ii.biological wastewater treatment.pdf

8/10/2019 ii.Biological Wastewater Treatment.pdf

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Wastewater Treatments 1

1. INTRODUCTION

1.1 GENERAL ABOUT WASTEWATER TREATMENT

Wastewater collected from urban areas ( cities, towns, villages ) and from

different industries, must ultimately be returned to receiving waters or to

the land. The complex question of which contaminants in wastewater mustbe removed to protect the environment and to what extent must be

answered specifically for each case. This requires analyses of local

conditions and needs, together with the application of scientificknowledge, engineering judgement based on past experience, and

consideration of federal, state and local requirements and regulations./1/

The presence of different pollutants in the wastewater makes it almostimpossible to treat all the wastewater in the same manner.

Some important contaminants ( pollutants ) of concern in wastewatertreatment are given in the Table 1./1/

In order to treat the wastewater it is generally necessary to combine anumber of basic processes, which may be physical, chemical or

biological in character and which have the effect of removing first of all

the suspended solids, followed by the colloids and any dissolved

inorganic or organic pollutants, and the elimination of pathogenic

organisms.Finally, some characteristics of water and wastewater need adjustment./2/

In addition to these, above mentioned, classical basic processes, some new

directions are also evident in various specific areas of wastewatertreatment, including :

- modification in treatment operations, processes andconcepts;

- the changing nature of the wastewater to be treated;

- the problem of industrial wastes;- wastewater treatability studies;

-

environmental and energy concerns;

- land treatment;

- small and individual onsite systems./1/


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Table 1 Important contaminants of concern in wastewater treatment

Contaminants Reason for importance

Suspended solids Suspended solids can lead to the development of sludge

deposits and anaerobic conditions when untreated wastewater

is discharged in the aquatic environment;

Biodegradable

organics

Composed principally of proteins, carbohydrates and fats,

biodegradable organics are measured most commonly in

terms of BOD and COD. If discharged untreated to the

environment, their biological stabilization can lead to thedepletion of natural oxygen resources and to the development

of septic conditions;

Pathogens Communicable diseases can be transmitted by the pathogenicorganisms in wastewater;

Nutrients Both nitrogen and phosphorus, along with carbon, are

essential nutrients for growth. When discharged to the waterthese nutrients can lead to the growth of undesirable aquatic

life. When discharged in excessive amounts on land they can

also lead to the pollution of groundwater;

Refractory

organics

These organics tend to resist conventional methods ofwastewater treatment. Typical examples include surfactants,

phenols, and agricultural pesticides;

Heavy metals Heavy metals are usually added to wastewater fromcommercial and industrial activities and may have to be

removed if the wastewater is to be reused;

Dissolved

inorganic solids

Inorganic constituents such as calcium, sodium, and sulfateare added to the original domestic water supply as a result of

water use and may have to be removed if the wastewater is to

be reused .

Source : Metcalf & Eddy, Wastewater engineering, /1/


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1.2 BIODEGRADABLE AND NON-BIODEGRADABLE WASTEWATER

The wastewater originating from various sources can be broadly divided

into two categories :

- biodegradable wastewater,

- non-biodegradable wastewater.

1.2.1 Biodegradable wastewater

The biological wastes in general have a predominance of biodegradable

organic matter, and are generally treated in a similar manner.

The stabilization of organic matter is accomplished biologically using a

variety of microorganisms. The microorganisms are used to convert the

colloidal and dissolved carbonaceous organic matter into various gasesand into cell tissue. Because cell tissue has a specific gravity slightly

greater than that of water, the resulting tissue can be removed from thetreated liquid as a sludge by gravity settling. /3/

Based on bacterial relationship to oxygen ( ability or inability to utilizeoxygen as aterminal electron acceptorin oxidation/reduction reactions ),

the microorganisms can be :

- obligate aerobes,

- obligate anaerobes,

- facultative anaerobes

-- denitrifiers.

The general term that describes all of the chemical activities performed bya bacterial cell is metabolism which is divided into catabolism andanabolism. Catabolism includes all the biochemical processes by which a

substrate ( food ) is degraded to end products with the release of energy.Anabolism includes all the biochemical processes by which the bacterium

synthesizes new cells.

The type of electron acceptor available for catabolism determines the type

of decompositionused by a mixed culture of microorganisms.

Decomposition of wastes and particularly of wastewater can be :

- aerobic decomposition,

- anaerobic decomposition,

- anoxic decomposition.


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Foraerobic decompositionthe molecular oxygen ( O2) must be present as

the terminal electron acceptor to proceed by aerobic oxidation. Thechemical end-products of aerobic decomposition are primarily carbon

dioxide ( CO2), water, and new cell material.

Anoxic decompositionoccurs when some microorganisms will use nitrate( NO3-) as the terminal electron acceptor in the absence of molecular

oxygen. Oxidation by this route is calleddenitrification.

In order to achieve anaerobic decomposition, molecular oxygen and

nitrate must not be present as terminal electron acceptors. Sulfate ( SO42-

),

carbon dioxide, and organic compounds that can be reduced serve asterminal electron acceptors. The end-products of anaerobic decomposition

are hydrogen sulfide ( H2S ), mercaptans, methane ( CH4 ), carbon

dioxide, ammonia and water./3/

1.2.2 Non-biodegradable wastewater

The non-biological wastesin general and the wastewater in particular are

rich in non-biodegradable matter consisting of solids and liquids in

suspended or dissolved form, including various inorganics and organics,many of which may be highly toxic.

Examples are domestic or industrial wastewater containing excessive

dissolved solids ( minerals ), inorganic or organic compounds or naturallyoccuring organics such as humic and fulvic acids.

Treatment processes are available for removing these contaminants. The

physical processes frequently used in engineered systems include

sedimentation, filtration and gas-transfer.

Chemical processes includes the usage of different chemicals for

wastewater treatment. Chemicals may be added to alter equilibrium

conditions and cause precipitation of undesirable species. It should be keptin mind that chemical processes are conversion processes and that actual

removal is accomplished by physically separating the solid, liquid, or

gaseous products of the chemical reactions. The chemical processesfrequently used in engineered systems include neutralization, coagulation,

flocculation, chemical precipitation and oxidation & reduction.

Some wastewater must be treated by means of highly sophisticated

processes and equipment, requiring highly skilled operators, and therefore

quite expensive. Such processes are physico-chemical processes and

include : demineralization, desalinization, ion-exchange, reverse osmosis,electro-dialysis, adsorption, evaporation, incineration, etc. /4/


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1.3 CHARACTERISTICS AND TYPES OF WWT

The contaminants ( pollutants ) in wastewater are removed by physical,

chemical and/or biological means, and the individual methods usually are

classified as physical-, chemical- and biological unit processes oroperations.

Treatment methods in which the application of physical forcespredominate are known as physical unit operations. Typical physical unit

operations are: screening, mixing, flocculation, sedimentation, flotation,

filtration and membrane filter operations.

