studges the viscosity decreases with time

[van Berlo, 1993]. The decline in viscosity is stronger for higher dosages of

flocculant. Unflocculated sludges show no rednetion of viscosity as a function of time.

• The change of the slope in the curve at a shear rate of 700 s·1 indicates the onset of

turbulence. The slope change depends, among other things, upon the type of sludge

used. CampbeU and Crescuolo [1982] determined a break in the rheogram of anaerob

ically digested sewage studges at a shear rate of 400 to 500 s·1•

• 1t is notabie that the deseending curve lies below the ascending curve. This loss in

shear strength of the sludge flocs is called thixotropy and is a typical sludge property.

The rheology of the floccnlated sludge suspension has been altered during the initia]

phase of the test. At increasing rates the floc network breaks down, whereas at


decreasing rates the flocs are allowed to regrow. However, the end stress does not

coincide with the yield stress 7 0• lt indicates that no full bnilt-up of the original floc

structure occurs. This irreversible loss in viscosity is called rheodestruction.

Thixotropie behaviour of anaerobically digested sewage sludges was also found by

Campbelland Crescuolo [1982].

The surface area between the two curves can be interpreted as the dissipated power

per unit volume and is a measure of both the degree of thixotropy exhibited by the

sludge, and floc strength of the aggregates. In tigure 4.24 the degree of thixotropy is

depicted as a function of the dosage of ferric chloride added to Veghel sludge.

Unconditioned sludges do not show thixotropie behaviour. The particles in unfloccu

lated sludges appear to be stabie and are relatively unaffected by shear. As the ferric

chloride dosage increases, the sludge sample becomes more susceptible to shear and

some breakdown in floc structure may be occurring. Studges flocculated with ferric

chloride/lime and polyelectrolyte exhibit a maximum value of the degree of thixotropy

(or dissipated power per unit volume) at a certain flocculant dosage. Different sludges

investigated exhibit different valnes of viscosity and degree of thixotropy [van Berlo,

1993].

Some problems arise when studges conditioned with polyelectrolyte are investigated

with the rheometer used. Relative large aggregates are formed (median diameter 500

to 2000 /Lm), which may be ruptured as a result of the small gap width (1750 /Lm). A

spindie having a smaller diameter should be used. It might be advantageous to study

this problem in more detail.

Partiele size analyses were conducted in conjunction with the rheological studies. The

partiele size distribution determined from the same sewage sludge sample has been

depicted as a function of the ferric chloride dosage in tigure 4 .18a. The partiele size

distribution and the changes that occur in the distribution as a result of chemical

conditioning will have a significant impact on the rheological behaviour. A maximum

degree of thixotropy (minimum floc strength of aggregates) corresponds with a

maximum median partiele size of the initially flocculated sludge sample. The maxi

mum degree of thixotropy is reached at the optimum flocculant dosage for dewatering.

This empirica! relationship has also been found for other sludge-flocculant combina

tions. The more fragile structure of larger flocs is due to their formation process in

which smaller more dense aggregates have collided which remained linked at their

points of contact, resulting in a more porons arrangement of lower overall density and

hence in a weaker structure [François and Haute, 1985].

100 Chapter 4

600 • •

ç;" a

400 ~ • t • 0 • b ~ 200 • :s

• 0 0 20 40 60 80 100 120


Fig. 4.24 Degree of thixotropy as a junction of the dosage of ferric chloride added to Veghel

sludge.

The general condusion can be drawn that the Searle type rheometer is a proper and

fust instrument to determine the degree of thixotropy (measure of floc strength) of

unflocculated sludges and sludges flocculated with ferric chloride/lime. The degree of

thixotropy is a maximum at the optimum flocculant dosage for dewatering. CampbeU

and Crescuolo [1989] developed an on-line measurement of rheological properties

( especially a rheogram) that could he used to control chemical conditioning at belt

presses.

4.9 Conclusions

In order to simulate the filtration-expression process at laboratory scale, four charac

terization tests for dewatering were used: Filtration-Expression cell, Modified

Piltration Test, Capillary Suction Time apparatus, and Compression-Permeability cell.

These tests provide information on the dewatering rate (in terros of average specific

cake resistance, permeability, vacuum suction time, and CST value) and final dry

solids content. The following sludge microproperties wbich were considered to he of


relevanee for a better understanding of the sludge dewatering process were measured:

composition (pH, electrical conductivity, and ATP content), partiele size distribution,

and rheological properties (floc strength, degree of thixotropy). The sludge macro- and

microproperties mentioned were determined as a function of the flocculant dosage.

Three types of flocculant were added to the sewage sludge samples: ferric chlor

ide/lime, the cationic polyelectrolyte KF975, and the cationic polyelectrolyte used at

the sludge treatment plant. It appeared that the flocculant dosage has a large impact on

the dewaterability of sewage sludges. The type of sludge and thus the design of the

plant and the operation of the waste water treatment have a minor influence on the

dewatering behaviour of sludges. At the optimum flocculation conditions (flocculant

dosage and mixing conditions) some characterization parameters show a minimum or a

maximum:

• mimimum specific cake resistance,

• minimum vacuum suction time,

• minimum CST value,

• minimum MFfid,

• minimum iron content in filtrate,

• maximum permeability,

• maximum median floc diameter,

• maximum degree of thixotropy.

At the optimum flocculation conditions, sewage sludges can be dewatered at the

highest rate and also the highest dry solids content is reached. A large median floc

diameter is accompanied by a small dewatering rate. Large-size particles!flocs

decrease the specific surface area with a rednetion in shear stresses during dewatering.

Larger aggregates are weaker in nature (high degree of thixotropy). This is due to the

level organization of the floc aggregates: primary particles, dense aggregates

(flocculi), flocs and floc aggregates. The minimum iron content in the filtrate at the

optimum dosage indicates a maximum occupation of ferric hydrolysis products which

specifically actsorb at the sludge solid particles.

Typical differences in dewatering behaviour of sludges flocculated at optimum

conditions between ferric chloride/lime and polyelectrolytes are:

• Both the average specific cake resistance and vacuum suction time of sludges

conditioned with ferric chloride/lime are five to ten times larger than those of sludges

flocculated with polyelectrolyte. Typical values for the specific cake resistance are in

102 Chapter 4

the range of 1013 to 1014 m/kg for s1udges flocculated with iron chloride/lime and in

the range of 1011 to 1013 mlkg for sludges flocculated with polyelectrolyte.

• The compressibility coefficient of sludges flocculated with polyelectrolyte (o=2.3)

is higher than that of sludges conditioned with ferric chloride/lime ( o = 1. 6).

• The median floc diameter of sludges flocculated with polyelectrolyte is 5 to 20

times higher than that of s1udges flocculated with ferric chloride/lime. Typical median

floc diameters are 8 to 100 J.tiD for flocs in s1udges flocculated with iron chloride, and

500 to 2000 J.tiD for sludges flocculated with polyelectrolyte.

The type (trademark) of polyelectrolyte may have a considerable influence on the

dewatering results.

Some of the characterization tests are considered to be useful tools in sludge treatment

plants. On the basis of the characterization research carried out, the following ranking

of dewatering tests recommended for application at sludge treatment plants can be

presented:

1. Filtration-Expression cell

2. Modified Piltration Test

3. Spectrophotometer to determine iron content in filtrate

4. Image analysing technique

5. Modified CST apparatus

6. Rheometer

7. Compression-Permeability cell

By making choices for these tests one should also consider:

• The significanee and usefulness of the information obtained for the mechanical

dewatering process. Tests may be used for diagnostic properties, for adjustment of the

flocculant dosage and for regulation of pressure control inchamber filter presses.

• The rapidity and simplicity of the method.

• Investment and maintenance costs.

The determination of dry solids content and ash content are useful characterization

tests, but are not incorporated in the order of tests. The pH, electrical conductivity,

and A TP content do not give useful information on the mechanical dewatering of

sludges.

5 MODIFIED CAPILLARY SUCTION TIME (CS1) APPARATUS

5.1 Introduetion

In section 4.4.3 the conventional CST apparatus is described. The conventional CST

apparatus bas, however, some disadvantages. Firstly, filter paper often differs in

structure, resulting in a different permeability and suction pressure. These differences in

paper negati vely influence the reproducibility of the measurement. Secondly, the validity

of a theoretical model descrihing the filtration process for a CST apparatus is difficult to

check when the position of the liquid front is only known at two times. Thirdly, the

CST value is dependent on the slurry concentration. CST valnes reported without the

slurry concentration could therefore be misinterpreted. The slurry concentration

influences the thickness of the formed cake and thus its resistance to flow. Since the

proposal of Baskerville and Gale [1968], several investigators have discussed CST

problems [Leu, 1981; Unno et al., 1981; Vesilind, 1988; Vesilind et al., 1988;

Dohänyos et al., 1988; Tilier et al., 1990; Lee and Hsu, 1992, 1993]. A theoretica!

model descrihing liquid flow in a CST apparatus, and some experimental results were

presentedinall of these papers.

The purpose of the research presented in this chapter can be subdivided into three parts:

• to deduce a theoretica! model that describes the position of the liquid front in a CST

apparatus. With the model an average specific cake resistance can be calculated from

experimental data;

• to develop a continuons CST apparatus with which reliable reproducible data can be

obtained;

• to carry out experiments to verify the model calculations and to be able to calculate

the average specific cake resistance of flocculated and unflocculated sewage sludges.

The advantage of a speci:fic cake resistance is that it is an intrinsic value, while the

conventional CST valnes changes, e.g. with the slurry concentration.

104 Cbapter 5

5.2 A theoretical model descrihing the liquid flow in a CST apparatus

The dewatering process in a CST apparatus is in fact a tiltration process in which the

capillary pressure of the filter paper is the main deiving force. The dewatering in a CST

apparatus consists of two consecutive processes, namely the tiltration of sludge in the

cylindrical tube, and the peneteation of filtrate into the filter medium. It is assumed that

the structnre of the filter medium is isotropie so that the shape of the liquid front will be

circular.

The model is based on four equations. Two equations describe the pressure difference

across the sludge layer and two describe the pressure difference across the filter

medium.

1. According to Darcy's law the following equation is obtained for the pressure

difference across the sludge cake.

1 dV f!U."C dV2

L\P. =A K"" dt = 2A 2 dt I

(5.1)

where L\P is the pressure difference across the sludge cake, A the area of the cross-s

section of the inner CST tube, 11 the viscosity of the filtrate, L the thickness of the

sludge cake, K the average permeability of the sludge cake, V the filtrate volume, C av

the cake mass deposited per unit flltrate volume, and a the average speci:fic cake av

resistance. The initial condition is: t=O; V =0.

2. The pressure difference across the sludge cake is the sum of the head exerted by the

sludge layer and the suction pressure exerted by the filter medium.

F' L\P = p gH+-• • A (5.2)

where p is the sludge density, g the gravitational acceleration, H the height of the s

sludge layer, and F' the suction force exerted by the filter medium under the CST tube.

3. As in the filter medium liquid flows in radial direction, Darcy's law must be

expressed in the following manner:

dP dr

dV r=r0 P=P0 (5.3)

Modilied capillary soction time (CST) apparatus 105

where P is the hydraolie pressore, P the hydraolie pressure at position r , r the position 0 0

of the liquid front at time t, r the position of the liquid front at time t=O (i.e. the 0

internal radius of CST tube), h the thickness of the filter medium, and ~ the

permeability of the filter medium.

Solving this differential equation gives:

p -P= .1.P = _11-.-1 ( !:._)dV ° F 2nhKF UCr0 dt

(5.4)

In this equation .1P F expresses the pressure difference across the filter medium.

4. The last ofthe foor equations is analogons to equation (5.2).

2ruh.1PF = 2nrhf3y cose- F' (5.5)

where y is the interfacial tension of the ftltrate, and f3 the reciprocal hydraolie radius.

The product j3ycose is equal to the capillary suction pressure P . The reciprocal cap

hydraolie radius f3 is introduced since the driving force for the flow of liquid in a

capillary medium depends on the measure of wetring of the surface by the liquid.

Combination of equations (5.1), (5.2), (5.4) and (5.5) , and where the area of the cross

section of the CST tube is introducedas nr02

, leads to:

21tlbf3y cose (5.6)

The liquid flow in the capillary medium is treated as a displacement process. The liquid

volume in the capillary medium is equal to the porosity times the wetted ceramic

volume. This is equal to:

(5.7)

Equation (5.6) can now be written as:

106 Chapter5

r~~}eh d(rz roz) p,gHro 2 = -'--=-.,---- --'------'-- + --'---...:....

KF dt 2mi3Ycose (5.8)

with the initial condition: t=O: r=r0.

If equation (5.8) is rewritten, the basic equation, describing the movement of liquid in a

CST apparatus, is obtained:

2r2

In(r0 }1,eh Îjdr KF dt

(5.9)

with the initial condition: t=O : r=r0

•

Equation (5.9) can only be solved numerically. A result of this numerical solution is

presented in tigure 5. 1.

The constauts in this numerical solution were chosen to be physically reasonable for

sludge and ceramics used in the modified CST apparatus (see section 5.5), i.e. p = 1030 ·3 -3 13 ·1 s

kg.m ,TJ=lO Pas,s=0.46,H=0.06m,a =10 m.kg ,P =50kPa,C=18 -3 ·15 2 av cap

kg.m , h=0.0017 m, ~=9.6•10 m, r0

=0.006 m.

The models presented by Unno et al. [1983] and Leu [1981] arebasedon the samebasic

equations. However, Unno et al. failed to give the right solution of the model, and Leu

never solved his model showing the position of the liquid front as a function of time.

0.024

:g 0.018 r:J'l ;:3

~ 0.006 -

0·000 ol----....,I;-;!oo;-;;:----:;;2±oo~---:;;3:\;-oo~--7.4oo!;-;;-----,s~oo

Time (s)

Figure 5.1 Example of a numerical solution of the CST model. Wetted radius as a junction of

time.

Modified capillary suction time (CST) apparatus 107

5.3 Parameter studies

The influence of the capillary pressure P , the filter medium porosity e, and the cap

average specific cake resistance a on the model calculations were investigated with the av

presented CST model. The constants used in these parameter studies were given in the

previous section.

The influence of the capillary pressure. The capillary pressure is a driving force for

liquid flow in the capillary medium. Therefore one should expect an increase of the

liquid front velocity with an increasing capillary pressure. This is shown in figure 5.2.

The capillary pressure was varled from 10 to 100 kPa.

The injluence of the porosity of the filter medium. The consequences of a variation of the

porosity of the filter medium E for the calculated radial position of the liquid front as a

0.030

k:Pa 70k:Pa 30k:Pa lOkPa

0.024

,-..., s 0.018 '-" rl:l ;:I ·-'"0 ro 0.012 ~

0.006

0.000 0 300 600 900 1200 1500

Time (s) Figore 5.2 The influence of the capillary pressure. Calculated wetted radius as a tunetion of

time.

function of time are not evident, because the capillary pressure P and the filter cap

medium permeability ~ both depend on the porosity E. The capillary pressure depends

on the redprocal hydraulic radius [3, the interfacial tension of the filtrate y, and the

contact angle e capillary medium/filtrate, according to the following formula:

108 Chapter 5

P •• p = l3y cose (5.10)

The redprocal hydraulic radius 13 is given by:

(5.11)

where a represents the specitic surface area of the ceramics. Substitution of equation V

(5 .11) into equation (5 .1 0) yields the dependenee of the capillary suction pressure on the

porosity:

a (1-e) P = v ycose

oap E (5.12)

The permeability can be calcnlated with the Kozeny-Carman equation. This equation has

been experimentally proved valid for the capillary medium used in the modified CST

apparatus [Uze~, 1992]. Results ofthe calculations are shown in tigure 5.3.

