A COMPARATIVE STUDY OF FREEZE—THAW PROCESSES FOR CONDITIONINGWASTEWATER AND WATER TREATMENT SLUDGES

by

•

Patti Gremillion Trahern

Dissertation submitted to the Faculty of theVirginia Polytechnic Institute and State University

in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

in

Environmental Engineering and Environmental Sciences

APPROVED:

W. R. Knocke, Chairman

gg „z„„„; K. (Z _¥_/¢—§_«,__4/G. D. Boardman P. C. Hoehn

gg gg 1,- ägäél

N R .. P. Wightman F. Sebba

g March, 1989Blacksburg, Virginia

A COMPARATIVE STUDY OF FREEZE-THAN PROCESSES FOR CONDITIONING

NASTENATER AND NATER TREATMENT SLUDGES

byPatti G. Trahern

N. R. Knocke, Committee Chairman

The Charles Edward Via, Jr., Department of Civil Engineering

Environmental Engineering and Environmental Sciences

(ABSTRACT)

This research effort involved the application of indirect- and

direct-contact, freeze—thaw conditioning techniques for improving the

dewatering characteristics of both wastewater and water treatment

sludges. Sludges tested included waste activated sludge, primary

sewage sludge, waste activated/primary sewage sludge mixtures and

alum sludge. The direct-freeze methods examined were the use of a

secondary refrigerant (butane) evaporated in the sludge and the use

of gas hydrate or clathrate formation by addition of Freon 12 under

appropriate temperature and pressure conditions. Sludges were also

frozen solid using indirect freezing methods, thawed and tested for

comparative purposes. Particle size distribution and floc density

measurements were used to determine changes in particle characteris-n

tics; specific resistance values and dewatered dry solids concen-

tration were used to assess dewatering characteristics. Results of

direct- and indirect-contact, freeze—thaw conditioning were compared

to the effects of polymer conditioning. The results indicated that

direct-freeze methods do not appear technically or economically

competitive with currently accepted conditioning methods. The

superior results obtained with the indirect-contact, freeze-thaw

process when compared to the direct—contact processes suggested that

the extent and rate of freezing may greatly influence the particle

characteristics of the conditioned sludge, and thus its dewatering

characteristics.

ACKNOWLEDGEMENTS

TABLE 0F CDNTENTS

PageI. INTRODUCTIDN 1

II. LITERATURE REVIEW 4Evaluating Conditioning Effectiveness 4Specific Resistance 4

Compressibility 11Blinding 12The Relationship of Particle

Characteristics to Specific Resistance 15

Freeze-Thaw Processes 18Natural Freeze-Thaw Conditioning 19Indirect—Contact, Freeze-Thaw Conditioning 21Direct-Contact, Freeze-Thaw Conditioning 22

Impact of Process and Handling variables onEffectiveness of Freeze-Thaw Conditioning 57

Rate and Extent of Freezing 57Chemical Addition 59Storage 60Thickening 61Handling and Mixing 62

Mechanisms of Freeze-Thaw Conditioning 63

Solids Migration and Compression 63Particle Dehydration and Cell Rupture 65Role in Direct-Contact Processes 66

III. METHODS AND MATERIALS 68 ßSludge Sources 68

Waste Activated Sludges 68Primary Sewage Sludge 69Alum Water Treatment Sludges 70

v

TABLE OF CONTENTS(continued)l

Page

Conditioning Methods 71

Direct-Contact, Freeze-Thaw Conditioningwith Butane 71

Direct-Contact, Freeze-Thaw Conditioningwith Dichlorodifluoromethane (Freon 12) 73

Indirect-Contact, Freeze-Thaw Conditioning 76Polymer Conditioning 76

Experimental Apparatus 79

Reactors 79Heat Exchange Systems 83Refrigerant Feed Systems 85Conditioning Agent Removal 89

Analytical Methods 90

Particle Size Distribution 90Specific Resistance, Initial and

Vacuum-filtered Solids Concentration,and Blinding Coefficient 93

Centrifuged Solids Concentration 93Floc Density 94Total Organic Carbon 94Sludge pH 95

IV. RESULTS 96

Stroubles Creek waste Activated Sludge 96

Polymer Conditioning 96Indirect-Contact, Freeze-Thaw Conditioning 101Direct—Contact, Freeze-Thaw Conditioning

with Butane 106Direct-Contact, Freeze-Thaw Conditioning

with Freon 12 115

vi

TABLE 0F CONTENTS Qcontinued)

Page

Stroubles Creek Primary Sewage Sludge 125

Polymer Conditioning 125Indirect-Contact, Freeze—Thaw Conditioning 130Direct-Contact, Freeze—Thaw Conditioning

with Butane 136Direct-Contact, Freeze-Thaw Conditioning

with Freon 12 140

Stroubles Creek Primary Sewage Sludge/wasteActivated Sludge Mixture 144

Polymer Conditioning 144Indirect-Contact, Freeze-Thaw Conditioning 148Direct-Contact, Freeze-Thaw Conditioning

with Freon 12 151

RAAP Alum Sludge 157

Effects of Dilution 157Polymer Conditioning 158Indirect-Contact, Freeze—Thaw Conditioning 166Direct-Contact, Freeze-Thaw Conditioning

with Butane 174Direct-Contact, Freeze-Thaw Conditioning

with Freon 12 183

V. DISCUSSION 196

Proposed Mechanism for Freeze-Thaw Conditioning 197

Disruption of Sludge Structure 197Formation of New Floc Structure 204

Evaluation of Direct-Contact Freeze-ThawConditioning

210Particleand Chemical Characteristics 210Dewatering Characteristics 215

vii

TABLE OF CONTENTS (continued)

Page

Comparison with Other Studies and ProcessViabiTity 219

Butane Conditioning 219Freon Conditioning 220Process Viabi1ity 221

VI. CONCLUSIONS 223

VII. LITERATURE CITED 226

viii

LIST 0F TABLES

TableNo. Title Page

1 Selected Results of Direct-Contact, Freeze-Thaw Conditioning of waste ActivatedSludges with Butane 37

2 Decomposition Temperature and Pressureof Hydrating Agents 51

3 Selected Results of Direct-Contact, Freeze-Thaw Conditioning of WastewaterSludges with Freon 56

4 Experimental Matrix 72

5 Butane Vapor Saturation Data 74

6 Freon 12 Vapor Saturation and Gas HydrateEquilibrium Data 77

7 Experimental Analyses and Determinations 91

8 Results of Analyses, Polymer-Conditionedwaste Activated Sludge 98

9 Results of Analyses, Indirect-Contact,Freeze-Thaw-Conditioned wasteActivated Sludge 102

10 Results of Analyses, Butane-Conditionedwaste Activated Sludge 108

11 Results of Analyses, Freon-Conditionedwaste Activated Sludge (Batch 117Experiments)

12 Results of Analyses, Freon-Conditionedwaste Activated Sludge (ContinuousExperiments) 118

13 Results of Analyses, Polymer—ConditionedPrimary Sewage Sludge 126

ix

LIST OF TABLES (continued)

TableNo. Title Page

14 Results of Analyses, Indirect-Contact,Freeze—Thaw-Conditioned PrimarySewage Sludge 133

15 Results of Analyses, Butane—ConditionedPrimary Sewage Sludge 137

16 Results of Analyses, Freon-ConditionedPrimary Sewage Sludge (BatchExperiments) 142

17 Results of Analyses, Polymer-ConditionedPrimary Sewage/waste Activated SludgeMixtures 146

18 Results of Analyses, Indirect-Contact,Freeze-Thaw-Conditioned PrimarySewage/waste Activated Sludge Mixtures 148

19 Results of Analyses, Freon-ConditionedPrimary Sewage/waste Activated SludgeMixtures (Batch Experiments) 153

20 Results of Analyses, Undiluted, Polymer-Conditioned Alum Sludge 160

21 Results of Analyses, Diluted, Polymer-Conditioned Alum Sludge 164

22 Results of Analyses, Undiluted, Indirect-Contact, Freeze-Thaw-Conditioned AlumSludge 167

23 Results of Analyses, Diluted, Indirect-Contact Freeze-Thaw-Conditioned AlumSludge 172

‘

24 Results of Analyses, Undiluted, Butane-‘

Conditioned Alum Sludge 175

x

LIST 0F TABLES (continued)E

TableNo. Title Page

25 Results of Analyses, Diluted, Butane-Conditioned Alum Sludge 184

26 Results of Analyses, Undiluted, Freon-Conditioned, Alum Sludge (BatchExperiments) 185

27 Results of Analyses, Diluted, Freon-Conditioned, Alum Sludge, (ContinuousExperiments) 186

28 Results of Analyses, Diluted, Freon-Conditioned Alum Sludge (BatchExperiments) 189

29 Results of Analyses, Diluted, Freon-Conditioned Alum Sludge (ContinuousExperiments) 190

30 Effect of Indirect—Contact, Freeze-Thaw Conditioning on FlocCharacteristics 198

31 Effect of Indirect-Contact, Freeze-Thaw Conditioning on ChemicalCharacteristics 202

32 Effect of Indirect-Contact, Freeze-Thaw Conditioning on ParticleCharacteristics 206

33 Effect of Indirect—Contact, Freeze-Thaw Conditioning on DewateringCharacteristics 208

34 Effect of Direct—Contact, Freeze-Thaw Conditioning on ParticleCharacteristics 211

35 Effect of Freon Conditioning onFloc Characteristics 213

xi

LIST OF TABLES (continued)

TableNo. Title Page

36 Effect of Direct-Contact, Freeze-Thaw Conditioning on DewateringCharacteristics 216

xii

LIST 0F FIGURES

FigureRg; Title Page

1 Characteristic Dewatering Response forUnblinded and Blinded Sludge Samples 10

2 Typical Process Flow Schematic forSecondary Refrigerant Desalination 28

3 Types I and II Gas Hydrate Structures 42

4 Generalized Gas Hydrate Phase Diagram 45

5 Butane Vapor Saturation Curve 75

6 Freon 12 Vapor Saturation and GasHydrate Equilibrium Curves 78

7 Schematic of Direct—Contact Freeze-Thaw Conditioning Apparatus 80

8 Particle Size Distributions, Polymer-Conditioned waste Activated Sludge 99

9 Particle Size Distributions, Indirect-Contact, Freeze-Thaw-Conditionedwaste Activated Sludge 104

10 Particle Size Distributions, wasteActivated Sludge Conditioned with400 mL/L Butane 109

11 Specific Resistance vs. Butane Dose,Butane-Conditioned waste ActivatedSludge 112

12 Particle Size Distributions, Freon-Conditioned waste Activated Sludge(Batch Experiments) 119

13 Particle Size Distributions, Freon-Conditioned waste Activated Sludge(Continuous Experiments) 120

xiii

LIST 0F FIGURES (continued)

FigureHg; Tit1e Page

14 Specific Resistance vs. Freon Dose, Freon-Conditioned waste Activated Siudge 124

15 Particie Size Distributions, Po1ymer—Conditioned Primary Sewage Siudge 129

16 Specific Resistance vs. Po1ymer Dose,Po1ymer—Conditioned Primary SewageSiudge 131

17 Particie Size Distributions, Indirect-Contact, Freeze-Thaw-ConditionedPrimary Sewage Siudge 134

18 Particie Size Distributions, PrimarySewage S1udge Conditioned with800 m1/L Butane 139

19 Particie Size Distributions, Freon-Conditioned Primary Sewage Sludge(Batch Experiment) 143

20 Particie Size Distributions, Indirect-Contact, Freeze-Thaw ConditionedPrimary Sewage /waste Activated SiudgeMixture 150

21 Particie Size Distributions, Freon-Conditioned Primary Sewage/wasteActivated S1udge Mixture (BatchExperiment) 155

22 Partic1e Size Distributions, Undiiuted,Poiymer-Conditioned A1um S1udge 161

23 Partic1e Size Distributions, Diiuted,PoIymer—Conditioned A1um Siudge 165

24 Partic1e Size Distributions, Undiiuted,Indirect-Contact, Freeze-ThawConditioned A1um Siudge 169

xiv

LIST 0F FIGURES (continued)

Figuregg; Tit1e Page

25 ParticIe Size Distributions, DiIuted,Indirect-Contact, Freeze-ThawConditioned Alum Sludge 173

26 ParticIe Size Distributions, UndilutedAlum Sludge Conditioned with 400 mI/LButane 178

27 Particle Size Distributions, UndiIutedATum Sludge Conditioned with600 mL/L Butane 179

28 Specific Resistance vs. Butane Dose,UndiIuted, Butane-ConditionedA1um Siudge 181

29 ParticIe Size Distributions, Diluted,Freon-Conditioned AIum Siudge (BatchExperiments) 191

30 Particle Size Distributions, Diluted,Freon-Conditioned AIum Sludge (ContinuousExperiments) 192

31 Specific Resistance vs. Freon Dose,DiIuted, Freon-Conditioned A1um Sludge 194

xv

LIST OF APPENDICES

Appendixgg; Title Page

A Oesignations for Halogenated Halocarbons 233

xvi

I. INTRODUCTION

Sludge production is an inevitable result of many wastewater and

certain water treatment processes. These solids must be stabilized,

sometimes thickened and conditioned, and then dewatered before

ultimate disposal. For most sludge generators, disposal methods are

limited to landfilling, incineration, and other processes, the costs

of which depend upon the volume of the sludge to be disposed. The

drier the sludge, the more efficient and less expensive the process

usually is.

Sludge conditioning is selectively undertaken for one of two

purposes: (1) to improve the dewatering rate of a sludge (i.e., to

decrease the amount of time required to remove a given amount of

water from the sludge); and (2) to improve the extent of dewatering

or thickening (i.e., to increase the amount of water removed from the

sludge.) In addition, conditioning is frequently necessary to obtain

an easily handled product.

Conditioning can decrease the resistance of sludge to dewatering

to one percent or less of the original resistance of the

unconditioned sludge. It typically reduces moisture content from

about 95 to 98 percent to about 60 ix: 75 percent for the final

dewatered productl._

Freeze-thaw conditioning occurs when sludge is allowed to come

into contact with air, water vapor, a secondary refrigerant, or other

1

freezing agent under temperature and pressure conditions conducive to

freezing. Freezing may occur naturally (for example, when lagooned

water sludges freeze during extended periods of cold weather) or may

be artifically induced by indirect or direct contact between sludge

and a refrigerant.

Freeze-thaw conditioning can produce an extremely dry, grainy,

odorless product composed of up to 60 to 70 percent solidsz. Such

conditioning often increases the solids handling capacity (e.g.,

filter yield) of mechanical dewatering equipment. In fact, certain

sludges drain so easily following freeze-thaw conditioning that there

is no need for thickehing or filtration after conditioning if

sufficient land is available for gravity dewateringß. Freeze-thaw

processing has been used to condition a wide variety of sludges,

including water treatment sludges as well as domestic and industrial

wastewater sludges.

With the exception of natural freezing, processes used to freeze

sludge are often technically complex, expensive, and energy

intensive. Natural freezing requires large areas of land for proper

lagooning and storage. Nevertheless, these disadvantages may be

offset by the unusually high solids concentrations made possible by

freeze-thaw conditioning, and the tractable nature of the product

sludge.

The results obtained from freeze-thaw conditioning must be

evaluated against the standards set by chemical conditioning. The

addition of inorganic metal salts (e.g., ferric chloride, aluminum

2

hydroxide, ferrous sulfate, and lime) as conditioning agents has a

long history of successful employment in water and wastewater

treatment practice. Likewise, polymer conditioning has become

increasingly popular.

The purposes of this investigation were:

1. to develop methods for direct-contact, freeze-thaw

conditioning processes;

2. to use these methods to condition a variety of sludges;

3. to compare the results of direct-contact, freeze-thaw

conditioning to indirect-contact, freeze-thaw conditioning

and polymer conditioning.

4. to develop a model of the freeze-thaw process.

3

II. LITERATURE REVIEW

A review of the literature was conducted to develop a technical

and historical context for the current investigation. This review

addresses the following specific goals:

— to evaluate methods of evaluating conditioning

effectiveness, i.e., improvements in the rate and

extent of dewatering of conditioned sludges, and to

determine which methods were appropriate to theE current investigation;

- to research process descriptions and histories for

freeze-thaw processes as background for selection of

freeze-thaw methods and agents;

- to assess the impact of process and handling variables

on the effectiveness of freeze-thaw conditioning in

past studies as background for development of

freeze-thaw techniques; and

- to present proposed mechanisms of freeze-thaw

conditioning. r

· 4

EVALUATING CONDITIONING EFFECTIVENESS

The parameters of interest in evaluating the effectiveness of

conditioning methods and agents are the rate and the extent of sludge

dewatering. For most sludges, these dewatering characteristics are

dependent upon factors such as particle size distribution, floc water

content, and other properties. Common measures of dewatering

characteristics--—specific resistance and compressibility---are

calculated using equations predicated on these variables. These

parameters and their derivations are introduced in this section. The

physical phenomenon of blinding and its matnematical relationship to

specific resistance are also presented. Finally, the relationships

of particle size and other sludge particle characteristics to

specific resistance are reviewed.

Specific Resistance

The specific resistance of a sludge is a measure of the relative

ease (or difficulty) with which filtrate flows through the cake

formed as sludge drains under an applied vacuum. The lower the

specific resistance, the easier the sludge is to dewater. This

parameter is defined as the resistance per unit weight of dry cake

solids per unit volume of filtrate collected and is expressed in

units of length per mass (m/kg).

Specific resistance is calculated from the data obtained from a

laboratory procedure known as the Buchner funnel test. The test is

conducted using an apparatus consisting of a Buchner funnel secured

5

atop a graduated cylinder. The desired vacuum (pressure

differential) is applied and the amount of filtrate removed from the

sample over the period of filtration is recorded at intervals. The

time required for the filter cake to crack is noted, and the final

cake solids concentration determined on a sample of the filter cake

as well.

Although the Buchner funnel procedure bears a closer resemblance

to vacuum filtration than to other methods of dewatering, its

accuracy in predicting vacuum filter _yields is arguable4'6. Its

usefulness in differentiating among conditioning methods and doses

for a given sludge is, however, generally accepted.

For a given sludge with a specific initial solids

concentration, the time required to obtain a specified volume of

filtrate, or the time required for the filter cake to crack, gives a

rough estimate of dewatering rate. The usefulness of this estimate

is limited, however, since these data do not predict sludge behavior

over a range of solids concentrations. In addition, fibrous or poorly

dewatering sludges may produce a cracked cake only after an

inordinately long filtration period, or not at all.

A more useful calculation obtained from the data collected in

the Buchner funnel test yields specific resistance. The concept and

derivation of this parameter is based upon equations fundamental to

filtration theory. The basic equation is Darcy's law, which statesP

dV = (P) (AK) [1]df TT

6

where

dV/dt = rate of flow (volume of filtrate

collected per unit time);

P = pressure difference

A = area

p = viscosity

K = permeability

L = thickness.

Stated simply, the change in filtrate volume with time, dV/dt,

is proportional to three variables: (1) the area of filtration, A,

(2) the pressure applied across the material filtered (medium and

sludge cake), P, and (3) the permeability of the medium and the cake,

K. The change is inversely proportional to the viscosity, u, and to

the thickness of the cake, L.

The resistance R is defined as the inverse of the permeability.

Substitution of R for 1/K in Equation 1 yields:

EUR5. [21dt uLR

where R is the combined resistance offered by the filter paper (Rm)

and the cake resistance (RC). These two resistances are sometimes

considered separately 6’7. It is argued that the initial resistance

offered by the filter before the cake begins to form is significant

when compared to the resistance offered by the sludge. The two

7

resistances can be considered, however, a single factor if filter

media resistance is always negligible when compared to cake

resistance5. Significant filter resistance is usually caused by

filter clogging by sludge particles, in which case the resulting

increase in resistance is due to the sludge itself.

This disagreement has led to differences in the method used to

acquire the data used to determine specific resistance. Gale7

recommended ignoring the first filtrate to pass through the filter

and noting volume only when the filter resistance relative to the

cake resistance is negligible. Notebaert5 recommended recording

filtrate volume as soon as the first filtrate is released. In this

study the protocol suggested by Notebaert's argument was used, and

the corresponding derivation is presented here.

The specific resistance r is defined as the resistance per unit

weight of dry cake solids. It can be expressed as:

r=By [3]w

where

R = combined resistance per unit volume of filtrate

v = volume of cake deposited per unit filtrate volume

w = filter cake deposited per unit volume of filtrate

The solids loading term vv may be determined from a rnaterial

8

balance:

w = Ckco1

[4]

where CO and Ck are the initial and final cake solids concentrations,respectively.

In order to introduce the specific resistance r into Equation I,

vV is substituted for AL:

area?. psidt p vVR

Substituting rw for Rv, Equation 3 becomes:

_ 2Q!·E./L I6]dt u Vrw

Equation 6 can then be rearranged to:

dt = (uVrw)dV [7](PA)

Assuming pressure changes during the test are negligible, Equation 7

can be integrated to obtain:

_ 2t — prwV [8]' 2PA2

Equation 8 can be rearranged in the form y = bx, as shown in Figure

Ia:

E = V BKHZ [9]V 2PA

‘ 9

II.I / _

//x

>\ .I-

slopa = b = ufw/2PA2

V18. SCHEHRTIC DIHGRHH OF TYPICRL BUCHNER FUNNEL F1LTR8•

TION O8T8 FOR SLLIISE EXHIBITING NO BLINOIPG PI·£NOHEN8

‘·* RT /LIJ /(_) /Z /

//

LD / /Rc

1-1

Ü ///uf /1_1 /’ Ru11.1 /CL(D

VII. SCHEHRTIC O186R8H OF r FOR 8 HEOIUHSLIMIO SLUOGE

DJLJZGIl•U')•—•

13 “}/Q , / Rc ·.1.1 ’Ü ’ : ..(D

V1C. SCHEHBTIC 0186888 OF r FOR 8 CRKE-BLINOED SLUDSE

FIGURE 1. CHFIRHCTERISTIC OEURTERING RESPONSE FORUNBL INDED RND BLINDED SLUDGE SRMPLE5.

10

The slope of the line t/V vs V is:

b = [10]

The specific resistance is then defined:

r = 2PA2b [11]pw

This resistance is an average value, being predicated upon an

average cake solids content, a constant pressure drop across the

cake, and the assumption that any filtrate squeezed from the cake

itself is small in proportion to the amount flowing through it7.

Although specific resistance is widely used as a shorthand

method of describing how well a sludge will dewater, Karr and

Keinathg noted that this test does not predict other characteristics

essential to good dewatering, such as pick-up and release

characteristics, shear resistance, and scrolling properties. For

example, an activated sludge with a very low specific resistance may

in fact be intractable, whereas a highly resistant anaerobic sludge

may handle very easily.

Compressibility

The compressibility of a sludge affects its response to vacuum— or pressure filtration. By measuring the specific resistance at

a

different pressures, the coefficient of compressibility can be

determined. It is defined as the slope of the line obtained when the

11

logarithm of the resistance is plotted against pressure:

log r = Solog P + constant [12]

An incompressible material has a coefficient of compressibility of

zero. Increasingly compressible materials have higher coefficients

of compressibilityö.

Blinding

The equations used to develop the concept of specific resistance

presuppose a straight line relationship between t/V and V. when

graphed according to this relationship, actual laboratory data often

produce a curve rather than a straight line. Notebaert5 suggested

that this nonlinearity occurs when the combined resistance (RC + Rm)

of sludges with heterogeneous particle distributions changes during

the course of filtration. Medium resistance changes when small

particles (equal in diameter to the medium pore diameter) lodge in

the pores of the medium, resulting in blinding. The rate of media

clogging decreases as the cake builds up and small particles are

retained in the cake itself (Figure 1b). Cake resistance changes

when small particles lodge in the pores of deeper layers of ‘the

filter cake, causing blinding of the cake itself. If these small

particles are larger than the pore size of the medium, specific

resistance tends to increase with time of filtration (Figure 1c).

The redistribution of particles results in deformation and

compression of the cake.

12

In order to model the resultant curvilinearity, r is redefined

as r':

r'= r (V)X [13]

Substituting, Equation_6 becomes:

dV = PA [14]— ;"—T—Tdt prw(V/A) 1+X

Integrating Equation 14 yields:

T = urw [15]

Taking the logarithm of both sides, Equation 15 becomes:

log T = lo (prw) + (2+x)log V [16]

which can be plotted as a line of the form

log T = log a + (2+x)log V [17]

where a is the average specific resistance.

The value of (2+x) can be used to characterize the response of a

given sludge to filtration. Notebaertß found that sludges with x

greater than or approximately equal to zero were more compressible,

with coefficients of compressibility of about 1. Sludges with values

of x less than zero were less compressible, with coefficients of

compressibility of about 0.3 to 0.5.

13

Huangg derived a similar concept from examination of specific

resistance data. He replaced the "x" in Notebaert's equation with 6

(the blinding coefficient), where 6 is related to change in cake

resistance which develops as cake deposition proceeds. Equation 15

can be transformed by replacing x with 6 and taking the logarithm of

both sides:

ln t = (6 + 2)ln V + ln (2pw) [18]IW?/—\

Blinding sludges (more compressible) have values of 6 greater

than zero. This corresponds to a slope of (2 + x) greater than 2 in

Notebaert's discussion. For perfectly incompressible sludges, B

equals zero. For sludges which do not resist filtration, 8 equals

-16.Karr and Keinathg approached the problem of blinding sludges by

comparing specific resistance values obtained at different solids

concentrations for a given sludge. The Blinding Index (BI) was

defined ix) range frmn zero (unblinded) to 100 percent (completely

blinded). It is defined as:

BI = 100(”1’”2) 62 [10]'T @1)

where rl and r2 equal the specific resistance values determined at

initial solids concentrations cl and c2, respectively.