Treatment methods in which the removal or conversion of contaminants is

brought about by the addition of chemicals or by other chemical reactions

are known as chemical unit processes.

Neutralization, oxidation, reduction, precipitation, gas transfer,adsorption, ion-exchange, electro-dialysis etc. are the most common

examples of these processes used in water and wastewater treatment.

Treatment methods in which the removal of contaminants is brought about

by biological activity are known as biological unit processes.

Biological treatment is used primarily to remove the biodegradable

organic substances ( colloidal or dissolved ) in wastewater. Basically, these

substances are converted into gases that can escape to the atmosphere andinto biological cell tissue that can be removed by settling. The most

common approaches in the biological wastewater treatments are: aerobic

processes such astrickling filters, activated sludge, oxidation ponds ( or

lagoons ) , and anaerobic processes such as anaerobic lagoons, sludgedigestion, etc.

Usually in the municipal wastewater treatment, but also in other

wastewater processing all the above mentioned unit operations and

processes are grouped together to provide what is known as primary,secondary and tertiary( or advanced) treatment.

The term primaryrefers to physical unit operations and in some cases tochemical unit processes; secondary refers to biological unit processes;

and tertiaryrefers to combinations of all three.

The principal combination of all alternatives for municipal wastewater

treatment are shown in Figure 1./3/

The contaminants of major interest in wastewater and the unit operationsand processes or methods applicable to the removal of these contaminants

are shown in Table 2./1/


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Figure 1 Categories of municipal wastewater treatment

RAW SEWAGE

Pretreatment

PumpPrimary Treatment Secondary Treatment

Tertiary Treatment

AdvancedWastewaterTreatment

Secon-darySettling

BiologicalTreatment

PrimarySettlin

EqualizationBasin

GritChamber

BarRack

Receiving body


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Table 2 Unit operations and processes and treatment systems

used to remove the major contaminants in wastewater

CONTAMINANTS Unit operation, unit process or treatment

System

Suspended

solids

Sedimentation

Screening and comminution

Filtration variations, Flotation

Chemical addition, Coagulation/Sedimentation

Land treatment systems

Biodegradable

organics

Activated-sludge variations

Fixed film: trickling filters, rotating biol.contactor

Lagoon variations

Intermittent sand filtrationLand treatment systems

Physical-chemical systems

Pathogens Chlorination, Hypochlorination

Ozonization


Nutrients :

- Nitrogen

- Phosphorus

Suspended-growth nitrification and denitrification

Fixed-film nitrification and denitrification

Ammonia stripping, Ion-exchange

Break point chlorinationLand treatment systems

Metal-salt addition

Lime coagulation/sedimentation

Biological-chemical phosphorus removal


Refractory

organics

Carbon adsorption, Tertiary ozonation


Heavy metals Chemical precipitation, Ion-exchangeLand treatment systems

Dissolved

inorganic solids

Ion-exchange, Reverse osmosis

Electrodialysis, Evaporation

Source : Metcalf & Eddy, Wastewater Engineering, /1/


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Questions to the Chapter . :

1. Which are the three main categories of processes in the

wastewater treatment ?

2. Distinguish between the biological and non-biological

wastes !

3. Describe the stabilization process of organic matter !

4. What is terminal electron acceptor ?

5. Distinguish between anabolism and catabolism !

6.

Describe short decomposition of wastes in wastewater !

7. Mention the treatment processes available for removing thenon-biodegradable matter !

8. Distinguish between unit operations and unit processes !


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2 WASTEWATER TREATMENT METHODS

2.1 PHYSICAL TREATMENT

Physical treatment or physical unit operations usually includes followingtreatment techniques : cooling, equalization, screening, comminuting, grit

removal, oil and grease removal, mixing, sedimentation and filtration.

The principal applications of these unit operations are summarized and

given in the next Table 3. /1/

Table 3 Application of physical unit operations in wastewater treatment

Operation Application

Screening Removal of coarse and settleable solids by

interception ( surface straining )

Comminution Grinding of coarse solids to a more or less uniformsize

Flow equalization Equalization of flow and mass loading of BOD andsuspended solids

Mixing Mixing of chemicals and gases with wastewater,

and maintaining solids in suspension

Flocculation Promotes the aggregation of small particles into

larger particles to enhance their removal by gravitysedimentation

Sedimentation Removal of settleable solids and thickening of

sludges

Flotation Removal of finely divided suspended solids and

particles with densities close to that of water. Also

thickens biological sludges

FiltrationMicroscreening

Removal of fine residual suspended solids

remaining after biological or chemical treatmentSame as filtration. Also removal of algae from

stabilization-pond effluents.

Source : Metcalf & Eddy : Wastewater Engineering, /1/


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2.1.1 Cooling

Some hot process effluents will require cooling treatment ( temperature

reduction ) before being fed to any biological or some other treatment or

disposed into the receiving body.

The devices like different types of heat-exchangers, cooling towers, etc.

are employed in cooling processes.

Some examples of heat-exchangers are shown in the next Figure 2./16/

A example of an cooling tower is shown in the next Figure 3 ./16/

2.1.2

Equalization

Wastewater from some industrial activities generally show variations inflow and some other characteristics like pH, BOD, COD, TSS, etc. These

could be regulated by retaining the wastewater in a basins so that theresulting effluent is fairly uniform in flow and characteristics.

2.1.3 Screening

This unit operation is envisaged to protect following equipment likepumps, valves, pipelines, etc. from damage or clogging by ranges and

large objects. The screening element may consist of parallel bars, rods orwires, grating, wire mesh or perforated plates.

Some typical screening devices are shown in the next Figure 4./1/

2.1.4 Comminuting

This is a process where devices like grinders, cutters or shredders are

employed to break up solid material into smaller sizes.

Figure 5 ./6/ gives a diagramatic sketch of comminutor installation.

2.1.5 Grit removal

This process is employed to remove sand, dust, gravel, stones, cinders and

other heavy inorganic settleable material.


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2.2.1 Neutralization

This is a unit process where the acid or alkaline wastes should be

neutralized before being discharged or transported to some additional

treatment.

The reagents usually used in the neutralization are caustic soda, lime and

sodium carbonate. Some other reagents are also employed for specific

applications ( ammonia, carbon dioxide, calcium carbonate, sodiumcarbonate, sulphuric acid, etc.)

An simplified flow sheet of neutralization process is shown in the nextFigure 15./9/

2.2.2 Coagulation and flocculation

In that unit processes the wastewater is treated with certain chemicalswhich form a flocs, that absorbs and entrains the suspended and colloidal

particles present.

In coagulation individual particles agglomerate or combine together. When

a coagulant is used in water, it forms a spongy gelatinuous precipitatewhich absorbs fine size particles in water and bind them together. The

whole process results into bigger particles which are easily settleable.