The porosity was varled from 0.3 to 0.8. The liquid front velocity is a maximumfora

capillary medium porosity of 0.5, when it is taken into account that the variation of the

0.030 r---------------------

0.8

0.006

0.000 L__ ___ ..__ ___ ..__ ___ _J__ ___ _J__ __ _

0 200 400 600 800 1000

Time (s)

Figure 5.3 The influence of the porosity & of the filter medium. Calculated wetted radius as a

junction of time.

Modilied capillary suction time (CST) apparatus 109

porosity bas consequences for the valnes of the capillary pressure and the filter medium

permeability. In the modified CST apparatus, ceramics is used as a capillary medium

(see section 5.4). The ceramic slab is characterized by the structure parameters of

porosity, permeability, and capillary suction pressure. The porosity can be determined

with mercury porosimetry, permeability with a permeability cell, and the capillary

suction pressure by conductinga CST experiment with demiwater (see section 5.5).

The irifluence of the average specific cake reststance a . The cake resistance a av av

represents the liquid flow resistance in the CST rube, since there has been a build-up of

a sludge cake. Therefore one should expect a decrease of the distance covered by the

liquid front at a certain time with an increase of the average specîfic cake resistance

a . Results of calculations shown in figure 5.4 conflfiD this expectation. av

The specific cake resistance a was varled from 1012 m.kg-1 to 1013 m.kg-1

. The av

calculated radial position of the liquid front at a time t decreases with an increasing cake

resistance aav· The value of~ used in this parameter study is 9.6•10-15

m2

• As can be

seen in figure 5.4, the differences between the curves are relatively small. To increase

this difference a filter medium with a higher permeability should be used. The

consequence of a smaller permeability of the filter medium is that the influence of the

specific cake resistance on the radial liquid front position as a function of time is

negligible.

0.020

~ s 0.015 . ..._, rn ;:I

:.0 ~ 0.010

0.005

0.000 0 60 120 180

Time (s)

12 --uo -----3.1012

12 : -----7.10 .

1.1013

240 300

Figure 5.4 The influence of the specifïc cake resistance aav . Calculated wetted radius versus

time.

110 Chapter 5

From these parameter examinations several conclusions cao be drawn. Firstly, in order

to calculate ao average specific cake resistance, the permeability of the sludge cake must

be small in comparison with the permeability of the filter medium. Secondly, it is

obvious that the capillary pressure is ao important driving force for the liquid flow in a

capillary medium. The hydrostatic driving force induced by the sludge head is negligibly 5

small (± 600 Pa) in comparison with the capillary driving pressure (± 10 Pa; see section

5.5). Finally, it cao be concluded that the porosity of the capillary medium has great

influence on the results of the model calculations aod experiments, because the porosity

influences the redprocal hydraulic radius 13 aod therefore the capillary pressure P , cap

aod at the same time the permeability of the filter medium Kp.

5.4 Modified CST apparatus

Several apparatuses to measure the CST are described in literature. The first apparatus,

developed by Baskerville aod Gale [1968] has been commercialized by Triton

Electronics. Alternative CST apparatuses are known with which e.g. CST must be

determined visually by a radial scale drawnon the capillary medium [Leu, 1981], or by

use of several electrodes [Lee aod Hsu, 1992, Unno et al. ,1983]. These instruments

have two important properties in common. Firstly, filter paper is used as a capillary

medium. Secondly, the position of the liquid front is only measured discontinuously.

Both properties imply several problems.

Filter paper is ao anisotropic material, which implies that the liquid front will move with

different veloeities in different directions. As a result no circular liquid front, on which

the model is based, will develop. Further, the reproducibility of the measurement will be

small, because of the differences between the filter papers.

To check the presented CST model on the physical reality more than two datapoints are

required. A continuous method of measurement is the best option.

Two typical differences between the conventional aod the modified CST apparatus cao

be distinguished.

Filter medium. Instead of filter paper, ceramics will be used as a capillary medium

(pressed Al20 3, initial partiele size ± 50 Jtm, pressing force 300 MPa, sintering

temperature 1500 °C). Since this capillary medium is isotropic, a circular liquid front

will develop during a CST experiment. One cao also expect the experiments to be

reproducible. The required thickness of the ceramic slab is 1-2 mm; this thickness

avoids slow saturation of the ceramic slab in the axial direction.


metal plate

Figure 5.5 Schematic diagram of the modified CST apparatus.

Method of measurement. The new continuous metbod of measurement is based on the

fact that the electrical resistance of the cernmies will decrease when the pores are fitled

up with water or any other liquid. The modified CST apparatus is schematically shown

in tigure 5.5. The ceramic slab is tightened between two copper plates. The copper

plates act as electrodes. A multimeter (measuring frequency 10 kHz), connected to the

electrodes, continuously measures the electrical resistance of the ceramic slice. The

experimental data are transferred to a computer. The sampling time of the computer is 1

second. When liquid is sucked into the ceramic slab, a decrease of electrical resistance

of the capillary medium will be measured. At the end of the experiment a constant

resistance has been reached, which depends on the type of ceramics used and the ionic

strength of the liquid. From the measured resistance as a function of time the radial

position of the liquid front as a function of time can be calculated. The situation at a

certain momentduringa CST experiment is also shown in tigure 5.5. The shaded area

represents the part of the ceramic slice that has been ftlled up with water and the white

area represents the dry ceramics. r0 equals the position of the liquid front at time t=O, r

the position at timet, and re represents the radius of the ceramic slab. This situation at a

time t can be interpreted as a parallel conneetion of the electrical resistances of the dry

part of the slice R d and the wetted part of the ceramic slice R , respectively. The ~ e~

total resistance of this system R can therefore be represented as: e,tot

R R R = e,d e,w e,tot R +R

e,VY' e,d

(5.13)

112 ChapterS

The position of the liquid front at an arbitrary time t can now be calculated from the

measured resistance R according to the following equation: e,tot

(5.14)

where p d and p w represent the specific electrical resistance of the dry and wetted

ceramics, respectively. It is essential that the whole experimental set-up is tightened in a

'clamping-screw', see tigure 5.6. This set-up assures good contact both between the

wetted ceramics and the copper plates and between the tube and the ceramics (no

leakage). To avoid pollution of the ceramics with small sludge particles, a small filter

paper (0 6 mm) with negligible flow resistance is placed between the tube and the

ceramic slab.

Figure 5.6 Schematic diagram for the whole experimental set-up for the modified CST

apparatus. 1) rubber, 2) copper electrode, 3) ceramic slab, 4) CST tube.


5.5 Experimental results and discussion

Detennination of capillary suction pressure. The capillary suction pressure P, of the -15 2 cap

circular ceramic slab (re =29 mm, h=l mm, E=0.46, ~=9.6•10 m) used in the

experimentsis an important system parameter. The capillary suction pressure is assumed

to be uniform in all directions of the ceramic slab and to be constant in time. 1n an

experiment a colonred water salution is poured into the CST tube and the position of the

wet front radius is measured by a ruler. 1n this way, the position of the wet front is

recorded directly. If only pure liquid is used in the CST experiment (i.e. there is no

sludge cake formation), the first two terms on the right-hand side of equation (5.9) can

b~ deleted. The mathernatical model is then used to obtain the capillary pressure. The

result of this experiment is shown in tigure 5. 7. A model calculation yields a value for 5 -2

the capillary suction pressure of 1.08•10 N.m . Another way todetermine the capillary

pressure is to carry out CST experiments with demiwater.

0.1130 ..

0.1128 :.

0.026

(1.024 .

11.022

c;;:? 0.020 .

6 0.018 .

Cf.) 0.018 ..

E3 o.m~ •.

~ ::: 0Jl06

O.OM

0.002 :·

* *

* * . '•·.

* . .. ·•

*

ODOO~~~~~~~~~~~~~~~~~~~~~~~

0 lOO 200

TIME (S)

Figure 5. 7 Wetted radius versus time for a CST experiment with water to determine the

capillary suction pressure; the position of the liquid front has been measured visually.

114 Chapter 5

O.OY

0.01.3

0.012

O.Ol1

0.010

~0.000 o.ooa

ril ~O.D07

~o.ooa * O.oo&

0.1103

D.002

0.001

0.000

0 10 20 30 40 50

TIME (S)

Figure 5.8 Wetted radius as a function of time for a CST experiment to determine the capillary

pressure; position of the liquid front determined indirectly by use of the electrical resistance of

ceramics.

The radial position of the liquid front as a function of time is determined indirectly by

measuring the electrical resistance of the ceramic slab as a function of time according to 11

equation (5.14). The specitic electrical resistance ofthe dry ceramics pd equals 10

nm. The specitic resistance of the wetted ceramics is calculated from the measured

resistance at the end of the experiment, which depends on the electrical conductivity of

the filtrate and the geometry of the slab. It is assumed that the ceramic slab is fully

saturated. The experimental result is fitted with the theoretica! model to calculate the 5

capillary suction pressure (fig. 5.8). The result of the calculation is Pcap=0.884•10 -2

N .m . lt can be concluded that both experiments showed satisfactory agreement. 5 -2

Henceforth in model calculations a value of the capillary suction pressure of 10 N.m

is assumed.

Reproducibility. In tigure 5. 9 three CST experiments carried out under the same

process conditions with unflocculated sludge are shown. In these experiments the same

ceramic slab was used. It can be seen that the reproducibility is acceptable. Model


15 -1 15 calculations yield values of tbe specific cake resistance of 4.39•10 m.kg , 3.64•10

-1 15 -1 m.kg , and 3.01•10 m.kg . One should notice tbat sewage sludges are biologica! in

O.o10

0.009

0.008

0.007

(/) ::J 0.005

0 <( 0.004

Cl:

0 100 200 300 400 500

TIME (S)

Figure 5.9 Results of three identical experiments carried out with unflocculated Eindhoven

sludge.

nature. Due to microbial activity tbe sludge composîtion may change witb time and

negatively înfluence tbe reproducibility. However, CST tests witb non-biological sludges

have not been carried out.

Determination of the specific cake resistance. CST experiments have been carried out

wîtb Eindhoven sludge. In these experiments a cylindrical perspex column witb an imler

radius of 3 mm was used. A larger column radius will increase tbe filtration area and

tberefore also tbe liquid flow through tbe cake and tbe capillary medium. Consequently,

differences in specific cake resistance will be hard to distinguish. At tbe start of an

experiment 3.5 ml of a sewage sludge sample (dry solids content 1.9 wt%) was poured

116 s

into the tube. The experiment ended when the ceramic slab was fully saturated. In tigure

5.10 two experimental results are shown.

0.020 re--:---:--:--:--:----,-;:--:--:--:--:--:--:--:-:--:--:--:-:--:--:-~

0.019 .

0.018 .· 0.017 ..

0,016 .

0.015 .

0.014 :

......... o.ou . ~ 0.012 : ......... . (/) 0.011 :

:::::> 0.010

Cl 0.009 <( 0.008

0::: 0.007

0.003

0.002

0.001

0.000 'r:-.'-r..,....,...,.;..,;..~,.;..":--rr;.,.;..,;...;....;...;,...",;-...:.,..:..r..,...:....,.;..,;.....,.;,.....;...--rr..,...:....,.;..,;..r-'~--rr..,...:....,-,' 0 100 200 300 500

TIME (S)

Figure 5.10 Experimental results of CST experiments, wetred radius as ajunetion qftime. Dots:

uriflocculated sludge; Stars: sludge flocculated with 100 g FeC/3/kg ds and 200 g Ca(OH)2/kg

ds. lines: results of the model calculations.

The stars represent a result of an experiment carried out with a sample flocculated with

100 g FeC~Ikg ds and 200 g Ca(OH)21kg ds. The dots represent the result of an

experiment carried out with uuflocculated sludge. It is clear that flocculation of a sludge

sample accomplishes a decrease of the time needed for the liquid front to move a certain

distance. The lines in the graph represent the results of the model calculations. A

specific cake resistance is determined from fitting the experimental results with the

theoretical model. Results of model calculations are: 15 -1

aav, uuflocculated = 1.24•10 m.kg 13 -1

<Xav, flocculated = 2.56•10 m.kg

As expected, the specific cake resistance of unflocculated sludge is much higher than the

cake resistance of a flocculated sample. At the beginning of the experiment, the model

overestimates the velocity with respect to the experiment. This is possibly caused by the

resistance of the small filter paper positioned under the perspex column. The modified

Modilied capillary suction time (CST) apparatus 117

CST device provides the possibility to determine a specitic cake resistance of flocculated

as wellas unflocculated sludges. Unflocculated sludges are hard-to-filter suspensions in

a batch filtration experiment.

0.012

0.009

0.006

0.003

1000 2000 3000

TIME (S)

Fig. 5.11 Wetted radius as a function of time in two CST experiments carried out with

unjlocculated sludge with two different slurry concentrations.

Effect of the slurry concentration. In figure 5 .11 the movement of the liquid front as a

function of time for two different slurry concentrations is shown. The movement of the

liquid front is slower when the slurry concentration increases. This is caused by the

formation of a thicker cake. The calculated average specific cake resistance was in both -15 -1

cases 2.7•10 m.kg . If only conventional CST values were reponed, a large

difference was found. However, since for the calculation of the specific cake resistance

the slurry concentration is needed, the cake resistance is equal for both slurry

concentrations. The intrinsic dewatering property bas not been changed. It may be

concluded that effects of slurry concentrations can also be investigated with the modified

CST apparatus.

118 Chapter5

To corre1ate the calculated specitic cake resistance from a CST experiment with normal

batch filtration cake resistances, it is useful to carry out filtration experimentsnot only at

100 kPa (capil1ary suction pressure of ceramics used) but also at other pressures. As

· sludge is very compressible, the average specific cake resistance depends on the

pressure drop over the sludge cake. In tigure 5.12 results of normal filtration and CST

experiments are shown. From tigure 5.12 it can be conclnded that there is an acceptable

corre1ation between normal filtration and CST experiments. The difference may be

caused by the varianee of the determined capil1ary pressure, possible sedimentation

effects, or wall friction effects which have more influence on a CST experiment than on

normal filtration experiments, because of the smaller dimensions of the CST tube.

However, only a few of these experiments have been carried out. To get a good idea

about the corre1ation between normal filtration and CST experiments more experiments

are needed.

70

- 60 'b Jol: • .s • 8 50 c • • flitration at --_CII

40 ..... el!!. 0 CST B • 8 30 Q

0 ;: "6 ID 20 Cl. liD •

10 0 100 200 300 400 500 600

(Thousands) pressure [Pa]

Fig. 5.12 Average specijic cake resistance versus pressW'e for normal flitration experiments

(filled dots) and a CST experiment (open dot).