For an unblinded sludge, the two specific resistances would be

equal, and thus BI = 0. For a completely blinded sludge, the two

I 14

terms are inversely equal. Their product is equal to 1, and thus BI

= 100. The validity of the relationship hinges on the dependence of

the specific resistance of a blinding sludge on solids concentration.

The Blinding Index is cumbersome, requiring two specific resistances

at differing solids concentrations.

The Relationship of Particle Size and 0ther Particle

Characteristics to Specific Resistance

The relationship between specific resistance and the particulate

structure of the sludge cake formed upon filtration was discussed by

Gale in 19677. He based his derivation upon the Kozeny equation:

r* = KSO2(1—e) [20]egpp

where

r* = specific resistance

K = Kozeny constant, or parameter

S0 = specific surface area of the particles

e = fractional voidage of the bed

pp = particle density.

The specific surface area, SO, is inversely related to the

equivalent diameter of° the particles in the sludge cake. As the

diameter of the particles increases, SO decreases. As can be seenfrom Equation 20, as SO increases (signifying a decrease in particle

15

size), r* increases as well.

This is, of course, based on the premise that the voidage

remains the same with changes in particle size. Gale noted that, in

fact, flocculated sludges often produced filter cakes of lower solids

content. The lower solids content resulted in a higher voidage and

thus even lower specific resistance. 0n the other hand, particle

compression during filtration would result in lower voidage and thus

higher specific resistance.

According to Equation 20, increases in particle density would

result in decreases in specific resistance. Gale noted that

increases in particle density would correspond to decreases in the

numbers of particles per unit. weight of solids. This decrease

would result in a decrease in the surface area across which filtrate

must flow. An increase in the weight of aggregates of particles,

i.e., floc density, might not correspond, however, to an increase in

particle density. Aggregates might grow in size without increasing

on a weight per mL basis. Thus, it is not surprising that Knocke and

Wakelandlo found that specific resistance did not vary consistently

with floc density.

Gale speculated that the presence of fines in a sludge increased

specific resistance by increasing the specific surface area and

reducing cake voidage. Research by others suggested that dilution

promoted release of fines. Roberts and Olssonll discovered that_

dilution of low solids activated sludge resulted in increased

capillary suction times (CST‘s). Ademiliuyilz found that specific

16

resistance increased with dilution of domestic sludge to about 2

percent and then decreased, while resistance of piggery and water

treatment sludges decreased with dilution to 4 and 3 percent,

respectively, and then increased.

Roberts and Olssonll found that the polymer dose for low solids

activated sludge depended upon the concentration of colloidal

particles. Similarly, Knocke and wakelandlo, finding that changes in

specific resistance with specific surface area were slight for poorly

dewatering biological sludges, suggested that conditioning dose for

these sludges was dependent upon the concentration of biopolymer in

the sludge.

The results of research reported by Karr and Keinathg showed

that changes in specific resistance with mixing, storage time, pH and

conditioning could be attributed in large part to corresponding

changes in particle size distribution. In these experiments, samples

of raw, activated, and anaerobically digested sludges were

reconstituted to approximately equal fractions of different size

ranges of particles. Buchner funnel tests were then conducted on the

samples.

In general, Karr and Keinath found specific resistance values

were closely related to particle size distribution. The relative

size of the supracolloid fraction (composed of particles of diameters

between 1 and 100 microns) determined how well a sludge would

dewater. The lower this fraction, the lower the characteristic

sludge specific resistance. Exceptions were attributed to different

17

distributions within particle size fractions, changes due to storage

durng testing, measurement errors, and other factors.

These results point out a main defect with utilizing Equation 20

for characterizing the filtration response of heterogeneous sludges:

that is, that the effective diameter, or other measures of inean

particle or floc size, can represent distributions of widely varying

shapes and physico-chemical characteristics. Representative measures

of floc or particle size (such as mean diameter) may not suffice in

predicting changes in specific resistance.

FREEZE-THAW PROCESSES

There are three main types of freeze-thaw processes used to

condition sludges: natural, indirect—contact, and direct—contact.

Natural processes depend upon extended periods of cold weather to

freeze sludge exposed in beds or lagoons. Indirect—contact processes

rely upon contact between sludge and a refrigerant across a heat

transfer surface. Direct-contact processes promote unmediated

contact between sludge and refrigerant. Direct-contact processes

include those in which water freezes in intimate contact with an

evaporating refrigerant as well as those in which solid gas hydrates

are formed.

Direct-contact processes are limited by certain authorsl3’l4 to

those in which freezing occurs as the result of water evaporation at

reduced pressures. These sources may refer to those processes using

18

an immiscible refrigerant as “indirect" freezing. In the current

study, processes which involve intimate contact between sludge and

any refrigerant, water vapor or secondary refrigerant, are classed as

direct-contact processes. Indirect-contact processes are those which

do not involve intimate contact between sludge and freezing agent.

Natural Freeze-Thaw Conditioning

In climates where freezing temperatures predominate for

prolonged periods, sludges exposed in beds or lagoons freeze slowly

and completely. Upon thawing, these sludges show much improved

dewatering characteristics.

Canfield and Sutphinlß described the results of natural

freeze-thaw conditioning of alum sludge from a high-rate direct

filtration plant in Onondaga County, New York. The sludge was

lagooned at 18-inch depths, and the supernatant drained off

regularly. After freezing and thawing, the solids content of the

decanted sludge increased from 8 to 25 percent. The product texture

varied from powdery to granular.

Bishop and Fultonlö reported the results of natural freeze-thaw

conditioning of alum sludge from a treatment plant near Rochester,

New York. Solids content increased from approximately 6 percent for

unfrozen lagooned sludges to 17 percent for frozen and thawed

sludges. The product sludge dewatered quickly when placed on a sand

filter.

In a study reported by Penman and Van Esl7, anaerobic sludge in

19

winnipeg, Manitoba, was settled throughout summer and fall in lagoons

to depths of 2 feet. The supernatant was drained off regularly.

Ouring winter, sludge was pumped into the lagoons to depths of 8

feet. Throughout the following spring, summer and fall, the thawed

supernatant was returned to the plant and the beds allowed to lie

dormant. They were allowed to freeze again the following winter.

Subsequent thawing produced a sludge with a solids concentration of

about 20 percent, suitable for land application.

Tilsworth gt_al.l8 reported the results of natural freeze—thaw

conditioning of extended aeration sludge in College, Alaska, where

freezing conditions prevail three to four months a year. Sludge

placed at 6-inch depths froze in about 3O to 50 days. Once the

sludge thawed, solids content increased by 225 to 920 percent, The

product was granular, had little odor, and was easy to transport and

place. a

In experiments in Ely, Minnesota, and Cincinnati, Ohio, Farrell

gt__al.19 found that complete freezing of alum wastewater sludge

increased initial dry solids content and improved specific resistance

(from 2.1 to 9.5 percent, and from 104 to 3.43 x 1011 m/kg).

Reed gt__gl.20 developed a sludge-specific equation for

predicting the total depth of sludge which could be conditioned by

exposure to subfreezing temperatures, based on application of sludge

in 8-cm layers. The method was verified by field applications in9

Hanover, New Hampshire, and from comparison with historical data from

Duluth, Minnesota.

20

Indirect-Contact, Freeze—Thaw Conditioning6

Indirect-contact, freeze-thaw conditioning processes rely on an

intermediate surface to transfer heat ebetween the sludge to be

conditioned and aa circulating refrigerant. The refrigerant either

has a large heat capacity, enabling it to absorb heat from the sludge

at high rates with no phase change (e.g., glycol or brine) or it

vaporizes during the process, absorbing the heat of vaporization from

the sludge.

Full- and pilot-scale indirect—contact processes for

conditioning alum water sludges at two plants in England (at Stocks

and at Fishmoor) were the subject of a number of articles in thel960's2l”23. The process at the Stocks water treatment facility

consisted of slowly freezing sludge at I5OF in 150·gallon tanks.

Compressed ammonia was pumped through 480 3/4-inch—diameter vertical

pipes immersed in the sludge. The liquid ammonia flowing through the

pipes vaporized as it was warmed by the sludge. The heat required to

warm and vaporize the ammonia was drawn from the sludge, causing the

sludge to freeze. In order to make the process economical, sludge

and ammonia were cooled prior to the freezing step by heat exchange

with the frozen product. Latent heat recovery minimized condensing

requirements and also thawed the frozen sludge. The operation was

performed in batch, with approximately 90 minutes required for

freezing and 45 minutes for thawing. The plant at Stocks processed

33,000 gpd of sludge. Prethickened sludge drained to 60 to 70 percent

solids after freezing.

21

Although the process resulted in a more easily handled product

and a °large increase in solids concentration, the systeni suffered

certain drawbacks. The main problem was the high cost of

construction and maintenance. Also, the repeated cycles of freezing

and thawing placed great strain on the tanks, which required frequent

repair or replacement. g

Indirect freezing of alum water treatment sludge was

investigated in the laboratory for the Milwaukee Sewerage

Commission24’25. In a bench-scale study, alum sludge was poured into

tall, thin rectangular pans, to depths of 1/2 to 2 inches. Contact

with circulating glycol or brine at operating temperatures of between

5 and 25OF froze the sludge in batch operations lasting from 20 to 60

minutes. Vacuum filtration produced a sludge cake of 67 to 75

percent solids from prethickened sludge. The same sludges without

treatment produced sludge cakes of 18 to 21 percent solids. The

encouraging results obtained in the laboratory led to a feasibility

study for· a pilot plant for the Milwaukee Sewerage Commissionza.

Activated sludge was to be frozen at 1/2—inch depths in pans on a

conveyer, in contact with chilled brine for 100 minutes. However,

cost analyses showed that the estimated expense of the system was too

great, and the pilot study was never implemented.

Direct-Contact, Freeze-Thaw Conditioningh

Much of the research and development of direct—contact,

freeze-thaw technology was conducted in the 1950's and 1960's under

22

the sponsorship of the Office of Saline water Research. Brianl3 andBarduhnzö published excellent reviews on the status of this

technology in 1968 and 1975, respectively. General process

descriptions were presented by Fraser and Gibson27, Fraser and

Johnson28, and weissgg. The flow of information declined sharply

upon the demise of the Office of Saline Water Research, which was

abolished in 1974 after several years of federal budget cuts26.

Research directed towards application of direct-contact,

freeze-thaw processes to wastewater treatment and sludge conditioning

has been limited. The AVCO Corporation26’28 developed a commercial

direct-contact process utilizing Freon 114 to recover recyclabe

constituents from plating wastewater. Kahn and other researchers at

Virginia Polytechic Institute and State University30'32 conducted

experiments to investigate the feasibility of conditioning wastewater

sludges primarily with butane (some experiments were conducted with

Freon 114 as well) in direct-contact processes. Molayem and

Bardaki53 used Freon 11 in a gas hydrate process to dewater

wastewater sludges.

Direct-contact, freeze-thaw processes were developed as

comparatively efficient alternatives to evaporative techniques of

desalination. The advantages of direct-contact freeze-thaw processes

include:

- absence of a metallic heat transfer surface;— minimal scaling, corrosion, and fouling; and

23

- minimal pretreatment requirements.

Energy savings are due mainly to the absence of a heat transfer

surface. Although the latent heat of evaporation is about seven

times the latent heat of fusion, this potential energy savings is

compensated for by the fact that the freezing point depression caused

by dissolved salts in seawater (and dissolved solids in wastewater)

is about seven times the boiling point elevation28.

Because of practical limitations of equipment efficiency and

control, however, processes usually operate at temperature

differences greater than the theoretical minima set by freezing point

depression (for freezing) or boiling point elevation (for

distillation)34. Energy requirements for freezing processes are

relatively low when compared to evaporative processes because of the

small temperature differentials which drive the reaction between feed

and refrigerant. For each degree that the minima are exceeded,

distillation processes suffer a sevenfold loss in efficiency compared

to freezing processes. The closeness of the temperature differential

used in freezing processes to theoretical temperature differences is

a direct result of the absence of a heat transfer surface26.

Because the processes are carried out at low temperatures,

scaling and corrosion are rare. The lack of a heat transfer surface

results in fouling being confined to reactor walls. In turn, the

absence of the potential for scaling, corrosion, and fouling

precludes the need for pretreatment.

24

These advantages are offset by two disadvantages. First,

refrigerants are expensive and removal to acceptable levels can be

difficult to achieve. Second, scale—up of desalination processes is

limited by the availability of efficient compression equipment

required to recover spent refrigerantlg.

There are three types of direct-contact, freeze-thaw processes:

- vacuum freezing, vapor compression processes;

- secondary refrigerant processes; and

- gas hydrate processes.

In the discussion that follows, feed and solution are used as

general terms to include seawater, wastewater, and sludge.

Vacuum Flash, Vapor Compression Processes - In vacuum flash,

vapor compression (VFVC) processes, precooled solution is fed into a

vacuum chamber where conditions are nmintained slightly below the

triple point of water. At this point, water evaporates and forms ice

simultaneously. The heat of vaporization is about seven times the

heat of fusion; thus, under ideal conditions, seven pounds of ice are

formed for every pound of vapor released.

The process has certain serious disadvantages. It requires

large mechanical compressors to condense the water vapor produced,

and a great deal of space. High volumes of low pressure water vapor

must be handled, with a pound of water vapor occupying 4600 cubic

feet at the operating pressure. To overcome some of these

25

difficulties, a variation of this process--—the vacuumg freezing

ejector process--·was developed. In it, ejectors replace some of the

compressors. The process is complex and both the original and the

variation suffer from large size and low efficiencyzg.

Several commercial VFVC desalination pilot plants were

constructed and installed in the United States and Israel during the

1960's, by Colt Industries and the Carrier Corporation. These plants

produced up to 240,000 gallons of fresh water from sea water daily.

By 1975, however, none of the U.S. plants were operating due to

various process and mechanical deficiencesl3’26’35.

Application of the VFVC process to treatment of paper pulp mill

effluent for dissolved solids and biochemical oxygen demand (B00)

reduction was suggested by Johnson in 196935. However, a review of

the literature uncovered no reports of utilization of this process

either for treating wastewater or conditioning sludge.

Secondary Refrigerant Processes - Secondary refrigerant freezing

processes rely on intimate contact between the solution to be

treated---the "primary" solution---and an agent with a low boiling

point at ambient or near ambient pressure---the "secondary"

refrigerant, introduced specifically for the purpose of heat

transfer. Initially developed as desalination technology, secondary

refrigerant processes have been applied in commercial ventures for

wastewater recovery. In addition, several research studies on the

effectiveness of secondary refrigerant processes have been

documented.

26

Process Description - In secondary refrigerant freezing,

precooled solution is fed into a reactor where it is intimately mixed

with an immiscible refrigerant. Conditions in the reactor are

maintained such that the refrigerant boils at 61 temperature lower

than the freezing point of water. As the refrigerant evaporates, it

absorbs the heat necessary for vaporization from the solution,

freezing the water.

A typical process diagram for secondary refrigerant desalination

is presented in Figure 227. The process comprises four unit

operations:

- heat exchange,

- freezing/crystallization,

- washing, and— melting/condensing.

These are discussed in further detail below.

Heat exchange - Feed is precooled prior to ·freezing by

contact with the ice slurry and waste concentrate produced

during desalination. Precooling avoids the use of

excessive amounts of refrigerant to lower temperature of

feed to process freezing levels._

Freezing/Crystallization - Precooled feed and liquid

27

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28

refrigerant are pumped into a reactor (crystallizer) under

pressure conditions such that the evaporating temperature

of the refrigerant is lower than the freezing temperature

of the feedlg. The refrigerant evaporates when it comes

into contact with the feed, lowering the water temperature

below its freezing point. Pure ice crystals (approximately

200 microns in diameter) form, excluding all other

· material, including solute and particleszö. The reactor

contents may be mixed by the turbulence created by

evaporating refrigerant; additional agitation may also be

provided.

Washing - The slurry from the reactor containing a mixture

of pure ice crystals and concentrate is washed, removing

the concentrate from the ice.

Melting/Condensing - Spent refrigerant vapors undergo two

compression stagesl3. Primary compression raises the

temperature of the vaporized refrigerant to the melting

point of ice. The vapor is pumped to the melter/condenser,

where it condenses in contact with the product slurry.

However, not all of the vapor can be condensed on the

product, due to incomplete heat exchange, heat leaks into

the system, and the work input of compressors, pumps, and

agitators. Some of the vapor is thus compressed to a

29

higher pressure to be condensed at a lmgher temperature

with cooling water.

In adapting the secondary refrigerant process for sludge

conditioning, the washing step is usually omitted as the object of

conditioning is not to produce pure water, nor to recover

concentrate, but to take advantage of the irreversible changes which

occur during freezing.

Thermodynamics and Kinetics of the Process — Discussions of the

thermodynamic principles involved in freezing processes have been

presented by Dodge and Eshayagö, Gilliland37, and wiegandtgg.

The minimum amount of theoretical reversible work necessary to

separate fresh water from seawater (or other solution) depends upon

the temperature of the process and the percent conversion attainedgö.

For example, the minimum amount of work required to separate 1000

gallons of fresh water from 2000 gallons c¤’ a 3.5 percent saline

solution at 2500, (a 50 percent conversion or recovery rate), is

variously calculated to be from 2.89 kw—hr to 4.15 kw-hr36’37’39. In

analyzing a continuous system in which the product water is in

equilibrium with the final concentrate alone, the minimum work rises

to 5.8 kw-hr/1000 gallons.

Although the work of separation is nnnimized at low recovery

rates, the work of pumping the feed through the process approaches

infinity as the yield of fresh water approaches zero. For example, to

recover 1000 gallons of fresh water at a 50 percent recovery rate

30

requires pumping 2000 gallons of solution. To recover 1000 gallons of

freshwater at a 0.5 percent recovery rate requires pumping 200,000

gallons of solution. At about a 50 percent recovery rate, the

pumping work becomes minimal with respect to the work required to

separate the fresh water from the solution, and this is the reason

the recovery rate is typically selected as 50 percent.

Although the amount of heat required to freeze and thaw an equal

amount of seawater are approximately the same, additional energy is

required for freezing because the freezing point of seawater is

depressed by salts below the melting point of the pure ice formed.

The minimum reversible work required by freezing processes depends

upon the freezing point depression, the process and ambient

temperatures, and the latent heat of fusion. It can be calculated

from the following equation:

w = 0 (Tl — T2) TO [21]

whereT2 T1

0 = heat of fusion

Tl,T2 = freezing temperature of pure water

and seawater, respectively

TO = ambient temperature.

work is required to pump the heat of fusion from the depressed

freezing temperature of the solution to the freezing point of water

and to discharge the heat from the work of pumping to ambient

31

temperature. The amount of minimum theoretical work required for 50

percent recovery from a 3.5-percent saline solution is approximately

6.3 kw-hr/1000 gallons of fresh water produced. However,

irreversible effects in actual processes raise the theoretical work

requirement to 40 kw-hr/1000 gallons, or more. These effects include

pressure drops from fluid friction and from throttling, heat leaks

into the system, and finite temperature and concentration gradients,

among others36.

Optimum temperature and pressure conditions for secondary

refrigerant processes depend upon the vapor saturation curve of the

refrigerant and the liquid-solid curve of the solution to be frozen.

A pressure is selected at which the temperature of the point on the

vapor saturation curve is below the freezing temperature of the

solution. The difference between these two temperatures is the

driving force of the reaction. y

The heat of' vaporization of butane (165 Btu/lb) is slightly

greater than the heat of fusion of water (144 Btu/lb). Theoretically,

1.15 lb of ice should form when water is contacted with 1 lb of

butane. In practice, refrigerant requirements are increased by

insufficient precooling of feed, incomplete mixing, and other

factors.

Process History — Several types of refrigerants have been used

in secondary refrigerant freeze-thaw processes, including butane andU

certain halogenated halocarbons. The attractiveness of butane as a

secondary refrigerant is that its boiling point at ambient pressure

32

is just below the freezing point of water. It also has a low

solubility in water, varying from 70 mg/L at 250 to 150 mg/L at 000.

At the temperatures maintained in secondary refrigerant processes, it

does not form gas or liquid hydrates. Finally, butane is relatively

inexpensive when compared to other refrigerants.

The major disadvantage of butane processes is poor refrigerant

recovery. Although relatively insoluble in water, liquid butane

tends to emulsify in impure solutions. Carryover of liquid butane in

slurry processes has been reported to vary from 1 to 10 g/L, and it

is estimated that freeze-thaw products may contain emulsified butane

concentrations up to 10 times the concentration of dissolved butane

in freeze-thaw products26. Loss of butane associated with the

product is not only expensive, but may prevent reuse or complicate

disposal of the contaminated product.

Various separation techniques which have been used to remove

butane from freeze-thaw products include stripping under vacuum and

at atmospheric pressure; stripping using vacuum sprays and packed

columnszö and charcoal filtration following vacuum strippinglg.

Application of any such separation method obviously increases the

overall cost of the process.

A secondary disadvantage of butane is its potential for

explosion at ambient temperature in the presence of oxygen. This

explosion hazard requires special consideration in the design of the

mechanical systems used for secondary refrigerant freezing and also

increases process cost.

33

Halogenated derivatives of hydrocarbons (which are also used

extensively in gas hydrate processes) are more familiar as Freons and

Arctons, the trade names under which they are manufactured. (An

explanation of numerical designations based on formula is included in

Appendix A.) They are generally nonflammable and nontoxic, with low

solubilities in water. Their densities, which are considerably

greater than water (e.g., Freon 114 has a specific gravity of 1.54)

make them easily separable.

As a group, the halogenated hydrocarbons have several

disadvantages. Their main drawback is that they are relatively

expensive in comparison with other secondary refrigerants. Also, in

gaseous form, they are heavier than air and can create a suffocating

environment if large amounts are released in confined spaces without

proper ventilation. Certain members of this group have been the

subject of controversy in the past decade, regarding potentially

destructive impacts on the atmospheric ozone layer.

Secondary refrigerant desalination projects have been developed g

and installed worldwide. Various desalination projects utilizing

secondary refrigerant processes were undertaken by commercial

concerns in the United States in the 1960's13. Among them, Blaw-Knox

Co. and Struthers Scientific and International Corporation installed

and operated 55,000 gpd and 15,000 gpd desalination plants,

respectively.N

The Japanese desalination industry has a long history of

research and development in secondary refrigerant processes as

34

evidenced by the large number of patents issued since the 1950's.

Unfortunately, little of the research associated with these efforts

has been reported in the English scientific literature. Articles by

Nagashima and Maedaao in 1979 and Nagashima gt_al.4l in 1981 reported

on the development of pilot- and full-scale direct contact

desalination plants using horizontal reactors and butane as the

secondary refrigerant in a standard process. The pilot plant

treated 45 m3/day (12,000 gpd) seawater and the full-scale plant had

a capacity of 1000 m3/day (264,000 gpd).

The British also have investigated the potential of butane

processes for desalination. In his report on the status of

desalination technology, Barduhnzö presented brief descriptions of

two bench-scale processes developed in England. The first, developed

and tested by the British Atomic Energy Authority at Harwell in the

years 1966 to 1972, produced as inuch as 200 lt> ice/hr/ft3. The

second, operated between 1965 and 1970, by the British firm Simon

Engineering, used a shell-and—draft tube reactor to produce ice at

the rate of 150 lb ice/hr/ft3.

Sludge Conditioning - Interest in secondary refrigerant

processes as alternatives to conventional methods of wastewater

treatment and sludge conditioning followed their development as

desalination technology, with most references appearing in the

1970's. Research conducted by Kahn on direct freeze-thaw

conditioning of wastewater sludges using butane was reported in

several sources in the 1970's3O-32. Experiments were conducted under

· 35

both batch and continuous operating conditions. Batch experiments ’

took place in a plexiglass reactor, with precooled butane

(temperature = -1.4506) sparged into 1500 mL of sludge at a flowrate

of 2.7 x 10-5cfs (46 mL/min) under a 3 cm (1.2 in) Hg vacuum.

Continuous experiments took place ‘Hi a 6-inch-diameter cylindrical

reactor, with a sludge depth of 6 to 12 inches. Sludge and butane

were precooled in a heat exchange system through which chilled saline

solution was circulated. Sludge and butane were fed to the reactor

at rates of 3 to 5 mL/min and 5 to 7 mL/min, respectively, under a

vacuum<M’

7 cm (2.8 in) Hg. The greater vacuum was necessary to

ensure proper conditions for butane vaporization throughout the

reactor. Butane vapors were collected and condensed by indirect

contact with the product slurry. Two compressors in series achieved

70 to 80 percent butane recovery.

Sludges tested included waste activated sludges from one

conventional and two extended aeration wastewater treatment plants.

Selected results are presented in Table 1. Batch experiments reduced

the specific resistance of the waste activated sludge from 4.7 to 1.8

x 1014 m/kg at a detention time of 60 minutes, and increased final

solids concentration from gravity drainage from 18 percent for the

unconditioned sludge to 77 percent for the conditioned sludge. It is

interesting to note that substantial improvements in dewatering

characteristics were achieved only at contact times of 40 minutes or

more, when freezing was complete. Also, the resulting treated

samples still indicated a poor dewatering sludge as evidenced by

36

TABLE 1

SELECTED RESULTS OF DIRECT·CONTACT, FREEZE·THAW SSNDITIONINGOF WASTE ACTIVATED SLUDGES WITH BUTANE

BATCH CONDITIONING

Tota1 Butane Contact Specific % ReductionDose, Time, Resistance, in Specific

mL/L min. x 10 m/kg Resistance

0 0 4,700 ·310 10 4,000 15610 20 4,400 6

1,230 40 2,000 571,840 60 1,800 62

CONTINUOUS CONDITIONING

Tota1 Butane Contact Specific % ReductionDose, Time, Resistance, in Specific

mL/L hr. x 10 m/kg Resistance

O 0 850 ·1,500 6 220 741,500 12 200 761,500 24 180 791,500 36 100 88

37

specific resistance values in excess of 1 x 1014 m/kg.