Some important factors have to be followed : proper coagulant and

flocculant selection, adequate mixing, and efficient detention time.

In order to determine approximately the dosage of coagulant, the usual test

which is performed in the laboratory, is the jar test.

Figure 16 /13/ shows process of coagulation and flocculation.

Reactor clarifier designed for mixing, coagulation and flocculation andclarification is shown in the next Figure 17. / 7 /

2.2.3

Chemical precipitation

Chemical precipitation in the wastewater treatment involves the addition

of chemicals to alter the physical state of dissolved and suspended solidsand facilitate their removal by sedimentation.

They are mainly used for removal of heavy metals, phosphates and certain

soluble anionic colour components.

2.2.4

Oxidation


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The toxicity of some contaminants can be reduced or destroyed byoxidation. The oxidizing agents used in the process are usually sodium

hypochlorite, hydrogen peroxide, ozone, etc.

The cyanides are generally treated in this manner.An typical cyanide oxidation system is shown in the Figure 18. /10/

2.2.5 Reduction

Usually chromium is treated by reduction where the hexavalent form ofchromium is converted into trivalent chromium by means of liquid sodium

bisulphite or disodium metabisulphite, subsequently precipitated as

chromium hydroxide, and removed by sedimentation.

An typical hexavalent chromate treatment system is shown in the nextFigure 19. /10/

2.2.6

Ion-exchange

This method is generally employed for the purification of raw water or for

upgrading the treated water to an acceptable level or for removal of

undesirable anions and cations from a wastewater.

The process involves the exchange of cations by hydrogen ions (H+) and

anions by hydroxil ions ( OH- ). To facilitate these exchanges the use of

certain resins is made.

To regenerate the used resins some acid or alkali will be necessary.

The principles of ion-exchange process is shown in the next Figure 20./9/

2.2.7 Carbon adsorption

This method is chiefly employed for removal of dissolved organics suchas saturated oils, alkanes and alkenes, dyes, phenols, etc. Certain colours

and odours are also removed by this method. The method is based on the

principle of the charcoal filter where the surface tension on the activatedcarbon particles causes molecules to adhere on it.

Activated charcoal filter which is usually used for carbon adsorption is

shown in the next Figure 21./9/


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2.2.8 Reverse Osmosis

This is the physico-chemical process that separates pure water from its

pollutants. This membrane filter operation is also called as hyperfiltration

or superfiltration. The method requires a high pressure to be applied to the

waste to force out the liquid (water only) through a semi-permeablemembrane, leaving behind the dissolved solids.

Principle of reverse osmosis is shown in the next Figure 22./9/

2.2.9 Electro-dialysis

It is a method of separation of dissolved solids from the solution based on

the difference in the rates of diffusion. The permeation through the

membranes is enhanced by the application of electrical energy. The

membranes commonly employed are cellulose nitrate, cellophane andparchment.

Principle of electrodialysis and the unit flow schematic are shown in the

next Figure 23. /16/

2.2.10 Other methods

Some other methods of physico-chemical treatments include : gas

stripping, evaporation, distillation, incineration, etc.


1. Which are the chemical WW treatments ?

2. Describe briefly all above mentioned chemical treatments !

3. Mention some of the reagents usually used in the neutralization !

4. Sketch the process of lime neutralization !

5. Distinguish between coagulation and flocculation !

6. What is jar-test ?

7. Sketch an example of the flocculator !


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2.3 BIOLOGICAL TREATMENT

Biological treatment as indicated earlier, the biological wastes are rich in

decomposable organic matter and can be treated by biological processes

involving microbial decomposition of organic matter, aerobically or

anaerobically.

The main difference between aerobic and anaerobic processes is in thepresence or absence of oxygen.

The common methods of biological wastewater treatment are :

a) aerobic processes such as trickling filters, rotating biological

contactors, activated sludge process, oxidation ponds and lagoons,

oxidation ditches,

b) anaerobicprocesses such as anaerobic digestion, and

c) anoxicprocesses such as denitrification .

The major biological wastewater treatment processes are shown in thenext Table 5./5/

2.3.1 AEROBIC PROCESSES

The basic principle of aerobic processes is the usage of aerobic

microorganisms which need oxygen for their metabolism in the process ofdecomposition of organic matter from the wastewater.

While the basic principles remain the same in all aerobic processes, the

techniques used in their application may vary widely, but may be broadlyclassified as either attached (film) growth or suspended growthprocesses.

A. ATTACHED GROWTH PROCESSES

Attached growth processes utilize a solid medium upon which bacterial

solids are accumulated in order to maintain a high population. Surface

growth processes include intermittent sand filters, trickling filters, rotatingbiological contactors, and a variety of similar devices.


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Table 5 Major biological treatment processes used for

Wastewater Treatment

Type Common name Use

AEROBIC PROCESSES

Suspended growth ACTIVATED SLUDGE PROCESS

- Conventional ( Plug flow ) Carbonaceous BOD

- Step aeration, Modified aeration - Contact Stabilization

- Extended Aeration, Oxidation Ditch

+ Nitrification

AERATED LAGOONS Carbonaceous BOD

AEROBIC DIGESTION HIGH

RATE ALGAL PONDS Carbonaceous BOD

Attached growth TRICKLING FILTERS

- Low rate Carbonaceous BOD

- High rate

ROTATING BIOLOGICAL

CONTACTORS

ANAEROBIC PROCESSES

Suspended growth ANAEROBIC DIGESTION

- Standard rate Stabilization

- High rate Single

Attached growth ANAEROBIC CONTACT PROC. Carbonaceous BOD

ANAEROBIC FILTER PROCESS

ANAEROBIC LAGOONS

ANOXIC PROCESSES - Suspended growth Denitrification- Fixed film

Source : NPC Report, /5/

2.3.1.1 Trickling filters


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This equipment consists of a bed of highly permeable media to whichmicro organisms are attached and through which wastewater is

percolated. The surface of the media that comes into contact with

wastewater develops a Zoogleal film rich in microorganisms. The

microorganisms in this film degrade the organic material present in thewastewater.

As the microorganisms grow, the slime layer increases. The adsorbedorganic matter is metabolised, before it can reach the microorganisms

near the media face. As a result of having no external organic source

available for cell carbon, the microorganisms near the media face enterinto an endogenous phase of growth and lose their ability to slime layer

stalks to grow. This phenomenon of losing the slime layer is called

sloughing and primarily a function of the organic and hydraulic loading

on the filter.

The trickling filter should also have an underdrain arrangement for

collecting the filtered effluent and a proper ventilation system tomaintain aerobic conditions.

A secondary settling tank should invariably follow the filter for removalof the sloughed off solids.

The principal flow scheme for an trickling filter treatment is shown inthe next Figure 24./11/

A typical trickling filter is shown in the Figure 25./12/

2.3.1.2 Rotating biological contactors

A rotating biological contactor consists of a series of closely spacedcircular disks of polystyrene or PVC, or other materials. The disks are

partially submerged in wastewater and rotated slowly through it.