Modified capillary suction time (CST) appara~_u_s _______ 1_19_

5.6 Conclusions

Witb tbe model presented and tbe modified type of CST apparatus average specific cake

resistances can be obtained for unflocculated as well as flocculated sludges. The

modified CST instrument is based on continuously measuring tbe electrical resistance of

a ceramic slab during filtrate penetration. The measured electrical resistance as a

function of time is converted to a radial position of tbe liquid front as a function of time.

Before any calculations can be made, several parameters of tbe ceramic porous medium

have to be k.nown. This implies tbe permeability, the capillary pressure, and tbe specific

electrical resistance of tbe ceramics. It is of great importance that tbe permeability of tbe

ceramics is high enough (10-14

to 10-15

m2) in comparison witb tbe permeability of tbe

-15 -17 2 s1udge cake (10 to 10 m ) in order to calculate a correct average specific resistance.

The required thick.ness of tbe ceramic slab must be equal to 1 to 2 mm to avoid slow

saturation in tbe axial direction. The slurry concentration does not influence tbe average

specific resistance of tbe cake formed in tbe CST tube because of tbe low

compressibility of tbe sludge at low applied pressures. However, tbe conventional CST

value depends on tbe initial solids concentration of the sludge suspension.

6 FLOCCULATION BEHA VIOUR OF SEWAGE SLUDGE

6.1 Introduetion

Material in waste water may comprise suspended and/or dissolved organic and

inorganic matter and numerous biologica! forms such as bacteria, algae and viruses.

Particles in the dimeosion range of 10·9 to 10·6 mare referred to as colloids.

Particles of colloidal or lesser dimensions are able to retain a dispersed state because

of eertam inherent characteristics which promote their stability. The term stability

describes the ability of individual particles to remain separate entities. The stability of

colloidal material arises from the predominanee of forces associated with the solid~

liquid interface.

If the size of particles becomes progressively smaller for a given total partiele mass,

the total surface area becomes extremely large. Colloidal material possesses a colossal

surface area to mass ratio. lt is apparent that for a given total mass, the smaller the

particles the more predominant the influence of phenomena associated with interfaces

becomes, and the lesser the influence of gravity effects associated with mass becomes.

Partienlate material of colloidal size may be removed from dispersions by methods

other than those relying on gravity effects. Finely dispersed colloidal material bas to

be converted into a form whereby separation from the dispersion is practicable.

Conversion of the stabie state of a given dispersion to an unstable state is termed

destabilization. Conversion processes could either alter the surface properties of

partienlate material, thereby increasing the adsorptivity of the particles to a given filter

medium, or generating a tendency for aggregation of small particles into larger units

or precipitate dissolved material, thereby creating partienlate material for which

separation by sedimentation and/or filtration is feasible.

A process which is able to accomplish destabilization of sludge particles is

flocculation. Flocculation is the process whereby small particles dump together and

thereby form larger agglomerates or flocs as a result of destabilization, and is a

generic term nsed in the waste water treatment indnstry. Chemical coagulants are

added to the dispersion to induce flocculation. In literature, a number of defmitions of

flocculation bas been presented. According to the La Mer system [1964], which is the

generally accepted standard, aggregation is used as the general term. The term

coagulation is reserved for those processes in which the London~van der Waals force

(see section 6.3) is the primary driving force causing aggregation.

122 Chapter 6

Destabilization reactions of colloids in an aqneons dispersion by chemical coagu]ants

are complex and arise from several mechanisms. Doderstanding of varions

destabilization phenomena involved in sludge tlocculation is essenrial to get insight

into the dewatering behaviour. In order to stndy sludge destabilization phenomena the

so-called electroacoustic technique (MA TEC) is used. With this technique the ESA

signal of a given sludge suspension is determined. The ESA signal is related to the

zeta potential. Zeta potentlal measurements are useful in stndying the colloidal stability

of dispersions, ion adsorption studies and characterization of partiele surfaces.

Before introducing this technique and carrying out experiments with it, the fust steps

are to consider the surface charge carried by sludge particles, the electrical double

layer around the charged surface, the effect of specific adsorption on both the effective

surface charge and its zone of intluence and destabilization phenomena of colloids

induced by metal coagulants and polyelectrolytes.

6.2 Tbe electrical double layer around a spberical sludge partiele

The charge of a partiele is predominantly produced by ionization of functional groups,

which are attached to the particle. Sewage sludge particles mainly consist of organic

matter. Sludge partiele surfaces are polysaccharide in nature, composed of neutral

sugars and glucuronic acid (monosaccharide CJI100 7, a sugar acid). Glucuronic acid is

one of the main ionogenic compounds at the sludge surface due to the presence of

ionized carboxyl groups [Steiner et al., 1976]. Carboxyl groups cao act as a proton

donor and proton acceptor group. looization of a carboxyl group is represented by:

As a consequence sludge particles are negatively charged. loos which arise from

dissociation of surface molecules and in this way causing the partiele surface charge

are called potential-determining ions. H+ and (OH)" ions are potential-determining ions

of sludge particles.

The partiele surface charge intluences the distribution of nearby ions in the liquid.

Due to electrostatle forces, ions of opposite charge (counter-ions) are attracted towards

the surface and ions of like charge (co-ions) are repelled away from the surface. This,

together with the random thermal motion and mutnal ionic repulsion or attraction,

leads to the formation of an electrical double layer. The electrical double layer

Flocculation behaviour of sewage sludge 123

consists of the charged surface and a neutralizing excess of counter-ions over co-ions

distributed in a diffuse manner in the nearby liquid.

The electrical double layer can be viewed schemarically made up of three regions:

1. A surface layer, having a electrical surface potenrial if;0 and a surface charge a0 •

2. A Stern layer with thickness ö.

3. A diffusedregion of ions having a charge ad.

In figure 6.1 a schematic illustrarion is given of the electrical double layer and the

variarion of the electrical potenrial with distance from a negarively charged wall in the

absence of specific adsorprion.

·~· ••• • .I •• . ' .. , .. stern diffuse ioyer tayer

• •

<llotance

Fig. 6.1 Electrical potenrial (absolute) as a tunetion of the distance from a negatively charged

wall. Due to electrastatic and van der Waals forces counter-ion adsorption predominafes over

co-ion adsorption. No specific adsorption [Shaw, 1969].

Ions cannot be assumed as point charges, but possess a finite size. This prevents the

eentres of the counter-ions approaching the surface closer than within a distance ö.

124 Chapter 6

Counter-ions are not easily dehydrated so that they retain their hydration shells in the

adsorbed state. The so-called Stern plane is located at about a hydrated ion radius

(distance o) from the partiele surface. The region between the eentres of the surface

charge and the distance o (Stern layer) is charge-free. The potenrial decay over the

Stem layer is linear. This is a consequence of Poisson's law (equation 6.4). The

electrical potenrial at the Stem plane is called the Stem potenrial 1/;0• Due to overall

electrical neutrality of the whole double layer, the charge of the diffuse double layer

ud is opposite to the charge of the partiele u0:

(6.1)

Ions or molecules can be attracted to the solid surfaces not only by repulsive forces

but also by van der Waals forces. Another mechanism is specific adsorption. All

interactions at the particle-solution interface which are not coulombic are designated as

specific. They may be of chemical or physical nature, such as dipolar, hydrogen,

entropie or covalent. Specific adsorption of co-ions or counter-ions at the surface

infl.uences the sign and magnitude of the Stern potenrial, in this way affecting

flocculation. The plane through the eentres of the specifically adsorbed ions inside the

Stem layer is called the 'inner Hehnholtz plane (iHp)'. The plane parallel to the

surface at distance ó is called the 'outer Hehnholtz plane (oHp)'.

Specific adsorption of counter-ions may result in a reversal of charge to take place

within the Stem layer, i.e. 1/10 and 1/lo have opposite signs, and is called superequivalent

adsorption (:figure 6.2). Specific adsorption of co-ions could create a situation in which

1/;6 bas the same sign as 1/;0 and is largerinmagnitude (see tigure 6.3). If the charge in

the Stern layer is called u., then because of electroneutrality in the electrical double

layer:

(6.2)

The diffuse part of the double layer plays an important role in colloidal stability of

dispersions. When two particles approach one other, the electrical double layers will

overlap and repulsive forces become operative. The electrical potenrial in the diffuse

layer decays exponentially from 1/;8 at the Stem plane to zero at infinity (see section

6.3).


distallee

l

Fig. 6.2 Reversal of charge may take place within the Stem layer due to specific adsorption of

counter-ions. Superequivalent adsorption of counter-ions.

Fig. 6.3 Specific adsorption of co-lons could increase the ejfective surface charge:

I u, I > I 11o I .

The Stem potenrial cannot be measured. A close approximation may be obtained by

measuring the potential at the plane of shear between a moving partiele and the

surrounding liquid. This electrical potential is known as the zeta m potential. The

126 Chapter 6

exact location of the shear plane is not known since it depends on the adsorbed ions in

the Stern layer and the degree of hydration. It is assumed that the shear plane is

located a small distance further away from the surface than the 'outer Helmholtz

plane' and that the zeta potentlal is, in general, marginally smaller in magnitude than

Vtö· The electroneutrality condition is then presented by:

(6.3)

where <rb is the charge between the surface and slipping plane and <rek the charge at the

slipping plane.

The description of the diffuse part of the double layer proposed by Gouy and Chap

man is based on an assumption of point charges in the electrolyte solution.

The electrical potential 1/t and the density p of the space charge in the solution

surrounding one single rigid sphere are related by Poisson's equation:

(6.4)

where Eo is the permittivity of vacuum and €r the dielectric constant of the medium.

The space charge is considered to be built up by un unequal distribution of pointlike

positive and negative ions:

(6.5)

where n+ and n_ are the local concentrations (number/m3) of positive and negative

ions, respectively, z the valency and e the elementary charge. Only one type of

positive and one type of negative ions are considered.

The statistical equilibrium of co-ions and counter-ions in the diffuse double layer sur

rounding a charged partiele is described by Boltzmann's law:

z e'ljt(r) n_(r) = n-Q exp(~)

(6.6)

(6.7)


where n+o and n_0 are the concentradons of co~ions and counter-ions at inftnity,

respectively.

Electrical neutrality of the bulk solution yields:

(6.8)

Substitution of equations (6.6) and (6.7) in (6.5) and the subsequent substitution of

(6.5) in (6.4) gives:

en+0z+( ( -z+el/t(r)) (z~el/t(r))) --- exp -exp ---

(V>r kT kT (6.9)

The boundary conditions are:

(6.10)

r=a 1/t = r (6.11)

a represents the sum of the sphere radius and the thickness of the Stem layer. An

analytical solution of the differentlal equation (6.9) is only possible when one assumes

that ezl/t < <kT. When the surface potenrial is sufficiently small ( if; 0 < 25 m V), this

inequality is valid. In that case, the Poisson-Boltzmann equation can be linearized by

expanding the exponential terms in the right-hand side and retaining only two terms.

This estimate is called the 'Debye-Hückel approximation'.

The solution of equation (6.9) is then given by:

a if; = t - exp( -K(r-a)) (6.12) r

where K·1 represents the double layer 'thickness' and is expressed by:

(6.13)

The double layer thickness is dependent on the electrolyte concentration. Increasing

the ionic strengthof the solution compresses the double layer.

128 Cbapter 6

6.3 Colloidal stability in terms of the electrical double layer

The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [Verwey and Overbeek,

1948] bas made it possible to study the stability of colloids quantitatively. The theory

involves estimations of the energy of attraction (London-van der Waals forces) and the

energy of repulsion (overlapping of electrical double layers) in terms of the distance

between particles.

The potenrial energy of interaction between two spherical double layers can be

calculated using Derjaguin's method, which is based on the interaction between two

flat double layers. The magnitude of the potenrial energy of repulsion between two

equal spheres of radius a is given by:

-lo in this equation is represented as:

expzet -1 2 2kT

'Yo = ---,---expze! +1

2kT

(6.14)

(6.15)

H0 is the shortest distance between spheres, a the partiele radius and llo represents the

concentration of ions in the bulk solution.

The London-van der Waals force between two atoms or molecules is a short-range

force and its potenrial energy varles inversely with the sixth power of the inter

molecular distance. Because of the additivity of this force, the London-van der Waals

force between macroscopie bodies becomes a long-range force.

For the interaction of two equal spheres with radius a and a distance R between their

centres, the London-van der Waals attraction potenrial is given by:

(6.16)

where

s R a

Flocculation behaviour of sewage sludge

H 2+-0

a

For H0 < < a equation (6.16) is approximated as

V = A

129

(6.17)

(6.18)

where AH is the Hamaker constant, which takes into account that particles of material

1 are embedded in a medium 2.

The stability of colloidal particles in aqueous media depends on the total potenrial

energy of interaction VT, that is

(6.19)

Figure 6.4 shows the total potenrial energy as a function of the separation distance H0•

Negative values of the potenrial energy correspond to attraction and positive values to

repuls ion.

vf

0

Fig. 6.4 London-van der Waals attraction potenrial VA (negative), potential energy of

repulsion VR (positive), and the sum of both V z; as a junction of the interpartiele distance H0•

The height of the potential harrier depends on the Stern potential 1/;ó and the electrolyte

concentration.

130 Chapter 6

Since V R varles exponentially with the distance and V A varles inversely with the

interpartiele distance, V A surpasses VR at short and long distances, thus producing

attraction between the particles. At intennediate distances, the plot of V1 shows

varlous shapes depending on the Stem potential I/la and electrolyte concentration. For

large values of I/la there is a potential barrier and for small values of I/la there is an

attractive potential. If the potential energy maximum A is large compared with the

thennal kinetic energy (kT), the system will be stabie and the potential barrier (curve

V n) will hinder two particles to stick together into the deep primary minimum,

appearing at a small separation distance Ho. The electrolyte concentration also has an effect on the potential barrier. Increase of the

electrolyte concentration (and thus decrease of the double layer thickness K-1) canses

the disappearance of the potenrial barrier. V 1 shows that there is attraction between the

particles and destabilization of the suspension takes place (curve V12). The potential

energy curve V 12 can be taken to require an expression for the critical coagulation

concentration. The critical coagulation concentration is defined as the electrolyte

concentration at which coagulation commences. From curve V 12 there is a certain

value of H0 at which

(6.20)

and

(6.21)

The equations mean that the maximum in the potenrial energy curve touches the

horizontal axis. Solution of equations (6.20} and (6.21) yields the critical coagulation

concentration n" (number of ions/m3):

(6.22)

The critical coagulation concentration C0 expressed in units of moles per liter is

related ton" according to:

Flocculation behaviour of sewage sJudge 131

(6.23)

where NA represems Avogadro's number.

The critical coagulation concentration is inversely dependent to z6• It states that the

higher the counter-ion valency, the lower the critica! coagulation concentration.