The continuous process resulted in a decrease in specific

resistance for one of the extended aeration sludges from 8.5 to 2.2 x1013 m/kg, with an increase in final solids concentration from one

percent to 20 percent, after six hours contact time. Further

improvements in specific resistance values and final solids

concentration were noted as detention times were increased up to 36

hours.

The Crystallex process, developed and promoted extensively by

AVC0 Corporation in the early 1970's, was touted as a means of

concentrating recoverable constituents in wastewater as well as

desalinating seawater. The Crystallex process utilized Freon 114 as

the refrigerant. In the process initially described by Fraser and

Johnson28, liquid Freon 114 and feed were introduced simultaneously

as a spray over a mechanically agitated pool contained in a

horizontal reactor. (The process was later modified such that the

refrigerant was sparged into the pool from the floor of the reactor.)

The ice slurry in the reactor was maintained in a fluid state by

recirculating concentrate26’42.

Slurry, concentrate, and any remaining liquid refrigerant were

separated in a hydrocyclone. The slurry was then washed in a

pressurized counter·current wash column, from whence it flowed to a

shell-and-tube melter-condenser. Freon 114 was condensed along the

outside of the melter-condenser tube. A refrigerated cold trap was

used to separate non-condensable gases from the spent refrigerant

38

after condensation. Vacuum stripping was used to remove traces of

refrigerant from the product. Additional stripping, if required, was

accomplished at atmospheric pressure26’42’43.

The Crystallex process was used, in addition to desalinating

seawater, in projects involving materials recovery from metal plating

baths and concentration of pulp and paper wastes. Preliminary

results from the treatment of a synthetic wastewater containing

nickel, cadmium, chromium, and zinc, in a 2500-gpd pilot plant,

showed that the process could reduce metal concentration in recovered

water to less than 0.5 mg/L26’28.Kahn3O also experimented with using Freon 114 to condition

sludges; results were deemed unsatisfactory due to poor settling and

dewatering characteristics of the conditioned sludge. Katz and

Mason34 and researchers working for the Sewerage Commission of

Milwaukee24 reported poor results from their attempts to condition

sludge with liquid nitrogen, methyl chloride, butane, isobutane, and

freon (type unspecified). v

Gas Hydrate Processes - Extensive research was conducted in the

1960's on the use of gas hydrates in direct-contact freeze-thaw

desalination processes. Basic studies were undertaken by Barduhn and

coworkers at Syracuse University for the Office of Saline water (0Sw)

of the United States Department of the Interior44-53. Their purpose

was to isolate hydrate-forming agents that could be employed in

economically competitive desalination processes; their efforts

resulted in the development of several pilot plants. As for other

39

freezing processes, however, research came to a halt and pilot plants

disappeared when the 0Sw was abolished in 197426.

Similar in concept to secondary refrigerant processes, hydrate

processes appear to have potential for use in wastewater treatment

and, specifically, in sludge conditioning. The technology is

virtually identical. Yet scant research has been conducted in this

area. Research on utilization of gas hydrate processes for sludge

conditioning has been limited; only. one study by Molayem and

Bardaki33 could be found via traditional literature review methods.

Gas hydrates are nonstoichiometric compounds of gas and water

characterized by a clathrate or cage structure. Molecules of gas or

gas mixtures known as "guests" inhabit a "host" crystalline lattice

of pure water54. The crystalline structure is maintained by hydrogen

bonding between water molecules and by Van der waals attraction

between host and guest55. The physical interactions between the host

and guest are relatively weak. Once the hydrate is formed, however,

the molecules of gas are trapped and cannot escape until the host

lattice breaks down54.

In desalination hydrate processes, salt-free hydrate crystals

are induced to form from seawater, then separated and decomposed to

form pure water and hydrating agent. The hydrating agent is then

stripped from the product water.

Hydrate Structure — The composition of gas hydrates has been

studied since the early 1800's. According to Cady55, the models

generally accepted today were elucidated in the 1950's by

40

M. von Stackelberg, w. F. Claussen, and L. Pauling and R. E. Marsh.

There are two structural types of gas hydrates (denoted as Type

I and Type II) consisting of 46 and 136 water molecules,

respectively. The Type I structure (Figure 3a) consists of two

pentagonal-dodecahedral cavities and six tetrakaidecahedral cavities,

with radii of 0.29 and 0.46 nanometers (nm), respectively. Corners

of the cavities are formed by hydrogen bonding of one corner oxygen

atom to an outside oxygen atom and three other corner atoms54. A Type

I hydrate (composed of 46 water molecules), can theoretically

encompass eight molecules of a hydrate—forming agent "X" with

diameter less than or equal to 0.58 nm. Thus, its theoretical

composition can be expressed as X8(H20)46, or X(H20)7•6.Experimental data show that molecules filling the small cavities

rarely exceed 0.5 nm in diameter, whereas the large cavities are

filled by molecules with diameters between 0.5 and 0.56 nm55.

The Type II structure (Figure 3b) consists of sixteen

pentagonal-dodecahedral cavities and eight hexkaidecahedral cavities,

with radii of 0.24 and 0.345 nm, respectively. The small cavities

are identical to the small cavities in the Type I structure. The

larger cavities accommodate molecules of 0.56 to 0.66 nm diameter.

If all twenty-four cavities of the 136-water—molecule framework are

occupied, the composition of the hydrate is X24(H20)136, or

X(H20)5•66. If only the larger cavities are occupied, the

composition is X8(H20)l36, or X(H20)l7.

41

OÜäüß}'Ü /'

·0*ghnvgyä

~

—

TYPE 1 HYORRTE

lf\ _ '.

' jf \$?$

·TYPEII HYORFITE~

FIGURE 3. TYPE I HNO TYPE II GHS HYORRTE STRUCTURES( FROM BERECZ. E. HNO BFILLFLFICHS. M. ( S1! 1

42

Experimental data on hydration numbers (the number <yf water

molecules per gas molecule) for both Type 1 and Type 11 structures

show considerable variation from these ideal compositions56. In

fact, it is normal for approximately ten to twenty percent of the

cavities to be unoccupied, thereby raising the water-to-hydrating

agent molecular ratio54. The fractional occupancy of a hydrate is

also affected by temperature, pressure, and the size of the guest

molecule.The bond angles and bond distances between water molecules in

both Type 1 and Type 11 hydrate structures are similar to those

between water molecules in liquid water. The hydrogen bonds are

approximately one percent longer, and the bond angles differ from

tetrahedral ice angles by 3.7 degrees in Type 1, and 3.0 degrees in

Type 1157.

Approximately thirty-five gases form hydrates with Type 1

structure; there are about sixty-five gases which form Type 11

structure hydrates57. Although there is a wide diversity in the

gases which form hydrates, they share certain common characteristics.

The shape and size of the molecules conform to the cavities in the

hydrate framework. The compounds are also typically homopolar, with

low solubility and high volatility. They experience moderate Van der

waals forces54.

Thermodyamics and Kinetics of the Process — The initial

conditions of hydrate formation (which occurs only if a hydrating

agent is present) are determined by the nature of the hydrating gas,

43

the state of the water, the pressure, and the temperature. These

conditions are best described by heterogeneous phase plots of

pressure vs. temperature, such as the generalized phase diagram from

Berecz54 shown in Figure 4. The curves depicted are:

I - the vapor pressure curve of the hydrating agent

saturated with water;

II — the vapor pressure curve of the hydrate;

III - the melting point curve of the hydrate; and

IV - the depression of the freezing point of water due

to solubilization of the hydrating agent in

water.

The hydrating gas exists in hydrate form in the regions to the upper

left of curve II and left of curve III.

Each gas hydrate is characterized by a quadruple point; that is,

a temperature and pressure at which ice, gas hydrate, liquid (or

solid) hydrating agent, and gaseous hydrating agent are in

equilibrium. This temperature and pressure are known as the critical

or decomposition values for the hydrate. This point (pk,l) is also

the pressure and temperature at which the vapor pressure of the

hydrate and the hydrating agent are equal. If the hydrate is

partially occupied, this point will occur at lower temperatures and

pressures54.

According to Davidson57, the lattice stability cyf the hydrate

44

III

I

ä Pai

_III

/1////

N

TEHPERFITURE

FIGURE 9. GENERRLIZED GHS HYDRHTE PHHSE DIHGRFIH( FROM BERECZ ( 51) 1

' 45

structure depends upon the degree <yf cavity occupancy. Since the

relative locations of guest and host molecules are known, the

fractional occupancy of the hydrate structure can be modeled using a

molecularly based, statistical theory of ideal localized

adsorption58. Cady55’56 has modeled the fractional occupancy of Type

I hydrates using a Langmuir-type adsorption isotherm equation.

Longer range interactions, asymmetry in gas—water interactions, and

restrictions of rotation and vibration of gas molecules in hydrate

cavities also must be considered, but are not usually specifically

addressed in simplified models.

John et_gl.59 addressed second neighbor effects and smoothing of

cavities due to asphericity of guest molecules. According to John

and his coworkers, stability depends upon the strength of the

physical interaction between host and guest. Thus, it follows that

the stability of a hydrate is related to the molecular size of the

hydrating agent, as attractive forces increase with increasing size.

The relationship between stability and size is proportional54.

The stability of a hydrate also depends upon the location of its

vapor pressure curve. The higher the temperature at which the vapor

pressure of the hydrate reaches 10.13 x 105Pa [145 psia), or

conversely, the lower the dissociation pressure at 006, the greater

the hyahete eteh1i1ty54.The thermal dissociation of hydrates is a four—step process54.

when the temperature drops sufficiently, the host lattice begins to

sublimate. As it does, its clathrate structure starts to

46

disintegrate. Guest molecules diffuse through the collapsing

structure, and finally desorb from its surface.

The rate of formation of a hydrate is difficult to model as the

system is complex, having two components and four phases54. It is

affected by nucleation rates, crystal growth rates, heat transfer

from the crystal to the aqueous liquid (or, in some cases, the

vaporizing hydrating agent), and mass transfer of salt or other

impurities away from the solid-liquid interface50.

Hydrates are formed primarily at the phase boundary between

liquid water and the gaseous hydrating agent54. There are two modes

of nucleation which occur: (1) homogeneous nucleation on the surface

of dispersed liquid agent droplets, and (2) heterogeneous nucleation

initiated by crystal fracturing6O. while hydrates can form inside

the liquid on dissolved gas particles, this requires greater

activation energy to reorient the surrounding water molecules. The

high molar ratio of gas to water in the vapor phase makes hydrate

formation in the vapor region unlikely as wellöl.

Factors which have been found in laboratory studies to

significantly affect the rate of formation of gas hydrates include

- 6T, the magnitude of the process driving force; that

is, the temperature differential between the

equilibrium temperature of the hydrate at the bulk_

salt concentration and the actual bulk aqueous

temperature;

47

- the degree of agitation or power input; and

- the salt concentration50.

The driving force 6T is expended primarily for (1) processes

taking place between hydrate crystals and the aqueous phase and for

(2) heat transfer from the aqueous phase to agent droplets,

especially for vaporization of the hydrating agentöl. Although

Barduhnöl suggested that the nature of the agent molecule may have

little effect on the kinetics of' hydrate formation, the relative

demand of these two processes seems to vary with the hydrating agent

used. working with Freon 31, Barduhn estimated that two-thirds of

the total driving force was consumed by agent evaporation, and

one-third by crystal growth. In contrast, the Office of Saline water

Research reported results of earlier work with methyl bromide5l which

assigned ten to twenty percent of the driving force to the former,

and eighty to ninety percent to the latter. In the formation of

hydrates from refrigerants, the relative amount of liquid agent

present surely affects the rate of formation as its presence enhances

heat transfer through evaporation.

Agitation of the hydrate-containing aqueous solution promotes

both homogeneous and heterogeneous nucleation by renewing contact

surfaces constantly and by breaking crystals apart. Barduhn's

group6O’6l developed empirical equations relating rates of formation

of certain hydrates to the power input and to the driving force, 6T.

48

Further research confirmed the importance of power input. Barduhnöl

noted a tenfold decrease in rate of production of Freon 31 and Freon

142a crystals brought to a steady state iri a continuously stirred

vessel when agitation ceased.

Mass transfer of salt and dissolved agent has been determined to

be relatively unimportant in affecting formation rate, although

increasing salt concentration does tend to cause a decrease in

reaction rate5l.

Process History - In its annual report for 1960, the Office of

Saline water published a list of hydrate-forming agents for potential

use in desalination processes, including several Freons,

cyclopropane, methyl bromide and vinyl chloride44. A number of

agents screened did not form simple hydrates. Others were considered

impractical because of their relatively high decomposition pressures.

In 1961, the OSN published a revised list of the most promising

agents for use in desalination45. Chlorine, methyl chloride, Freon

12, and propane had been added to the list, while cyclopropane, Freon

12B2 and vinyl chloride had been dropped.

This initial research on hydrating agents specifically suited

for desalination was carried out by Barduhn and coworkers44’45. Over

the next twenty years, they produced a considerable body of

thermodynamic and kinetic data for many of these agents, including

Freons 12, 21, 31 and 142b46’48’5l-53’6l. inain amaria for anaevaluation of a compound as a suitable hydrating agent included

49

- moderate decomposition temperature and pressure;

— high rate of hydrate formation;

- low losses due to solubilization or hydrolysis; and

- reasonable cost.

Secondary considerations included crystal separability and

refrigerating capacity. More recent work by Kubota58 produced

thermodynamic data on Freon 13, Freon 23, and Freon 152a.

Hydrating agents of particular interest are discussed in more

detail below. Their decomposition pressures and temperatures, as

well as their hydration numbers, are given in Table 2.( Freon 12 (CCl2F2) forms a Type 11 hydrate, with an approximate

composition of Freon 12(H20)l7. It has a decomposition temperature

of 12.1OC and a decomposition pressure of 4.58 x 105Pa (64.5 psia).

Freon 12 was used to start up the Koppers Co. propane hydrate

pilot desalination plant, built for the 0Sw at wrightsville Beach

Test Facility, South Carolina, in 196650-52. Freon 12 was also

tested at the same location from March, 1967, to July, 1968, in a

pilot program conducted by the Mason—Rust Co. Efforts concentrated

on optimizing wash—separation processes rather than hydrate

formation, as Freon 12 hydrate crystals proved difficult to separate.

Production rates were low and failed to offset the high capital and

operating expenses, and the plant finally closed down for economic

reasons, although technically it worked well.

Freon 31 (CH2ClF), a Type I hydrate, has a high decomposition

50

· TABLE 2E

DECOMPOSITION TEMPERATURES AND PRESSURES OF HYDRATING AGENTS

Decompos.Decompos. Presägre, Hydration

Agegg TENE.2 °C x 10 Pa Number Bei;

Carbon Dioxide 31.1 73.73 6 54

Chlorine 28.7 6.09 7.27 45

Cyclopropane 17.1 5.96 - 44

Freon 12 12.1 4.58 17 45

Freon 1281 10.0 1.70 17 44

Freon 1282 9.9 2.68 17 44

Freon 13 8.5 24.2 19.3 58

Freon 21A

8.7 1.01 16.6 44

Freon 2281 9.9 2.68 17 44

Freon 23 19.9 40.4 7.9 58

Freon 31 17.9 2.86 8.0 44

Freon 1428 13.1 2.32 7.2 45

Freon 152A 15.3 4.51 17 44

Methyl Bromide 14.7 1.53 7.9 44

Methyl Chloride 21 5.07 7.2 45

Propane 5.5 5.67 18.4 58

Vinyl Chloride 1.2 1.82 · 44

51

temperature (17.906) and a moderate decomposition pressure (2.86 x105 Pa, or 41.5 psia). It has an extremely low hydrolysis rate and a

high rate of formation. Results of kinetic studies showed that the

Freon 31 hydrate ·formed six to ten times ·faster ‘than hydrates of

propane, methyl bromide, or Freon 1252. Crystals were granular and

easy to wash53. Unfortunately, Freon 31 is no longer commercially

available.

Freon 142b (6H366lF2) produces a Type II hydrate which

decomposes at 13.106 and 2.32 x 105 Pa. Freon 142b was used in

Barduhn's kinetic studies on the rate of formation of hydrates as

function of temperature differential44.

Propane (63H8) forms a Type II hydrate with a relatively low

decomposition temperature (5.706) and a high decomposition pressure

(5.67 x 105 Pa, or 82.2 psia). Despite these unfavorable

characteristics, it has been the subject of continued laboratory and

pilot studies because of its ready availability44’45’48°5l.

In 1960 and 1961, the results of laboratory research conducted

by Koppers Co. on a bench—scale propane hydrate desalination process

were reported44’45’48-51. Hydrate yield varied from 3 to 21 weight

percent of feed. Agitation proved critical; problems with crystal

separation were also noted.

The laboratory bench-scale work undertaken by Koppers resulted

in the design and construction of a 10,000-gpd pilot plant at the~

wrightsville Beach Test Facility, which was completed in 196648-51.

The seawater was first pretreated by filtering suspended materials,

52

chlorinating, and deaerating, and then prechilled. The seawater was

introduced into a reactor where propane was added and hydrates were

formed. Following hydrate formation, the hydrate crystals were

washed, liquid propane was decanted from the waste brine and the

product water, and the hydrate slurry was decomposed. Gaseous

propane was stripped from the waste brine and product water streams.

The ultimate failure of the pilot operation was attributed to the

poor washing characteristics of the propane hydrate crystals.

In the early 1960's, the Sweet water Development Company also

tested a propane hydrate desalination process46-48. Pilot tests were

conducted at a small scale on a 700-gpd pilot plant in St. Louis,

Missouri. The encouraging results obtained from this plant led to

the design and construction of a 20,000-gpd pilot unit at the 0Sw's

Wrightsville Beach Test Facility. The process resembled that

developed by Koppers. However, the washing operation relied on a

cocurrent displacement system which employed a cyclone to separate

propane, water, and hydrate crystals based on differences in

densities. The cyclone did not provide satisfactory performance and

was eventually replaced with a pressurized wash column.

Chlorine (Cl2) produces a Type I hydrate that has one of the

highest decomposition temperatures (28.7OC) of the group of hydrates

actively considered for use in desalination processes. Unfortunately,

it also has a high decomposition pressure (6.09 x 105 Pa or 88.3

psia), is extremely corrosive, and is highly toxic. Because of its

wide availability, however, it is still considered a potential

53

desalination agent62. No references to its use in bench-scale, pilot

or full-scale projects were found.

Carbon dioxide (CO2) has a very high decomposition temperature

(31.1OC) and a very high decomposition pressure (73.73 x 105Pa or

1069 psia). Despite its high decomposition temperature and pressure,

it is attractive as a desalination agent because it is nontoxic,

nonflammable, and readily available at reasonable cost.

In 1964, it was reported that the „Jacobs Engineering Company

investigated the use of carbon dioxide as a hydrating agent for

desalinationag. The process used multiple stages of hydrate

formation, applying the carbon dioxide step-wise to avoid flash

hydrate formation, which apparently can have the same deleterious

effects as flash freezing in freezing processes. Neither direct or

indirect use of carbon dioxide as a refrigerant was practical, and

ammonia was used for all heat transfer processes instead. Carbon

dioxide recovery was accomplished by vacuum stripping. Crystal

separation took place in a continuous classifier.

Methyl bromide (CH3Br) and methyl chloride (CH3Cl) have both

been seriously considered as hydrating agents55’58. Methyl bromide

is a Type I hydrate with a high decomposition temperature (14.7OC)

and low decomposition pressure (1.53 x 105 Pa or 22.2 psia).

Kinetic studies on rate of formation of methyl bromide hydrate

were conducted in a continuously fed, stirred vessel, and compared

favorably with other agents. Methyl bromide was eliminated from

consideration as a desalination agent, however, when it was

54

discovered that its relative cost, due to a high rate of hydrolysis,

made it economically unattractiveäl.

Methyl chloride, also a Type I hydrate, has a high decomposition

temperature (21.0OC) and an acceptably low rate of hydrolysis. Its

high decomposition pressure (5.07 x 105 Pa or 73.5 psia) has

discouraged researchers from pursuing further kinetic studies.

Sludge Conditioning - The literature search for this study found

only one reference regarding the application of the gas hydrate

process to sludge conditioning. Molayem and Bardaki33 dewatered

samples of primary and activated sludge as well as mixtures of the

two sludges with excess quantites of Freon 11. 500-mL doses of Freon

11 were applied to 20 mL samples in 10-minute stages. Sludge solids

were physically separated from Freon 11 hydrate crystals during the

course of the experiments. As can be seen from the selected results

presented in Table 3, high concentrations of final solids were

obtained in certain instances.

The practical significance of this study is questionable for the

following reasons:

1. Results were extremely variable. Percentage increases in

final solids concentration varied from 13 to 94 percent for

primary sludge; from 43 to 98 percent for activated sludge,

and from 55 to 91 percent for mixtures.

2. Estimated values were used in calculating final solids.

55

TABLE 3

SELECTED RESULTS OF DIRECT-CONTACT, FREEZE-THAWBSONDITIONINGOF WASTEWATER SLUDGES WITH'FREON 11

ONE-STAGE CONDITIONING

Initial FinalSolids, Solids,

Sludge % %

Primary 7.0 5#.0

Primary 7.0 1#.7

Secondary 1.8 #.0

Secondary 1.8 52.6

Mixture 5.3 11.5

Mixture 5.7 #0.5

TWO—STAGE CONDITIONING

Initial FinalSolids, Solids,

Sludge % %

Primary 7.# 20.3

Primary 9.3 10.6

56

Final solids concentrations were calculated from equations

which required estimates of the amount of water remaining

associated with the hydrates formed during the experiment.

3. Excessive mnounts of' Freon 11. were used in experiments.

The applied Freon dose varied from 12.5 to 25 L Freon per L

of sludge.

IMPACT OF PROCESS AND HANDLING VARIABLES ON CONDITIONING

EFFECTIVENESS OF FREEZE—THAN CONDITIONING

Factors which affect the conditioning effectiveness of

freeze-thaw conditioning processes include the rate and extent of

freezing; type, amount, and point of chemical application; length and

condition of storage; sludge solids content; and the amount and

intensity of handling and mixing. These factors are discussed in the

following subsections.

Rate and Extent of Freezing

It is generally accepted that effective freeze-thaw conditioning

of sludges is only achieved by slow, complete freezing. Studies of

natural and indirect freeze-thaw conditioning have, in general, borne

out this beliefl8’l9’34’63. Experiments conducted using secondary‘

refrigerant processes have shown that slurry freezing could also be

used to successfully condition sludges.

57

Katz and Mason34 conducted indirect freezing experiments with

waste activated sludge. They found that deep freezing caused the

solids to separate from the liquid and form layers. Rejection of

solids occurred at a critical freezing rate of 0.0005 mL/sec (0.03

mL/hr). At freezing rates greater than 0.001 mL/sec (0.06 mL/hr),

sludge particles and unfrozen liquid were trapped in the advancing

ice matrix. Rapidly frozen sludges did not dewater as well as

sludges which had been frozen at a slower rate.

In experiments to determine the relationship between freezing

speed and conditioning effects, Logsdon and Edgerly64 progressively

froze samples of barium sulfate sludge, iron and aluminum hydroxide

sludges, and alum water treatment sludge. The 350-mL samples were

placed in plastic bags and lowered into an antifreeze bath at a fixed

rate. As freezing progressed, solids tended to migrate upwards and

consolidate. The lower the freeze rate, the greater were the

migration and consolidation. Critical velocity of the freezing

isotherm was about 30 mm/hr, above which no migration occurred. Fast

freezing of iron and alum sludges resulted in local segregation of

particles. Although segregation patterns were different in frozen

samples of the two different kinds of sludge, both sludges showed

improved dewatering characteristics, as evidenced by decreases in the

equivalent volume of settled solids in settled samples.

Ezekwo gt_al.62 subjected partially digested secondary sewage

sludge to progressive freezing by lowering sludge-filled glass tubesthrough a chilled metal block. Freezing rate varied from 0.8 to 8.8

58

x 10-4 cm/sec (0.05 to 0.53 mm/hr). Although there were

morphological differences observed in cross and longitudinal sections

of sludge frozen at different rates, solids obtained upon gravity

drainage did not vary appreciably with freezing rate.

In experiments in which biological sludges were frozen by

contact with butane, ‘Khan3O and Randall, et_Jal.,32’65 found that

slurry freezing improved dewatering characteristics. In both batch

and continuous processes, increasing the contact time between sludge

and butane improved conditioning. The improvement in the specific

resistance of the activated sludge freeze-thaw conditioned in the

batch process corresponded to a first—order reaction curve,

approaching a maximum at 60 minutes. Contrary to results produced by

solid freezing, filtrate quality improved with conditioning time,

although supernatant quality decreased.

Chemical Addition

Several studies have reported that the addition of inorganic

chemicals or polymers to freeze-thaw conditioning improved dewatering

characteristics. The addition of ferric chloride to activated sludge

prior to freezing improved dewatering rates24. Preaddition of alum to

activated sludge increased solids production rate and resulted in

more efficient solids capturego, and improved dewatered solids

concentration for a variety of wastewater treatment sludges66.

Polymer addition prior to freezing also decreased sedimentation

volumes of iron and alum sludges63; polymer addition to digested

59

sewage sludge during the freezing process dramatically improved

solids migration67. In contrast, chemical addition after

conditioning has been found to hinder dewatering24.

Baskerville68 has suggested, that preconditioning with these

agents strengthens the delicate flocs which constitute some

biological and chemical sludges. The improved structure of the floc

is more able to withstand the potentially destructive pressures of

the freeze—thaw process.

Storage

Gale gt_gl;69, in their basic studies on the effects of handling

variables on the dewatering characteristics of raw sewage sludge,

found that storage of unconditioned and alum-conditioned sludges over

a period of days resulted in large increases in specific resistance.