In operation, biological growths become attached to the surfaces of the

disks and form a slime layer over the entire wetted surface area of the

disks. The rotation of the disks alternately contacts the biomass with theorganic material in the wastewater and then with the atmosphere for

adsorption of oxygen.

A principal scheme of an rotating biological disc plant is shown in thenext Figure 26./1/

B. Suspended growth


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Suspended growth processes maintain an adequate biological mass insuspension within the reactor by employing either natural or

mechanical mixing. In most processes the required volume is reduced

by returning bacteria from the secondary clarifier in order to maintain a

high solids concentration. Suspended growth processes includeactivated sludge and its various modifications, oxidation ponds, and

sludge digestion systems.

2.3.1.3

Activated sludge process

It is a treatment process in which biologically active mass, called

activated sludge is continuously mixed with the sewage flow in an

aeration basin in the presence of abundant oxygen. The liquid in the

aeration basin is called mixed liquor. The oxygen is supplied to the

mixed liquor either by diffused compressed air or by mechanicalagitation. The activated sludge is subsequently separated from the mixed

liquor by sedimentation process in a clarifier and a part of this sludge isre-circulated to the aeration basin. The rest of the sludge, which is the

excess production of biological cell material, is disposed-off.

A principal scheme of an activated sludge process is given in the next

Figure 27 ./5/

Many modifications in basin configuration and aeration techniques have

been made in past. The more important types of activated sludge processare described below :

- theconventional process (known also as the plug flow type )

consists of a aeration basin, a clarifier and a solid-return linefrom the clarifier bottom. The return solids are mixed with the

incoming wastes and pass through the reactor in a plug flow

fashion.Air is provided uniformly along the aeration basin.

Conventional activated sludge process is shown in the nextFigure 28./13/

- tapered aerationprocesses attempt to match the oxygen supplyto demand by introducing more air at the head end. The process

is otherwise the same as described for conventional aeration

process. The main advantage of tapered aeration is the optimum

use of air.

- step aerationprocesses distribute the incoming flow to a number


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of points along the aeration basin, thus avoiding the locally high

oxygen demand encountered in conventional and taperedaeration process.

A principal scheme for an step aeration is shown in the next

Figure 29./13/

- contact stabilizationorbiosorptiontakes advantage of the

observed adsorptive properties of activated sludge.The sewage is mixed and aerated with activated sludge for a

short period ( - 1 hours) and the mixed liquor then passes into

a clarifier, where final effluent and sludge are separated bygravity. The settled sludge in the clarifier is re-aerated in a

separate sludge re-aeration ( stabilization ) tank for a period of

some 3 6 hours. After the aerated sludge is fed into contact

tank for intimate mixing with the inflow. This process is suitable

for high organic loadings.

A principal scheme for an contact stabilization is shown in thenext Figure 30./13/

- completely mixedor ahigh rateaeration processes disperse theincoming waste and return sludge uniformly throughout the

aeration basin. The main objective of high rate aeration is to

reduce the cost of construction.

A principal scheme of an completely mixed high rate aeration isshown in the next Figure 31./13/

- extended aerationis a completely mixed process operated at a

long hydraulic retention time and a high sludge age. The processis limited in application to small plants where its inefficiency is

outweighed by its stability and simplicity of operation. The

advantage of this process is that usually there is no provision forexcess activated sludge necessary.

A principal scheme of an extended aeration process is shown inthe next Figure 32./12/

- short-term aerationorhigh-rate activated sludge is apretreatment process where retention times and sludge age are

low, which leads to a poor effluent and relatively high solids

production. A possible application of this process is as the first

stage of a two-stage nitrification process.

- High purity oxygen activated sludge systemshave been


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developed in an attempt to permit easier matching of oxygen

supply to oxygen demand and higher-rate processes throughmaintenance of higher concentrations of biological solids. / 7 /

A principal scheme for an oxygen activated sludge system is

shown in the next Figure 33./1/

2.3.1.4 Waste Stabilization Ponds

Waste stabilization ponds are low cost wastewater treatment units whichusually depend on natural processes. These systems are sometimes also

called oxidation ponds or lagoons. In these ponds the organic matter

from the wastewater is degraded by natural oxidation processes by the

action of microorganisms, both algae and bacteria.

If these processes take place in a stream water then it will be known as aself-purificationprocess.

Waste stabilization ponds or lagoons are classified according to the

biological process taking place as aerobic, anaerobic, facultative andmaturation ponds . /3/

The general reactions which occur in aerobic and facultative ponds are

illustrated schematically in the next Figure 34./14/

Aerobic ponds are shallow ponds, less than 1 m in depth, wheredissolved oxygen is maintained throughout the entire depth, mainly by

the action of photosynthesis.

Facultative pondsare ponds between 1 to 2.5 m depth, which have ananaerobic lower zone, a facultative middle zone, and an aerobic upper

zone maintained by photosynthesis and surface reaeration.

A schematic presentation ( diagram ) of a facultative pond operation is

given in the next Figure 35./14/

Anaerobic ponds are deep ponds that receive high organic loadings

such that anaerobic conditions prevail throughout the entire pond depth.

Maturation or tertiary ponds are ponds used for polishing effluents

from other biological processes. Dissolved oxygen is furnished through

photosynthesis and surface reaeration. This type of pond is also known

as a polishing pond.


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Aerated lagoons are these ponds which are oxygenated through the

additional action of surface or diffused air aeration.

A typical aeration system for the aerated lagoons is a static tube aerator

as shown in the next Figure 36./7/

Oxidation ditchesare simple artificial ditches, dug in the ground for the

purification of wastewater using a very simplified technique. The basic

form of the plant is a ring shaped circuit or ditch. Oxygen as well ascirculation is provided by mechanical rotors . In fact, oxidation ditch is

a modified form of extended aeration of activated sludge process.

Some typical oxidation ditches are shown in the Figure 37./12/

2.3.1.5. Natural systems

Natural treatment systems are designed to take advantage of physical,chemical and biological processes which occur in the nature when water,

soil, plants, microorganisms and the atmosphere interact.

The natural treatment systems are :

- the soil-based or land-treatment systems, (infiltration, irrigation, etc.),

- the aquatic-based systems (constructed and natural wetlands andaquatic plant treatment systems.

Aquatic plant systems are schematically presented in the Figure 38./11/

2.3.2 ANAEROBIC PROCESSES

The anaerobic waste treatment is an effective method for the treatmentof highly concentrated organic wastes. In the absence of oxygen,

anaerobic bacteria convert organic material into gaseous end-products

such as CO2, CH4and H2S.

This biological process is called asanaerobic digestion and is one of the

oldest processes used for the stabilization of sludges.