Theoretica! critica! coagulation concentrations of indifferent electrolytes, yielding

counter-ions with a valency z = 1 ,2,3 should be in the ratio of 729: 11: 1. This phenom

enon is known as the rule of Schulze and Hardy. In practice it is found that a trivalent

ion is 700 to 1 000 times as effective as a monovalent ion in destabilizing a colloid of

opposite charge. The rule of Schulze and Hardy is in reasonable agreement with

experimental evidence on the coagulation of colloids by non-specifically adsorbable

ions. Typical valnes for the critical coagulant concentration are very small. For

example, to flocculate negative As2S3 sols, about 9·10·5 moles.(liter)-1 of trivalent

cation and 7·10·4 moles.(liter) 1 of divalent cation are neerled [Hiemensz, 1986].

lt should be noted that the expression for the concentration of indifferent electrolyte

relies on a model of double layer depression according to the Gouy-Chapman treat

ment. It does not take the specific adsorption of roetal coagulants into account. In the

next chapters we wi11 deal with the destabilization of a dispersion due to specific

adsorption of metal coagulants and polyelectrolytes.

6.4 Specific adsorption flocculation by metal coagulants

Inorganic salts, e.g. ferric chloride, are commonly used as a flocculant in the

treatment of waste waters. Ferric salts, when in solution, immediately dissociate to

form hydrated reaction products. The ferric ions form coordination compounds with

water molecules to give Fe(H20)63+.

The high positive charge on the central metal ion canses some polarization of the 0-H

honds and there is a tendency for protons to dissociate, giving one or more hydrolyzed

species, thus

Fe(H20)6.0(0H)

0

3-n + H20 ;;::± Fe(H20)6-n-t(OH)n+t3·n-t + H30+ (n=O up to and

including 5)

132 Chapter 6

In this way, six mononuclear hydrolysis products are present in the aqueous solution.

The equilibrium reacrions are characterized by a particular equilibrium constant, which

depends on the nature of the metal ion. Small and highly charged ions have a great

tendency to release protons and hence are acidic in a neutral pH medium, e.g. an

unconditioned sludge suspension. As the pH of a sludge suspension is increased, the

equilibria are driven to the right. Figure 6.5 presents a distribution diagram for the

various mononuclear ferric hydrolysis products in an equilibrium state at different pH.

1.0

I f'e(OH);"'

0.6 I

·o l 0.6'

i: .. ~ .. ~ c ·§ 0.4

2

0.2 I i

Fe(OHl, Ft(O~);-

I

I 0

0 2 10 pH

Fig. 6.5 Distribution diagram for iron hydralysis products for iron concentration of 1()5 M [Singley and Sullivan, 1969].

The diagram is valid for a molar iron concentration of 10-s M. The distribution

diagram is basedon calculations carried out by Singley and Suilivan [1969].

Besides mononuclear hydrolysis products also multinuclear hydrolysis products,

collectively given by FexOH/x-y, can be formed. Multinuclear hydrolysis products are

not incorporated in calculating the distribution diagram presented in tigure 6.5.

Mononuclear and multinuclear iron hydrolysis products show enhanced adsorption

characteristics. Specific adsorption of hydrolysis products is possibly another way to

destabilize a sludge suspension. Through adsorption of charged coagulant species of

opposite sign to the partiele surface, the effective charge is reduced and, as a conse

quence, the extent of the double layer repulsive interaction between adjacent particles


is reduced. When treated with an excess of counter-ions, a charge reversal of particles

may even occur (see tigure 6.2). A secoud mechanism considered, again as aresult of

adsorption of coagulant species at the particle-solution interface, is that described as

the bridging mechanism. During hydrolysis reactions, metal coagulants have a

pronounced tendency to polymerization. On adsorption of such polymerie species to

particles, a coagulant bridge spanning between adjacent particles is formed, thereby

promoting destabilization.

Another mechanism of destabilization by ferric salts is that of precipitate entrapment.

Under appropiate conditions of the coagulant concentration and pH, ferric coagulants

in an aqueous solution form insoluble ferric hydroxide precipitates Fe(OH)3 (see tigure

6.5), initially as a fine colloidal dispersion. These particles then aggregate to form

hydroxide flocs which enmesh the colloidal particles originally present in the water.

This destabilization process is called sweep flocculation [ Gregory, 197 8].

In practice, after an addition of ferric chloride, calcium hydroxide is mixed with the

s1udge suspension. As a consequence, the suspension pH increases to 12. The formed

ferric hydroxide flocs precipitate (sweep flocculation process). Since lime bas a low

solubility it stays undissolved in the sludge cake and can be regarded as an ordinary

filling material. The formed fllter cake will be less compressible and the dewatering

properties of the sludge cakes are improved [Saunders, Holmes, 1987; Janssen et al.,

1994]. Other effects of lime addition are:

• The elimination of offensive odours.

• The removal of sulphide, sulphate, bi-carbonate, and ammonium by precipitation

(calcium sulphide, calcium sulphate, calcium carbonate) and gas escape (ammonia).

• Desinfection: dying of (pathogenie) rnicroorganisms, bacteria, and viruses.

• An increase of the amount of dry solids, which is regarded as a disadvantage. The

amount of lime to condition sewage sludge is high in practice: 10 to 20 kg per m3

sludge.

6.5 Polymerie adsorption flocculation

Polyelectrolytes are widely used as sewage sludge conditioners. The term 'polyelectro

lyte' is often used to describe all polymerie flocculants. Polyelectrolytes contain

functional groups which may or may not carry a charge. If the polyelectrolyte is

charged, the groups may be such as to give an anionic character, a cationic character

or an ampholytic character to the chain, where both anionic and cationic charged sites

134 Chapter 6

are present. The intensity of the charge carried by the polyelectrolyte is dependent on

the degree of ionization of the functional groups and on the degree of

copolymerization. Besides the possibility of functional groups carrying a charge, also

sites are present along the polyelectrolyte cbain which possess the property of being

adsorbed. Destabilization by polyelectrolytes could involve a mechanism combining

both charge effects and effects due to adsorption.

The extent of polymerization of the polyelectrolyte is characterized by the molecular

weight. High molecular weights signi:fy long chains, whereas low molecular weights

indicate short polyelectrolyte chains.

Especially in the fields of water and waste water treatment, it appears that the only

effective polymerie flocculants are those of opposite charge to the negatively charged

particles: cationic polyelectrolytes.

The basic elements (monomers) of cationic polyelectrolytes are derivatives of poly

(meth)acrylamides. Dissociation of the derivatives in water yields positively charged

polymerie ions. Monomer molecule structures of these derivatives are presented as

[Röhm, 1991]:

The monomeric ions are characterized by the presence of tertiary (RNH+) or

quarternary (RN+) ammonium groups. The polymerization is started by the reaction of

the monomer with a radical.

For polymers which are neutral or similarly charged to the partiele surfaces, one or

more of the following mechanisms of interaction can lead to adsorption: a) hydrogen

bonding, b) dipolar and van der Waals interactions, c) linkage of similarly charged

polymerie groups and the partiele surface of a divalent and trivalent ion of opposite

sign. When the polymer and the surface are oppositely charged, general electrostatic

forces act in addition to one or more of the above-mentioned mechanisms,


In this section we will discuss in detail three different mechanisms of destabilization of

charged particles by polyelectrolytes of opposite charge [Levine and Friesen, 1987]:

charge neutralization, bridging mechanism and electrostatic patch mechanism.

Charge neutralization

Cationic flocculants interact strongly with surfaces of opposite charge and are

adsorbed, at least up to the point of charge neutralization (iso-electric point). In this

way electrostatic repulsion between particles is eliminated, attractive forces become

effective and flocculation may occur. Because of the strong electrostatic interaction,

polyelectrolyte ebains should adopt a rather flat contiguration on oppositely charged

surfaces. At excess counter charged polymer concentrations, surfaces become

saturated which may result in charge reversal. This phenomenon is called restabiliz

ation of particles by the adsorbed polymer.

Bridging mechanism

Bridging flocculation is dependent on the adsorption of polymer segments onto

colloidal particles. The adsorption should not be too strong, since a fair proportion of

segments must remain unattached and available for adsorption on other particles. So

bridging flocculation requires the attachment of the adsorbed polymer to vacant sites

on other particles, thereby creating increasingly larger flocs.

The contiguration of the adsorbed polymer depends on the degree of electrostatic

attraction between the ebains and the surfaces. The amount of electrostatic attraction is

influenced by:

• The charge density of the polyelectrolyte.

At high charge densities the polymer adopts a nearly flat conformation on the

surface. At low charge densities the polymer-particle attraction is weak and the

contiguration of the polymer consists of more loops and tails.

• The ionic strength of the solution.

Bridging is possible when the adsorbed polymer spans the distance over which

double layer repulsion operates k 1). At higher indifferent electrolyte concentration,

the diffuse layers are less extensive and bridging is more pronounced. When more

loops and tails tend to extend further into solution, more segments of the chain will

permitadsorption.

The contiguration of the adsorbed polymer also depends on the flocculation rate. In

bridging flocculation the following rate processes are involved:

136 Chapter 6

• Mixing of polymer solution and sludge dispersion.

• Collisions between polymer and particles, leading to attaclnnent. The rate of this

process is determined by the concentration of polymer and particles and by the

migration speed of both species in solution. Encounters between particles and

polymer molecules may be brought about by a diffusion-controlled process (peri

kinetic flocculation) and by fluid motion (orthokinetic flocculation).

• Reconformation of polymer molecules at the surface of the particles.

• Partiele collisions during which bridges may be formed.

We consider two possibilities: (1) the adsorption is slow with respect to the reconfor

mation process and (2) the adsorption is faster than this reconformation (figure 6.6).

In case (1) the attached polymer ebains are within the bounds of the double layer.

Flocculation can be induced by salt addition (depression of the double layer) and

effective bridges are created. This type of bridging flocculation is called 'equilibrium

flocculation'.

__ .--~ .. altachment

·-~.

low altachment rate

1

no jlocculation

I low collision rate

~

high co Dision

rate

non-equilibrium jlocculoiion

M-.·> salt addiiÎ~~./~

···· ....

equilibrium floccukJtion

Fig. 6.6 Schematical representation of the mechanisms of bridging flocculation in charged

systems [Pelssers, 1988].


In case (2) enough polymer is attached and still available in the extended state so that

effective bridging is possible. The partiele collision rate determines whether or not a

stabie bridge is formed. At slow partiele collision rates the polymer ebains will have

flattened before a collision occurs. Equilibrium flocculation may only occur if salt is

added. In the case of a high attachment rate and high partiele collision rate the

polymers stay in the random-coil configuration and enough polymer is available for

bridging to take place during collisions. This phenomenon is called 'non-equilibrium

flocculation'.

Electrastatic patch model

This model arises from a consideration of unlikely charged densities for partiele

surfaces and polyelectrolyte chains. Consequently, it would not be possible for each

charged site on a partiele surface to be neutralized individually by a charged polymer

segment. Even though suftkient polymer may be adsorbed on particles to give zero

net charge, regions of positive and negative charge would still remain. A collision

between two such particles could occur so that positive and negative 'patches' come

into contact and adhere as a result of electrostatic attraction. In this way flocs are

formed.

6.6 An experimental technique to determine the ESA signal and zeta potential

In flocculation behaviour the electrical potenrial of the partiele plays a crucial role.

The zeta potential can be determined, for instance, by the standard techniques electro

phoresis, electroosmosis and streaming potential. Electrophoresis is the most widely

used of the three classica! teclmiques. In a specific experimental teclmique, called

microelectrophoresis, the colloidal suspension is contained in an enclosed cell of small

dimensions and the movement of the particles in an applied electric field is observed

directly. The velocity of the partiele divided by the electric field strength is the

electrophoretic mobility of the particle. The magnitude of the electrophoretic mobility

is a function of the zeta potential. In this stndy, a new electroacoustic teclmique for

the application of electrokinetic measurements of colloidal suspensions is used.

Let us consider a typical probe which can perform electroacoustic measurements (see

figure 6.7). The colloidal system is placed between the electrodes. The delay line,

which is made from a solid nonconductive material, separates the piezocrystal trans

ducer from the electrodes.

138 Chapter 6

U2

ele<l1rodes

Fig. 6. 7 The principles of Ultrasound Vibration Potenfiat (UVP) and Electrokinetic Sonic

Amplitude (ESA) measurements.

When a voltage U2 is applied at the transducer, a sound wave of the same frequency

propagates through the delay line and electrode into the colloid. For particles more

dense than the continuous phase, the motion of the particles will lag bebind the motion

of the liquid. Tbis leads to a relative motion between the particles and the liquid. If

the colloidal particles are charged, the resulting motion creates a periodic polarization

of the electtic double layers and an alternaring dipole moment at the frequency of the

applied field. The alternating dipoles sum up to a potential U1 that can be detected by

placing a pair of electcodes in the suspension.

Tbis effect is termed the Ultrasonic Vibration Potential (UVP) and was first predicted

for electrolyte solutions by Debye [1933]. UVP is measured in units of volts per unit

velocity amplitude of the applied acoustic field. In 1938, Rutgers and Hermans pointed

out that the effect would also be present in colloidal suspensions.

In the case of an applied alternating electtic field, the relative motion between the

charged particles and the surrounding liquid generates a sound wave at the frequency

of the applied field. Each partiele vibrating in the electric field radiates sound which


sums up to a coherent sound wave when many particles are present. The sound wave

is detected as a voltage U2 •

This effect was discovered by Matec [1985] and has been termed the Electrokinetic

Sonic Amplitude or ESA of the colloid. The magnitude of ESA is the pressure

amplitude per unit electric field generated by the colloid and has SI units of pascals

per volt per meter. Both the ESA and UVP effects can be used to determine an

electrophoretic mobility of the particles, where the mobility in this case is a dynamic

or AC mobility.

The electroacoustic technique for application of electrokinetic measurements is

marketed by Matec lnstruments Inc. The MATEC ESA system uses O'Brien's theory

[1988] for electroacoustic effects in a dilute suspension of spherical particles to

calculate the suspension zeta potenrial from the measured ESA. O'Brien's equation

relating the dynamic mobility P-iw) to the zeta potenrial r of particles suspended in

aqueous systems is given by:

where

and

P-iw) = 2ef (1 +t) G(a) 3'7

G(a)

(X =

-I

where: w = angular frequency (s-1)

fJ.p = density difference between the particles and the liquid (kg.m-3)

a = partiele radius (m)

E = dielectric permittivity of the suspension (C. v-'.m-1)

" = kinematic viscosity of the liquid (m2.s-1)

'7 = viscosity of the liquid (Pa. s)

f = constant

(6.24)

(6.25)

(6.26)

140 Chapter 6

The factor fin equation (6.24) can be assumed to be equal to 0.5 for most cases in

ESA measurements (frequency 1 MHz) where the ionic strength is at least 10'3

moles.(liter)·1 and the zeta potentialis less than 75 mV. This then yields the following

equation for the dynamic mobility:

(6.27)

The equation given by O'Brien for G(cx) is expressed as a complex quantity. When

converting ESA magnitudes to dynamic mobility, it is necessary to calculate the

magnitude of G(cx). The equation for converting the ESA amplitude to the dynamic

mobility is given by:

ESA = ~td<fiLlQCGf = re I ~(ex) I cfiáQCGf (6.28)

where cfi equals the volume fraction of particles, Gr represents a correction factor for a

given electrode geometry and c represents the velocity of sonnd in the suspension. For

the case of a parallel plate electrode geometry, Gr equals one.