Other researchers have confirmed that storage of sludge prior to and

after freeze-thaw conditioning generally resulted in deterioration of

dewatering characteristics. In a study for the Sewerage Commission

of Milwaukee24, samples of activated sludge stored for periods up to

24 hours, before and after indirect—contact, freeze-thaw

conditioning, resulted in deterioration of dewatering

characteristics. Khan30 observed that storage of biological sludges

both prior to and after direct-contact conditioning with butane

hindered settling, filtration, and drainage.

Doe gg_gl.22 found that the dewatering characteristics of alumsludges deteriorated appreciably after E3 to IO hours of thaw time

60

following indirect-contact, freeze-thawconditioning.Thickening

.

The effect of initial solids concentration, and, thus, the

usefulness of prethickening in freeze-thaw conditioning of sludges is

disputed. Certain studies have shown that conditioning improves with

increased initial solids concentration while others have shown the

opposite effect. This is an important consideration as prethickening

can decrease the volume of sludge freeze—thaw conditioning processes

must handle, resulting in substantial cost savings.

Doe gt_al.23 prethickened alum water treatment sludges to 2.4

and 4 percent prior to successful indirect—contact, freeze-thaw

conditioning. In 61 study conducted for the Sewerage Commission of

Milwaukee24 activated sludge was prethickened to 4 percent with no

adverse effect on the dewatering characteristics of the

indirect-contact, freeze—thaw conditioned sludge.

Katz and Mason34 found that the dewatering time of

indirect-contact, freeze-thaw conditioned activated sludge decreased

as feed solids concentration increased from about 1 to 1.5 percent to

about 2 to 2.5 percent.

Ezekwo gt_al.64 compared the solids concentration of

progressively frozen, partially digested sewage sludge after thawing

and gravity drainage. Undiluted sludge (4 percent) dewatered to a

higher concentration than diluted sludge (2.2 percent).

In contrast to these studies, Tilsworth et_al.18 reported on

61

natural freezing of extended aeration sludges, noting that the lower

the solids content of the sludge, the more efficient the process.

Randallgz, in slurry freeze—thaw conditioning of biological

sludges, also found that higher solids concentrations required longer

detention times. The relationship could be modeled according to the

equation:

YCT = axfz + 6 [24]where

YCT = conditioning timexf = feed solids concentrationa,b = constants.

Handling and Mixing

Handling and mixing have been shown to have an adverse effect on

the dewatering characteristics of certain sludges22’23’69. The

effect depends upon the type of sludge and the intensity and duration

of handling and mixing. Farrell7O noted the negative effects of

stirring on both sewage and alum hydroxide sludges which been

freeze-thaw conditioned; the effect was more pronounced for sewage

sludge. Khan30, in his direct-contact, freeze-thaw conditioning

experiments with biological sludges, found that unixing during the

freezing process was necessary to maintain a pumpable slurry.l

Vigorous agitation resulted ‘Hi a poorly settling sludge, but its

effects could be partially allayed by subsequent slow stirring.

62

MECHANISMS OF FREEZE·THAw CONDITIONING

Freeze-thaw conditioning produces irreversible changes in the

nature of some sludges, making them, in many cases, easier to

dewater. Several mechanisms have been proposed to explain these

changes. A discussion of the mechanisms involved in freeze-thaw

conditioning of sludges is difficult to approach, as various theories

put forth are based upon the extent and rate of freezing as well as

the type of sludge conditioned. Mechanisms can be roughly divided

into two categories:

- solids migration and compression, and

- particle dehydration and cell rupture.

Solids Migration and Compression

The segregation of particles and layers of particles (termed

micromigration and nacromigration, respectively, by Logsdon and

Edgerly63) which occur when sludges are deep frozen has been noted in

a number of studies. The irreversibility of the changes wrought by

freeze-thaw conditioning has led some researchers to suggest that

the forces which are responsible for segregation are large enough to

destroy cells and to compress particles and flocs, dehydrating and

concentrating them.

The behavior of solids in a freezing liquid is a result of the

process of crystallization and of the nature of the solids

themselves. Ice crystals nucleate and, if unimpeded, proceed to grow

63

in planes. If surrounded by solution, the crystal interface is

destabilized by the concentration gradient of the solute. In

response, the crystal sends out projections and loses its planar

characteristics, becoming dendritic. As the solution is dehydrated

at the ice interface, colloidal solutes lose some of their mobility.

The interface pushes‘ particles ahead of it, excluding them by

surface—energy effects. Solids thus tend to localize at the grain

boundaries of the ice crystals64’67.

whether or not a particle in the path of the advancing interface

is trapped depends upon the size and roughness of the particle, the

solids content, and the rate of crystal growth63’64’67. The larger

the particle, the lower is the velocity required to trap it. while

polymers increase the size of particles, they improve the

concentration of solids by lowering the resistance of flow through

them, thus preventing entrapment of solids. The larger the surface

area relative to particle size (i.e., the rougher the particle), the

greater is the tendency of the particle to be carried along by the

interface.

Ezekwo gt_al.64, from their work with partially digested sewage

sludge, noted differences in the behavior of samples of diluted and

undiluted sludges upon thawing. The solids from the undiluted sludge

(4 percent) formed a rigid mass which retained its longitudinal form,

collapsing along the side of the tube in which the sludge was frozen

and thawed, during melting and draining. This facilitated the flow of

water from the mass. On the other hand, the diluted sludge solids

64

formed a fragile mass which broke apart during collapse, and settling

at the bottom of the tube during melting hindered drainage.

From these observations, the researchers hypothesized that slow

freezing permitted development of elongated crystals which drained

well. Rapid freezing resulted in small ice crystals enveloped by a

matrix of dissolved and suspended solids which drained poorly.

Migration of solids was hindered by increased initial solids

content, as evidenced by decrease in the size of ice domains. The

higher the solids content, the lower the rate at which impurities

diffused in the system. At high solids concentrations (greater than

80 percent), nucleation and crystal growth were severely reduced.

Logsdon and Edgerly63, found that freeze-thaw conditioning had a

decreased effect on sludges which had relatively poor dewatering

characteristics before conditioning. They suggested that the

segregated solids formed a cake similar to that formed during the

Buchner funnel test, with ice as the supporting medium. Highly

flow-resistant sludges were inclined to entrap solids during

freezing.

Particle Dehydration and Cell Rupture

Silvares et_al.71 developed a model on freezing injury to cells

based on two effects:

— solution effects, predominant in slow freezing,

wherein cells are immersed in a concentrated salt

65

solution as ice forms outside the cell. The

concentration gradient exerts an osmotic effect upon

the water in the cell, drawing water from the cell and

thus dehydrating it.

- intracellular freezing, predominant in more rapid

freezing, wherein ice forms inside the cell.

Several researchers have suggested that freezing has a

dehydrating effect on sludge particles. As water freezes, the

unfrozen solution surrounding the crystals becomes increasingly

hypertonic. This imbalance causes cellular or bound water to duffuse

to the immediate environment of the cells or flocs. Rapid or deep

freezing causes cells and flocs to rupture, releasing their contents.

Upon thawing, the filtrate contains a higher percentage of dissolved

solids because of the material extracted from the particles3O’34.

Farrelllg speculated that freeze-thaw conditioning removed water

from hydroxide flocs, whereas its basic effect on sludge particles

was to enhance agglomeration. Sewage sludge particles were thus

susceptible to resuspension upon mixing.

Role in Direct-Contact Processes

The role of either of these mechanisms in direct contact

conditioning processes is not clear. Large scale migration of

particles is not possible in well-mixed systems in which the sludge

66

is maintained as a slurry. whether local compression and dehydration

at the ice-liquid interface is sufficent to cause the irreversible

changes noted in previous studies is not known.

67

III. METHODS AND MATERIALS

Samples of water and wastewater treatment sludges were

conditioned using a variety of methods and analyzed to determine the

relative effectiveness of direct-contact, freeze-thaw processes.

This section describes sludges, conditioning methods, equipment and

process variables, and analytical methods used in this study.

SLUDGE SOURCES

Samples of waste activated sludge, primary sewage sludge, and

alum water treatment sludge were used in this study. General

characteristics of each sludge and the handling protocols developed

for each are detailed in this section.

waste Activated Sludges

Samples of waste activated sludge from the Roanoke wastewater

Treatment Plant (Roanoke, Virginia), and from the Christiansburg

wastewater Treatment Plant (Montgomery County, Virginia) were only

used in preliminary experiments. Samples of waste activated sludge

from the Stroubles Creek wastewater Treatment Plant (Montgomery

County, Virginia) were used throughout the study.

Samples of Stroubles Creek waste activated sludge were drawn

from two sources. Sludge for same-day testing was bled through a tap

68

on the main line returning settled sludge from the secondary

clarifiers to the aeration basins. Sludge for use over the course of

several days was taken directly from the aeration basin. In both

cases, the sludge was allowed to settle and excess supernatant

decanted before transport to the laboratory. Sludge from the

aeration basin was transferred to 30—liter plastic batch reactors,

stored at 200 Celsius and aerated. Samples were withdrawn from these

basins, settled, and decanted, as needed. The batch reactors were

fed approximately 400 mg/L Bactopeptone and supernatant decanted and

replaced with tap water daily.

The initial solids concentration of the settled sludge was about

1 percent (varying frmn 0.6 to 1.6 percent). Specific resistance

values generally ranged from 4 to 70 x 1011 m/kg. Cake solids

concentrations resulting from Buchner funnel tests varied from 9 to

15 percent. Sludge characteristics remained stable over the periods

in which the sludge was maintained in the batch reactors. These

periods did not exceed four days.

Primary Sewage Sludge

Samples of primary sewage sludge were obtained from the primary

clarifier underflow lines at the Stroubles Creek wastewater Treatment

Plant. Sludge was collected during waste pumping and screened

through a 1/4-inch plastic mesh to remove fibrous materials. Samples

were permitted to settle before testing and excess supernatant

decanted.

69

Initial dry solids concentrations and specific resistance values

of primary sludge samples varied greatly. Initial solids

concentrations of the settled sludge varied from 1 to 6 percent.

Specific resistance values ranged from 80 to 7100 x 1011 m/kg, and no

cake was formed during Buchner funnel vacuum-filtration tests.

Primary sludge samples were conditioned and tested within 8 hours of

collection, as specific resistance of the sludge increased greatly

with prolonged storage.

Alum water Treatment Sludges

Samples of alum water treatment sludges were obtained from two

sources. 0ne set of samples (used only in preliminary studies) was

obtained from an industrial water treatment plant in Philadelphia,

Pennsylvania. The other set was collected from the water treatment

plant at the Radford Arsenal Ammunitions Plant (RAAP) in Fairlawn,

Virginia.

The RAAP alum sludge was drawn from the thickening tank at the

solids handling building at the plant. The suspended solids

concentration of the thickened sludge was ranged from 1 to 5 percent.

The sludge was diluted with tap water to obtain lower initial solids

concentrations when necessary.

Specific resistance values for the undiluted sludge ranged from

160 to 310 x 1011 m/kg. while cake solids concentrations resulting

from Buchner funnel tests varied from 7 to 26 percent, they generally

were between 20 and 26 percent. Specific resistance values for

70

samples diluted with tap water to about. 1 percent initial solids

concentration ranged from 100 to 160 x 1011 m/kg; cake solids

concentrations varied from 23 to 24 percent.

Storage did not appreciably affect sludge dewatering

characteristics for the RAAP sludge. Undiluted sludge settled very

slightly during storage periods of over a nmnth, and solids were

easily resuspended and stabilized by mixing. Diluted sludge settled

moderately over the course of several hours.

CONDITIONING METHODS

Four methods were used to condition samples of Stroubles Creek

waste activated and primary sludges, samples of a mixture of these

sludges, and samples of RAAP alum sludge to gauge the comparative

effectiveness of direct-contact, freeze-thaw conditioning. These

included direct-contact, freeze-thaw conditioning with butane and

with Freon 12; indirect—contact, freeze—thaw conditioning; and

polymer conditioning. The experimental matrix for these studies is

presented in Table 4. Information on conditioning agents and

equipment and process variables is given below.

Direct—Contact, Freeze-Thaw Conditioning with Butane

Sludge samples were mixed with butane under temperature and

pressure conditions conducive to the formation of ice. All

experiments were continuous. Predetermined doses of butane were

71

TABLE 4

EXPERIMENTAL MATRIX

CONDITIONING METHODIndirect Butane Freon 12Freeze- Freeze- Freeze-Thaw

Sludge Polymer Thaw Thaw Batch Continuous

Stroubles Creek X X X X XWaste Activated (WA)

Stroubles Creek X X X XPrimary Sewage (PS)

Stroubles Creek (WA/PS) X X X

RAAP Alum SIudgeUndiluted X X X X XDiIuted X X X X X

72

introduced into 1-L sludge samples at a set flow rate of 20 mL/min.

Butane was introduced into the sludge either as a pure liquid or in

the form of colloidal liquid aphrons.

The butane used in this study was "cp" grade, compressed in

30-lb cylinders, and was obtained from an industrial source. Vapor

saturation data for butane are given in Table 5. The graph of these

data in the relevant temperature range is shown in Figure 5.

Direct-Contact, Freeze—Thaw Conditioning with Dichlorodifluoro-

methane (Freon 12)

Sludge samples were mixed with dichlorodifluoromethane (marketed

as Freon 12 by E.I. DuPont de Nemours & Co., and referred to as such

henceforth) under temperature and pressure conditions conducive to

the formation of gas hydrates. Two types of experiments were

undertaken: batch and continuous. In the batch experiments,

selected doses of Freon 12 were rapidly introduced into 1-L sludge

samples at pressures too high for hydrate formation. The sample and

the refrigerant were mixed vigorously and the pressure lowered to an

appropriate level for hydrate formation. Certain samples were

subjected to repeated slug doses of Freon 12 for two- and three-stage

conditioning. In the continuous experiments, predetermined doses of

Freon 12 were introduced into the sample continuously at a set flow

rate of 10 mL/min, at temperatures and pressures suitable for hydrate

formation.Freon 12 was obtained compressed in 30-lb disposable jugs from a

73

TABLE 5

BUTANE VAPOR SATURATION DATA

English Units _ SI UnitsT€|T\p€|'atU!‘€ Pressure T€IT|p€|‘ötUf'€ P|'€SSgf'8

°F gsia °C x 10 Pa

0 7.30 -18 0.5035 8.20 -15 0.565

10 9.21 -12 0.63515 10.33 -9 0.71220 11.56 -7 0.797

25 12.90 -4 0.88930 14.35 -1 0.98931.1 14.70 -0 1.01335 15.95 2 1.10040 17.62 4 1.215

74

80

§_20LJ1DLDLDääO- 10

0 0 10 20 30 40TEMPERFITURE. ° F

1.5

IIL1E, 1.0•-I

><LJ1äUJ 0.5LLI10.

0.0-18 -18 -7 -2 4TEMPERRTURE. °C

FIGURE S. BUTHNE VHPOR SRTURRTION CURVE.

75

commercial source. Vapor saturation and gas hydrate eguilibrium data

for Freon 12 are given in Table 6. Graphs of these data in the

relevant temperature range are shown in Pigure 6.

Indirect-Contact, Freeze-Thaw Conditioning

Indirect-contact, freeze-thaw conditioning was used to estimate

the maximum potential response of each sludge to other freeze-thaw

processes. Indirect freezing of sludge samples was accomplished by

placing samples in a conventional deep—freeze unit and allowing them

to freeze completely. Two-liter samples initially at room

temperature when placed in the unit cooled to o°c in two hours, and

froze completely in 24 hours. Completely frozen samples were removed

from the unit and thawed at room temperature to approximately 2000

before analyses were performed.

Polymer Conditioning

Results of polymer conditioning were used as a conventional

standard against which results of freeze-thaw conditioning were

compared. Samples of sludge (volume = 500 mL) were dosed with

varying amounts of a 0.1-percent solution of Betz 1160, a high—weight

cationic polymer, in a standard jar test apparatus. Samples were

mixed at 100 rpm for approximately 10 to 15 seconds, and then

flocculated for 30 to 60 seconds at 40 rpm.

76

TABLE 6

FREON 12 VAPOR SATURATION AND GAS HYDRATE EOUILIBRIUM DATA

VAPOR SATURATION DATA

English Units SI UnitsTemperature Pressure Temperature Pressgre

°F gsia °C x 10 Pa

10 29.33 -12 2.02215 32.42 -9 2.23520 35.74 -7 2.46425 39.31 -4 2.71030 43.15 -1 2.975

35 47.26 2 3.25940 51.67 4 3.56245 56.37 7 3.88750 61.39 10 4.23355 66.74 13 4.602

HYDRATE EOU|LlBRIUM DATA

English Units SI UnitsTemperature Pressure Temperature Pressgre

°F gsia °C E x 10 Pa

33.4 6.38 0.8 0.4435.6 7.83 2.0 0.5437.0 8.85 2.8 0.6137.6 9.57 3.1 0.6638.7 10.88 3.7 0.75

39.9 13.20 4.4 0.9141.7 15.52 5.4 1.0743.2 18.27 6.2 1.2644.4 21.17 6.9 1.4645.1 22.92 7.3 1.58

45.9 25.82 7.7 1.7846.8 28.14 8.2 1.94 _48.4 35.24 9.1 2.4349.6 41.04 9.8 2.8351.6 42.49 10.9 2.9352.9 60.62 11.6 4.1853.4 64.25 11.9 4.43

77

75Ü VRPÜR SGTURRTIÜN l® HYDRQTE EGUILIBRIUM il]

Y 0

äso u“

Q E 5 0; °L?} Y 0ua , YEIla25 dy

99_9.g=°

6 9

010 20 30 40 50 60TEMPERHTURE,%”

SE VHPÜR SGTURGTIONO HYÜRGTE EGUILIBRIUME __ =

E 4 9”

•-4 ·,X g

A

LJ _, 5 0; ”Q «•

ä P 55 0;U) 2 II ,0;L•J '·Q: 0:4O. _9

6 9,0%%

L: ’•‘

0-12 -3 G 15TEMPERHTURE,T

I-'IGURE S. FREDN 12 VRPOR SRTURFITION BND GHSHYDRFITE EQUILIBRIUM CURVES.

78

EXPERIMENTAL APPARATUS

The experimental apparatus used in the experiments conducted as

part of' this study is shown schematically in Figure 7. The nmin

components of the system were the reactor, heat exchange systems, and

refrigerant feed systems, which are described in detail in this

section.”

Reactors

The original reactor used in this study was constructed of

I/4—inch-thick Plexiglas. It had a rectangular 6-inch by 10-inch base

and 8-inch side walls. The reactor base had an integral diffuser

composed of shallow I/8—inch-wide channels grooved between two layers

of Plexiglas. The channels were laid out iri a rectangular grid.

Tiny holes (I/32-inch in diameter) were drilled through the top layer

of the Plexiglas base to allow liquid refrigerant to bubble up from

the channels into the reactor.

The reactor was fitted with a sealed cover. The cover included

several ports for vacuum and temperature gauges and for a line

through which a vacuum could be drawn and maintained. The cover also

had a removable plate fitted with a bearing for a mixer shaft.

Trial experiments using water proved the reactor configuration

to be inadequate. Ice formed directly over the diffuser, and mixing

was neither sufficiently intense nor the mixer properly positioned to

break up the ice and intimately mix the refrigerant with the reactor

contents.

79

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80

A cylindrical reactor was designed and built to replace the

original reactor. The cylindrical reactor was also constructed of

1/4-inch-thick Plexiglas. It was 5 inches in diameter and 11-1/2

inches tall. Four 1/2-inch diameter ports for temperature

measurement, vacuum measurement and maintenance, and nüscellaneous

other purposes were located along the sidewall. An integral diffuser

similar in construction to the diffuser in the rectangular reactor

was tooled in the base of the reactor. The channels formed a spiral

instead of a rectangular grid.

Another four ports, 1/4-inch in diameter, were located in the

reactor cover. Like the original reactor, the new reactor cover also

supported a mixer shaft bearing plate.

The reactor was modified several times during the course of

preliminary studies. Four 1/4-inch wide baffles running the height

of the reactor, equally spaced along the side wall, were added to

improve mixing. Modifications were also made to the mixer bearing to

accomodate a larger mixer.

Both reactors were originally equipped with integral diffusers.

However, these diffusers were prone to clogging. Distribution of

refrigerant was uneven, with most issuing from openings along the

wall of the reactor, promoting flash freezing at the wall. when the

original diffusers were found to be unsuitable, a number of different

diffuser types and configurations were tested. An aluminum base was

constructed with channels similar to those in the original reactor

base. Instead of entering the reactor through small holes, the

81

refrigerant flowed into the reactor through fine, spherical porous

diffusers. However, combined head loss through the channels and the

diffusers proved too great, causing liquid butane to vaporize before

emerging from the diffusers. Similarly, discs of finely divided

brass, set into the base, created too much head loss in the system.

Since the aluminum base was a large cold sink, it was ultimately

abandoned. The original reactor base was replaced by an unperforated

Plexiglas disc. Refrigerant was fed into the reactor through one of

the sidewall ports. 0n the inside of the reactor, a short length of

1/4—inch-diameter Tygon tubing connected the port fitting to a

pyramidal coarse bubble diffuser. This diffuser tended to ice,

especially at long detention times, and was replaced in later

experiments by a coil of I/4-inch copper tubing perforated with

0.012-inch holes.

Mechanical mixing was provided in all reported experiments to

promote intimate contact between sludge and refrigerant or hydrating

agent. In initial studies, a small laboratory mixer with a maximum

speed of I00 rpm was used. This proved insufficient to prevent the

formation of solid ice on the reactor surfaces, along the mixer shaft

and blades, and on the diffuser itself.

This mixer was eventually replaced with a Sears I3-inch,

5-speed, 3.0-hp drill press. The drill press not only unade high

speed mixing up to 2000 rpm possible, but also provided a stable‘

support for the reactor. However, while higher speeds reduced

diffuser icing significantly and prevented formation of solid ice

82

along the reactor walls and mixer shaft, they also resulted in

deterioration of sludge dewatering characteristics. A constant

mixing speed of 400 rpm was selected as a result of a series of

mixing tests on samples of RAAP alum sludge. At this speed, both the

negative effect on dewatering characteristics and the formation of

ice on surfaces were minimized.

Heat Exchange Systems

Heat exchange systems were necessary to maintain reactor and

refrigerant temperatures at optimum levels for butane and Freon

conditioning during experiments. Sludge samples were precooled to

minimize the use of refrigerant for this purpose. Systems for

temperature and vacuum/pressure maintenance and methods of sludge and

refrigerant precooling are discussed in the following subsections.

Temperature and Vacuum/Pressure Maintenance - To maintain

desired temperatures, the reactor was wrapped with uninsulated Tygon

tubing and encased by a two-inch-thick styrofoam jacket. Cooled

ethylene glycol was pumped from the well of a refrigerating bath

(Master Scientific Formaline Model 2095 Bath and Circulator) through

Rubatex-insulated tubing, around the reactor and back to the

reservoir of glycol.

Process temperature was maintained between -1 and +100 during

butane experiments. The temperature generally dropped over the

course of an experiment from the range of +0.5 to +100, to the range

of -0.5 to -100 as the sludge froze. A 4-cm Hg vacuum was

83

maintained in the reactor during butane experiments using a Gast air

compressor (Model DOA-P106-AA), piped to produce a vacuum. The

vacuum insured that the butane evaporated at operating temperatures.

For Freon 12 experiments, precooling the reactor in a commercial

refrigerator to 4 to 806 was sufficient to maintain the temperature

at desirable levels. Pressure—temperature conditions during Freon 12

experiments were maintained at points slightly below the vapor

saturation curve (Figure 6). (Hydrates form at all points beneath

this curve and above the gas hydrate formation curve at temperatures

less than 1206. However, reactions rates are more rapid at points

close to the vapor saturation curve.) Process temperature was

generally maintained between 7 and 1006; process pressure was varied

accordingly from 56 to 62 psia (3.9 to 4.3 x 105 Pa), using a Gast

Super Series air compressor (Model ROA-P101-AA). .

After conditioning, the temperature of the sludge was allowed to

rise above 1406 in order to decompose the hydrates. The pressure in

the reactor was subsequently increased to compress the Freon 12 to a

liquid state, as allowing it to bubble through the sludge resulted in

foaming. The liquid Freon floated to the surface and was then

allowed to evaporate.

Refrigerant Precooling - Butane was precooled to temperatures

ranging from -10 to -506 by feeding it through a 1/4-inch-diameter

copper coil (approximately two meters total length) submerged in a

bath of ethylene glycol and ice. Low temperatures were necessary to

prevent premature volatilization of butane due to heat gains in the

84

refrigerant feed system. Excessively low temperatures were avoided

to prevent exacerbating diffuser icing.

Freon 12 was precooled to 2-1/2 to 300 by immersing the copper

coil in ethylene glycol contained in the well of the refrigerating

bath.

Sludge Precooling - Samples of sludge were precooled before

introduction into the reactor. For butane experiments, two-liter

samples of sludge were precooled in a conventional commercial

deep-freeze unit for approximately four hours, until the sludge

temperature dropped to between +0.5 and +100. For Freon 12

experiments, samples were precooled to 4 to 800 in a commercial

refrigerator.

Refrigerant Feed Systems

Two types of refrigerant feed systems were utilized in

preliminary experiments: a conventional liquid system and a

polyaphron system. The polyaphron system was not used in later

experiments due to mechanical difficulties and foaming problems.

Both methods are discussed in the following subsections.

Conventional Liquid Systmn - Butane and Freon 12 were pumped

through the feed system using the compressive force of the

pressurized containers in which the refrigerants were purchased.

Butane pressure varied from 22 psi to 37 psi (1.5 to 2.6 x 105 Pa),

and Freon 12 pressure from 70 to 95 psi (4.8 to 6.6 x 105 Pa),

depending on room temperature. The tanks were tilted or turned

85

upsidedown, placing the feed valve below the bulk of the container

contents to insure that the refrigerant emerged from the cylinder as

a liquid.U

The feed system was constructed of 1/4-inch-diameter Polyflo

tubing, insulated with Rubatex. Liquid refrigerant flowed from the

tank through a precooling copper coil to a Gilmont rotameter for flow

measurement. Butane flow rate was adjusted with a union bonnet

metering valve. Freon 12 flow rate was adjusted with an ultra-fine

low-flow metering valve. A check valve in the line just prior to the

reactor prevented backflow of the reactor contents into the feed

line.