Anaerobic digestion is a fermentation process which involves the

decomposition of organic and inorganic matter in the absence of

molecular oxygen. The process stabilizes the organic matter byconverting it as completely as possible into methane and carbon-dioxide

gas through the two stages:


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3. The purification of sewage in oxidation ponds is carried out by :

a)algae; b) bacteria; c) both ; d) non of these .

4. What is zoogleal film ?

5.

In the trickling filter process : a) much skilled labour is required;b) operation cost is higher than in ASP unit; c) has low adaptability

with varying load; d) power requirement is low as compare to ASP;

6. Sketch a principal schematic of a trickling filter !

7. Distinguish between suspended- and attached- growth processes !

8. Describe short RBC systems !

9. Sketch a principal scheme of a RBC plant !

10. Describe short an ASP !

11. Sketch a principal scheme of an ASP !

12. Which types of ASP do you know ?

13. Describe short a process of an extended aeration !

14. Distinguish between the diffused aeration and surface aeration !

15. Explain short the terms step aeration and tapered aeration !

16. When is usage of pure oxygen in ASP required ?

17. What are stabilization ponds ?

18. How are the stabilization ponds classified ?

19. Sketch different types of stabilization ponds !

20. Sketch an oxidation ditch !

21.

What is anaerobic digestion ?

22. Distinguish between anaerobic and anoxic processes !

23. Explain the denitrification process !


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3. SLUDGE TREATMENT

The main objective of any type of wastewater treatment is to get clear

effluent and good sludge. The sludge which is in the form of slurry is

produced in bulk quantities.

It is essential to classify a sludge in order to select the tratment methodapplicable to it, and to forecast the performance of the equipment to be

used.

The Table 7./2/ shows the most usual sludge treatment process chains.

The main objectives of sludge treatment are, as follows :

- reduction in the volumeof sludge for disposal by removing

some of the water,- stabilizationof the organic matter contained in the sludge,

- destructionof pathogenic organisms,- collectionof by-products which maybe used ( or ) sold to get

some revenue,

- disposal of the sludge in a safe and aesthetically acceptablemanner.

Sludge treatment and disposal comprise any following method orcombination of methods :

a) Concentration it is the reduction in the volume of sludge to be

treated by pressing the sludge to a higher solids content

b) Treatment it is intended to stabilise organic matter , destroypathogens or to get by-products of the process

c) Dewatering and drying - it is the removal of water from sludge,which decreases the sludge volume.

d) Disposal if the receiving environment is legally, aesthetically andecologically acceptable, the sludge can be disposed on the land or into

the water.


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TABLE 7LIQUID SLUDGE DS 0.5-5%

(possibly DS up to 20%)

CHEMICAL ANAEROBIC AEROBIC

(Pasteurization)

THERMAL (175-200C)

CHEMICAL - inorganic

- polymer

OTHER FORMS - freezing

Open air drying beds- sand

- improved

>

Pressure : belt filter

filter-press Continuouscentrifuge

Vacuum : rotary filter

- remov. cloth

- precoat

INORGANIC S L U D G E

(VS < 30%) DS 4 40%

F R E S H ORGANIC SL UDGE

(30 < VS < 90%) DS 2 15%

THICKENING THICKENING

STABILIZATIONDisposal in liquid

form

authorized farm-

land land

DEWATERING

CONDITIONING

MECHANICAL

FILTRATION CENTRIFUGING

DRYNESS 15 - 80 % DS

DRYNESS 10 60 % DS

INCINERATION DRYING COMPOSTING DRYING INCINERATION

DISCHARGE

NATURAL

THICKENING

AGRICULTURE


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The principal methods used for sludge processing and disposal arereported in the next Table 8. /1/

Table 8 Sludge processing and disposal methods

Processing, disposal

function

Unit operation, unit process, or treatment method

Preliminaryoperations

Sludge pumping and grinding,Sludge blending and storage

Thickening Gravity thickening, Flotation thickening,

Centrifugation, Classification

Stabilization Chlorine oxidation, Lime stabilization,

Anaerobic digestion, Aerobic digestion,

Pure-oxygen digestion, Heat treatment

Disinfection Disinfection

Conditioning Chemical conditioning, Elutriation

Dewatering Centrifuge, Vacuum filter, Pressure filter,

Horizontal belt filter, Drying bed, Lagoon

Drying Dryer

Composting Composting, Co-composting

Thermal reduction Multiple hearth incineration, Fluidized-bed incineration

Flash combustion, Co-incineration, Co-pyrolysis,

Pyrolysis, Wet-air oxidation, Recalcination

Ultimate disposal Landfill, Land application

Reuse

Source : Metcalf & Eddy : Wastewater Engineering, /1/


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Vacuum filtration - this is the most widely used type of mechanical

dewatering techniques. The vacuum filters most commonly used forwastewater treatment sludge are of the rotary drum type. Other types of

filters, used in some industries, are disc filters, vacuum belt filters,

horizontal table filters and paper filters.

The principles and the main phases of a vacuum-filtration cycle are shown

in the next Figure 43. /2/

Pressure filtration - pressure filters can apply a large pressure

differential across the septum to give economically rapid filtration with

viscous liquids or fine solids. The most common type of pressure filtrationequipment are filter presses.

Aplate-and-framefilter press is shown in the next Figure 44. /13/

Centrifuges- are used both to thicken and to dewater sludges. Thickeningor dewatering by centrifugation involves the settling of sludge particles

under the influence of centrifugal forces. The three basic types ofcentrifuges currently available for sludge thickening and dewatering are

nozzle-disk, solid-bowl, and basket centrifuges.

Centrifuges used for the processing of sludges are shown in the next

Figure 45./4/

3.3 Sludge Stabilization

Sludge stabilization or conditioning includes a variety of processes such as

anaerobic and aerobic digestion, chemical coagulation, and heat

treatment.

Anaerobic digestionprocess is described in the Chapter 2.3.2 .

Aerobic digestionmay be employed to treat the sludge solids particularly

in small installations. The end product is very stable, and organic contentof the supernatant liquor is very low. Moreover, the digested solids are

odourless and can be disposed off easily. Cost of installation of such units

is low, but due to the requirement of power for the supply of air, therunning cost may be more compared to that in anaerobic digester.

Anaerobic digestion is usually accomplished in open tanks equiped with

diffused-air or mechanical aerators.


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Chemical conditioning may be applied to sludges which have been

digested or to raw sludges. The chemicals used include the metallic andpolymeric coagulants, and lime.

Heat treatment actually destroys the cell structure releasing even the

bound water.A schematic of an thermal sludge conditioning process is presented in the

next Figure 46. /7/

3.4 Sludge Disposal

Whatever processes are used in handling the solids from wastewater

treatment a residue will still remain which must be disposed of. This

material may range from raw solids at a moisture content of over 95

percent to incinerator ash, and its handling will depend, in part, upon its

nature.