This calculation is only valid nnder the condition that the electrical donbie layer is thin

relative to the partiele radins (Ka> 50). In this case ESA is linearly dependent on the

volume fraction. At high volume fractions (cfi > 0.1), hydrodynamic and electrical

double layer interactions lead to a non-linear dependenee on the volume fraction. Gen

erally, non-linear behaviour can be expected when the double layer thickness is

comparable to the interpartiele spacing.

The lntrasonic Vibration Potential (UVP) is related to the Electrokinetic Sonic

Amplitude (ESA) of the colloid by:

ESA(w) <fiLlQCG~d(w) UVP(w) = = ----

K* K• (6.29)

where K* represents the magnitude of the complex conductivity.

The complex conductivity is used to characterize electrical properties of colloids,

which are neither dielectrics nor conductors. The magnitude of the complex conductiv

ity needs to be known for conversion of the UVP signal to the dynamic mobility. 1t is

not easy to measure the complex conductivity and this is one of the limitations of the

UVP mode analysis. In this study, experiments with the MA TEC ESA system have

been carried out in the ESA mode.


1. Sladie 1ample l. Titraa.t 3. Stlrrer 4. Platla.D.IIl ltTD temp. probe 5. Coada.c:ttrity electrode 6. pH eleetrode 7. ESA ultrUG.D.lc probe 8. Tefloa. 1euor .._bly

Fig. 6.8 Schematic diagram of the MATEC sample eelt.

In figure 6.8 a schematic diagram is presenred of the sample cell. The vessel is stirred

by a paddie type agitator and includes sensors for pH, temperature, electrical conduct

ivity and electroacoustic measurements. Electroacoustic measurements are carried out

with Matec's ESA probe sensor which fits directly in the vessel. The electroacoustic

properties that are determined are the ESA amplitude and the phase angle between the

applied electric field and the response signal. The system includes a syringe pump

burette for carrying out volumetrie titrations with simultaneons electrokinetic measure

ments (concentration series measurements). The user specifies the total volume to be

titrated, the volume increment, and a time delay between increments. Tbe concentra

tion series module is suitable to study destabilization phenomena of a sludge suspen

sion induced by flocculants.

142 Chapter 6

6. 7 Results and discussion

Experiments with inorganic jlocculants

In order to study the flocculation behaviour of sewage sludge in detail, only experi

ments were carried out with secondary sludge suspensions originating from the

Eindhoven waste water treatment plant. Experiments with other sludges were carried

out incidentally.

Figures 6.9 and 6.10 showtheresult of a duplex experiment in which a ferric chloride

solution was added to 250 m1 of an Eindhoven sludge suspension. The dry solids

content of the sludge suspension was 2 wt%. The volume increment of the ferric

chloride solution used was 1 ml, the time between two additions 30 seconds and the

total of titrated volume 25 ml. The sludge suspension was agitated with a propellor

typed stirrer having a speed of 500 rpm. The total of added volume (25 ml) of the

ferric chloride solution corresponded with a dosage of 300 g FeC13/kg ds. In tigure

6.9 the ESA amplitude and the phase angle are given. as functions of the ferric

chloride dosage.

0.10

0.09

0.08

0.07

~ 0.06 0.05

i 0.04

0.03

~ 0.02

j::l::l 0.01

0.00 -0.01 -

-0.02

-0.03

0

180 ------ ~-----~: :~-,--

r-"""""'"" . ".r .. '

160

140

120

100

80

60

40

20

0

-20

30 60 90 120 150 180 210 240 270 300 g FeC13 /kg els

.......... ~ ~ ~ -1

Q)

j ~

Fig. 6.9 ESA amplitude (-;---) and phase angle (- -;- --) as functions of the FeC/3 dosage.

Result of a duplex experiment.

0.10 O.o9 0.08 0.07

~ 0.06 0.05

1 0.04 0.03

~ 0.02 0.01 0.00

-0.01 -0.02 -0.03

3


. "" ' I \ . \•

\ .. '

'•

'· ·"...:_...,:,_

4

.,, ;~

5 6

pH

Fig. 6.10 ESA signa/ as a function of the sludge suspension pH.

143

7 8

In figure 6.10 the ESA amplitude is depicted as a function of the suspension pH.

Conversion of the ESA signal to the zeta potential is not practicabie in the sewage

sludge/FeC13 system. During an experiment carried out with a sludge suspension, the

zeta potential, partiele radius, and volume fraction of particles will change and all

influence the magnitude of the ESA signal (equation 6.28). The exact changes of

partiele radius and volume fraction in time are unknown, so it is not possible to

convert the ESA signal to zeta potential. Moreover, we are not interested in the

absolute magnitude of the zeta potential but in the course of the ESA signal in time.

At the beginning of the experiment (FeC13 was not yet added to the sludge suspen

sion), the ESA signal sign was negative, which confirms the negative charge of the

sewage sludge particles (see section 6.2).

In an experiment, the suspension pH decreases due to both the weak acidic nature of

ferric ions (see section 6.4) and the increase of ferric ion concentration in the

suspension. At a certain FeC13 dosage and pH, ESA reversal and consequently charge

reversal (- to +) occur in both experiments. The ESA reversal is sharp and discon

tinuous and occurs in a narrow pH range. Marlow and Fairhorst [1988] attributed this

144 Chapter 6

discontinuity to the presence of an undesired electrolyte signal, which is appreciably

comparable to the colloid signal.

Debye [1933] predicted that an ultrasonic sound wave passing through an electrolyte

solution would result in different displacement amplitudes and phases between anions

and kations. The relative displacement of anions and kations produces a separation of

charge accompanying the sound wave resulting in potential differences. This mechan

ism is called the 'Debye effect' and the resulting alternaring potential is termed the ion

vibration potential (IVP). The displacement of a charged partiele in a colloidal system

from its surrounding 'ion atmosphere' induces a similar alternaring potential termed

the colloid vibration potential (CVP) [Hermans, 1938; Rutgers, 1938]. The IVP is

generally out of phase with the CVP. CVPs are normally orders of magnitude larger

than ion vibration potentials (IVPs).

Bruil [1983] measured the ESA signal of a titanium dioxide suspension (density 4000

kg/m3) as a function of the solution pH. The suspension was titrated with HCl. ESA

amplitudes were in the order of magnitude of 0.1 mPa.m/V. At the isoelecttic point

(pH=7.8), the phase angle sudddenly changed from oo to 180°, which means that

partiele charge reversal occurred. It can be concluded that for heavily dispersed

materials and in the absence of salt in the system the acoustopboretic mobility agrees

with the electrophoretic mobility.

However, when the particles have a low zeta potential, volume fraction, and density

relative to the medium and the continuons phase has a high ionic strength, the

magnitude of the CVP approaches that of IVP [Marlow and Fairhurst, 1988]. This

also occurs when the colloid is near its isoelecttic point.

The partiele concentration (2 wt%), the density difference (30 kg/m3), and the ESA

amplitude (0.01 mPa.m!V) are relatively small in the sludge/FeCl3 system. Possibly,

salt is present in the sewage sludge suspension. Under these conditions the resultant

measured signal is the vector sum of the CVP and the IVP.

In the experiments the phase angle gradually changes from oo to 180° (see figure

6.9). This is also due to the presence of the electrolyte signal [Marlow and Fairhurst,

1988]. However, for most colloids this results in errors in determining the isoelecttic

point that are insignificant [Marlow and Fairhurst, 1988]. At a phase angle equal to

90° the polarity of the CVP changes as well. As a consequence, the polarity of the

sludge partiele changes as well. The charge reversal is attributed to an abrupt increase

of specific adsorption of positively charged hydrolysed ferric ions on active surface

sites of sludge particles. These hydrolysis products react to produce polymers. These


polymerie species span bridges between adjacent particles, thus forming flocs. After

charge reversal the ESA signal increases due to the continuing polymerization of

hydralysis products and, as a consequence, both the zeta potenrial and partiele radius

increase as well. Obviously, positively charged hydralysis products are present in the

sludge suspension for pH valnes smaller than 6. Moreover, at this critica! pH value

electronentral ferric hydroxide species are formed (see figure 6.5) and precipitation of

the insoluble Fe(OH)3 takes place. Precipitation of Fe(OH)3 only occurs over a small

pH range. At pH valnes larger than the critica! value, negatively charged polymerie

hydralysis complexes are predominant species present in the sludge suspension.

Electrastatic repulsion between sludge particles and complexes binders destabilization

of the sludge suspension.

In the two experiments charge reversal occurs at FeC13 dosages of 34 and 50 g/kg ds.

These dosages are much higher than the critica! coagulation concentration of non

adsorbable trivalent ions: 0.12 g/kg ds. The amount of metal coagulant needed for

polymerization and interpartiele bridging is much higher than the amount needed for

coagulation by double layer depression. lt can be concluded that coagulation does not

play a role in the destabilization of a sludge suspension. The dominant mechanism of

destabilization is specific adsorption of positively charged hydrolysed ferric ions.

Theoretically, the dosage at which ESA equals zero (t-potential equals zero) corres

ponds to some suspension properties: maximum dewaterability, minimum colloidal

stability and minimum sediment volume. Wakeman et al. [1992] showed that the most

rapid filtration of anastase (moderately compressible material) occurred at the point of

zero charge, whereas at greater (either positive or negative) zeta potentials the

filtration rate was reduced.

Charge reversal was also observed in sludges originating from other waste water

treatment plants: Mierlo, Amsterdam, and Lage Zwaluwe.

Determination of the point-of-zero-charge of sludge particles

H+ and (OHY ions are potential-determining ions, because they are the only species

through which the solid could be charged. At a certain concentration of potential

determining ions the sludge surface charge equals zero. Kations and anions adsorb to

sludge particles to an equivalent amount. This situation is known as the point-of-zero

charge (PZC).

146 Cbapter 6

H the conditions in the solution are such that t=O, the system is at its isoelectric point

(IEP). In the absence of chemical adsorption, the point-of-zero-charge and isoelecttic

point coincide. Due to specific adsorption, the charge between surface and slipping

plane ub is higher than it would be if no such specific effects were present. H some

cations adsorb specifically at the surface of the sludge particles, the new IEP is

attained by compensating the positive charge between surface and slipping plane ob by

(OHt adsorption, i.e. by increasing the pH. So specific adsorption of kations shifts the

IEP to higher pH values. In this way the occurrence of specific adsorption is tested.

o.oa O.o&

I 0.04

0.02

0.00

~ -0.02

-0.04

-0.08

1

't\' 1· .. I' . ','

' .. '

I I I

.I. l l l

'

2 3 4

pH

5 6 7

Fig. 6.11 Determination of the point-of-zero-charge of a sewage sludge suspension. Results of three experiments in which sludge suspensions were titrated with a HCl solution. The

continuons line (-) is the result of an experiment carried out with a sludge suspension adjusted with ]()'4 M KCl. The dotted lines (--) and (· · ·) are the results of experiments

carried out with a pure sewage sludge suspension.

In order to determine the point-of-zero-charge of an Eindhoven sludge suspension,

three experiments were carried oot with the MA TBC ESA system. A sludge suspen

sion was titrated with a HCl solution in two experiments (duplex experiment). In the

third experiment a HCI solution was added to a sewage sludge suspension adjusted

with KCI toa concentration of 1(}'4 M. KCI is assumed to be an indifferent electrolyte.

The point-of-zero-charge is found as the intersection of the three ESA versus pH

curves. At the intersection point the surface charge density is independent of the

Flocculation behaviour of sewage slndge 147

indifferent electrolyte concentration and equals zero. In tigure 6.11 the results of the

experiments are shown. The suspension pH decreases during the experiments. Charge

reversal (- to +) is observed in the experiments. The pH at which ESA equals zero (t

potential 0) corresponds to the isoelectric point of sewage sludge particles.

Because of the absence of specitic adsorption in the experiments, the isoelecttic point

coincides with the point-of-zero-charge.

Theoretically the three curves must interseet at the point-of-zero-charge. This is not

the case, although the differences are very small. It can be concluded that the pH of

zero partiele charge (PZC) and thus the pH at the isoelectric point (IEP) both equal 2.

The determined PZC is in good agreement with the PZC of the Si02 , which is the

main inorganic material present in sewage sludge solids (about 30 wt%). The

isoelecttic point of the sludge/FeCl3 system equals 6 (see tigure 6.10). The isoelectric

point of the sludge particles shifts from 2 to 6 due to the presence of ferric chloride in

the sludge suspension. It physically means that specific adsorption of positively

charged hydrolysed ferric ions takes place. The results of the experiments presented in

tigure 6.11 are an additional proof of the occurrence of chemica! adsorption in the

system.

+

CR 1 CR2 CR3

pH

Fig. 6.12 Schematic illustration of the general electrophoretic mobility behaviour of colloid

systems in the presence of hydrolysable metal ions.

148 Chapter 6

James and Healey [1972] studied the adsorption of hydrolysable metal ions at the

oxide-water interface. They showed that hydrolysable metal ions are able to reverse

the charge of anionic colloidal substrates. The study was focused on the adsorption of

Co(ll) by Si02 and Ti~ colloids. The electrophoretic mobility behaviour, supple

mented by streaming potentlal data, was determined as a function of the pH.

0.10 9

0.05 ,... ""': ..... 8 I I I

i ' 0.00 I 7 I I 1', I I

' I I

' -6.05 I' ' ' 8 i ' ' ' .\

~ ' ' ' -0.10 ., ' 5

" "' -~ ' --~

~

-0.15 "', _",

4 >-· .... .:....:. ~ .,. .... '

-0.20 3

0 10 20 30

do8ap (ml)

Fig. 6.13 ESA signal as a junction of the dosages of FeCl3 and KOH solutions. lnitially, the

suspension was titrated with a ferric chloride solution ) until the occurrence of charge

reversal (- to + ). The suspension pH ( .. .) decreased. Afterwards, the suspension was titrated

with a KOH solution (--). Charge reversal occurred (+ to -). The suspension pH ( ... )

increased.

The various charge reversals observed are, in order of increasing pH (see figure

6.12), shown to represent the point-of-zero-charge on the Si02 substrate (CRI),

surface precipitation of hydrolysable metal ions (CR2) and the point-of-zero-charge of

the metal hydroxide solid itself {CR3).

CRI and CR2 were also observed in experiments carried out with a sludge suspension.

An experiment in which a sludge suspension was used as a colloidal substrate was set

up to check the occurrence of CR3. Initially, a ferric chloride solution was added to

the sludge sample until the occurrence of charge reversal CRl (- to + ).

Flocculation behaviour of sewage slndge 149

Subsequently, the sludge suspension was titrated with a KOH solution. The experimen

tal result is shown in tigure 6.13. Charge reversal CR3 ( + to -) occurred at a pH of

4.5. If sufficient metal ion is adsorbed to yield a complete coating of adsorbed metal

hydroxide, the charge reversal corresponds to the point-of-zero-charge of the metal

hydroxide. The point-of-zero-charge of ferric hydroxide is presented by the reaction:

The PZC of ferric hydroxide occurs at pH=8.5 [Parks and de Bruyn, 1962]. In the

experiment CR3 occurred at a lower pH value than the PZC of ferric hydroxide. This

is possibly caused by incomplete coating of adsorbed metal hydroxide on particles.