Butane flow rates between 5 and 35 mL/min were used in

preliminary experiments. Based on the results of these experiments,

a flow rate of 20 nm/min was selected for the remaining studies.

Higher flow rates resulted in flash freezing, while it proved

difficult to maintain a constant vacuum at lower flow rates.

Freon 12 flow rates between 5 and 25 mL/min were used in

preliminary experiments. A flow rate of 10 mL/min was selected for

the remaining studies. Higher flow rates resulted in an excessively

foamy product, while lower flow rates resulted in very long detention

times.

Polyaphron System — As an alternative method of introducing

butane into the sludge, experiments were conducted to incorporate

liquid butane into colloidal aphrons. Colloidal aphrons are finedroplets of water—insoluble gas or liquid, each encapsulated in a

86

thin surfactant film72. They exhibit unusual foam-like properties,

including great stability, and have been shown effective in

separation techniques73. Potential benefits in using liquid butane

in a polyaphron included storage and feeding at ambient pressure and

improved contact between butane and sludge.

The standard procedure used to produce aphrons is to add an oil

to a surfactant in solution. Initially one or two drops are added at

a time, with vigorous shaking between additions. Once a milky foam

begins to form, indicating the presence of aphrons, larger quantities

of oil are added.

The ultimate ratio of oil to water is approximately 10:1. Thus,

if 500 mL of oil aphrons are desired, 500 mL of the selected oil are

added to 50 mL of the surfactant solution.

Certain new techniques had ·u¤ be developed to produce butane

aphrons since liquid butane vaporizes at ambient temperature and

pressure. To maintain butane as a liquid at atmospheric pressure,

its temperature must be maintained below 000. Glycerol must be added

to the surfactant solution to prevent it from freezing during

production and storage.

The procedure developed was as follows:

1. Three to four gallons of ethylene glycol were chilled to

-2000. A plastic container and a dropper bottle were also·

chilled. The plastic container was weighted so that itcould not float when submerged in the glycol.

87

2. Two coolers were prepared to hold the glycol. One gallon

of the chilled glycol was poured into one cooler, and the

rest into the second cooler.

3. A solution of 5 g/L sodium dodecyl benzene sulfonate (SDBS)

was prepared. To make 250 mL of oil aphrons, 25 mL of 5

g/L SDBS, 10 mL of glycerol, and 2.5 mg of lauric acid were

added to the plastic container. Solution temperature was

allowed to decrease to -1OOC in the larger cooler.

4. Liquid butane was fed into the dropper bottle until it was

three-quarters full. The dropper bottle was then attached

to the side of the cooler that contained the gallon of

glycol, with the bottle immersed in the glycol.

5. The butane was added to the surfactant solution with the

dropper one drop at a time, with vigorous shaking

maintained for about one minute between drops. After five

drops were added in this manner, two to three drops could

be added at a time. The amount of butane added was

gradually increased to 5 mL. However, this much was not

added until the solution had a thick, milky appearance.

Adding too much butane too soon caused the aphrons to

destabilize and fall apart.

88

6. Once formed, the aphrons were stored in a commercial

deep-freeze unit.

It was discovered that too low a storage temperature caused the

surfactant solution to freeze and settle to the bottom, leaving the

liquid butane floating at the surface. If the solution was

vigorously shaken and warmed so that the solution thawed, the aphrons

reformed, as long as the temperature did not rise above OOC.

Based on this discovery, a new method was devised to produce the

aphrons. The surfactant solution, glycerol, and lauric acid were

added to a plastic container. Butane was added gradually as

described above until four to five drops of butane were added at a

time, with vigorous shaking between additions. The solution was then

put in a freezer and the surfactant solution allowed to freeze. The

aphrons reformed after five to ten minutes of vigorous shaking.

Relatively large amounts of butane (up to 1O mL per addition) were _

then added to produce the desired quantity of aphrons.

The butane aphrons proved difficult to pump. Contact with the

pump and tubing, which could not be maintained satisfactorily below

OOC, caused the aphrons to volatilize prematurely. Similar problems

with premature volatilization were encountered with gravity feed.

Conditioning Agent Removal

Various techniques were used during the early stages of this

study in an attempt to remove spent butane and Freon 12 from the

89

product sludge. Neither vacuum stripping nor aeration resulted in

any observable improvement in dewatering characteristics for either

butane- or Freon—conditioned sludges.

Overnight storage at 40OF, on the other hand, improved the

dewatering characteristics of both butane- and Freon-conditioned

sludges. Product sludges, which in general exhibited very poor

settling characteristics immediately after thawing, usually separated

into sludge and supernatant fractions with storage. Certain

butane-conditioned samples produced a third fraction, a stable,

high-solids foam with poor dewatering characteristics.

ANALYTICAL METHODS

Table 7 lists analyses and determinations used to quantify the

results of conditioning experiments. Descriptions of analytical

methods are presented in this section.

Particle Size Distribution

Particle size distributions of sludge and supernatant samples

were analyzed to gauge any changes which occurred during conditioning

which might impact dewatering characteristics. Particle counts were

made using a HIAC Particle Size Analyzer Model PC-32074. The sensor

used in this study was a HIAC Standard Sensor Model CMB-300, which is

capable of detecting and counting particles in the range of 5 to 300

microns (um).

90

TABLE 7I

EXPERIMENTAL ANALYSES AND DETERMINATIONS

Analysis/Determination Reference

Total Particle Counts 74

Particle Size Distributions 74

Weight Fractions 75

Specific Resistance 6

Blinding Index 9

Vacuum-Filtered Cake Solids Concentration 75

Initial Solids Concentration 75

Centrifuged Solids Concentration 75

Floc Density 76

Supernatant TOC 75

Sludge pH 75

91

Samples were diluted, if required, to meet the particle

concentration limit of the sensor. If available, filtered

supernatant was used to dilute samples. If not, tap water was used

for dilution. Usually sludge samples were diluted to 1/1000, and

supernatant to 1/10, if at all. Three 10-mL aliquots of each sample

were analyzed for particle size distribution, and the results of the

three averaged.

Results are reported as totals per "mean particle diameter

(MPD)." MPD was calculated by averaging the end points of each size

range (e.g., if the particle sensor limits for a given range were set

at 35 and 45 um, the MPD is 40 um.) (This nomenclature is not

intended to imply that particles are spherical or that the

measurement reported is a characteristic length as determined by

Knocke and wakelandlo. Rather it is a convenient term to describe

the dimension of the particle sensed by the analyzer.)

Prior to analysis, samples were filtered through a brass screen

(250 um pore openings) to prevent clogging of the sensor. A sample

of the filtrate was weighed before and after drying at 103OC and the

percentage of solids passing through the filter determined (in

accordance with Section 209A of Standard Methods75. This calculated

value was reported as percent of sludge solids passing a 250-um

sieve.

92

Specific Resistance, Initial and Vacuum-filtered Solids

Concentrations, and Blinding Coefficients

Specific resistance was calculated using the results of time vs.

volume data obtained using Grade 40 filter paper (8 um particle

retention) in a Buchner funnel apparatus as described in Vesilindö.

A vacuum of 20 in Hg (508 mm Hg) was drawn on the filtering apparatus

during the procedure. At cake failure, a sample of the sludge cake

was scraped from the filter, and the dry solids concentration

determined according to procedures described in Section 209A of

Standard Methods75. If no cake was produced within 30 minutes, a

representative cake solids concentration was used to calculate

specific resistance. Generally, this value was that determined for

the cake produced at the lowest conditioning dose for which a cake

formed. Initial dry solids concentration was determined according to

the same procedure.

The blinding coefficient, as defined by Huangg, was determined

by plotting the natural logarithm of the time and volume data from

the Buchner funnel test and calculating the slope of the resultant

line.

Centrifuged Solids Concentration

Samples of sludge were distributed equally between 2 plastic

50-mL test tubes and placed in a Beckman Model J-2IC centrifuge.

Samples were centrifuged at 9000 rpm for ten minutes. Supernatant was

then decanted, the solids removed from the tube, and the dry solids

93

concentration of the resultant cake determined by drying at 1030075.

Floc Density

The floc density of selected sludge samples was measured to

determine if conditioning had any effect on the relative water

content of flocs. Density was analyzed using the method described by

Vollrath-Vaughn76. Small droplets of sludge were introduced beneath

the surface of stratified layers of various concentrations of Percoll

contained in a 10-mL test tube. (Percoll is a polymer-coated silica

sol manufactured by Pharmacia Fine Chemicals, which can be

concentrated to specific gravities of up to 1.3.) The test tubes were

centrifuged in an International Clinical Centrifuge Model CL at 850

RPM for two minutes. Upon removal, the samples were observed and the

location of flocs noted. Flocs tended to concentrate either within a

particular gradient step or between two adjoining gradient steps.

Commonly, particles concentrated in bands at several locations,

indicating a range of densities. The samples were returned to the

centrifuge for another two minutes and then removed. Location of

floc bands was again noted.

Total Organic Carbon

The total organic carbon (TOC) content of supernatant samples

was measured to assess the (effect of conditioning on supernatant

quality. TOC was analyzed using a Horiba Total Organic Carbon

Analyzer Model PIR-2000. This analyzer utilizes the

94

persulfate-ultraviolet oxidation method described in Section 505A of

Standard Methods75.

Sludge pH

The pH of sludge sampies was measured using a Fischer Scientific

Accumet pH meter Model 610, according to the procedure described in

Section 423 of Standard Methods75.

95

IV. RESULTSl

The results of direct-contact, freeze-thaw conditioning

experiments are presented together with the results of polymer and

indirect—contact experiments in this chapter. Each of the following

sections is devoted to a specific sludge.

STROUBLES CREEK WASTE ACTIVATED SLUDGE

Samples of unconditioned waste activated sludge obtained from

the Stroubles Creek Wastewater Treatment Plant dewatered rapidly to a

moderate solids concentration. As the specific resistance of the

sludge was already low prior to conditioning, the main benefit

anticipated from conditioning was an increase in dewatered solids

concentration.

Polymer conditioning typically increases filtration rates while

having little effect on final dewatered solids concentrations.

Freeze—thaw conditioning, in contrast, was expected to produce

significant increases in solid concentration. Certain improvements

in filtration rates were also anticipated.

Polymer Conditioning

Samples of Stroubles Creek waste activated sludge were

conditioned with 10, 20, 30 and 50 mg/L Betz 1160. There was little

96

change in dewatering characteristics for any polymer dose. The

primary conditioning effect of polymer addition was an increase in

sludge floc size. The limited improvements observed when polymer was

added to this particular sludge suggested that an increase in floc

size does not automatically result in significant improvements in

sludge dewatering characteristics.

The results of analyses and determinations on

polymer-conditioned samples of waste activated sludge are presented

in Table 8.

Particle and Chemical Characteristics - The main effect of

polymer conditioning on waste activated sludge was a shift in

particle size distribution to size ranges with larger mean particle

diameter (MPO). As can be seen from Figure 8, conditioning reduced

the total number of particles with MPD less than 55 pm, especially in

the size ranges with MPD less than 30 pm. Reduction in the

percentage of small particles varied consistently with polymer dose.

The greatest reduction was achieved at the highest polymer dose: the

number of particles with MP0 less than 55 tun was reduced froni 95

percent for the unconditioned sample to 84 percent for the sample

conditioned with 50 mg/L polymer. Also, 10 percent of the particles

in this sample were retained when filtered through a 250-pm sieve,

compared to no measurable percentage for all other samples. These

results indicate a substantial shift towards larger sizes in the4

distribution of particles.Polymer conditioning had little effect on the supernatant, other

97

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99

than slightly reducing the total number of particles with MP0 less

than 250 um.

Dewatering Characteristics - The overall effect of polymer

conditioning on the dewatering characteristics of waste activated

sludge was minimal. The dewatered solids concentration obtained from

both vacuum filtration and centrifugation decreased slightly for all

conditioned samples. Contrary to expectations, specific resistance

did not decrease with increasing polymer dose or with increases in

particle size, showing instead only slight variations from the value

calculated for the unconditioned sludge. All calculated specific

resistance values were less than 100 x 10ll m/kg.‘ The altered appearance of sludge flocs observed during floc

density analyses suggested that; the decreases in dewatered solids

concentration resulted from retention of previously free water within

the polymer-enhanced floc structure. when polymer was added to the

sludge, formerly fine, isolated flocs aggregated together into large,

amorphous clumps. As small amounts of free water were incorporated

into the growing aggregates, the amount of moisture available for

extraction by dewatering was reduced, resulting in decreased

dewatered solids concentration.

Other parameters were unaffected by the restructuring of

particle groups. Free water still drained through the new structures

at similar rates, as measured by the specific resistance, and changes

in the amount of water associated with the particle were not

sufficient to affect floc density.

T 100

Changes in blinding coefficient supported the indications of

increased sludge floc size observed in particle size data. Three of

the four conditioned sludges had negative blinding coefficients in

contrast to the unconditioned sludge, which had a blinding

coefficient of zero. The decrease in this parameter may have

resulted from an increasing homogeneity in particle size, as

particles were bound together by polymer, as suggested by Notebaert5.

Indirect Contact, Freeze-Thaw Conditioning

The major effects of indirect-contact, freeze-thaw conditioning

of Stroubles Creek waste activated sludge on parameters of interest

were large increases in initial and vacuum-filtered solids

concentration and hi floc density. These changes signalled major

alterations in floc structure and substantial decreases in floc water

content. There were also moderate increases in specific resistance,

most likely related to increases in solids content.

Results of these and other analyses and determinations on the

sample of indirect-contact, freeze—thaw conditioned waste activated

sludge are presented in Table 9.

Experimental Observations — Certain distinct changes in the

appearance and gross settling characteristics of indirectly frozen

waste activated sludge were observed. After thawing, the conditioned

sludge settled rapidly, and a large volume of supernatant was

decanted easily. The freeze-thaw solids were large, grainy, and

dense, similar in appearance to freeze—dried coffee granules.

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Particle and Chemical Characteristics - The overwhelming

difference between the particle distributions of indirect-contact,

freeze-thaw conditioned and unconditioned samples of waste activated

sludge, depicted in Figure 9, was in the total number of particles

represented by the distributions. The conditioned sludge sample had

20 percent fewer particles with MP0 less than 250 um. Similar

reductions in the number of particles in the supernatant with MPD

less than 250 pm were noted.

The small particles eliminated by conditioning from the

distributions measured by the particle counts constituted part of the

mass of larger particles retained on a 250-um sieve. Particles with

MPD greater than 250 um represented 76 percent by weight of the total

solids of the conditioned sludge sample, compared to the

unconditioned sample, which had no measurable percentage in that size

range.

Changes iri waste activated sludge ini with freezing indicated

that physical disruption of particles during freezing may have

occurred. Sludge pH increased from 7.0 for the unconditioned sample

to 9.3 for the conditioned sample. Chemically neutral particles

might have broken apart during freezing to release alkaline

components into the supernatant, causing the sludge pH to increase.

Indirect freezing resulted in a large increase in floc density,

from 1.03 to 1.05 g/L. The increase in density was readily apparent

from the greatly improved settling characteristics of the freeze-thaw

solids, as well as the appearance of individual particles. Mass

103

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FIGURE 5. PFIRTICLE SIZE DISTRIBUTIONS. INDIRECT-CONTRCTFREEZESTHFIU CONDITIONED HRSTE HCTIVFITE0 SLUDGE.

104

balance calculations showed that this represented the release of 53

percent of the floc water.

Dewatering Characteristics - Indirect-contact, freeze-thaw

conditioning of waste activated sludge resulted in a large increase

in initial and vacuum-filtered dry solids concentration and moderate

increases in specific resistance. The process released a large

amount of water as supernatant, increasing the initial solids

concentration of the sludge over 300 percent. Vacuum-filtered solids

increased by 51 percent, and centrifuged solids posted a smaller but

still sizable increase of 15 percent.

The concomitant increases in dewatered solids concentration and

floc density were probably related. Arundel77, in her study of the

dewatering behavior of sludges exposed to high pressures, observed

increases in floc density with increased dry solids above a

sludge-specific concentration. She concluded that internal water was

released from the flocs, causing floc density to increase, as

pressure deformed the floc matrix. The intense pressures developed

during freezing have been suggested as possible causes of similar

results by other researchers63’78.

Specific resistance more than doubled with indirect freezing,

although the value calculated for the conditioned sample was still

moderate (120 x 1011 m/kg). Increased solids content has been shown

to increase specific resistancea, and it is likely that the large

change induced by freezing in initial solids affected the sludge's

response to filtration.

105

The change in blinding coefficient with indirect-contact

freeze-thaw conditioning suggested that conditioning enhanced the

tendency of the sludge cake to blind,. The blinding coefficient

increased from zero for the unconditioned sludge to a positive value

for the conditioned sludge. The usual interpretation of a positive

blinding coefficient---the blocking of cake pores by small

particles---was not suggested by particle size analysis data, which

showed a definite increase in particle size. Considered with the

increased specific resistance, the positive blinding coefficient may

have indicated changes in cake structure. Under vacuum, larger flocs

may have compressed, resulting in a more tightly packed cake less

conducive to drainage.

Direct-Contact, Freeze—Thaw Conditioning with Butane

One-L samples of Stroubles Creek waste activated sludge were

conditioned with 20 mL/min butane for contact times of 20, 30, and 40

minutes, for total butane doses of 400, 500 and 800 mL/L,

respectively. Direct-contact freeze-thaw conditioning <yf Stroubles

Creek waste activated sludge with butane produced no significant

improvements in dewatering characteristics. 0nly small increases in

centrifuged solids were observed. Changes in vacuum-filtered solids

were variable, and specific resistance increased for all butane

doses.

Indirect freezing of waste activated sludge also resulted in

increased centrifuged solids and increased specific resistance.

106

Evaluation of particle size distributions suggested, however, that

different mechanisms were responsible for similar results.

Analyses were performed on unconditioned and conditioned samples

of waste activated sludge conditioned with butane to determine

particle and dewatering characteristics. Results of analyses and

determinations are presented in Table 10.

Experimental Observations — Conditioning produced a foamy,

poorly settling product sludge. The foam, high in solids content and

difficult to dewater, was stable, dissipating only slightly with

gentle mixing.

Particle and Chemical Characteristics - The main result of

direct-contact, freeze-thaw conditioning of' waste activated sludge

with butane was an increase in the number of small particles in the

product sludge. The changes in particle size distribution were not

likely caused by the conditioning mechanisms of the freeze-thaw

process alone, however, but influenced by other factors.

The particle size distribution for the sludge conditioned with

400 mL/L butane, shown in Figure 10, is representative. The

distribution of the conditioned sludge sample, similar in shape and

magnitude to that xyf the unconditioned sample, comprised a nmch

larger number of particles in size ranges with mean particle diameter

(MPO) less than 25 pm. The total number of sludge particles with MPD

less than 250 pm nearly doubled for the 600 mL/L and 800 mL/L butanel

doses.

Conditioned supernatant samples also had more small particles

107

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FIGURE 10. PRRTICLE SIZE DISTRIBUTIONS. WRSTE RCTIVHTEDSLUDGE CONOITIONED WITH 100 ¤•L1L BUTFINE.

109

than unconditioned supernatant samples. The particle size

distribution of the conditioned samples comprised a. much greater

number of particles in size ranges with MPD less than 30 pm, and

exceeded the unconditioned distribution in all size ranges with MPD

less than 50 um-

The changes observed in the particle size distributions of

conditioned samples may have been caused by (1) the reduction of

particle size during freezing, either through compression or

disruption; and/or (2) floc shear induced by mixing applied during

experiments. Compression of particles was not substantiated by floc

density measurements, which did not change with conditioning.

Certain evidence of particle disruption was observed in the results

of indirect freezing of waste activated sludge, although the particle

size data from those experiments seemed to indicate that these

fractionated particles reaggregated upon thawing. If the butane

conditioning experiment likewise resulted in particle disruption, it

is possible that reaggregation was prevented by the foam generated

during the experiment. Mixing could also have contributed to reduced

particle size through floc shear or by preventing smaller particles

created by freezing from reaggregating. (Mixing tests of equal

intensity and duration conducted in the experimental apparatus used

in this study produced similar changes in particle size

distributions.)

Increases in supernatant TOC were noted for all conditioned

samples. The increases for the samples conditioned with 600 mL/L

110

butane were moderate, while the increase for the sample conditioned

with 400 mL/L was excessively large. Moderate increases in TOC

concentration might be attributed to any of several causes, including

the release of organics as particles ruptured during freezing, the

suspension of fines in the supernatant as a result of mixing, or

entrainment of butane. The large TOC increase observed at the lowest

butane dose was likely due to the entrainment of excessive amounts of

butane in the sludge during this experiment, which generated large

amounts of foam.

Dewatering Characteristics - There was no overall improvement in

the dewatering characteristics of butane-conditioned waste activated

sludge. while centrifuged solids increased slightly for all butane

doses, the response to vacuum filtration was poor. Specific

resistance increased for all butane doses, Final vacuum-filtered

cake solids increased only slightly with the application of 400 mL/L

butane and decreased with the application of 600 mL/L and 800 mL/L

butane.

The deterioration of specific resistance with conditioning is

shown in Figure 11. The increase in specific resistance in the

indirect·contact, freeze-thaw experiment was attributed to the

increased solids content, which resulted from the release of internal

water. Thus the change in specific resistance for indirectly frozen

sludge, though undesirable, was assumed to be part of the

conditioning effect of the freeze-thaw process.

111

300I.! ··

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FIGURE 11.. SPECIFIC RESISTFINCE V5 BUTRNE DOSE.BUTRNE-CONDITIONED VRSTE RCTIVFITED SLUOGE.

112

In contrast, changes in particle size distribution in the

direct-contact, freeze-thaw experiments suggested that the increase

in specific resistance for butane-conditioned sludge was related to

process variables independent of. or extraneous to the actual

conditioning process, i.e., mixing and foaming.

In considering the causes of the increases in specific

resistance, it is interesting to note that higher butane doses

resulted in relatively lower specific resistances. In addition, a

positive blinding coefficient was calculated for the sludge treated

with 400 mL/L butane, in contrast to the other samples, conditioned

and unconditioned, which showed no evidence of cake blinding. Since

higher butane doses were administered over longer contact times, and

thus longer mixing times, specific resistance would be expected to

increase with butane dose if deterioration was due to mixing alone.

The decrease observed in specific resistance with high butane dose

suggests that (1) deterioration of dewatering characteristics was not

caused by mixing alone, but by some other factor as well; and (2)

higher butane doses and/or longer mixing times mitigated the negative

effect of this other factor.

Certain observations suggest that the foam generated during the

process was the factor. First, the foam, produced whether mixing was

provided or not, tended to dissipate at longer mixing times.

Therefore, foam would be more likely to affect specific resistance at

shorter contact times. Second, the characteristics of foam make it a

likely cause of poor dewatering.

113

The changes in vacuum-filtered solids---an increase at 400 mL/L

butane and decreases at 600 mL/L and 800 mL/L butane---suggested that

the extent to which conditioned sludge dewatered upon vacuum

filtration depended less upon the foam content of the sludge than did

filtration rate. The variability in size and direction of these

changes precludes any conclusions as to their meaning.

The slight increase in centrifuged solids probably resulted from

the separation of previously bound water from sludge solids as flocs

were broken apart by freezing pressures or by inixing turbulence.

Certain factors, such as altered particle size distributions and the

flow—inhibiting characteristics of the foam, which prevented the

removal of this water by vacuum filtration, did not affect its

removal by centrifugation, which tends to enhance foam drainage. ·

In summary, butane conditioning of waste activated sludge

resulted in a poorly dewatering product. The effects of mixing and

of foaming were severe enough to counteract any beneficial effect of

the freeze—thaw process. Longer conditioning times resulted in less

deterioration of specific resistance values, but no overall

improvement in dewatering characteristics.

Results were disappointing when compared to the increases in

vacuum-filtered solids obtained with indirect freezing, but it must

be recalled that neither polymer conditioning nor indirect freezing

improved specific resistance, and that polymer conditioning actually

resulted in small decreases in the vacuum-filtered cake solids

concentration.

114

Direct-Contact, Freeze-Thaw Conditioning with Freon 12

0ne-L samples of Stroubles Creek waste activated sludge were

conditioned with Freon in batch and continuous experiments. In the

batch experiments, samples were treated in one- and two-stage

conditioning, with 250 mL Freon applied at each stage. In the

continuous experiments, samples were fed 10 mL/min Freon for contact

times of 25, 50, and 75 minutes. Thus the samples treated in

one-stage batch and 25-min continuous experiments were fed 250 mL/L

Freon; the two-stage batch- and 50-min continuously conditioned

samples, 500 mL/L; and the three-stage batch- and 75-min continuously

conditioned samples, 750 unL/L. It should be noted as well thatC batch-conditioned samples were mixed for very short periods at the

beginning of each experiment to insure intimate contact with the

refrigerant, while continuously conditioned samples were mixed

throughout the defined contact time.

The effects of direct-contact, freeze-thaw conditioning of

Stroubles Creek waste activated sludge with Freon 12 on dewatering

characteristics were varied. Conditioning resulted in large

increases in vacuum-filtered solids and smaller, but still sizable,

increases in centrifuged solids. Increases in specific resistance

were very large. Changes in particle distributions were not

straightforward, and were likely influenced by handling

inconsistencies.

Although increases in solids concentration were similar to those

obtained in indirect freezing, changes in particle characteristics

115

for Freon-conditioned samples suggested that mixing and foaming

effects interfered with the rate of dewatering.