Sanitary landfills are the preferred disposal technique for solid wastes ingeneral. This method of disposal is most suitable if it is also used for

disposal of the other solid wastes of the community.

Land disposal of sludges is subject to biological,chemical and physical

constraints. Disease transmission, odours, heavy metal accumulation, and

social and aesthetic problems must be considered before land applicationis selected.

Ocean disposal has been commonly practiced by coastal cities, but

nowadays is being phased out because of changes in water pollution

control regulations.

Dumping , such as in an abandoned mine quarry, is a suitable disposal

method only for sludges and solids that have been stabilized so that no

decomposition or nuisance conditions will result.

Questions to the Chapter :

1.

What is the objective of a sludge treatment ?

2. What are the methods for sludge treatment ?

3. Describe short every of, above mentioned, sludge treatmentmethods !


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4. Describe sludge thickening !

5. What types of sludge thickeners do you know ?

6. Sketch a sludge thickener !

7. Describe briefly the methods for sludge dewatering !

8. Distinguish between vacuum-filter and filter-press !

9. Give sketch on a drying bed !

10. What is sludge stabilization ?

11. Differentiate between effluent, supernatant and sludge !

12.

What is the necessity of sludge disposal ?

13. Describe briefly the various methods of sludge disposal.


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Table 9 Typical data on the unit loading factors and expected wastewater

constituent concentrations from individual residences

ValueItem Unit loading

factor,

lb/capita d

Unit Range Typical

BOD5

SSNH3 as N

Org. N

TKN as NOrg. PInorg. P

Grease

Total coliform

0.180

0.2000.007

0.020

0.0270.0030.006

mg/l

mg/lmg/l

mg/l

mg/lmg/lmg/l

mg/l

Number/100ml

216-540

240-6007-20

24-60

31-804-106-17

45-100

107-10

10

392

43614

43

57712

70

108

TemperaturepH

Funitless

59-795-8

707.2

Note : lb/capita d x 0.4538 = kg/capita d0.555 ( F 32 ) = C

Source : Metcalf&Eddy ; Wastewater Engineering; /1/


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b) systems for clusters of homes and small communities thatare to be sewered or are already sewered.

Commercially available prefabricated treatment plants, known

as package plants, are often used for the treatment ofwastewater for individual properties and small communities.

Although package plants are available in capacities up to 4000

m/d , they are used most commonly for wastewater flows inthe range from 40 to 1000 m/d .

The most common types of package plants are : extendedaeration, contact stabilization, sequencing batch reactors,

rotating biological contactors and physical/chemical plants.

Typical example for a RBC package unit is shown in the next

Figure 46d./1/

A package WWT-plant with extended aeration is shown in nextphotograph in Figure 46e.

Some typical design criteria for package treatment plants andother treatment systems for small communities are presented in

the next Table 11./1/


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Table 11. Design criteria for package treatment plants and other

treatment systems for small communities

Value

Design parameter Unit Range Typical

Plant loadingsBOD5

SS

TKN NNH3

PO4 P

Extended aeration processPretreatment

Detention time (aerationtank)

BOD5 loading

MLSS (aeration)Oxygen requiredAverage at 20C

Peak at 20C

Excess sludgeSettling tank overflow rate

Aerobic digestion

Solids detention timeVSS loading

Sand drying beds

Equalization basin volume

Rapid sang filterChlorination

Dosage at peak flow

Detention time at peak

Contact stabilization processPretreatment

Detention time (contact t.)Detention time (reaeration)

BOD5loading

MLSS (contact tank)MLSS (reaeration)

Oxygen requiredAverage at 20C

Peak at 20CExcess sludge

Settling tank overflow rate

lb/capita d

lb/capita d

mg/lmg/l

mg/l

h

lb BOD/lb MLVSS d

mg/l

lb/lb BOD applied

(value) xavg. flowlb/lb BOD remov.

gal/ft d

d

lb/ft d

ft/capita% of avg. flow

gal/ft d

mg/l

min

min

h

lb/10ft dmg/l

mg/l

lb/lbBOD applied

(value) xavg.flow

lb/lbBOD removed

gal/ftd

0.13-0.24

0.13-0.25

15-505-25

5-15

Bar screen,

18-36

0.05-0.15

1500-5000

2-3

1.25-2.0

0.3-0.75600-1000

10-300.05-0.25

1.5-2.5

25-100

4-6

15-40

15-45

20-4020-36

1000-30004000-8000

2-3

1.25-2.00.3-0.75

600-1000

0.18

0.20

2515

10

communition

24

0.10

2500

2.5

1.5

0.4800

150.15

2.0

50

5

25

30

3024

18005000

2.5

1.50.4

800

(continued)


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Table 11 - continued

Value

Design parameter Unit Range Typical

Sequencing batch reactor

PretreatmentDetention time

BOD5 loading

MLSSOxygen required

Average at 20CPeak at 20C

Rotating biological contactorsPretreatment

Surface loading

Total BOD5 loadingOther factors

minlb BOD/lb MLVSS d

mg/l

lb/lbBOD applied

(value) xavg.flow

gal/ft d

lb/10ft dsee extend. aeration

Bar screen16-36

0.05-0.15

1500-5000

2-31.25-2.0

bar screen

1-2.5

6-10

communition24

0.10

2500

2.51.5

communition

1.5

8

Source : Metcalf&Eddy : Wastewater Engineering; /1/


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5. DESIGN & ANALYSIS OF WWTS

Treatment-plant and detail/specific plant-objects design is one of the most

important aspects of the planing of the wastewater treatment plant.

The proper planing includes : treatment-process flowsheets, tables of

design criteria, solids balances, hydraulic profiles, and plant layouts.

5.1 Flow sheets for WWT systems

Depending on the pollutants that must be removed from wastewater, a

great number of different treatment-process flowsheets can be developed,

using preliminary and/or primary, secondary and tertiary treatments ( unit

operations and unit processes ).

To develop the proper treatment flowsheet it is necessary a detailed

process analysis of the suitability of every possible individual treatmentmethod to be prepared.

The detailed process analysis of the suitability of an individual treatmentmethod ( unit operations and processes ) is given in the next Table 12./1/

Apart from the process analysis and the evaluation of treatment, givenbefore in the previous Table, the exact flowsheet configuration selected

will also depend on following factors :

c)

engineers experience,

d) company and regulatory agency policies,

e) availability of equipment,f) the maximum use of existing facilities,

g) initial construction costs,

h) future operation and maintenance costs.

Some typical flowsheets for the wastewater treatment are shown in the

next Figures 47 , 48 , 49 , and 50./1,2,3/

Some simplified examples of flow-diagram for WWT of different

industrial wastewater are shown in Figures 50a to 50f.