Simulation of the flocculation process in practice

0.14 14.0

0.12 12.5

I 0.10 11.0

0.08 9.5

0.06 8.0 a ~

0.04 ' ., . 6.5 ' '

r.:.::l 0.02 5.0

0.00 --- 3.5

-0.02 2.0

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

felTic chloride dosage (ml)

Fig. 6.14 Simu/ation of the jlocculation process induced by ferric chloride/time in practice.

ESA signal (- ) and suspension pH (---) as a function of the ferric chloride dosage. After

addition of 20 ml of the FeCl3 solution, JO ml of time dispersion (350 glkg ds) was dosed in

one step.

150 Chapter 6

Lower concentradons of metal or higher concentradon of colloidal substrate will

reflect surface-coated and uncoated areas. The formed insoluble ferric hydroxides will precipitate and eumesh the sewage sludge particles (sweep flocculation).

An experiment was carried out to simulate the flocculation process induced by ferric

chloride/lime in practice. In figure 6.14, the result of the experiment is shown. First,

250 m1 of a sludge dispersion (dry solids content 2 wt%) was titrated with a ferric

chloride salution up to a total dosage of 25 ml, which corresponded with a dosage of

375 g!kg ds. During the addition of ferric chloride the suspension pH decreased. At a

pH of 6.3 the expected charge reversal (- to +) occurred. After 20 m1 of the ferric

chloride salution (300 g!kg ds) was dosed, 10 m1 of a lime dispersion (350 glkg ds)

was added to the sludge suspension in one step. As a consequence both the ESA signal

and pH sharply increased (figure 6.14). The sudden increase in ESA signal is possibly

attributed to specific adsorption of calcium ions at sludge particles covered by ferric

hydrolysis complexes. Other processes may occur simultaneously, such as the

formation of negatively charged ferric hydralysis products and the precipitation of iron

hydroxide due to the strong increase in pH.

Experiments with organic jlocculants

In order to study the mechanism of cationic polyelectrolyte flocculation of a sewage

sludge dispersion, two different polyelectrolytes were used in experiments carried out

with the MATEC ESA system: Röhm KF 975, which is the successar of Praestol

444K and Röhm KF 945. Both polymers are widely used as sewage sludge condi

tioners. Röhm KF 975 is a very strongly cationic polyelectrolyte, having a high

molecular weight (viscosity 2000-5000 mPas). Röhm KF 945 is a strongly cationic

polyelectrolyte, having a viscosity of 50-200 mPas.

Figures 6.15 and 6.16 show the results of two experiments in which a 250 m1 sludge

dispersion (dry solids concentradon 1.75 wt%) was titrated with the polyelectrolyte

solution Röhm KF 945 and Röhm KF 975, respectively.

The ESA signal (and thus the t potential) of the flocculated sludge sample changed

sign from negative to positive with an increase in the amount of cationic polyelectro

lyte, after which it approached a constant positive value. It means that positively

charged polymers were adsorbed at the sludge partiele surface. The polymerie ebains

adopted a nearly flat conformation on the partiele surface due to the strong electra

static attractive forces between polymer and particle.

0.012 0.010 0.008 0.006

0.002

Flocculation bebaviour of sewage sludge 151

i 0.004

0.000 +-+---'---.;.__--,----'----------'------'----'-------1

-0.002 ............. -0.004

~· -0.006 ~ -0.008

-0.010 -0.012 -0.014

-0.~ ~~~~~~~ .. ~~~~~~~~~~~~~~~~

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

polyelectrolyte dosage (g KF 945/kg ds)

Fig. 6.15 Measured ESA signal (related to the zeta potential) as a function of the dosage of

polyelectrolyte KF 945. Charge reversal occurred at a dosage of 0.03 g KF945/kg ds.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

polyelectrolyte dosage (g KF 975/kg ds)

Fig. 6.16 Measured ESA signal (related to the zeta potential) as a function af the dosage af

polyelectrolyte KF 975. Charge reversal accurred at a dosage of 0.12 g KF 975/kg ds.

152 Cbapter 6

At the cationic polyelectrolyte dosage wbereby the zeta potential becomes zero, the

repulsive forces between sludge particles decrease and tlocculation due to charge

neutralization may occur. The pbenomenon of charge reversal caused by adsorption of

polyelectrolytes was found by many investigators. Gregory [1969] made a detailed

analysis of the flocculation of anionic polystyrene latex particles by cationic polyelec

trolytes. The electrophoretic mobility of the particles cbanged sigmoidally from

negative to positive with an increase in polymer dosage. Black et al. [1965] investig

ated the use of polyelectrolytes in tbe destabilization of a kaolin clay suspension. The

isoelecttic point corresponded witb tbe optimum condition for destabilization. At the

optimum dosage the lowest supernatant turbidity was found. Igarashi and Nishizawa

[1992] measured tbe zeta potentlal of digested sewage sludge samples witb a new

streaming potentlal measurement device. The zeta potential cbanged sigmoidally from

negative to positive when tbe cationic polyelectrolyte was increased. When tbe

colloidal charge was zero, tbe gravity tiltration speed reached a maximnm and the

moisture content in dewatered cakes reached a minimum. Roberts [1978] observed that

the optimal cationic polyelectrolyte (Zetag 92) dosage for dewatering of an activated

sludge sample occurred at zero electropboretic mobility.

Charge neutralization is not necessarily the main mechanism teading to destabilization

wben sludge particles and polyelectrolytes are of opposite signs. The polyelectrolytes

used possess a high molecular weight. Bridging is more efficient witb polyelectrolytes

of a high molecular weight since loops will tend to extend further into solution.

Moreover, tbe high charge density of the polyelectrolytes used results in strong

electrostatle repulsion between adjacent ebains and consequently the polyelectrolyte

ebains are stretched. This also leads to a more effective bridging mechanism.

Destabilization of a sewage sludge suspension by the bridging mechanism is possible

at polymerie concentrations higher than tbe concentration at which charge reversal

occurs (point-of-zero-charge).

The degree to which a sludge suspension will be stabilized is difficult to determine.

Many factors are involved in the destabilization process by polyelectrolytes: partiele

concentration, inter-partiele interactive forces, ionic strengtb, polymer contiguration in

tbe adsorbed state and intensity and duration of mixing. The influence of these

parameters on the destabilization process of a sewage sludge dispersion was not

investigated in tbis study.

The phenomenon of charge reversal was only observed in a few experiments. In many

experiments the expected charge reversal did not occur. Possible explanations are:


• The sludge flocs formed could not be detected by the ESA probe. The sludge flocs

could not pass the small gap between the two electrodes.

• The time available for reconformation was large and equal to the time between two

dosages (30 s). The fust attached polymers could flatten to an extension smaller

than the 'Debye length' K-1 before further adsorption took place. Electrostalie

repulsion between the particles hindered flocculation.

Discussion about the experiments carried out

In some experiments carried out with various sewage sludge samples, the expected

charge reversal due to specific adsorption of hydrolysed ferric ions on the surface of

sludge particles did not occur. The real reason is unknown. However, several factors

which are inherent in the sewage sludge/FeC13 system could disturb the measuring

result:

• The volume fraction of particles il> in the sewage sludge suspension is small (1 to 4

wt%). Consequently the ESA signalis small (equation (6.28)).

• The density difference tJ.p between sludge particles and the liquid phase is small

(!J.p ::::::30 kg/m3). A small density difference yields a small ESA signa!, too (equa

tion (6.28)).

• As already stated in this section, an undesired electrolyte signal plays an important

role in the sewage sludge/FeC13 system. A few experiments were carried out to

determine the relative importallee of the electrolyte signa! in the sewage

sludge/FeC13 system.

Two sewage sludge samples and demi-water were titrated with a ferric chloride

solution. The results of the experiments are presented in tigure 6.17. It can be

concluded that the ESA signal measured in the demi-water/FeCl3 system is of the

same order of magnitude as that measured in the sewage sludge/FeC13 system. The

electrolyte signalis caused by growing ferric hydrolysis complexes.

154 Cbapter 6

0.12 .---------------------------------------------~

0.08

0.04

-0.04 L..L->---c.__._......._,..._._......___._._..J......J.__,_.i..-L_._ ......... _,__~.....~.. .......... ._._ ............ __._ .......... _.__.___._.._.~..-~....J

0 50 100 150 200 250 300 350

dosage of FeCl3 (g/k.g ds)

Fig. 6.17 ESA signa! as a function of the dosage af FeC/3• The continuous line represents the

ESA signal of the demi-water-FeCl3 system. The dotted lines are the results of experiments carried out with the sludge-FeCl9 system.

6.8 A model to describe hydrolysable metal ion adsorption at the sludge solid

lValer interface

James and Healey have presented a model for the adsorption of hydrolysable metal

ions, which provides a general onderstanding of the phenomena observed in Co(ll)

adsorption by SiO:z or Ti02 colloids. The model is used as a base for the description of

adsorption of hydrolysable ferric ions at the surface of sludge solid particles.

James and Healey measured adsorption densirles of co2+ on Si02 colloids and

concluded that these metal ions characteristically do not adsorb until a critical pH is

obtained. At pH valnes smaller than this critical pH the adsorption is close to zero.

The most likely interaction that could prevent adsorption of (hydrolysed) cations on

negative surfaces against electrostatic attraction below the critical pH is a solvation

term [James and Healey, 1972]. Changes in solvation energy areexpressedas changes

in primary and secondary hydration. James and Healey concluded that the adsorbed

species are separated from the solid surface by at least one layer of water molecules so

that direct chemical bonding is precluded. Consequently the meta1 ions are not

required to lose their primary hydration shells. The solvation energy term refers to the


removal of the second hydration layer of a cation. The total work of cation adsorption

is separated into a simple coulombic term and a secondary solvation term. The

coulombic term will be corrected with a 'chemical' free energy term where necessary.

The adsorption of metal ions at the sludge solid-liquid interface is treated in terms of

competing energy changes as the ion approaches the interface. The change in free

energy of adsorption ..1Gads [J.mot-1] equals the sum of the change in coulombic energy

..1Gcoul [J.mot'], the change in secondary solvation energy .dGsoiv [J.mot'] and a

specitic adsorption energy contribution ..1Gchem [J.mol-1], i.e.

(6.30)

The energy terms .dGcoui and ..1Gchem are favourable to adsorption, whereas .dGsoiv is

unfavourable. Adsorption may occur when:

(6.31)

The change in coulombic energy by brioging an ion charge ze up to a surface where

the potenrial is t/lx is zet/;x and equals the binding energy. The change in specitic

adsorption is equal to ze4>, where 4> is termed a superequivalent adsorption potenriaL

An expression for the change in secondary solvation energy was derived by James and

Healey [1972].

In the scope of this thesis we present some results which are of relevanee in this

study.

In the region adjacent to charged interfaces electtic fields of considerable magnitude

exist. Water molecules adsorbed on the interfaces will undergo parrial or complete

electrical saturation, in which case the dielectric of these molecules is reduced from

the bulk value of 78.5 to the value of 6.

156

.tl 80 '"" iS

;l ., l " 60 '"' .....

j 40

20

0

~ /

/ (A) I f I I I 13 I I I

I

8

Distance 1 Ä

Cbapter 6

1

2 -

12 0

I I I I I I

Distance 1 Ä

(B)

12

Fig. 6.18 Actual continuous (A) and approximate discontinuous (B) representations of the

variation af the dielectric constant as a tunetion of distance for: (1) a surface of low electric

field; (2) a surface of high electric field; (3) a hydrated ion or a very high electric field

[James and Healey, 1972].

The varlation of the dielectric of water er,x as a function of elistance from the interface

is expressed by [Anderson and Bockris, 1964]:

€bulk -6 6 er.x = [ dY, 2] +

1 + B (-) dx

(6.32)

where the constant B bas a value of 1.2·10-17 m2.V'2 and (dY,/dx)x represents the

electtic field strength estimated from the Gouy-Chapman model of the double layer. It

is possible to make a good approximation to the system by dividing the interface into

three regions and using a constant value of the dielectric in each of the regions (see

figure 6.18).

The regions are:

1. The solid ( e, = Eson.J.

2. The layer of adsorbed water at the interface (er=eintr= 6).

3. Water outside the adsorbed layer (er=ebulk=78.5).


(a)

te---: r·"': 1on

157

(b)

Fig. 6.19 Representation of the solid-liquid intelface showing two of the possible locations of

an adsorbed ion: (a) in the diffuse double layer; and (b) near the inner Helmholtz plane

[James and Healey, 1972].

There will be a discontinuity in the dielectric of the medium at each boundary of the

three regions. Becanse of this artificial division in the model there are two general

locations for the hydrated metal ion ( radins rion + 2r w) with respect to the interface. The

fust is for the case where the primary hydration sheath of the ion and the water

adsorbed on the solid do not overlap, as is shown in tigure 6 .19a. The secoud case

covers the possibility that the primary hydration shell of the ion may include the

adsorbed water ( tigure 6.19b). For case 1, .:lGsoiv [J.ion-1] is given by [James and

Healey, 1972]:

(6.33)

For case 2 the change in secondary solvation energy is represented as [James and

Healey, 1972]:

158 Chapter 6

r. ) 2(r. +2r \2

ton wl

1 1 1 - _1) Eintf

(6.34)

Both equations represent the change in secondary solvation free energy in moving a

hydrated metal ion from the bulk solution to the inner Helmholtz plane at the solid

liquid interface. The dielectric constant of sludge solids is assumed to be the dielectric

constant of SiÜ:l, the main inorganic material present in sewage sludge solids. The

dielectric constant of Si02 equals 4.3. From equations (6.33) and (6.34) it can be

concluded that .1.Gso1v > 0, so work must be done to remove the second hydration

layer of a cation and reptace it by interfacial water with a very low dielectric constant.

At the start of the continuons series experiment, presenled in figure 6.9, in which a

sludge suspension is titrated with a ferric chloride solution, only negatively charged

hydrolysed ferric ions are present. For suspension pH valnes between 7 and 6, no

adsorption occurs due to electrostatle repulsion between particles and negatively

charged hydrolysed complexes:

(6.35)

During the experiment the suspension pH decreases until the predominant form is

Fe(HzOMOH)2 + (at pH equals 6). Adsorption of these monovalent complexes is

energetically favourable, becanse of the validity of the inequality:

(6.36)

If more salt is added, higher positively charged (twovalent and trivalent) hydrolysed

complexes are formed. The equilibrium as presenled in section 6.4 is driven to the

left. As a result, the average ionic charge increases and the suspension pH decreases.

The increase of the average ionic charge ze increases the coulombic, the specific

adsorption, and the solvation energy. However, because both the conlombic and the

specific adsorption energy vary with the valency z, and the solvation energy change

varles with z2, the increase of charge due to hydrolysis increases the magnitude of the


solvation term much more than the som of the conlombic and specifïc adsorption term.

As a consequence, above a certain average ionic charge and thus below a certain pH:

(6.37)

Thus, adsorption of higher charged hydrolysed products is energetically unfavourable.