The results of analyses and determinations from batch and

continuous experiments on Freon-conditioned samples of waste

activated sludge are presented in Tables 11 and 12, respectively.

Experimental Observations - The Freon-conditioning process

resulted in a foamy product sludge; the foam, however, was not

produced in the large quantities noted in the butane-conditioning

experiments, nor was it stable, tending to disperse with overnight

storage.

Particle and Chemical Characteristics - Unlike the changes

noted as a result of polymer addition, indirect freezing, and direct

freezing with butane, changes in particle size distributions for

Freon-conditioned waste activated sludge samples followed no distinct

trend.

In general, particle size distributions for both batch- and

continuously conditioned samples were very similar in shape and

magnitude to those of unconditioned samples, as can be seen from

Figures 12 and 13. All continuously conditioned samples had a

greater number of particles with mean particle diameter (MPO) of 10

pm, suggesting that, as for butane conditioning, there was some

mixing-induced floc shearing.

The total number of particles with MPD of 250 pm or less in

Freon-conditioned samples generally decreased, however, in contrast

to the sizable increases observed in butane-conditioned samples.

116

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119

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120

This suggests that there may have been an increase in the number of

particles with MPD between 55 and 250 um. (There was no measurable

solids fraction with MPD greater than 250 pm.)

Batch-conditioned supernatant samples had a very large total

number‘ of particles, up to seven times that <rf the unconditioned

samples. Most continuously conditioned samples also had large

numbers of particles in the supernatant.

Supernatant particle size distributions had two distinct shapes.

The first decreased steadily, at a decreasing rate, as particle size

increased, and was similar to distributions observed in other

experiments for both conditioned and unconditioned samples. The

second was relatively flat for size ranges with MPD less than 35 um,

and then decreased in a similar manner to the first distribution.

Each of the two distributions were observed for both conditioned and

unconditioned samples, and the second was characteristic of

supernatant which has been allowed to settle, suggesting that these

differences were the result of unintentional inconsistencies in

sample preparation prior to particle analysis.

J There were small decreases in floc density noted for both batch

and continuously conditioned samples. These decreases would imply

the association of previously unbound water with sludge flocs, if

that conclusion were not contradicted by the increase in dewatered

solids concentrations discussed below. Given the increase in

dewatered solids, it is more likely that any decrease in floc density

resulted from the association of Freon bubbles with the flocs, giving

121

rise to the foam observed in all samples.

TOC increased moderately with conditioning, from values of 8 to

12 mg/L for unconditioned samples, to values of 34 to 54 mg/L for

conditioned samples. TOC tended to increase with increasing total

Freon volume applied. TOC had also increased for butane-conditioned

samples, and it was speculated that the source of a certain fraction

of the TOC was entrained butane. Unlike butane, however, Freon is

not measured by TOC analyses. The increase in organic carbon must

have originated from a different source, either from particles

ruptured during freezing or as fines produced by mixing.

Dewatering Characteristics - There were increases in

vacuum-filtered and centrifuged solids respectively for certain

batch- and continuously Freon-conditioned samples of waste activated

sludge. Vacuum-filtered solids increased 63 percent for one-stage

conditioning, approaching the level of improvement established by

indirect conditioning. Continuous conditioning with 250 and 500 mL/L

Freon resulted in smaller increases. In contrast, the sample

conditioned with 750 mL/L Freon produced no cake at all in 30 minutes

of dewatering.

Centrifuged solids concentration increased by percentages

similar to that obtained with indirect freezing. Smaller increases

were obtained with higher doses, whether administered continuously or

in stages. Improvement in dewatered solids for indirectly frozenO

sludge was attributed to the removal of floc moisture. As water was

squeezed from the floc, the floc became more compact and, thus, more

122

dense. According the the alternative interpretation, particle

structuress were broken apart, releasing water, and then recoalesced

in new floc structures of higher density. In either case, increases

in dewatered solids were accompanied by increases in floc density.

As suggested above, the decrease in floc density observed in

Freon-conditioned samples was likely the result of the foam formed by

the Freon bubbling through the sludge during the freeze-thaw process.

The conditioning effect of the Freon might have resulted in net

increases in density, had the conditioning process not been

compromised by the effects of foaming.

As can be seen from Figure 14, Freon conditioning of waste

activated sludge resulted in large increases in specific resistance,

which varied directly with Freon dose. Increases were very large for

Freon doses of 500 mL/L or more. The order-of-magnitude increases in

specific resistance, which worsened with higher Freon dose for both

batch- and continuously conditioned samples, were not likely the

results of changes in particle size distributions, which were

minimal, but more likely caused by the inclusion of Freon bubbles in

the sludge as foam. The increase was greatest for the two-stage

batch-conditioned sample, mixed only intermttently, than for the

continuously conditioned sample also treated with 500 mL/L, mixed

throughout the contact time. This is consistent with the

interpretation of the interplay of mixing and foaming developed in

the results of butane conditioning of waste activated sludge reported

in the preceding section; that is, longer mixing times ameliorated

123

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00 200 400 600 600

FREON 005E. ¤L/L

FIGURE 11. SPECIFIC RESISTFINCE VS FREON DOSE.FREONCONDITIONED URSTE RCTIVRTE0 SLUDGE.

124 _

the negative effects of foaming.

STROUBLES CREEK PRIMARY SEWAGE SLUDGE

Samples of unconditioned primary sewage sludge from Stroubles

Creek wastewater Treatment Plant dewatered poorly. Not only did the

sludge filter very slowly, with high specific resistance, but no cake

formed during the Buchner funnel test. Conditioning was expected to

improve filtration rates and cake quality.

Polymer Conditioning

Samples of dry solids Stroubles Creek primary sewage sludge werel conditioned with 10, 20, 75, 100, 150, and 200 mg/L Betz 1160.

Polymer conditioning was successful in greatly decreasing specific

resistance, as well as producing a cake of moderate solids

concentration upon vacuum filtration at higher polymer doses. The

results of analyses and determinations on conditioned and

unconditioned samples of primary sewage sludge are presented in Table

13.

Particle and Chemical Characteristics - Polymer conditioning of

Stroubles Creek primary sewage sludge resulted in reductions of the

total number of particles with mean particle diameter (MPD) less than

250 um, as determined by particle counts, and in decreases in the

relative number of very small particles (MPD = 10 um).

As can be seen from Table 13, there was a steady decrease with

125

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increasing polymer dose in the total number of particles represented

by the particle distributions of sludge samples. There were only

slight changes in the weight fraction of total solids retained on a

250-um sieve in samples conditioned with less than 100 mg/L polymer;

however, samples conditioned with 100 mg/L or more showed large gains

in this fraction. Successively greater doses of polymer resulted in

fewer numbers of particles in each analyzed size range, especially in

size ranges with MPD less than 25 pm, as can be seen from Figure 15.

Like sludge samples, conditioned supernatant samples had many

fewer particles than the unconditioned supernatant sample, especially

in the size range with MPD equal to 10 um. The total number of

particles with MPD less than 250 um in most conditioned supernatant

samples did not decrease strictly with increasing polymer dose,

however. Anomalies may have resulted from handling inconsistencies

prior to settling. Large reductions in the number of small

particles (MPD less than 20 pm) in the supernatant at all polymer

doses were noted.

Floc density cM’ the primary sewage solids increased slightly

with polymer conditioning, with the bulk of particles increasing in

density from 1.05 and 1.06 g/L for the unconditioned sample and

samples conditioned with 100 mg/L polymer or less, to 1.07 g/L for

samples conditioned with 150 and 200 mg/L polymer.

Dewatering Characteristics - Dewatering characteristics of

Stroubles Creek primary sewage sludge improved greatly with polymer

conditioning. Specific resistance decreased for all polymer doses,

128

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FIGURE 15. PRRTICLE SIZE DISTRIBUTIONS. POLYMER-CONDITIONED PRIMRRY SEUFIGE SLUDGE.

129

as shown in Figure 16. Also, higher polymer doses succeeded in

producing a vacuum-filtered cake of moderate solids content, and

specific resistance was much reduced. It was likely that the

increasing improvement in dewatering characteristics of primary

sewage sludge conditioned with increasing amounts of polymer resulted

from the changes in particle size distribution; specifically, the

reduction in the number of small particles in sludge and supernatant.

Small particles are generally responsible for cake blinding, and

their presence and interference in the unconditioned sample was

signalled by a positive blinding coefficient.

The reversal of the sign of the blinding coefficient for all

conditioned samples as well as the ability of samples conditioned

with higher polymer doses to form a cake during vacuum filtration

probably resulted from an increase in the size of the particles

constituting the sludge.

Centrifuged solids, dewatered by different mechanisms and less

susceptible to changes in particle size distribution, remained stable

for all polymer doses. Had changes in floc density been associated

with changes hi the water balance, centrifuged solids might have

varied accordingly.

Indirect-Contact, Freeze-Thaw Conditioning

Indirect-contact, freeze-thaw conditioning of Stroubles Creek

primary sewage sludge resulted in a substantial decrease in specific

resistance. Although initial solids concentration more than doubled,

130

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äQ"° 6000xE"x¤uiLJZEan 4000•-•U')I.n.|C!LJI-OLL•-•L'] 2000D.U]

00 25 50 75 100 125 150 175 200POLYMER DOSE. ng/L

FIGURE 1B.” SPECIFIC RESISTFINCE VS POLYMER DOSE.POLYMER-CONDITIONED PRIMRRY SEUFIGE SLUDGE.

131

and particle characteristics showed a shift towards larger, more

dense particles, cake solids concentrations showed no improvement.

In general, indirect freezing was not as effective as polymer

addition in improving the dewatering characteristics of primary

sludge.Results of analyses and determinations for an indirectly frozen

sample and an unconditioned sample of Stroubles Creek primary sewage

sludge are presented in Table 14.

Particle and Chemical Characteristics - Indirect-contact, freeze

thaw conditioning of primary sewage sludge resulted in reduction of

the total number of particles represented by the particle size

distribution and in a decrease of the relative number of very small

particles (mean particle diameter (MPO) = 10 um). As can be seen

from Table 14, the total number of particles represented by the

distribution decreased sharply with conditioning, as did the weight

fraction of total solids passing through a 250-um sieve. As can be

seen from Figure 17, there were many less particles with MP0 equal to

10 mn in the conditioned sludge and supernatant samples, with the

conditioned samples also having fewer particles than the

unconditioned samples in each size range.

Average floc density of indirectly frozen primary sewage sludge

increased from 1.045 to 1.06 g/mL. The range of densities broadened,

with the lower limit extended from 1.055 to 1.03 g/mL, and the upper(

limit from 1.10 to 1.11 g/mL.

The changes in floc density suggested that the increase in

132

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FIGURE 17. PRRTICLE SIZE DISTRIBUTIONS. INDIRECT-CONTRCTFREEZE-THFW CONDITIONED PRIMRRY SEHRGE SLUDGE.

134

particle size was not a simple amalgamation of existing flocs, as

occurred during polymer conditioning. Particle size changes were

more likely caused by destruction of existing floc structures during

freezing, resulting in the freeing of previously bound water, and

ensuing consolidation of released particles during thawing and

storage. This interpretation would explain the change in the range

of floc densities, which increased to encompass both less and more

dense flocs, created as old particle attachments were broken and new

ones formed, and the general increase in floc density, resulting from

the release of floc water.

Dewatering Characteristics — Indirect-contact, freeze-thaw

conditioning had varying effects on the dewatering characteristics of

Stroubles Creek primary sewage sludge. Initial dry solids

concentration increased over 100 percent, from 2.1 to 4.8 percent

solids, with indirect freezing, Neither the conditioned nor the

unconditioned sample, however, produced a cake after 30 minutes of

dewatering. Centrifuged solids actually decreased over 30 percent

with conditioning, from 17 to 11 percent solids.

There was a substantial decrease in specific resistance for the

conditioned sludge. As for the polymer-conditioned sludge, the

decrease was related to the increase in particle size. The decrease

was not, however, as large as that obtained by the higher doses of

polymer in comparative experiments. Changes in floc structure would

account for the decrease in specific resistance and the increase in

initial solids content. Likewise, the change in the blinding

135

coefficient from negative to positive could result from a more

tightly packed, and hence more compressible, cake structure,

facilitated by flocrestructuring.Direct-Cor

act, Freeze-Thaw Conditioning with Butane

A 1-L sample of Stroubles Creek primary sewage sludge was fed 20

mL/min butane for a contact. time of 40 uninutes, resulting in a

cumulative butane dose of 800 mL/L. Direct-contact, freeze—thaw

conditioning of primary sewage sludge with butane resulted in a

moderate reduction in specific resistance. 0ther parameters showed

only slight changes. A shift in floc density range was observed; it

was, however, dissimilar to the change in floc density which occurred

upon indirect freezing.

Results of analyses and determinations on samples of primary

sewage sludge conditioned with butane are presented in Table 15.

Particle and Chemical Characteristics - The major effect of

butane conditioning on the particle size distributions of primary

sewage sludge samples was a change in distribution shape. Although,

as can be seen from Table 15, there was little difference in the

total number of particles with mean particle diameter (MPD) less than

250 um, as determined by particle counts, the conditioned sample had

more particles than the unconditioned sample in size ranges with MPD

less than 20 um, and fewer particles in size ranges with larger MPD.

In addition, the conditioned sample had a smaller percentage of

sludge solids retained on a 250-um sieve.

136

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The quality of the conditioned supernatant sample as evaluated

by particle concentration improved. The distribution of the

conditioned supernatant sample represented fewer particles than that

of the unconditioned supernatant sample in all size ranges, as can be

seen from Figure 18. The number of particles in the size range with

mean particle diameter of 10 um were very close, however, with the

major difference in magnitude noted in size ranges with mean particle

diameter between 15 and 35 um.

The average floc density of primary sewage sludge samples

changed little with butane conditioning, but the distribution of

density was altered. The unconditioned sample had particles ranging

in density from 1.055 to 1.10 g/mL, with most particles forming a

heavy band at 1.045 mg/mL, while particles in the conditioned sample

were concentrated at densities of 1.035 and 1.045 mg/mL. This

narrowing of the range of floc density was in contrast to the

opposite effect resulting from indirect freezing.

Certain changes in particle characteristics, including

deviations from the results of indirect-contact, freeze-thaw

conditioning, can be explained as the influence of rnixing, which

served to mask changes that could be directly attributed to

conditioning.

Dewatering Characteristics - Direct—contact, freeze-thaw

conditioning with butane resulted in a moderate decrease of 22

percent in the specific resistance of primary sewage sludge. Initial

solids content did not increase, as it had for indirectly frozen

138

60000i‘

EJ UNCONDITIONEDQ; O CONDITIONEDE 60000an E3L1J.1L1P-I

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2O .2000 “m 2 2E 2 2 _Z 00 10 20 60 40 50 60

MEQN PQRTICLE 0I6METER„ MICRONS

FIGURE 18. PRRTICLE SIZE DISTRIBUTIONS. PRIMHRYSLUDGE CONDITIONED UITH 800 •L1L BUTRNE.

139

sludge. (Initial solids actually showed a slight decrease, but it isprobably that the difference between the conditioned and

unconditioned samples resulted from the difficulty in obtaining a

representative sample.)

The blinding coefficient increased from zero for the

unconditioned sample to a positive value for the conditioned sludge.

However, the blinding coefficient varied from negative to zero to

positive for different samples of unconditioned primary sludge, so

the change may not have been significant.

The effects attributed to mixing and its effects on parameters

of interest were discussed in detail in reporting earlier results of

experiments with waste activated sludge. Like particle changes,

changes in dewatering characteristics caused by conditioning may have

been masked or neutralized by the effects of unixing. while floc

restructuring caused by the freeze-thaw process probably resulted in

reduced specific resistance, the decrease was not accompanied by the

increase in initial dry solids content and strong change in blinding

coefficient that characterized the indirectly frozen sample.

Direct-Contact, Freeze-Thaw Conditioning with Freon 12

A 1-L sample of Stroubles Creek primary sludge was subjected to

1—stage batch conditioning with 250 mL/L Freon. The main effect of

direct-contact freeze-thaw conditioning with Freon 12 on primary

sewage sludge was a large increase in specific resistance, Other

measurable changes in dewatering and particle characteristics were

140

small, with the exception of a large increase in the total number of

particles in the supernatant.

Results of analyses and determinations on samples of primary

sewage sludge conditioned with Freon 12 are presented in Table 16.

Particle and Chemical Characteristics - There was little change

in sludge particle size distributions of primary sewage sludge

samples conditioned with Freon 12, as can be seen from Figure 19. As

can be seen from the data presented in Table 16, the total numbers of

particles with mean particle diameter (MPD) less than 250 um for the

conditioned and unconditioned samples were similar. As for the

butane-conditioned sludge, however, the number of particles in the

Freon-conditioned sample in the size ranges with MPD less than 20 um

was greater than that in the unconditioned sample, and smaller in

size ranges with larger mean particle diameter. As for

butane-conditioned samples of primary sludge, this change probably

resulted from combined effects of freeze-thaw conditioning and

mixing.

0ddly, the supernatant from the conditioned sample had an

excessively large number of particles, about 10 times that of the

unconditioned sample, reflected in all size ranges of the

distribution.

As it had for indirect-contact, freeze-thaw conditioning with

butane, the range of floc densities of sludge conditioned with Freon

12 increased from a single band at 1.03 g/L, to two bands at 1.02 and

1.04 g/mL. This change, similar to that noted for indirect freezing

141

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MEHN PQRTICLE DI8METER„ MICRONS

FIGURE 13. PFIRTICLE SIZE DISTRIBUTIONS. FREON-CONDITIONED PRIMRRY SLUDGE (BFITCH EXPERIMENTS1

143

of primary sludge samples, was the only indication that some

conditioning effect had occurred.

Dewatering Characteristics - There was a threefold increase in

specific resistance with conditioning, from 7100 to 23000 x 1011

m/kg. The general resistance to dewatering, evidenced by the

decrease in both rate and extent of filtration, may have been caused,

in part, by the increased number of small particles in the

supernatant, which is generally recognized as a factor in poor

response to dewateringg. Since mixing was minimal in batch

experiments, it is likely that these small particles were generated

by the freeze-thaw process.

Neither the conditioned nor the unconditioned sample produced a

cake upon vacuum filtration, while centrifuged solids decreased

slightly (less than 10 percent) with Freon addition.

STROUBLES CREEK PRIMARY SEWAGE SLUDGE/WASTE ACTIVATED SLUDGE MIXTURES

Unconditioned samples of a 50/50 mixture of Stroubles Creek

primary sewage sludge and waste activated sludge showed dewatering

characteristics similar to those of primary sludge. Specific

resistance was high, and no cake formed upon vacuum filtration,

Polymer Conditioning

Samples of a 50/50 mixture of Stroubles Creek primary sewage and

waste activated sludges were conditioned with 10, 30, and 50 mg/L

Betz 1160. Polymer conditioning of the sludge mixture resulted both

144

in greatly decreased specific resistance and in cake formation upon

vacuum filtration. The major effect of polymer addition on particle

characteristics was an increase in floc size and a small increase in

floc density.Results of analyses and determinations on mixtures of Stroubles

Creek sludges are presented in Table 17.

Particle and Chemical Characteristics - Polymer conditioning

increased the size of flocs constituting the mixture of Stroubles

Creek primary sewage and waste activated sludges. As can be seen

from Table 17, the total numbers of particles with mean particleC

diameter (MPD) less than 250 pm decreased with increasing polymer

dose, as did the percent of sludge solids passing through a 250-pm

sieve. The distributions for all sludge samples, conditioned and

unconditioned, were similar in shape, with samples conditioned with

increasing polymer doses having fewer particles in each size range.

The same held true in general for the supernatant samples.

Floc density distributions shifted to represent increased

average densities for polymer—conditioned solids. Particles in the

unconditioned sample banded at densities of 1.02 and 1.04 g/mL.

Samples conditioned with 30 and 50 mg/L showed a slight increase in

particle density, with particles banding at 1.03 and 1.05 g/mL. The

particles in the sample conditioned with 50 mg/L concentrated in a

single band at 1.04 g/mL.

Dewatering Characteristics - The specific resistance of mixtures

of Stroubles Creek primary sewage and waste activated sludges was

145

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greatly reduced by polymer addition, and in contrast to the

unconditioned samples, conditioned samples formed a cake upon vacuum

filtration. Centrifuged solids concentrations decreased slightly.

It is interesting to note that while sludge specific resistance

continued to decrease with increasing polymer dose, the corresponding

cake solids concentration decreased slightly. This decrease was

thought to be related to incorporation into the floc structure of

previously free water by the amalgamation of flocs. (This concept

was introduced and discussed in reporting results of experiments with

waste activated sludge.)

The change in the blinding coefficient from positive to

negative, considered in conjunction with the particle size increase,

suggested that small, potentially cake-blinding particles were

coagulated by the polymer into large, more homogenous flocs.

Indirect-Contact, Freeze—Thaw Conditioning

Indirect-contact, freeze—thaw conditioning resulted in major

changes in the dewatering characteristics of a sample of a 50/50

mixture of Stroubles Creek primary sewage and waste activated

sludges. Sludge specific resistance was substantially reduced, and a

cake of moderate solids content formed upon vacuum filtration.

Indirect freezing increased both the characteristic size and density

of sludge particles.A

Results of analyses and determinations on the indirectly frozen

sample of the sludge mixture are presented in Table 18.

147

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Particle Characteristics - Indirect-contact, freeze—thaw

conditioning of a mixture of primary sewage and waste activated

sludges resulted in a shift to larger particle size. As can be seen

from particle totals in Table 18, the number of particles with mean

particle diameter (MPO) less than 250 um decreased, and,

correspondingly, the weight fraction of sludge solids retained on a

250-um sieve increased. The reduction in particle concentration with

conditioning spread across the particle size distribution (Figure

20), with the conditioned sample having fewer particles than the

unconditioned sample in each size range.

The density distribution of sludge mixture flocs changed with

conditioning. Flocs from the unconditioned sample concentrated at

1.04 g/mL, while conditioned flocs were divided into a light band at

1.05 mg/mL and a heavy band at 1.07 g/mL. The increase in average

density and widening of range were also noted for indirectly frozen

samples of primary sludge.

Supernatant TOC increased substantially with conditioning from

31 to 152 mg/L, while pH decreased from 7.6 to 6.7. The changes in

supernatant TOC and sludge pH suggested, as mentioned in reporting

results of experiments with other sludges, that the freeze-thaw

process disrupted existing flocs, resulting in the release of

intrafloc components.

Dewatering Characteristics — Indirect-contact, freeze-thaw

conditioning of the mixture of primary sewage and waste activated

sludges resulted in a nmch improved response to vacuum-filtration.

149

10000 uT} [I1 UNCONOITIONE0

8000 O CONOITIONE0QU')LuÖ 6000 N1: SUPERNFITQNTCZ 0Q¤ 4000Q 0OCI c 0

ä 2000EE ° „ = _

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FIGURE 20. PFIRTICLE SIZE DISTRIBUTIONS. INDIRECT-CONTFICT FREEZE-THFIU CONDITIONED PRIMRRY SEVFIGEUFISTE FICTIVRTED SLUOGE MIXTURE.

~ 150

The conditioned sampled dewatered with little resistance toia cake of

moderate solids content. Conditioning reduced specific resistance to

less than 10 percent of its value for the unconditioned sample, from

1500 to 130 x 1011 m/kg. The cake formed upon vacuum filtration was

24 percent solids, significantly higher than that produced with any

polymer dose. (Centrifuged solids concentration, on the other hand,

decreased 41 percent, from 13 percent solids for the unconditioned

sample to 8 percent solids for the conditioned sample.)

These improvements, as well as the reversal of the blinding

coefficient from a positive to a negative value, were attributed to

the changes in floc structure drawn from the particle data. Freezing

produced a denser, larger floc structure. This in turn resulted in a

greater amount of water being outside the floc in the free water

fraction, where it was drained more rapidly via vacuum filtration.

Direct-Contact, Freeze-Thaw Conditioning with Freon 12

Samples drawn from two mixtures of equal volumes of Stroubles

Creek primary sewage and waste activated sludges were direct-contact

freeze-thaw conditioned with Freon 12. The mixtures differed in

solids content because of daily variablity in the primary sewage

sludge. The first sludge mixture ("A"), with an initial dry solids

concentration of about 1 percent, was subjected to 250 mL/L Freon 12

administered in a one-stage batch experiment. The second sludge

mixture ("B"), with an initial dry solids concentration of about 3

percent, was subjected to Freon doses of 250 mL/L and 500 mL/L in

151

one- and two-stage batch experiments, respectively.

The primary effect of Freon conditioning on these sludge

mixtures was an increase in specific resistance. The main change in

particle characteristics was an increased number of small particles

and an increase in the range of floc densities. These changes were

likely caused by a combination of conditioning and mixing effects.

The results of analyses and determinations on 1-liter samples of

these 50/50 mixtures of Stroubles Creek primary and waste activated

sludges are presented in Table 19.

Particle Characteristics - In general, Freon conditioning

increased the concentration of small sludge particles in the primary

sewage/waste activated sludge mixture. As can be seen from Table 19,

the concentration of particles with mean particle diameter (MPD) less

than 250 um increased for each conditioned sample, although this

increase was negligible for sludge mixture A. As a rule, the

conditioned samples had more particles in the size ranges with mean

particle diameter less than 20 pm, and fewer particles in size ranges

with larger mean particle diameter (Figure 21). Otherwise, the

particle size distributions for the conditioned and unconditioned

samples were similar in shape. There was an increase with

conditioning in the weight fraction of sludge solids retained on a

250-um sieve for sludge mixture A, and a decrease for sludge mixture

B.As

reported for other freeze—thaw experiments with primary

sludge and primary/waste activated sludge mixtures, changes in floc

152

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FIGURE 21. PFIRTICLE SIZE DISTRIBUTIONS. FREON-CONDITIONEDPRIMRRY SEHFIGE/URSTE HCTIVFITED SLUOGE MIXTURE(BHTCH EXPERIMENTS1

155

density for sludge mixture A indicated increased heterogeneity in

floc characteristics. while flocs in the unconditioned sample

concentrated at a single density, flocs from the conditioned sample

separated into two distinct bands. The unconditioned sample of

sludge B already showed a range of floc densities, and there was no

observable change with conditioning. The difference in the

characteristic floc density profile for the two mixtures probably

stemmed from the varying ratio of solids contributed by the primary

fraction, with inixture B, having the larger percentage of° primary

solids, showing the greater variation in floc density.