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Table 12 Important factors that must be considered when selecting and

evaluating unit operations and processes

Factor Comment

1. Process

applicability

2. Applicableflow range

3. Applicableflow variation

4. Influent-

wastewatercharacteristics

5. Inhibiting

and unaffectedconstituents

6. Climaticconstraints

7. Reaction

kinetics and

reactor

selection

8. Performance

9. Treatmentresiduals

The applicability of a process is evaluated on the basis of past

experience, data from full-scale plants, and pilot data from plantstudies. If new or unusual conditions are encountered, pilot-plant

studies are necessary.

The process should be matched to the expected flow range. Forexample, stabilization ponds are not suitable for extremely large

flows.

Most unit operations and processes work best with a constant flowrate, although some variation can be tolerated. If the flow variation

is too great, flow equalization may be necessary.

The characteristics of the influent affect the types of processes to

be used (e.g. chemical or biological) and the requirements for theirproper operation.

What constituents are present that may be inhibitory, and under

what conditions? What constituents are not affected duringtreatment?

Temperature affects the rate of reaction of most chemical andbiological processes. Freezing conditions may affect the physical

operation of the facilities.

Reactor sizing is based on the governing reaction kinetics. Data

for kinetic expressions usually are derived from experience, the

literature, and the results of pilot-plant studies ( e.g. the complete

mixing reactor with continuous flow, or plug-flow reactor) .

Performance is most often measured in terms of effluent quality,

which must be consistent with the given effluent-dischargerequirements.

The types and amounts of solid, liquid, and gaseous residualsproduced must be known or estimated. Often, pilot-plant studies

are used to identify residuals properly.

( continued )


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Table 12 ( continued )

Factor Comment

10. Sludge-

handling

constraints

11. Environmental

constraints

12. Chemicalrequirements

13. Energy

requirements

14. Other resource

requirements

15. Reliability

16. Complexity

17. Ancillaryprocesses

18. Compatibility

Are there any constraints that would make sludge handling

expensive or infeasible ? In many cases, a treatment method

should be selected only after the sludge processing and handlingoptions have been explored.

Nutrient requirements must be considered for biological treatment

processes. Environmental factors, such as the prevailing winds andwind directions, may restrict the use of certain processes,

especially where odors may be produced.

What resources and amounts must be committed for a period oftime for the successful operation of the unit operation or process ?

The energy requirements, and probable future energy costs, must

be known if cost-effective treatment systems are to be designed.

What, if any, additional resources must be committed to the

successful implementation of the proposed treatment system using

the unit operation or process in question ?

What is the long-term record of the reliability of the unit operation

or process under consideration ? Is the operation process easilyupset ? Can it stand periodic shock loadings ? If so, how do suchoccurrences affect the quality of the effluent ?

How complex is the process to operate under routine conditionsand under emergency conditions such as shock loadings ? What

level of training must the operator have to operate the process ?

What support processes are required ? How do they affect theeffluent quality, especially when they become inoperative ?

Can the unit operation or process be used successfully withexisting facilities ? Can plant expansion be accomplished easily ?

Can the type of reactor be modified ?

Source: Metcalf &Eddy,Inc.: Wastewater engineering /1/


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Figure 47 General flowsheet (diagram) for municipal WWT

RAW SEWAGE

Bar

Rack

Grit

Chamber

Pump

EqualizationBasin

Secon-darySettling

BiologicalTreatment

Primary

Settling

AdvancedWastewater

Treatment

Receiving body (river)

Source : M.L.Davis, D.A.Cornwell, Environmental Engineering /10/


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Figure 48 WWT plant flow chart ( CETP for tanneries in

Pallavaram, Chennai )

Raw effluent from tanneries

Tanneries

CETP

Pre-treatment unit(Cr-recovery)

Grit chamberMechanical screen

Receiving sump

Collection wells

Treated effluent

Equalization tank Flash mixer Clariflocculators

Aeration tanks Secondary clarifiers

Belt filterpress

Sludge

drying bed

Sludge thickener

To the landfill

Source : Pallavaram Tanners Industrial Effluent Treatment Company Ltd.,

Chromepet, Chennai


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Figure 49 Process flowsheet for an WWT in USA

Influent

wastewater

Sludge to processing

Grit removal

Excess sl.

Recycle sl.

Primarysettling

Aeration Aeration

Disinfection

Finalsettling

Finalsettling

Primarysettling

Final effluent

Source : Metcalf&Eddy; Wastewater Engineering, /1/


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5.2 Process Design Criteria

Determination of the sizes of the treatment plant components and physical

facilities needed is the next step in the planing and developing the wastewater

treatment plant.

The sizes of each plant component are depending on the process design

criteria that are adopted.

It is necessary that all the key design criteria should be listed in a summarytable. A typical example of such a table ( for the process flowsheet shown in

Figure 49) is presented in the next Table 13./1/

Table 13 Example of summary table listing basic design data for WWT

Item Design value or description

Year 1985

Population served

- sewered 29,000

- unsewered 13,000

Per capita contributions

- sewered

Average daily flow, [l/capitad] 450

5-d BOD, [g/capita d] 100Suspended solids, [g/capita d] 120

- unsewered5-d BOD, [g/capita d] 18

Suspended solids, [g/capita d] 45

Total flows, [m3/d]

Average daily 15,000Maximum daily 30,000

Minimum daily 4,000

Maximum hourly 45,000Total loadings, [kg/d]

5-d BOD, (average daily) 3,100

Suspended solids,(average daily) 4,100Type of treatment Secondary (activated sludge)

Expected average removal efficiencies:

Percent BOD5removalPrimary 30

Overall 90

Percent SS removal

Primary 60Overall 90

(Continued)


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Table 13 (continued)

Item Design value or

description

Plant components

Communition equipment

Number of units, [pcs] 2Size, [mm] 900

Maximum unit capacity,[m3/d] 57,000

Bypass rack yes

Main pumping station

Number of variable speed pumps 3

Unit capacity range, [m3/d] 0-24,000

Type of variable-speed drive Wound rotor

Method of pump control Liquid rheostatFlow metering equipmentType Flow tubeSize, [mm] 600

Flow range ,[m3/d] 2,500 50,000

Aerated grit chamber

Number of units 1

Length,[m] 9

Width,[m] 3.5Average water depth,[m] 3

Detention time at max. hour flow,[min] 3.0Air-supply range, [m

3/ m h] 10 35

Air blowers

Number of units 2

Type CentrifugalUnit capacity range, ,[m

3/h] 100 350

Method of grit removal Clam shell bucket

Primary settling tanks

Number of tanks 2

Diameter, [m] 18

Sidewall water depth, [m] 3Bottom slope, [mm/m] 150

Detention period, [h]

At average flow 29.5At maximum hour flow 88.4

(continued)


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Table 13 (continued)

Item Design value or

description

Chlorination systemChlorinators

Number of units 2

Unit capacity range, [kg/d] 450Dosage, [mg/l] with largest unit

out of service

At average flow 30At max. hour flow 10

Chlorine contact tank

Number of units 2

Length, [m] 12

Width, [m] 6Depth, [m] 6.5

Detention time at max. hour flow, [min]Contact tank 11.5

Outfall 4.5

Total 16.0

Sludge dewatering units

Type Vacuum filters

Number of units 2Unit filtration area, [m

2] 35

Unit filtration capacity, [kg/h] 650Total filtration capacity, [kg/h] 1,300

Estimated operating period, [h/wk] 24

Source : Metcalf&Eddy; Wastewater Engineering, /1/


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5.3 Solids balance

After the design criteria are established, solids balances should be prepared for

each process flowsheet.