The general condusion can be that within a certain pH range chemica! adsorption of

hydrolysed ferric ions at active sites of sludge particles occurs. Outside this pH range

speciiic adsorption does not occur. Since total ion concentrations are different no exact

calculations of ~Gads can be made. However, in tigure 6.20 a schematic presentation

of the free energy of adsorption as a function of the suspension pH for Eindhoven

sludge is given.

+

4 5

pH

Figure 6.20 Free energy of adsorption as a function of the suspension pH for Eindhoven

sludge.

6.9 Conclusions

The flocculation behaviour of sewage sludge was studied with the MA TEC ESA sys

tem, an electroacoustic technique for application of electrokinetic measurements. With

this system the Electrokinetic Sonic Amplitude (ESA) of a (flocculated) sewage sludge

suspension was measured, which is related to the zeta potenrial of sludge particles.

160 Chapter 6

The study was focussed on the various destabilization phenomena of sludge suspen

sions indoeed by the flocculants ferric chloride and polyelectrolytes.

The dominant mechanism of destabilization of a sludge suspension induced by ferric

chloride is specific adsorption of mainly monovalent positively charged hydrolysed

ferric ions at active sites of the surface of sludge particles. Specific adsorption of

positively charged hydrolysis products results in partiele charge reversal (- to + ).

Specific adsorption of these monovalent complexes on sludge particles is energetically

favourable within a certain pH range. As the hydrolysed ferric ion approaches the

sludge particle, the sum of change in conlombic energy and specific adsorption energy

(both favourable to adsorption) is larger than the solvation energy (unfavourable to

adsorption) and adsorption may occur. The solvation energy refers to the removal of

the secoud hydration layer of a ferric ion.

The electrok:inetic behaviour of a sewage s1udge colloid was determ.itled in the

presence of various hydrolysable ferric ions as a function of the pH. Three charge

reversals were observed and listed as CRI, CR2 and CR3. CRI (- to +)is the point

of-zero-charge (PZC) of the colloidal sludge particles and occurred at pH equal to 2.

The measured PZC corresponds with the PZC of Si02 , which is the main inorganic

material present in sewage sludge solids. CR2 (- to + ), occurring at pH equal to 6,

indicated specific adsorption of principally monovalent hydrolysed ferric ions. Charge

reversal CR3 ( + to -) reflected a partial coating of ferric hydroxide on the sewage

sludge particles and occurred at a pH equal to 4.5. Typically, the charge reversals are

sharp and discontinuous, occurring within a very small pH range. This can be

attributed to the presence of an undesired electrolyte signal in the sludge/FeCl3

system. However, errors in detennining the isoelectric point due the electrolyte signal

are insignificant.

The MA TBC ESA system is not suitable to study the destabilization phenomena of

sewage sludge particles induced by cationic polyelectrolytes. Only in a few experi

ments the expected charge reversal occurred. Due to the high charge densities of the

cationic polyelectrolytes used (Röhm KF975 and KF945), the zeta potential becomes

zero at very small polyelectrolyte dosages. The main destabilization mechanism is

charge neutralization. At polymerie dosages higher than the concentration at which

charge reversal occurred, the bridging mechanism is responsible for destabilization.

Partiele bridging is efficient because the polyelectrolytes used possess a high molecu

lar weight and loops tend to extend further into solution.

7 CONCLUDING REMARKS AND PERSPECTIVES

The relevant dewatering properties have been determined of four sewage studges

originating from four different waste water purification plants in the Netherlands. The

design and operation of these waste water treatment plants are different, including

different dewatering techniques (chapter 2). In this way, a great variety of sewage

studges could be studied.

The vacuum exsiccator metbod for water vapour isotherms and thermal analysis

techniques (TGA and DT A) for isothermal drying curves are suitable measuring

methods to determine the water bond enthalpy (chapter 3). The TGA-DTA drying

model can be regarded as an acceptable model for the drying behaviour of sewage

sludges and the determination of the water bond enthalpy as a function of the sample

moisture content. Both measuring methods show that the water bond enthalpy differs

significantly from zero at sample moisture contents smaller than 0.3 to 0.6 kg water

per kg dry sollds. The sludge type, and type and dosage of flocculant do not influence

this critica! moisture content. Water having a bond enthalpy larger than l kJ/kg is

categorized as 'bound water' . and cannot be removed in a mechanica! dewatering

process at applied pressures smaller than 10 bar. So the theoretica( maximum dry

solids content that can be reached is about 65 to 75 wt%. In practice dry solids

contents of about 15 to 35 wt% are obtained. A lot of 'free' water remains enclosed in

the sludge filter cake during the mechanica! dewatering process. Higher mechanica!

pressures are needed to remove more 'free' water out of the sludge filter cake. Water

vapour sorption isotherms measured at different temperatures with the vacuum

exsiccator technique can bedescribed very well with the S-shaped G.A.B. equation.

The dewatering behaviour of sewage sludges was tested with different techniques: the

filtration-expression cell, the conventional Capillary Suction Time test, the compres

sion-permeability cell, and the Modified Piltration Test (chapter 4). The most appro

priate instrument to study the dynarnic dewatering behaviour of sludges is the

fûtration-expression cell. The influence of various process parameters, such as

pressure, slurry concentration (cake thickness), flocculant type and dosage can be

investigated. In the characterization research, floc microproperties, which are of

relevanee for a better understanding of the dewatering process, were determined too.

Sludge floc microproperties of interest are composition (dry solids content, ash

content, A TP content, pH, electrical conductivity), zeta potential, partiele size

distribution and rheological properties (thixotropy). At the optimum flocculation

162 Chapter 7

conditions (flocculant dosage and mixing conditions) some characterization parameters

show a minimum or maximum: minimum specific cake resistance, minimum vacuum

suction time, minimum CST value, minimum iron content in the filtrate, maximum

dry solids content, maximum permeability, maximum median floc diameter, maximum

degree of thixotropy. By definition, the dewaterability of sewage sludges shows a

maximum at the optimum flocculation conditions.

The modified CST test, which continuously measures the position of the liquid front in

a ceramic slab, provides information on the dynamic dewatering behaviour of both

non-flocculated and flocculated sludges (chapter 5). The model, which describes the

position of the liquid front as a function of time, enables the possibility to calculate an

average specific cake resistance from the experimental data. The modified CST test is

an improvement of the conventional CST apparatus, which only measures the position

of the üquid front at two different times.

Prior to dewatering, flocculants are added to the sludge suspension in order to

improve the dewatering behaviour. The addition of flocculants induces the destabiliza

tion of the suspension (chapter 6). The so-called electroacoustic technique is suitable

to study the destabilization mechanisms of sewage sludges induced by ferric chloride.

The dominant destabilization phenomenon is specific adsorption of mainly monovalent

positively charged hydrolysed ferric ions at the active sites of the sludge particles.

Specific adsorption manifests in the reversal of the partiele charge from negative to

positive and occurs within a certain pH range. The pH range can he determined with a

model that presents an energetic consideration of the hydrolysable metal ion adsorption

at the sludge-solid-water interface.

Future areas of research

It is obvious to study the practical applicability of some characterization tests at sludge

treatment plants. The filtration-expression cell and the compression-permeability cell

are to he considered for this study. Nowadays, there is no question of an unambiguous

determination of sewage sludge dewatering properties at sludge treatment plants.

Another important consumer market is the industries which produce sludges as a part

of their waste products. The filtration-expression cell can he nsed to diagnose and

optimize the flocculation process, which has a large influence on the dewatering

hehaviour. The introduetion of the filtration-expression cell may lead to a better

insight into the dewatering process, a better cantrolling mechanical dewatering, and

Concluding remarks and perspectives 163

possibly a smaller use of the amount of flocculants. Achieving higher dry solids

contents yields a rednetion of expenses for the total sludge processing. In the Nether

lands, the costs of the dewatering of sewage sludge rougWy amount to 300 to 400

guilders per ton of dry solids. The annual production of sewage sludge dry solids is

about 300,000 tons of dry solids. Consequently, the total costs of sludge dewatering is

approximately 100 million guilders. A saving in one percent on the sludge dewatering

costs means a saving of one million guilders per year. Mechanica! dewatering is

foliowed by other processing steps, such as drying, incineration, and dumping. The

rate for sludge processing amounts to 150 guilders per ton of sludge cake. The

average dry solids content is 23 wt% , so the rate equals 650 guilders per ton of dry

solids. Consequently, the total expenses for sludge processing annually amounts to 200

million guilders. The increase of the average dry solids content from 23 to 24 wt%

annually yields a cost sa ving of about 7.5 million guilders.

NOTATION

a sphere radius plus thickness of Stem layer [m]

al constant H <ly specitic surface area [mz.m-s]

~ water activity H A cross-sectional area of the filter medium [mz]

Ac area of the cross-section of the inner CST tube [mz]

AH Hamaker constant [J]

Ar heat transferring surface area of reference cup [mz]

A, heat transferring surface area of sample cup [mz]

A .. sample surface area for moisture transport [mz]

ATP = Adenosine-triphosphate

bi constant [-]

B constant in equation (6.32) [mz.v-1]

BOD= biologica! oxygen demand

c velocity of sound in suspension [m.s-1]

Cr calibration factor [W."'v-lJ

Ctc conversion factor [/kV.K1]

Cr GA correction factor [-]

Cv concentration of solids in suspension [k:g.m-3]

c cake mass deposited per unit filtrate volume [k:g.m-3]

Co critica! coagulation concentration [mol.lite11]

Cs BET adsorption constant [-]

CB,O BET adsorption constant [-]

cg Guggenheim constant [-]

Cg,O Guggenheim constant H cp.d specific heat of water vapour [J.kg·l.KI]

cp,ds specitïc heat of dry sludge solids [J .kg·l.Kl]

cp.r specific heat of reference cup [J .kg·l.K-1]

cp,w specitic heat of water [J.kg·l.Kt]

CR charge reversal

CST Capillary Suction Time

CVP colloid vibration potentlal

~ mean partiele diameter [m]

166 Notation

d. = density of dry solids [kg.m-3]

ds dry solids

d,. density of water [kg.m-3]

DTA= differential thermal analysis

e elementary charge [C]

Et difference between molar sorption entbalpy in fust layer and in the mth layer [J.mol1

]

~ difference between molar sorption entbalpy in mth layer

and condensation entbalpy [J.mot-1]

ESA= electrokinetic sonic amplitude

f = parameter defined by equation (6.24) [-]

fractional distance in the temperature boundary layer [-]

fw fugacity of water [-]

e fugacity of pure water at standard temperature (-]

p' suction force exerted by filter medium under CST tube [N]

g gra vitational acceleration [m.s-2]

.6.G Gibbs free energy [J]

.6.Gads = change in adsorption energy [J.mot1]

.6.Gchem= change in chemica! adsorption energy [J.mol1]

.6.GcouF change in conlombic energy [J.mot-1]

Gr = correction factor for a given electrode geometry [-]

.6.Gso1v= change in secondary solvation energy [J.mol-1]

h thickness of cernmie slab [m]

H = height of sludge layer in CST tube [m]

.6.H = enthalpie change [J]

Ho distance between two spheres [m]

.6.Hb bond entbalpy of water in slndge cake [J.kg-1]

.6.Hcow:F condensation enthalpy [J.mot1]

Hds entbalpy of sludge dry solids [J.kg-1]

.6.Hexc= excess entbalpy of sorption [J.mot1]

.6.H = m adsorption entbalpy in the mth layer [J.mot1]

H,. = enthalpy of reference cup [J.kg-1]

H, enthalpy of sample cup [J.kg-1]

.6.Hsor = sorption entbalpy [J.mol1]

.6.H= V entbalpy of evaporation of pure water [J.kg-1]

Notation 167

.ó.Hv,o= enthalpy of evaporation of water at 273.25 K and 1 bar [J.kg-1]

HW enthalpy of water [J.kg-1]

.ó.Hw differential enthalpy of wetring [J.mot1]

i.e. inhabitant equivalent

IEP isoelectric point

iHp inner Helmholtz plane

IVP ion vibration potenrial

jw moisture flux [kg.m·2 .s 1]

k Boltzmann constant [J.Kl]

GAB constant [-]

ko GAB constant [-]

kKc Kozeny constant [-]

K permeability [mz]

Ko permeability at p,=O [mz]

K,.v average permeability [m2]

KP permeability of filter medium [m2]

Koo equilibrium permeability [m2]

K• complex conductivity of suspension [-]

length of inner cylinder of rheometer [m]

L cake thick:ness [m]

11\is mass of dry solids [kg]

m,. mass of reference cup [kg)

m. mass of sample cup [kg]

illw water mass [kg]

illw,oo = water mass at the end of a CP-cell experiment [kg]

.:lm., = loss of water mass [kg]

.:lm.,,.,.;= total loss of water mass [kg]

MCC= microcrystalline cellulose

MFT= Modified Piltration Test

llo concentration of ions in bulk solution [number.m 3]

n+ local concentration of positive ions [number.m 3]

n+o concentration of counter-ions at infmity [number.m-3]

n. local concentration of negative ions [number.m-3]

ll.o concentration of co-ions at infinity [number.m 3]

llç critica! coagulation concentration [number.m 3]

168 Notation

NA Avogadro's number [-]

oHp outer Helmholtz plane

Pa constant [Pa]

p.e. polyelectrolyte

p, compressive pressure [Pa]

Pw = partial vapour pressure of water [Pa]

p~ vapour pressure of pure water [Pa]

p pressure [Pa]

äP = applied pressure difference [Pa]

Po = hydraulic pressure at position r0 in ceramic slab [Pa]

pcap capillary suction pressure [Pa]

.ó.Pp pressure difference across the filter medium [Pa]

.ö.Pt = hydraulic pressure drop [Pa]

äP, pressure difference across the sludge cake [Pa]

PZC = point of zero charge

Q heat flow [J.s-1]

Q = liquid flow [m3.s-l]

r radial distance [m]

= position of liquid front at time t [m]

ro position of liquid front at time t=O [m]

re radius of ceramic slab [m]

re distance from centre of ion to adsorbed water layer [m]

rion radius of ion [m]

fw radius of water molecule [m]

R general gas law constant [J.mol1.K1]

R.v average radius [m]

~ = radius of outer cup of rheometer [m]

Re distance between eentres of two spheres [m]

Re = Reynolds number [-]

Reent= critical Reynolds number [-]

Re electrical resistance [0]

Re.d = electrical resistance of dry part of ceramic slab [0]

R.o.tot = total electrical resistance of ceramic slab [0] R.,,w electrical resistance of wetted part of ceramic slab [0] RH = relative humidity [-]

Notation 169

R", ftlter medium resistance [m-I]

R. radius of inner spindie of rheometer [m]

s ratio between distance between eentres of two spheres (R.,)

and sphere radius (a) [-]

s thermocouple signa! [t-tV]

AS entropy change [J.Kl]

t time [s]

T temperature [K]

torque [N.m]

Tgas gas temperature [K]

TGA thermogravimetrie analysis

Tr reference temperature [K]

Tr,exp measured reference temperature [K]

T, sample temperature [KJ

Ts,exp measured sample temperature [KJ

Tw water temperature [K]