The supernatant from conditioned sludge mixture A had an

excessively high number of particles in each size range (Figure 21),

with almost nine times the total number of particles when compared to

the unconditioned sample. Supernatant TOC also increased. These

results were similar to those reported for Freon-conditioned primary

sludge, and seemed to point to combined effects from inixing and

conditioning. The increase in the number of small particles in

sludge inixture /\ outweighed the potential benefits of' the general

increase in particle size, as evidenced by the increase in the weight

fraction of larger particles for sludge mixture A.

Dewatering Characteristics - Direct-contact, freeze—thaw

conditioning of samples of the mixture of primary sewage and waste

activated sludge with Freon 12 resulted in large increases in

resistance to dewatering. Specific resistance values increased with

conditioning from 1500 to 4000 x 1011 m/kg for the sludge mixture A

156

sample, and from 2900 to 11000 and 15000 x 1011 m/kg for the sludge

mixture B one- and two—stage samples, respectively. Neither the

conditioned nor the unconditioned sample produced a cake upon vacuum

filtration, while centrifuged solids decreased slightly with

conditioning.

As for Freon-conditioned primary sludge, deterioration of

dewatering characteristics could be ascribed to higher particle

concentrations in the supernatant resulting from conditioning.

RAAP ALUM SLUDGE

Unconditioned samples of undiluted RAAP alum sludge dewatered to

high solids concentration upon vacuum filtration and centrifugation,

although they were moderately resistant to filtration. It was

anticipated that polymer addition would improve filtration rate, and

that freeze-thaw conditioning might improve both rate and extent of

dewatering.

Effects of Dilution

Diluting different sludges has been observed to induce varying

effects on dewatering characteristics. Ademiluyi et_al.l2 found that

specific resistance increased or decreased with increasing dilution,

depending upon the sludge type and specific range of solids_

concentration. For example, specific resistance gradually increased

for domestic sewage sludge diluted from 7 percent to 3 percent

157

solids, and then decreased rapidly at higher dilutions. water

treatment plant sludge, in contrast, showed a decrease in specific

resistance ·for dilutions frmn 8 to 3 percent solids, and then an

increase at higher dilutions. It; was speculated that at certain

concentrations, conditions were favorable for release of fine

particles, which then blinded the sludge upon filtration.

In this study, certain samples of RAAP alum sludge was diluted

approximately 3 to 1 with tap water prior to conditioning to reduce

solids concentration to about one percent. Dilution had little

effect on the dewatering characteristics of unconditioned sludge.

Specific resistance remained moderate, and the sludge samples

dewatered to a high dry solids concentration. The sludge showed no

tendency towards cake blinding, either with cn· without dilution.

Particle size distributions varied, but there was no evidence of an

increase in the number of smaller particles with dilution.

Polymer Conditioning

Samples of undiluted RAAP alum sludge were polymer-conditioned

with 50, 75, 100, and 125 mg/L Betz 1160. Polymer addition resulted

in substantial decreases in specific resistance and in slight changes

in dewatered solids concentration. Likewise, increases in the

characteristic size and density of sludge flocs were also observed.

The results of analyses and determinations on

polymer-conditioned samples of undiluted RAAP alum sludge are

158

presented in Table 20.

Particle and Chemical Characteristics - The main effect of

polymer conditioning on particle characteristics <rf undiluted alum

sludge was an increase in particle size. As can be seen from Table

20, the weight fraction of sludge solids retained on a 250-um sieve

increased, and the number of particles with mean particle diameter

(MPD) less than 250 um decreased. This shift towards larger particle

size was also apparent across the size ranges represented by the

particle size distributions depicted in Figure 22. For polymer doses

of 100 mg/L and less, there was a decrease in the number of particles

with MPD less than 20 pm, and an increase in the number of particles

with MPD greater than 20 um.

The 125 mg/L dose seemed to represent a threshold value for

substantive conditioning. The total number of particles per mL with

MPD less than 250 um decreased sharply while the weight fraction of

sludge solids retained on a 250-um sieve increased to 75 percent.

Like particle size, floc density increased substantially with

polymer conditioning. Mass balance calculations indicated that the

increase reflected a 20 percent loss of floc water. Floc structure

observed during floc density analyses also changed. The small,

uniform flocs observed in unconditioned sludge were replaced by

large, bulky aggregates.

The polymer could have caused the apparent release of floc water

by reducing surface charge, thus permitting particles to draw closer

together, and/or by physically trapping particles in a more tightly

159

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161

packed structure. water originally measured as integral to the floc

would have been displaced as particles drew, or were drawn, closer

together.

Dewatering Characteristics - Changes in dewatering

characteristics of polymer-conditioned samples of alum sludge were

typical. Large decreases in specific resistance were observed, while

improvements in dewatered cake solids concentration were slight.

Polymer conditioning resulted in only small changes in

vacuum—filtered solids concentration. The unconditioned sample had a

final cake solids concentration of 20 percent; the greatest

improvement, noted for the sample conditioned with 100 mg/L polymer,

was a 15 percent increase to 23 percent solids.

The specific resistance of all polymer-conditioned samples was

reduced by at least 63 percent, to values of 85 x 1011 m/kg or less,

when compared to the unconditioned sludge, which had a specific

resistance of 230 x 1011 m/kg. Reductions in specific resistance did

not vary with polymer dose, nor with the number of particles with MPD

less than 250 um, although there did appear to be a general inverse

relationship between specific resistance and the weight fraction of

sludge solids retained on a 250-pm sieve.

Effects of Dilution - Samples of diluted RAAP alum sludge were

conditioned with 10, 50, and 100 mg/L Betz 1160. Polymer

conditioning resulted in decreased specific resistance at all

doses.Vacuum-filteredcake solids concentration remained stable, except at

the highest polymer dose, where it decreased, while centrifuged

162

solids concentration increased slightly for all polymer doses.

Floc density and particle size increased with increasing polymer

dose. The largest change in particle characteristics was the sharp

decrease in the number of small particles in the supernatant.

Results of analyses and determinations on samples of diluted

alum sludge conditioned with polymer are presented in Table 21.

As can be seen from Figure 23, a major effect of polymer

addition on the particle characteristics of diluted alum sludge was

the decrease in the number of particles with mean particle diameter

(MPU) of 10 pm. Particle size shifted towards more particles in the

25 to 35 pm range for the 10 and 50 mg/L polymer doses. As can be

seen from Table 21, the 100 mg/L dose resulted in a more definite

shift towards larger particles, with the weight fraction of sludge

solids retained on a 250-um sieve increasing from zero for the

unconditioned sample to 50 percent.

The unost striking result of polymer conditioning was in the

reduction of the number of particles in the supernatant.

Concentrations dropped at least two order of magnitude for all

polymer doses, to less than 4 percent of the numbers in the

unconditioned samples.

The 100 mg/L dose produced substantially different results in

other areas as well. whereas TOC had decreased and the floc density

and sludge pH had remained virtually unchanged for lesser doses, the

sample treated with 100 mg/L polymer showed an increase in TOC and

floc density, and a decrease in pH.

l 163

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165

The effects of polymer conditioning on the dewatering

characteristics isf samples of diluted aluni sludge iwere similar to

those observed for undiluted sludge. There was a large decrease in

specific resistance for all polymer-conditioned samples, while

variations in dewatered solids concentrations were small.

Specific resistance decreased to approximately the same level at

each polymer dose, from an unconditioned value of 160 x 1011 m/kg to

13 x 1011 m/kg and less for conditioned samples.

Indirect-Contact, Freeze-Thaw Conditioning

Indirect—contact, freeze-thaw conditioning greatly improved the

dewatering characteristics of undiluted RAAP alum sludge,

outperforming polymer conditioning in all areas. Specific resistance

decreased greatly, while vacuum-filtered and centrifuged solids

concentrations increased 200 and 150 percent, respectively.

Decreased specific resistance was associated with large, concommitant

increases in particle size and floc density.

The results of analyses and determinations for an indirectly

frozen sample of undiluted RAAP alum sludge are presented in Table

22.Experimental Observations - Thawed alum sludge solids had the

granular, gritty appearance typical of freeze-thaw conditioned

sludge. A large amount of supernatant was decanted from the product

sludge, resulting in a fivefold increase in initial dry solids

content.6

166

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Particle and Chemical Characteristics - The large increase in

particle size resulting from indirect—contact, freeze-thaw

conditioning of undiluted alum sludge was apparent from the large

amount of sludge solids retained on a 250-um sieve for the

conditioned sludge. No measurable fraction was retained upon sieving

the unconditioned sample, while 76 percent of the conditioned sludge

solids fell in this size range.

The similarities in distributions of particles with mean

particle diameter (MPO) less than 250 pm (Figure 24), as well as the

total number of particles represented by these distributions, for

conditioned and unconditioned samples were belied by comparison of

the weight fractions of total sludge solids that the distributions

represented. The particle distribution depicted for the

unconditioned sample represented approximately 100 percent of sludge

solids, while the distribution depicted for the conditioned sample

represented a much smaller fraction of solids--—approximately 24

percent-—-from a more concentrated sludge. The number of particles

in these size ranges would have decreased with conditioning, had

supernatant not been removed from the sample, thereby increasing

initial solids content.

Floc density increased greatly with conditioning, from 1.17 g/mL

to more than 1.3 g/mL, representing the release of up to 46 percent

of floc water.

Dewatering Characteristics - Indirect-contact, freeze-thaw

conditioning of undiluted alum sludge resulted in large increases in

168

15000 ·11 Q] Ü UNCÜNÜITIONEÜQ 12000 1 O CÜNDITIONEÜE \U') ••u..I.1U 9000

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MEHN PQRTICLE DI9METER„ MICRONS

FIGURE 2%. PHRTICLE SIZE DISTRIBUTIONS. UNDILUTED INDIRECT-· CONTFICT FREEZE-THHH CDNDITIONED FILUM SLUDGE.

169

initial and dewatered solids concentrations and a large decrease in

specific resistance. Initial solids content increased 450 percent,

from 3.2 to 14.7 percent solids. Conditioned vacuum-filtered cake

solids concentration nearly doubled, from 21 percent solids for the

unconditioned sludge to 40 percent solids. Centrifuged solids almost

tripled, from 10 to 26 percent. The change in specific resistance

was correspondingly large, with a decrease from 190 x 1011 m/kg for

the unconditioned sample, to less than 1 x 1011 m/kg for the

conditioned sample.

These results were the classic response to indirect-contact,

freeze-thaw conditioning: a dramatic increase in particle size and

density, accompanied by large improvements in specific resistance and

in initial and dewatered solids concentration. All evidence

indicated that a large amount of water previously unavailable for

removal was released by the freeze-thaw process, and that the removal

of this water, reduction of surface charge, compression or rupture of

particles, and/or other changes resulting from the freeze-thaw

process, enhanced the formation of large, dense flocs with remarkable

settling and dewatering properties.

Effects of Dilution - The dewatering characteristics of diluted

RAAP alum sludge were greatly improved by indirect-contact,

freeze-thaw conditioning. Specific resistance was reduced to less

than one percent of its initial value, and there were large increases

in initial and dewatered solids concentrations. Floc density and

particle size increased, and there was a large reduction in the

170

number of particles in the supernatant.T

The results of analyses and determinations for an indirectly

frozen sample of RAAP alum sludge diluted to one percent are given in

Table 23.

The main change in particle characteristics, as can be seen from

Table 23, was an increase in the percent of sludge solids larger than

250 um. Approximately 78 percent of the conditioned sludge solids

were retained on a 250-um sieve, as compared to none for the

unconditioned sample. Particle size distributions, while apparently

similar, as can be seen from Figure 25, were fundamentally different,

in that the initial solids concentration of the conditioned sludge

was 15 times that of the unconditioned sludge. These results were

similar to those observed for undiluted sludge. Initial solids for

conditioned samples of diluted and undiluted sludges were practically

identical, as were weight fractions of solids retained on a 250-um

sieve.

Supernatant distributions for the two samples were also quite

different. There were very few particles in the indirectly frozen

supernatant sample, less than five percent of the total number of

particles counted for the unconditioned sample. In contrast, the

undiluted sludge had shown no such improvement. Also, conditioning

of diluted sludge resulted in a moderate decrease in supernatant TOC,

from 2O to 8 mg/L, while there had been no improvement for undiluted

sludge.

The changes in dewatering characteristics of diluted alum sludge

I7I

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173

with indirect-contact freeze-thaw conditioning were striking, as they

had been for the undiluted sludge. The increases in solids

concentration, both initial and dewatered, for the indirectly frozen

sample of diluted alum sludge were very large. Initial dry solids

concentration increased from 1 to 15 percent. Vacuum-filtered cake

solids concentration‘ nearly doubled with conditioning, from 22

percent dry solids for the unconditioned sample to 41 percent dry

solids for the indirectly frozen sample. Centrifuged dry solids

concentration nearly tripled, from 10 to 28 percent dry solids. The

specific resistance of the sludge decreased sharply following

freeze-thaw conditioning.

Direct-Contact, Freeze-Thaw Conditioning with Butane

One-L samples of undiluted RAAP alum sludge were conditioned

with 20 mL/min butane for contact times of 20, 30, and 40 minutes,

resulting in butane doses of 400, 600, and 800 mL/L, respectively.

The only improvement noted in dewatering characteristics was a slight

increase in vacuum-filtered solids concentrations at the highest

butane dose. Specific resistance increased to similar values for all

conditioned samples. The only consistent change in other parameters,

including particle characteristics, was a moderate decrease in pH.

Results of analyses and determinations on samples of

butane-conditioned, undiluted alum sludge are presented in Table 24.

Experimental Observations - The butane conditioning process

generated large amounts of foam in alum sludge samples. In general,

y 174

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the foam had a higher initial solids content (about 4 percent) and

dewatered to a lower vacuum-filtered solids concentration (about 20

percent) than either the unconditioned or conditioned sludge.

Particle Characteristics - Changes in particle characteristics

varied considerably and inconsistently; there were no general trends

observed which could be related to increasing butane dose. The cause

of certain changes were suggested by the results of previous

experiments with other types of sludge.

There was little change in the distribution of particles with

mean particle diameter (MPD) less than 50 um for the sample

conditioned with 400 mL/L butane (Figure 26), and the total number of

particles per mL with MPD less than 250 pm also remained about the

same. However, the weight fraction of sludge solids retained on a

250-pm sieve increased slightly and the floc density distribution

narrowed, shifting towards a typically more dense particle. The

changes in floc density and weight fraction reflected an increase in

the percentage of larger, more dense particles, suggesting that some

freeze-thaw conditioning might have occurred.

The changes in the particle characteristics of the undiluted

alum sludge sample conditioned with 600 mL/L (Figure 27), while

different from those resulting from the 400 mL/L dose, also indicated

that certain freeze—thaw effects might have occurred. Initial solids

content more than doubled, accounting for a certain part of the~

increase in the total number of particles per mL with MPD less than

250 pm. Although most of the increase was in the size ranges of MPD

( 177

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178

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179

of 10 um or less, suggesting mixing-related changes, particle

compression may have been responsible for a fraction of these smaller

particles. The supernatant also contained a large number of small

particles, most with MPD less than 10 pm.

The only noticeable change in particle characteristics for the

sample conditioned with 800 mL/L butane was a decrease in the amount

of solids retained on a 250-um sieve, which may have resulted from

the production of fines through freezing.

Dewatering Characteristics - The effects of butane conditioning

on the dewatering characteristics of alum sludge were variable, with

certain changes in dry solids concentration obtained upon vacuum

filtration and centrifugation and increases in specific resistance.

For butane doses of 400 and 600 mL/L, there was a small increase and

decrease, respectively, in vacuum-filtered cake solids concentration,

while cake solids concentration increased 13 percent for the 800 mL/L

dose. Centrifuged dry solids concentration of the sample conditioned

with 400 mL/L butane showed a large (perhaps anamolous) increase, but

did not change for the sample conditioned with 600 mL/L butane, and

decreased for the 800 mL/L dose.

Changes in specific resistance with butane conditioning were

more consistent, as shown in Figure V28. Specific resistance

increased for all butane-conditioned samples, from 160 to 170 x 1011

m/kg for the unconditioned samples, to more than double that value.

Conditioning had no effect on the sign of the blinding coefficient,

which was negative for all samples.

180

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FIGURE 28. SPECIFIC RESISTRNCE V5 BUTRNE 00SE.UNDILUTED BUTRNE-CONOITIONE0 RLUM SLUDGE.

181

Contact times for these experiments were relatively short and

the degree of freezing taking place indeterminate. This may explain

the absence of any clear trends. The large increase in floc density,

in the number of particles with MPD greater than 250 pm, and in

centrifuged solids concentration for the sample treated with 400 mL/L

butane, and the increase in initial solids concentration for the

sample treated with 600 mL/L may have been indications of the

conditioning effect of the freeze-thaw process. As suggested in

reporting results of earlier experiments, the vacuum filtration

process might have been more sensitive to interference from the foam

generated during application of the butane, thus explaining the

limited response in vacuum filtered solids and the increase in

specific resistance. Improvement may also have been obscured by the

detrimental effects of mixing.

The only trend consistent with the results of indirect freezing

was a decrease in sludge pH for all conditioned samples.

Effects of Dilution - A one—liter sample of diluted RAAP alum

sludge was conditioned with 20 mL/min butane for a contact time of 75

min, resulting in a butane dose of 1500 mL/L. This experiment

represented an attempt to determine the effects of larger butane

doses. The results were very similar to those obtained for smaller

doses applied to samples of undiluted sludge. There were slight

increases in vacuum-filtered and centrifuged solids concentrations,Q

while the specific resistance of the sludge doubled. The range of i

floc density increased, while other particle characteristics remained

182

unchanged.

Results of analyses and determinations on samples of

butane-conditioned, diluted alum sludge are presented in Table 25.

Direct-Contact Freeze-Thaw Conditioning with Freon 12

One-L samples of undiluted RAAP alum sludge luere conditioned

with Freon 12 in batch and continuous experiments. In the batch

experiments, samples were treated in one and two stages, with 250 mL

Freon applied at each stage. In the continuous experiment, the

sample was treated with 10 mL/min Freon for a contact time of 50 min.

Thus, the sample treated in the one-stage batch experiment was

exposed to 250 mL/L Freon; and the samples treated in the two-stage

batch experiment and in the continuous experiment were exposed to 500

mL/L Freon.

In general, the application of Freon 12 to undiluted alum sludge

samples had little effect on particle or dewatering characteristics.

Specific resistance increased, whether the Freon was applied

continuously or as a slug dose. The increase in specific resistance

was smaller for batch experiments than for the continuous experiment,

suggesting mixing effects. Vacuum-filtered solids concentration

decreased for all conditioned samples. Centrifuged solids

concentration increased slightly for samples batch-conditioned with

Freon, but decreased for the continuously conditioned sample.

Results of analyses and determinations on samples of undiluted

alum sludge conditioned with Freon are presented in Tables 26 and 27

183

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Particle Characteristics - Effects of Freon conditioning on the

particle characteristics of samples of undiluted alum sludge were

small. Average floc density increased slightly with Freon addition

in both batch and continuous experiments, while there were no notable

changes in other particle characteristics. Supernatant quality

deteriorated considerably, with an order—of—magnitude increase in the

number of particles with mean particle diameter (MPD) less than

250 um in the supernatant from the sample conditioned in two stages

with 500 mL/L total Freon, and the absence of any removable

supernatant at all from the sample conditioned with 500 mL/L in the

continuous experiment.

Dewatering Characteristics - Freon conditioning of undiluted

alum sludge resulted in slight increases in centrifuged solids for

batch experiments. The response to vacuum filtration, however, was

uniformly poor. Specific resistance increased for all doses and

conditioning regimes, and vacuum-filtered solids decreased. The

continuously conditioned sample failed to form a cake upon

filtration,

A few tenuous relationships can be drawn from the experimental

data. For batch experiments, increased centrifuged solids

T concentrations were accompanied by increases in floc density. This

relationship suggested that the addition of Freon to the sludge had

some conditioning effect. This relationship was not observed for the

semicontinuously conditioned sample, which had an increase in floc

187

density but no change in centrifuged dry solids concentration. Since

this sample was mixed for a much longer time than the

batch—conditioned samples, this results might have been caused by the

detrimenal effects of mixing. It is interesting to note that this

sample was also the only one not to form a cake upon vacuum

filtration, and had the highest specific resistance.

Effects of Dilution ·- 0ne-liter samples of RAAP alum sludge

previously diluted to a 1 percent dry solids concentration were

conditioned with Freon, in batch and continuous experiments. In the

batch experiments, samples were treated with 1-, 2-, and 3-stage

conditioning, with 250 mL Freon applied at each stage. In the

continuous experiments, samples were fed 10 mL/min Freon for contact

times of 25, 50, and 75 rninutes. The samples treated in 1—stage

batch and 25-min continuous experiments were fed 250 mL/L Freon; the

2—stage batch- and 50-min continuously conditioned samples, 500 mL/L;

and the 3-stage batch- and 75-min continuously conditioned samples,

750 mL/L. Results of analyses are presented in Tables 28 and 29 for

batch and continuous experiments, respectively.

Particle size distributions for conditioned and unconditioned

samples of diluted alum sludge were similar in shape and magnitude,

with the exception of the samples batch-conditioned with 500 mL/L

Freon (Figure 29) and continuously with 250 mL/L (Figure 30). These

samples had greater numbers of particles in the size ranges with mean

particle diameter less than 40 um. The total number of particles per

mL represented by the distribution of the batch—conditioned sludge

188

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191

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FIGURE 30. PRRTICLE SIZE UISTRIBUTIUNS. UILUTEO. FREUN-CONDITIUNEU RLUM SLUUGE LCUNTINUOUS EXPERIMENTS1

192

was double that of any other sample.

Floc density increased slightly for batch conditioned samples.

Supernatant TOC increased with conditioning from 10 mg/L for the

unconditioned sample up to 38 mg/L for conditioned samples, with no

trend. The pH of batch-conditioned samples decreased with Freon

dose, from 8.4 for the unconditioned sample to 7.4 for the sample

from the 3-stage experiment, while there were only slight variations

in pH for continuously conditioned samples.

The major effect of Freon conditioning on the dewatering

characteristics of diluted alum sludge was a substantial increase in

vacuum-filtered solids concentration for batch experiments, a change

that was not noted for continuously conditioned samples.

Vacuum-filtered dry solids concentration increased for all

batch-conditioned samples, from 23 percent solids for the

unconditioned sample to 27 to 28 percent solids for conditioned

samples. This change represented 17 to 22 percent increases in

solids concentration. There were only slight variations in final cake

solids concentration for continuously conditioned samples, which

ranged from 23 to 24 percent, as did the final cake solids

concentration for unconditioned samples.

whereas there were only slight variations in specific resistance

values of batch-conditioned samples, there was a clear upward trend

in specific resistance values with increasing contact time for

continuously conditioned samples, as shown in Figure 31. Specific

resistance values increased from 100 to 140 x 1011 m/kg for

193

400

E1 BQTCHC) CONTINUOUS

90°”° 300X

.2*g 1uiUZCEI-LD 200 9I-1U)W asIILJ•-• •:

G I;I.1..I 100 ¤= “ ¤O.U')

ss

0 0 1 00 200 300 400 500 600 700 600FREON DOSE. nL/L

FIGURE 31. SPECIFIC RESISTHNCE V5 FREON DOSE.DILUTED FREON-CONDITIONED FILUM SLUDGE.

194

unconditioned samples, to 270 x 1011 m/kg for the sample

semicontinuously conditioned with 750 mL Freon. This sample had very

poor supernatant quality.O

Dewatering characteristics of undiluted sludge had generally

deteriorated for samples freeze-thaw conditioned with Freon. More

promising results were obtained for diluted sludge, particularly for

those conditioned in batch operation.

195

V. DISCUSSION

The purpose of this study was to develop and evaluate

direct-contact, freeze-thaw processes for conditioning wastewater

and water treatment sludges in the context of conventional

conditioning methods and to develop a theoretical model for the

freeze-thaw process. This chapter presents the following:

- a proposed mechanism for conditioning effects achieved

during freeze-thaw processes, based upon the results of

indirect-contact, freeze-thaw experiments;

- a comparative evaluation of direct-contact, freeze-thaw

conditioning; indirect-contact, freeze-thaw conditioning;

and polymer conditioning; and

- a comparison of the results of the current investigation

with other studies and a discussion of process viability.

PROPOSED MECHANISM FOR FREEZE-THAw CONOITIONING

In reviewing the particle and floc characteristics of sludgesl

treated with direct-contact, freeze-thaw processes, no clear trends

were discovered which could be used independently to formulate a

196

mechanistic explanation of freeze-thaw conditioning. Results

obtained with indirect-contact, freeze-thaw conditioning of the

various sludges used in this study suggested a two-stage mechanism

for conditioning: the disruption of both the sludge floc and

particle structural components, and the formation of a rmw sludge

floc arrangement. The effects of the freeze-thaw process

ondewateringcharacteristics were dependent upon these sludge—specific

changes in particle and floc characteristics.

Disruption of Sludge Structure

The first hypothetical stage in the freeze-thaw process, the

disruption of existing flocs and particles, was evidenced in these

experiments by changes in floc density ranges, in sludge pH, and in

supernatant TOC concentration. The basic conditioning effect of this

stage was the release of water previously incorporated in the

original floc structure.