Such information must be available for the following reasons :

i) to assess the need for sludge storage facilities and their

capacity,j) to determine the proper size of the sludge piping and

pumping equipment.

The preparation of a solids balance for a flowsheet given in the Figure 50 is

illustrated in the next Figure 57./1/

5.4 Hydraulic profile

After the flowsheet has been selected and the size of the correspondingphysical facilities and interconnecting piping is determined, hydraulic profiles

should be prepared for both average and peak flowrates.

These profiles are prepared for following reasons :k) to ensure that the hydraulic gradient is adequate for the

wastewater to flow through the treatment facilities by

gravity,l) to establish the head requirement for the pumps where

pumping will be needed,m) to ensure that the plant facilities will not be flooded or

backed up during periods of peak flow.

Hydraulic profiles for the treatment plant given before as flowsheet in theprevious Figure 49 are shown in the next Figure 58./1/

5.5 Plant lay-out

Plant lay-out refers to the spatial arrangement of the physical facilitiesrequired to achieve a given treatment objective. The overall plant lay-out

includes all treatment plant units, infrastructure, the location of the control and

administrative buildings and any other necessary buildings and objects.

Among the factors that must be considered when laying out a treatment plant

are following :

n) geometry of the available treatment-plant sites,o) topography,

p) soil and foundation conditions,


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q) location of the influent sewer,

r) location of the point of discharge,s) transportation access,

t) types of processes involved,

u) effects of the length of process piping on treatment,

v)

process performance and efficiency,w) reliability and economy of operation,

x) aesthetics,

y) environmental control, andz) an additional area for future plant expansion.

Some examples of treatment plant layouts are shown in the next Figures 59,60 /1/ , and photographs in Figures 61 to64.

5.6 Civil works considerations

Preparation of construction plans, detailed drawings for plant-facilities, and

specifications of material used and necessary works which has to be provided,is the final step in the design of an treatment plant.

These plans and specifications become the official documents on which

contractors base their bids for the construction of the facilities. They are alsothe documents under which construction administrators hold the constructor

responsible for the completion of the project as specified.

5.7 Mechanical engineering considerations

Preparation of specification lists ( sometimes called also Bills of quantity )

of the selected equipment for the treatment plant units ( facilities ) is the next

step in the design of an wastewater treatment plant.

An example of Bill of quantities for a wastewater treatment plant is

presented in the Table 14.

These specification lists become later together with civil work plans and

specifications the official documents for construction of an wastewatertreatment plant.

5.8 Electrical engineering considerations

The operation of facilities accounts for the major component of energy

consumption at wastewater treatment plants.Because energy consumption of different unit processes and operations varies

greatly and because there are innumerable combinations of process


53/56


flowsheets, electrical engineering data must be available for each prospective

treatment operation or process.The basis of comparison of the wastewater treatment plant alternatives is the

consumption of primary energy per unit of time.

The computed energy requirements are important elements of the cost-analysisand of the environmental impact assessments.

5.9 Cost Analysis

Of major significance in the selection and design of alternative wastewatertreatment facilities is the question of costs not only initial construction

costs but also annual operation and maintenance costs.

When preparing the cost estimate, the same basis of comparison should be

used to evaluate all the alternatives.

5.10 Environmental Impact Assessment

An environmental impact assessment ( EIA ) is required for any activitythat significantly affects the quality of the human environment, and that is

supported by federal grants, subsidies, loans, permits, or licenses.

The preparation of environmental impact assessment reports is an

interdisciplinary activity ( environmental engineering, ecology, biology,geology, soil science, economics, sociology, etc. ).


1. What are components of WWT-planing ?

2. Explain short the analysis of treatment methods to be used in

planing of WWT-plant!

3. Draw a flowsheet for an municipal WWT-plant !

4. What are the process design criteria ?

5. Describe the preparation of solids balance sheet !

6. What are the reasons for preparation of the hydraulic profiles ?


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5. REFERENCES

/1/ METCALF & EDDY, INC.: Wastewater engineering

Treatment, Disposal and Reuse ; McGraw-Hill,Inc. USA,

1989.

/2/ DEGREMONT : Watern Treatment Handbook ; Paris,

1979.

/3/ Mackenzie L. Davis, D.A.Cornwell : Introduction to E

nvironmental Engineering ; McGraw-Hill, Inc., Singapore,1991.

/4/ H.S.Peavy, D.R.Rowe, G.Tchobanoglous : Environmental

Engineering ; McGraw-Hill Inc., Singapore, 1986.

/5/ National Productivity Council : Training Programme on

Water Pollution Prevention and Control ; Madras; 1996 .

/6/ A.K.Chatterjee : Water Supply, Waste Disposal and

Environmental Pollution Engineering ; Khanna Publishers,New Delhi, 1996.

/7/ E.W.Steel, T.J.McGhee : Water Supply and Sewerage ;McGraw-Hill Inc., Singapore , 1988.

/8/ S.C.Rangwala : Water Supply and Sanitary Enineering ;

Charotar Publishing House, Anand, 1997 .

/9/ W.W.Eckenfelder: Industrial Water Pollution Control ;McGraw-Hill Inc., Singapore , 1989.

/10/ H .F.Lund : Industrial Pollution Control Handbook ;McGraw-Hill Co., New York, 1984.

/11/ S.J.Arceivala : Wastewater Treatment for Pollution Control;Tata McGraw-Hill Publ. Co. Ltd., New Delhi, 1998.

/12/ M.N.Rao, A.K.Datta: Wastewater Treatment ; Oxford &IBH Publishing Co., Pvt.,Ltd., New Delhi , 1987 .

/13/ P.N.Cheremisinoff : Wastewater Treatment ; Pudvan

Publishing Co., Northbrook (USA), 1987 .


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/14/ B.S.N.Raju : Water Supply and Wastewater Engineering ;

Tata McGraw-Hill Publ.Co.Ltd., New Delhi , 1995 .

/15/ ATV : Lehr- und Handbuch der Abwassertechnik ; Band III

und IV, Verlag von Wilhelm Ernst & Sohn, Berlin

Mnchen, 1983.

/16/ W.L.McCabe, J.C.Smith, P.Harriott : Unit Operations of

Chemical Engineering; McGraw-Hill Inc., Singapore, 1993.

ii.biological wastewater treatment.pdf

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