Twall furnace wall temperature [K]

u solid based moisture content [kg w. kg ds-1]

u' bound water content [kg w. kg M 1]

UI solid based moisture content in monolayer [kg w. kg ds-1]

UVP ultrasound vibration potential

Vw molar volume of water [m3.mol-1]

V ftltrate volume [m3]

VA London-van der Waals attraction potential [J]

VR potential energy of repulsion [J]

VST vacuum suction time

VT total potential energy of interaction [J]

x mole fraction [-]

diameter [m]

distance co-ordinate [m]

Xo maximum diameter [m]

z valency [-]

170 Notation

Greek symbols

(l' = parameter defined by equation (6.26) [-]

Cl'av average specitic cake resistance [m.kg-1]

Cl'eff = effective heat transfer coeffident [J.s·1.m·2.K"1]

Cl'r convective heat transfer coeffident for reference cup [J.s·1.m·2.K1]

a, convective heat transfer coefficient for sample cup [J .s·1 .m-2 .K1]

6 compressibility coefficient [-]

redprocal hydraulic radius [m-1]

'Y interfacial tension of water or filtrate [N.m-1]

i' shear rate [s-t]

'Yo = parameter defined by equation (6.15) [-]

0 Stem layer thickness [m]

compressibility coefficient [-]

ÓT thickness of the temperature boundary layer [m]

€ porosity [-]

emissivity [-]

dielectric permittivity of suspension [C. v-•.m-1]

Eo = permittivity of vacuum [N-1 .m-2. C2]

porosity at p.=O [-]

€." = equilibrium porosity [-]

€bulk = dielectric constant of bulk material [-]

Eintf dielectric constant of adsorbed water [-]

Er dielectric constant of medium [-]

Es solidosity [-]

Esolid = dielectric constant of solid [-]

E,_o = solidosity at p, = 0 [-]

Es,oo equilibrium solidosity [-]

r zeta potenrial [V]

fJ viscosity of filtrate [Pa.s]

fla apparent viscosity of suspension [Pa.s]

'f/p plastic viscosity [Pa.s]

() = contact angle (filter medium/filtrate) [rad]

(), sample temperature [OC]

K. = redprocal double layer thickness [m-1]

Notation 171

À compressibility coefficient [-]

f.'d dynarnic mobility [mz. y-t.s-1]

V kinematic viscosity of the liquid [nt.s-1]

p density of space charge [C.m-3]

Äp density difference between particles and liquid [kg.m 3]

Pd specific electrical resistance of dry cerarnics [O.m}

Pn specific electrical resistance of wetted cerarnics [O.m]

p, sludge density [kg.m-3]

(J Stefan-Boltzmann constant [W.m-2.K4]

(Jo surface charge [C]

(Jb charge between surface and slipping plane [C]

(Jd charge of diffuse double layer [C]

(Jek charge at slipping plane [C]

(J, charge in Stem layer [C]

7 shear stress [Pa]

7o yield stress [Pa]

q, superequivalent adsorption potential [V] .p volume fraction of particles [-]

"" electrical potential [V]

% surface potentlal [V]

""~ Stem potentlal [V]

w angular velocity [rad.s 1]

angular frequency [s-t]

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::sl IJ.CI

> .... ~ (")

~ "" "" g.

~ ~ s. "" ~ §=

~ ~ ~

~ ~ ... ;;-; ;:,

I l

,/·

bo ~ !ooo6

INGANG·-·-·-·-·-·-· ·-·--·-·-

i I ! ~ 9 i s. S' 1'1)

l 1'1)

~ a ~

[ "" = ~ ~ i 1'1) a "=

I

~ ~ ~ ....

186 Appendix 1

"chenu.f::al

tond<:tám<'hf -

e~ ta.w..t.,. r'hovt9>

/k:e.; - s-che~ne o/ ihe rÛ<1"" P.~a/me-;/ ;0/anl

1 m(er-6.

Fig. A2 Process scheme of the Mierlo sludge treatment plant.

Appendix 1 187

JJJ

Fig. A3 Map of the Amsterdam-Oost waste water treatment plant.

1 Ontvangput 2 Roosterinstallat1a 3 Hoo!dgemaal-Bedrijtsgebouv 4 Zandvanger 5 Mangcontaottank 6 Oxydatietank 7 Habednktank

-

8-8a Retour- en surplusslibgemaal BY. Bio-filter 9 Ringaloot

10 ludikkèr r 11 Sliblagunea ( 6+ ""- 1,~dt.-l v.tRa.J;c jwec&c..,/ J} 12 Slibmengtank I 1' Slibverwerking 14 Laadplaats slihcontainers

RIJ.z.T

Appendix 1 189

\ I ,

..... i;l ...

::::: i <->

"" ç=> ...., ;;1!

"' ' "" !~ i 'o i ,o

'""

Fig. A5 Process scheme of the oxidation ditch system De Hooge en Lage Zwaluwe.

~ ~ "'cl d <')

~ "" g.

~ ~ s. "'

I <::>' ;::

ti: g.

~ ~ ::tl

<.::::: ~

!lli.lL l=~====--AANVOER' VREEM05tl6~

HOOGHEEMRAADSCHAP WES]'_:!\:~~NT ~-~!::!· ,~··!J .• §: .. JBo.•!:!-:J·~-"!:!-"";:?CC:==l hslMUil Projwtl_..,

RWZI RIJEN

411f((ltHl ..... ..,..,...., .. , ludtfAO fllWtt n trltkitll: tn-•HMt "*~'

:i

(AfiROUSfl

~

1 ~ ....

1?1.150

(ffLIJtiH

....

.....CAO td .... n ""*--dil ·-- •l!ltrM do ~•t.

APPENDIX2

Output of MAPLE® program

Determination of the analytica! expression for the differential enthalpy of wetting i\Hw.

cg: Cg; c:=Cg,o; e: = f:J; f. = E1 ;

kO:=ko; m: = u1 ;

r: R; y: 1/T;

cg:= c*exp(f*y/r) ;

k: kO*exp(e*y!r) ;

a : ln (aw);

a:= ln((cgA(0.5)*(cg*uA2-2*m*n*(cg-2) +mA2*cgn0.5) +n*(cg-2)-m*cg)/ ((2*u*k)*(cg-1 )));

a:= ln(l/2 (

.5 f y .5 I f y 2 I f y \ 2 f y \.5 c exp(---) Ie exp(---) u 2 m u I c exp(---)- 21 + m c exp(---) I

r \ \ r I rl

I fy \ fy I ey I fy \ + n I c exp(---) 21 - m c exp(---)) I (n kO exp(---) I c exp(---) 11))

\ I rl r\ r I

b : (à In <Iw )1(8 1/T) AHw /R

b: =diff(a,y);

.5 fy.5 .5 c exp(---) %2 f

r b 2 ( 1/2 (. 5 ------------------------

r

192 Appendix2

I fy 2 fy 2 fy \ Ie fexp(---) u m u e fexp(---) m e fexp(---) I

.5 f y .5 I r r r I c exp( --) 1-------- -2 ---------- + -------------1

r \ r r r I + .5 -----------------------------------------------------------

fy ue fexp(-)

fy me f exp(---)

.5 %2

r r I ey I fy \ + --------- - --------)I (u kQ exp(-) Ie exp(---)- 1 D

r r I r\ r/

%3e - 112 ---------------------

ey I fy \ u kQ exp(-) Ie exp(---)- ll r

r \ r I

fy %3 e fexp(-)

r ey I fy \ -1/2 -----------------------)u kQ exp(-) Ie exp(--)- ll/%3

ey I fy \2 r \ I ukQexp(--) leexp(-) -11 r

r \ r I

fy %1:= qexp(---)-2

r

fy 2 2 fy %2:= cexp(--)u 2mu%l+meexp(--)

r

.5 fy .5 .5 fy %3:= c exp(--) %2 +u%1-mcexp(--)

r r

APPENDIX 3

Working scheme of sludge characterization

1. Sludge dewatering properties

Used flocculants: ferric chloride/lime, polyelectrolyte KF 975 and the polyelectrolyte

used at the sludge dewatering plant.

First day

Coneetion of sewage sludge sample at the sludge treatment plant

Measurements:

dry solicts content

- ash content

- loss of ignition

- ATP content

CST </>10

(unconditioned sludge)





44 MFT experiments to determine optimal flocculation conditions; 16 MFT experi

ments per combination sludge-polyelectrolyte and 12 experiments for the conditioning

with ferric chloride.

Secoud day

ATP content

- CST </>10

- MFT&CST </>10

- constant pressure filtration

pH

electrical conductivity

Third day

- A TP content

CST </>10

partiele size distribution

rheological properties

Fourth day



( conditioned sludge)

( conditioned sludge)

(conditioned sludge)

(conditioned sludge)



( conditioned and unconditioned sludge)

(conditioned and unconditioned sludge)

- ferric ion concentration in filtrate (conditioned sludge)

polyelectrolyte concentration in filtrate (conditioned sludge)

194 Appendix 3

2. Sludge solid-to-water bond strengtb properties

First day

- Constant pressure :filtration; use of effective fungicide

- Water vapour sorption isotherms

Secoud day

-Constant pressure :filtration

- TGA/DTA: isothermal drying curves (3x)

Tbird day

- TGA/DTA: isothermal drying curves (3x)

Fourth day

TGA/DTA: isothermal drying curves (3x)

APPENDIX4

Shift of the absorption maximum of cobaltphthalocyanine due to increasing added

amounts of polyelectrolyte.

In the eight figures presented, the absorbance of a 212 mg/1 CoPc(NaS03) 4 solution is

given as a function of the wavelengtb. A solution of 100 mg KF975/l was stepwise

added to the cobaltphthalocyanine solution. The absorption maximum shifts from 662

nm to 628 nm. A volume of 56 J.Ll polyelectrolyte solution is needed to shift the

absorption maximum. A further increase of the added amount of polyelectrolyte does

not shift the absorption maximum .

... .. t: ~~

,IS

i .I

.os

VAVElEUGTH CrwJ)

... :1: llo;J

.12

.I

I ·"" ... ... ·""

IIAVEt EIIGlH in.)

196 Appendix 4

... 3: s""/''(

.!Z

..

I .oa

...

...

... 't: n_rJ

.I

...

I .... ...

• !l ll ~ il ~ IL D i I lil f!

WAVELEIIGlU lmt)

.u --------r~-----r-

.12 >:>.z~

..

I ... • 1111

••• • oz

0

~ H ~ M M a D i I i I! V4VElEHiïnt (na)

Appendix 4 197

... .12

t; s~r

.I

I .OB

·"" .o•

.oz

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ WAYELEJICHI (rw)

·" 7' '~

.I

.DB

I ,06 .

. o•

.oz

~ ~ ~ ~ ~ ~ ~ ~ ~ Ij ~ WAVEtEilGlit (n~~)

·" --------.----,.-

8: fPtJ/.J

.I

.OB

i • DO

.Dl

.oz

0

!l---~---t---~- --~--~ ~ ~ ~ i ~

VN.IElfUGTii (o•)

CURRICULUM VITAE

De auteur werd geboren op 19 februari 1958 te Eindhoven. In 1976 behaalde hij het

Atheneum-B diploma aan het Bisschop Bekkers College te Eindhoven. Aansluitend

studeerde hij Technische Natuurkunde aan de Technische Universiteit Eindhoven. Het

afstudeerwerk werd verricht bij Prof.dr.ir. G. Vossers in de vakgroep transportfysica.

In 1984 behaalde hij zijn doctoraal examen. Van 1985 tot medio 1987 was hij

werkzaam als procestechnoloog bij Vitrite te Middelburg. De Vitrite fabriek vormt

onderdeel van Philips Ligbting. Gedurende de periode van medio 1987 tot 1990 was

hij als procesontwikkelaar in dienst van Silenka te Hoogezand. In de periode 1990 -

1994 was hij werkzaam als senior-onderzoeker in het Laboratorium voor Scheidings

technologie, vakgroep Chemische Proceskunde van de Technische Universiteit

Eindhoven. Het in dit proefschrift beschreven onderzoek werd uitgevoerd in de groep

van Prof.dr.ir. P.J.A.M. Kerkhof. Vanaf januari 1995 is hij vennoot van het bedrijf

Herwijn & Janssen sludge technology vof en sedert februari 1996 directeur van het

bedrijf Herwijn & Janssen dewatering bv i.o. Het laastgenoemde bedrijf is een

vennootschap onder firma aangegaan met Witteveen +Bos Raadgevende ingenieurs bv

te Deventer. Deze vennootschap draagt de naam Sludge Consultants.

Stellingen

behorende bij het proefschrift van A.J.M. Herwijn

1. Het fysisch/chemisch gebonden watergehalte in slib draagt slechts in geringe

mate bij tot de vochtretentie bij het mechanisch ontwateringsproces.

dit proefschrift, hoofdstuk 3

2. De compressie-permeabiliteitscel kan ook gebruikt worden voor de bepaling van

de hoeveelheid gebonden water in slibkoeken. dit proefschrift, hoofdstuk 4

3. De resultaten van een "Capillary Suction Time" test worden niet alleen bepaald

door de ontwateringseigenschappen van de slibkoek, maar ook door de structuur

eigenschappen van het filtreerpapier, waardoor meetwaarden verkeerd kmmen

worden geïnterpreteerd. dit proefschrift, hoofdstuk 5

4. In de literatuur wordt ten onrechte aangenomen dat de diffusiecoëfficiënt van

vochttransport in poreuze materialen tijdens een droogproces een eenduidige

functie is van het vochtgehalte.

IsotheJJllal vapour and liquid transport inside clay during drying, Zanden, van der, A.J.J. et al.,

accepted for pubHeation in Drying Technology, vol. 14, no. 3, 1996.

5. De oorzaak van een slechte slibontwatering in de praktijk wordt vaak ten

onrechte toegewezen aan het purificatieproces.

6. Omdat de chemische industrie Nederland dreigt te verlaten, dienen de hoorcolle

ges voor het scheikunde-curriculum in de Engelse taal te worden gegeven.

7. De kostenbesparing die bedrijven kmmen boeken door reductie van de slibstroom

wordt vaak onderschat of in het geheel niet onderkend.

8. Door het uitschrijven van een toenemend· aantal prijsvragen en competities voor

afstudeerscripties of andere wetenschappelijke resultaten, worden studeren en

wetenschap bedrijven steeds meer een topsport.

9. De steeds verdergaande commercialisering van de Nederlandse televisie pleit

voor afschaffing van het luister- en kijkgeld.

10. Loonmatiging is slechts gericht op behoud van bestaande banen en niet op het

creëren van nieuwe banen zodat het gangbare standpunt dat loonmatiging goed is

voor de werkgelegenheid in twijfel kan worden getrokken.

11. Het doel om door een nieuw aan te leggen woon-werk-recreatiegebied in Oost

Groningen genaamd "Blauwe Stad" het imago en de leefbaarheid van dit deel van

Nederland te verbeteren, wordt door de naamgeving enigszins voorbij gestreefd.

12. Om de tijd, die verloren gegaan is met het gekrakeel rond de aanleg van de

tracés van de hogesnelheidslijn, weer in te halen moet de TGV wel heel hard

kunnen rijden.

fundamental aspects of sludge characterization

Documents