Changes in floc density indicated that the freeze-thaw process,

at a minimum, altered sludge structure at the floc level. As can be

seen from Table 30, freeze-thaw conditioning resulted in wider

density ranges for certain sludges and in increased average density

for each type of sludge.

The increase in floc density range observed for conditioned

samples containing primary sludge included flocs of lesser and

greater density than the original range. The appearance of less

dense flocs in these samples suggested that the particles

197

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constituting flocs were forced apart from one another during the

freeze-thaw process. The observation of this phenomenon only in

samples containing primary sludge suggested that primary sludge

flocs, characteristically more heterogeneous than either waste

activated or alum sludge, were more susceptible to disruption during

freezing than the other sludges.

Alternatively, changes in floc density range may actually have

occurred in other sludges, but may have been obscured by the

limitations of particle analytical methods, i.e.:

1. The amount of water which must be lost or gained to affect

a measurable change in floc density depends upon the dry

density of the sludge solids. For example, a much larger

amount of water loss is required to increase the floc

density from 1.02 to 1.03 g/mL for a sludge with a dry

density of 1.10 g/mL than for a sludge with a dry density

of 2.0 g/mL. Since waste activated sludge dry solids have

a low characteristic density, a relatively large change in

the amount of water associated with the floc would be

necessary to be reflected by changes in measured floc

density.

2. The density of indirect—contact, freeze-thaw-conditioned

alum sludge flocs was greater than the maximum density that

could be achieved via concentration of the Percol media

199

(maximum density = 1.30 g/mL).

It is also possible that amalgamation of particles in waste activated

and alum sludge samples upon thawing may have obscured fundamental

changes in particle characteristics induced by freezing.

As shown in Table 30, average floc density increased for all

sludges. (Average floc density for each sludge was taken as that

density gradient at which the greatest amount of sludge was

deposited. If sludge was concentrated more or less equally at more

than one gradient, the corresponding densities were averaged to

obtain the mean value.) This densification can be attributed to the

loss of floc water during the freeze-thaw process. To determine the

percentage of floc water hypothetically released, a general mass

balance was written around the floc as:

pf = <¤d„},(pd„y) + <¤w<¤w) [231

where

odry = fraction of floc volume occupied by dry

solids (dimensionless) 2

owet = fraction of floc volume occupied by water(dimensionless)

pdry = dry solids density (g/mL)

pwet = density of water (1.0 g/mL)

Based upon observed changes in floc density conditions, the

200

percentage of floc water released was calculated as:

% released = [Qwu ”©wc(¢dryu/¢dryc)](l00) [24]

Qwu

where the notations "u" and "c" denote unconditioned (control) and

conditioned volume fractions, respectively.

As can be seen from the data presented in Table 30, the

percentage of floc water released by the indirect-contact,

freeze-thaw process varied from 28 to over 50 percent.

Changes in the fundamental chemical characteristics of

conditioned sludges suggested that particles as well as flocs may

have been disrupted by the freezing process. As can be seen from the

data presented in Table 31, changes in sludge pH and supernatant TOC

concentrations were significant. The pH of primary sludge, primary

sludge/waste activated sludge mixtures, and alum sludge decreased,

while the pH of waste activated sludge increased. Changes in

supernatant TOC concentration for primary sludge and for primary

sludge/waste activated sludge mixtures were large, with increases of

over 100 percent.

Due to the described limitations in particle analyses, the

extent and nature of particle disruption is not evident from these

observations. There are at least three mechanisms which may singly

or together be used to explain the data: .

1. Dehydration and Rupture - Mechanical stresses and/or

201

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physical/chemical imbalances produced during freezing could

result in dehydration and rupture of cells and particles.

Discussing the results of his study of butane conditioning

of waste activated sludges, Khan3O hypothesized that large

mechanical stresses placed on the cell during freezing

caused the release of internal water, which subsequently

froze outside the cell. Irreversible changes resulted from

contact of cellular protein formerly isolated by internal

water. Mechanical stress could similarly disrupt

non-biological particles.

Silvares71 found that cells in a slowly freezing

solution are immersed in an increasingly saline

environment. The resulting concentration gradient across

cell boundaries elicits water from the cells, resulting in

cellular dehydration. Ice unay also forna with the cell,

which could ultimately lead to the rupture of cells into

smaller particles30’34.

The occurrence of particle/cell dehydration and

rupture in the current investigation is supported by the

data which indicated increases in floc density range and

average floc density and changes in supernatant TOC

concentration and sludge pH.

2. Particle Compression - The intense pressures generated

during freezing may deform particles without significant

203

loss of water or destruction of cell boundaries. —In

freeze-thaw experiments with water treatment sludges,

Flint78 used X—ray diffraction techniques to determine that

the size of aluminum hydroxide "crystallites" was actually

halved by the process. Flint suggested that the particles

were compressed by the expansion of external freezing

water. In the current study, compression may have enhanced

for the densification of floc observed for all sludges.

3. Change in Particle Surface Charge - Changes in particle

surface charge may also occur upon freezing. Flint78T hypothesized that the readiness with which the aluminum

hydroxide "crystallites" produced by freeze-thaw

conditioning precipitated was due to reduction of surface

charge. The large electric potential which may develop

across the ice-water interface may impart a negative charge

to the liquid surrounding the particles, thereby

neutralizing them. Such changes could also explain in part

the increases in average floc density observed for all

sludges.

Formation of New Floc Structure

Many of the particle and dewatering data collected in this study

support the hypothesis that the second stage in the freeze-thaw

process is the formation of new flocs. whether flocs and particles

204

experienced dehydration and rupture, compression, and/or a change in

surface charge, the basic effects of the freeze—thaw process on the

sludges in this study were large increases in floc density and

particle size. The flocs comprising the conditioned sludge were thus

fundamentally different from those originally constituting the

unconditioned sludge.

The changes in floc density were previously discussed in the

context of floc disruption. Conditioned flocs were, on the average,

considerably denser than unconditioned flocs, as can be seen from

data in Table 30. Large changes also occurred in particle size

distributions for each sludge. Data in Table 32 document the

reductions in the number of particles with mean particle diameter

less than 250 pm per mg of sludge solids for each sludge.

Concentrations of particles (number/mL) were normalized by dividing

by initial solids concentration to account for differences in solids

content among sludges and between conditioned and unconditioned

samples. As can be seen from the data in Table 32, reductions in the

number of small particles per unit weight of sludge solids were

substantial, ranging from 80 to 96 percent.

Increases in the weight percent of particles greater than 250 um

in size were correspondingly large. For unconditioned samples of

waste activated and alum sludges, there was no fraction of particles

retained on a 250-pm screen. with conditioning, the weight percent

retained on a 250-pm screen increased to 73 percent or greater for

each sludge.L

205

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The increases in floc density and particle size suggest that

particles were simply forced together by intense pressure during

freezing ‘u> fomn larger ones, squeezing out excess water. Other

results suggest a inore complex process combining cell disruption,

particle compression, and surface charge reduction, as discussed in

the previous section.

The general effects of the conditioning-induced changes in

particle and floc characteristics were increases in initial and

vacuum-filtered solids concentrations, and variable, sludge—specific

changes in centrifuged solids and specific resistance. Data

included in Table 33 show that the change in initial,

gravity-thickened solids concentration with conditioning was

substantial, with increases ranging from 62 to 1600 percent. This

increase corresponds well with the calculated values for floc water

content lost due to conditioning (Table 30). Much of the floc water

released by the freeze-thaw process completely dissociated from the

floc and was easily drained off as supernatant.

The improvements in dewatered solids concentrations for certain

sludges can also be related to the release of previously bound floc

water. As can be seen from data in Table 33, the solids obtained

following vacuum filtration showed increases of 51 to 90 percent,

where measurable. (Primary sludge and primary sludge/waste activated

sludge mixtures did not produce cakes before conditioning; thus

extent of improvement could not be determined). Indirect freezing

showed increases of 15 to 190 percent in the solids concentration

207

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obtained following centrifugation, except for samples containing

primary sewage sludge. Conditioned samples comprised by or

containing primary sewage showed a decrease in centrifuged solids

concentration. This decrease was assumed to result from the change

in floc density range, with less dense particles remaining suspended

after centrifugation,

Arundel77 investigated the dewatering behavior of sludges

exposed to long—term applications of high pressure. She

characterized both the floc density and the dewatered solids

concentration obtained following high pressure application. The

observed increase in floc density was concluded to be related to floc

deformation and the release of internal floc water. This likewise

correlated to the generation of significantly greater cake solids

concentrations. Likewise, the intense pressures developed during

freezing have been suggested as producing similar water release

during sludge conditioning63’78.

Indirect freezing produced a marked improvementa in sludge

dewatering rate as evidenced by a significant decrease in sludge

specific resistance (data in Table 33). This result was predictable

based upon the increase in the characteristic particle size

distribution observed following conditioning. The only anomaly was

with respect to the conditioning of the waste activated sludge

sample; in this instance, an increase in specific resistance was

observed. The reason for this response was not immediately evident

upon analysis of the sludge characterization data collected.

209

EVALUATION 0F DIRECT-CONTACT, FREEZE-THAW CONDITIONING(

There are three main advantages of direct-contact, freeze-thaw

conditioning processes when compared to indirect-contact processes:

(1) reduced energy requirements, (2) flexibility in reactor

configuration, and (3) maintenance of a pumpable slurry, thus

permitting continuous operation. These advantages must be weighed

against the main disadvantage of direct-contact processes, that is,

the reduced potential for improvement in dewatering characteristics.

The following sections discuss the observed effects of direct-contact

freeze-thaw conditioning on particle, chemical, and dewatering

characteristics and compare them to the results of the

indirect-contact process.

Particle and Chemical Characteristics

The decrease in the number of particles with mean particle

diameter (MPO) less than 250 pm per mg of sludge solids, observed for

all sludges conditioned with the indirect-contact process, provided

evidence of the general increase in particle size and change in floc

structure resulting from this freeze-thaw process. In contrast,

normalized concentrations of particles present in each sludge sample

conditioned with a direct-contact process increased significantly in

comparison to the control sample. (Selected data are presented in

Table 34.) These values reflected an increase in the number of

particles; in fact, most of this increase was due to the production

of very fine ((20 pm) size particles.

210

TABLE 34

EFFECT OF DIRECT-CONTACT, FREEZE-THAW CONDITIONINGON PARTICLE CHARACTERISTICS

No. of Particles With MPD(250 um per mg6of Sludge Solids

Sludge Conditioning Method (x 10 ) Range

Waste Activated (WA) None (Control) 10.4Direct Freeze (Butane) 14.5

None (Control) 18.6Direct Freeze (Freon) 24.3

Primary Sewage (PS) None (Control) 10.6Direct Freeze (Butane) 15.4

None (Control) 17.8Direct Freeze (Freon) 25.7

Alum, Undiluted None (Control) 17.9Direct Freeze (Butane) 28.2

None (Control) 7.4Direct Freeze (Freon) 9.0

Alum, Diluted None (Control) 17.0Direct Freeze (Butane) 18.2

None (Control) 20.9Direct Freeze (Freon) 38.7

211

The general effect of indirect—contact, freeze-thaw conditioning

on measured sludge floc characteristics was an increase in the

average sludge floc density. For certain sludges (alum sludge, in

particular), this increase was large. In contrast, minimal changes

in floc density ranges and averages for both butane- and

Freon-conditioned sludges were observed. The direction of change

varied, as can be seen from the selected floc density data presented

in Table 35. Two trends were noted:

1. Average floc density increased for alum sludge conditioned

with Freon, as it had for sludges conditioned with the

indirect-contact process.

2. Floc density range increased for all sludges

batch-conditioned with Freon, as it had for

indirect-contact conditioned samples consisting totally or

partially of primary sludge. The latter was interpreted as

a sludge-specific phenomenon, as the increase was not

observed for other sludges. In this case, the result

appeared to be refrigerant-specific.

Changes in sludge pH were observed in sludges conditioned with

the indirect-contact process. These changes were interpreted to

signal disruption of the sludge at the primary particle level.

Similar changes were noted in butane- and Freon—conditioned samples

212

TABLE 35

EFFECT DF FREON CONDITIONINC ON FLOC CHARACTERISTICS

~Floc Density g/mL

Sludge Conditioning Method Average Range

Waste Activated (WA) None (Control) 1.035 1.03 — 1.04Direct Contact (Freon) 1.02 1.01 - 1.05

Primary Sewage (P5) None (Control) 1.03 ·Direct Contact (Freon) 1.03 1.02 - 1.04

WA/PS Mixture None (Control) 1.04 -Direct Contact (Freon) 1.04 1.03 - 1.05

Alum, Undiluted None (Control) 1.17 1.16 - 1.18Direct Contact (Freon) 1.18 1.16 - 1.18

Alum, Diluted None (Control) 1.17 1.16 - 1.18Direct Contact (Freon) 1.18 -

213

of alum sludge. Minimal change in pH was noted for other sludges.

Moderate changes in supernatant TOC were also observed in

sludges conditioned with the indirect-contact process. In general,

increases of similar magnitude to these changes occurred for sludges

conditioned with the direct-contact process. Two deviations were

noted. First, the supernatant TOC concentration of the waste

activated sludge conditioned with 400 mL/L butane was several times

greater than the supernatant TOC concentration for any other

conditioned or unconditioned waste activated sludge sample. Also,

the supernatant TOC concentration of certain butane- and

Freon—conditioned alum sludge samples increased, whereas there had

been no such increase for the samples conditioned with the

indirect-contact process.

As suggested in the presentation of results, these increases may

have been caused by the suspension of fines in the supernatant as a

result of mixing. Butane-conditioned supernatant samples might also

have contained significant concentration of dissolved or emulsified

butane. For example, based upon butane solubility at experimental

temperatures, up to 50 percent of the total increase in TOC

concentration noted for the waste activated sludge sample treated

with 400 mL/L butane could be attributed to solubilized butane. All

or part of the remaining increase could be accounted for by the

presence of emulsified butane in the supernatant, which has been

reported in amounts up to ten times that of dissolved butane in

freeze—thaw process products26.

214

DewateringCharacteristicsThe

indirect-contact, freeze-thaw process generally resulted in

higher cake solids concentrations and 'lower specific resistance

values for the sludges studied. As for changes in particle

characteristics, the direct-contact processes resulted in variable

changes in dewatering characteristics, usually of lesser magnitude.

Initial and vacuum-filtered solids concentrations obtained upon

indirect-contact, freeze-thaw conditioning were typically higher than

those noted for unconditioned samples. Similar results were noted in

solids concentrations of certain sludges conditioned with

direct-contact processes. Selected results are presented in Table

36.

Certain samples of butane- and Freon-conditioned alum sludge and

butane-conditioned waste activated sludge showed slight increases in

initial solids concentration. The dewatered solids concentrations of

certain samples of waste activated and diluted alum sludges

conditioned with direct-contact processes improved,

Changes in specific resistance with indirect-contact,

freeze-thaw conditioning were sludge specific. The specific

resistance of alum and primary sewage sludges and of primary

sewage/waste activated sludge mixtures greatly improved, while

increasing with indirect-contact conditioning for waste activated

sludge. In contrast, specific resistance deteriorated for almost

every sludge sample conditioned with a direct-contact process.

(Selected data are presented in Table 36.) The exceptions were

215

TABLE 36

EFFECT OF DIRECT-CONTACT, FREEZE—THAW CONDITIONINGON DEWATERING CHARACTERISTICS

Solids SpecificConcentration, % Resistance,

Sludge Conditioning Method Initial Vacuum·Filtered m/kg x 10

Waste Activated (WA) None (Control) 1.0 12.9 36Direct Contact (Butane) 1.2 13.3 290

None (Control) 0.8 8.3 210Direct Contact (Freon) 0.6 13.5 360

Primary Sludge (PS) None (Control) 2.1 · 4,100Direct Contact (Butane) 1.4 - 3,200

None (Control) 1.5 - 7,100Direct Contact (Freon) 1.1 - 23,200

Alum, Undiluted None (Control) 1.3 24.0 160Direct Contact (Butane) 2.8 23.7 290

None (Control) 3.3 16.1 150Direct Contact (Freon) 3.6 13.1 260

Alum, Diluted None (Control) 1.1 21.3 125Direct Contact (Butane) 1.2 23.1 205

None (Control) 0.8 23.4 110Direct Contact (Freon) 0.8 28.0 120

216

butane—conditioned primary sludge, for which there was an

improvement, and Freon-batch—conditioned alum sludge, for which there

was virtually no change in specific resistance with conditioning.

The increases in specific resistance, which were very

significant in certain instances (for example, the Freon-conditioned

primary sludge), were probably due to the production of large numbers

of very fine particles during conditioning and to the foam and scum

layers typically generated during the direct-contact process.

In summary, the benefits observed from indirect-contact

conditioning, such as particle compression, increased floc density,

higher cake solids concentrations, and lower specific resistance

values, were not generated by the direct-contact processes. A

combination of causes is likely responsible for these results,

including the rate and extent of freezing and process-specific

factors, such as mixing and foaming.

The rate of freezing was relatively rapid in the direct-contact

experiments. The detention time for freezing was typically less than

45 minutes, with virtually instantaneous freezing occurring in

certain instances (i.e., batch conditioning with Freon). In

comparison, in the indirect-contact process, solid freezing occurred

over two to four hours.

Other researchers have noted that the rate of freezing affects

sludge characteristics. Logsdon and Edgerly63 found that slow

freezing permitted development of elongated, crystalline solids,

which drained well upon thawing. Rapid freezing resulted in small

217

ice crystals enveloped by a matrix of dissolved and suspended solids,

which drained poorly. Martel and Vesilind79, subjecting samples of

wastewater sludges to variable rates of freezing and different final

temperatures, found that dewatering characteristics (as evaluated by

capillary suction time) showed the greatest improvement at the

slowest rate of freezing. (Although the final freezing temperature

also had an effect on dewatering characteristics, with lower tempera-

tures resulting in improved dewatering, the rate of freezing appeared

to be the significant factor.)

The direct-contact methods utilized in this study generally

resulted in rapid freezing, which appeared to promote cell rupture

and the production of fine particles, as evidenced by particle size

distribution data. The aggregation and densification noted as a

result of the indirect-contact process did not take place in the

direct-contact process.

The extent of freezing also differed between the direct-contact

and indirect-contact processes. In this study, the direct-contact

process was designed and conducted to produce approximately a slurry

composed of equal parts of ice and sludge, typical of the mixture

produced in salt water desalination. In comparison, sludges condi-

tioned with the indirect-contact process were allowed to freeze

solid. From the comparative results, it appears that the higher

fractions of ice formation (approaching a near total freeze) achieved

with the indirect-contact process may be necessary to achieve signif-

icant internal water removal and densification of sludge floc.

218

COMPARISON WITH OTHER STUDIES AND PROCESS VIABILITY

The purpose of this study was to develop a process model for

direct-contact, freeze-thaw processes within the existing context of

previous research and to gauge general process viability. The

following sections present comparisons of the results of this study

with earlier research and discuss the potential of future application

of direct-contact freeze-thaw conditioning processes.

Butane Conditioning

Selected results from research conducted by Khan30 to determine

the feasibility of conditioning waste activated sludge with butane in

a direct-contact process were previously presented in Table 1.

Khan3O observed substantial reductions in specific resistance and

large increases in dewatered solids concentration using high butane

doses (up to 1800 mL/L) in batch experiments and long contact times

(6 to 36 hours) in continuous experiments. Comparable results were

not obtained in the current study for waste activated sludge for any

freeze-thaw method, including the indirect-contact process. Several

factors may have contributed to this difference:

l. The dewatering characteristics of the unconditioned waste

activated sludge used in this study were far superior to

those of the unconditioned sludge used in Khan's study.

Typical specific resistance values for unconditioned waste 7

activated sludge in Khan's study ranged from 850 to 4700 x

219

1011 m/kg; in the current study, typical specific

resistance values for waste activated sludge ranged from 40

to 70 x 1011 m/kg. Therefore there was less room for

improvement in the current study.

2. Khan's best results from batch experiments were achieved at

butane doses that were at least 50 percent greater than

those used in this study. His best results from continuous

experiments were achieved at butane doses that were almost

twice as large as those used in this study and at very long

contact times. The extent of freezing achieved was likely

greater in Khan's study than in the current investigation,

and, in the continuous experiment, the rate of freezing

slower. Both of these factors would serve to improve

conditioning.

3. Khan limited mixing speeds to 45 rpm or less. This

moderate agitation probably minimized foaming, to which a

certain part of the deterioration of sludge quality

observed in this study has been attributed.

Freon Conditioning

Selected results from Molayem and Bardaki's33 feasibility study

utilizing Freon 11 to direct-contact freeze-thaw condition wastewater

Sludges were presented in Table 3. Large increases in final solids

220

were observed in certain experiments. As noted, however, Freon doses

were extremely large and results variable. Gains in solids

concentrations observed in the current study, while not as

substantial as the best results of Molayem and Bardaki's research,

were more consistent.

Process Viability

Although the results of the current study did not warrant

development of the experimental apparatus to larger scale or to

include refrigerant or heat recovery, several observations may be

made with respect to process viability:

1. External factors, e.g., mixing and reactor configuration,

appeared to have a significant effect on process results.

A viable process must include means of intimate contact

between sludge and refrigerant without disturbing floc

structure. In this study, mixing resulted in deteriorating

sludge quality by reducing particle size and exacerbating

refrigerant foaming. Possible alternative solutions exist,

such as higher refrigerant doses, application of sludge in

thin layers, and longer contact times; however, these

options tend to increase the cost of the operation and

reduce its potential for full-scale applications.w

221

2. Foaming is more likely to occur when back pressures in feed

systems cause partial volatilization30; but it also appears

to be depend upon the type of sludge. Since it is worst

for waste activated sludge, it may be enhanced by

biomass-produced surfactants.

3. Refrigerant hazards must be addressed. For example, butane

is both toxic and explosive; Freon 12 is toxic at high

concentrations and may become obsolete, as its potential as

an ozone-destroying agent is better defined.

4. Mechanical and operational complexity must be considered in

comparing direct-contact, freeze-thaw processes with other

alternatives. Temperature, pressure, mixing, and

refrigerant and sludge flowrates must be tightly

controlled. To be cost effective, the process must include

refrigerant and heat recovery, requiring sophisticated

compression and heat exchange systems.

5. Refrigerant and power costs are among the several

market-sensitive variables upon which the cost-

effectiveness of direct-contact, freeze—thaw processes

depend. The economic feasibility of these processes is

enhanced by market climates wherein energy costs are low.

222

VI. CONCLUSIONS

The results obtained in this study indicate that direct—contact,

freeze-thaw conditioning processes do not offer a panacea for

producing marked improvements in sludge dewatering properties. Long

conditioning times at slow rates of freezing may be necessary to

produce significant changes in floc size and water content,

parameters which ultimately affect the rate and extent of sludge

dewatering achieved. The rapid rate of freezing which characterized

the direct-contact conditioning investigated in this study produced

significant concentrations of fine particles, possibly as a result of

floc rupture. In contrast, the slow freezing of sludge to a solid,

as occurred in the indirect—contact process utilized in this study

(and as achieved by natural freezing colder climatic conditions)

resulted in the release of internal floc water and a corresponding

aggregation and densification of the floc structure, producing

excellent sludge dewatering characteristics.

The following specific conclusions were reached:

I. As evidenced by the results of the indirect-contact

process, freeze-thaw conditioning has the potential to

greatly increase dewatered solids concentration and reduce

specific resistance for both wastewater and water treatment

sludges.

223

2. The conditioning effects of freeze-thaw processes can be

explained by a two-stage model in which disruption of

sludge structure at the floc and particle level is followed

by formation of a new, denser floc structure.

3. Variability in the response of alum sludge, waste activated

sludge, primary sewage sludge, and mixtures of waste

activated and primary sewage sludges to freeze-thaw

conditioning can be attributed to inherent differences in

sludge characteristics. Freeze-thaw conditioning was most

effective in improving the dewatered solids concentration

of alum water treatment sludge.

4. The rapid and incomplete freezing characteristic of

direct-contact, freeze-thaw processes was not conducive to

significant changes in the particle and dewatering

characteristics of treated sludges.

5. The reduced performance of direct-contact, freeze-thaw

processes when compared to indirect-contact results may

also be attributed in part to the interfering effects of

mixing-induced changes in particle size distribution and of

refrigerant foaming.

224

Although significant improvements in dewatering characteristics

were not observed for sludge conditioned with direct-contact,

freeze-thaw processes, the results obtained in this study should not

necessarily be interpreted to show that future investigation of

direct-contact, freeze-thaw processes is unwarranted. Many other

agents form gas hydrates or clathrates with water; further testing is

appropriate with different compounds. The results of this study have

indicated which design parameters (e.g., rate of freezing, particle

properties) are critical to the potential success of such

conditioning methods.

225

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232

APPENDIX A

DESIGNATIONS FOR HALOGENATED HYDROCARBONS

Halogenated hydrocarbons used as secondary refrigerants share

the general formula °CaHbClCFd, where b+c+d = 2a + 2. Theserefrigerants are designated by a two- or three-digit number,

calculated from the variables above. The first digit equals (a-1),

unless this is zero, in which case, it is omitted. The second digit

equals (b+l), and the third, d.

If isomers occur, the most symmetrical is assigned the basic

designation, while "a" and successive lower case letters are appended

to isomers with decreasing symmetry. Brominated compounds are

designated by aui upper case B, followed by the number of bromine

atoms. The identifying number is preceded by either an F or an A

(indicating "Freon" or "Arcton") or by an R (for the general term

"Refrigerant"), or by the term for which the letter stands. For

example, monochlorodifluoroethane (CH3CClF2) is marketed as Freon

142b, where a = 2, b= 3, c = 1, d = 2, and a-1 = 1, b+1 = 4, d = 2,

and it is the third most symmetric isomer. It can be designated as

F-142b or R-142b.

[After Gosney, w. B., Principles of Refrigeration. Cambridge

University Press, New York, New York (1982).]

233

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