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Dr.Vandana

Environmental Pollution Control ETZ C362

Chapter 9 : Wastewater Treatment

Lecture 12

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Summary of Lecture 11

Remove the contaminants from water so that the treated water can meet the acceptable quality

standards.

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Treatment

Physical

Biological

Chemical

Screening, Sedimentation,

flotation and filtration

Precipitation,Coagulation and

disinfections

Biofiltration and Activated

Sludge Process

Physical &Chemical

Air stripping, Carbon

adsorption, Oxidation andreduction, Ion exchange, and

membrane Processes

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ET ZC362: Environmental Pollution Control

Another classification of waste water treatment processes

Primary Treatment

Removes identifiable suspended solids and floating matter

Secondary Treatment Also known as biological treatment (organic matter that is soluble or in the

colloidal form is removed

Tertiary Treatment

Also known as advanced waste treatment (may involve physical, chebiological processes)

Pretreatment

Primary Treatment Process

FloSedimentation

Discrete Settling Flocculent Settling Zone Sett

4

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Primary Treatment

5

Pretreatment

To remove large floating and suspended solids .Also include flow measureand prechlorination to prevent odours.

Consist of screening and grit removal.

Fixed bar screens are the most common types screens used in domestic w

water treatment facilities.

Bar Screen

- catches large objectssuch as bricks, bottles,pieces of wood, etc.

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Sedimentation

6

The settleable solids are removed by gravitational settling unde

quiescent conditions. The sludge formed at the bottom of the tank is removed either

by vacuum suction or by raking it to a discharge point at the

bottom of the tank for withdrawal.

The clear liquid produced is known as the overflow and it shou

contain no readily settleable matter.

The sedimentation may be carried out in rectangular horizontaflow, circular radial flow or vertical flow basins.

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Flotation

7

Used in place of sedimentation, for treating industrial waste

waters containing finely divided suspended solids and oilymatter.

This technique is used in paper industry to recover fine fibres

from the screened effluent and in the oil industry for the

clarification of oil bearing waste.

Two types of flotation : Dispersed air flotation and dissolved air

flotation.

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8

In dispersed air flotation, air is introduced directly into the

liquid through a revolving impeller or through diffusers.

This technique is not a favored tech. in the treatment of

municipal waste waters.

In dissolved air flotation, air is brought into contact with the

waste water at a pressure of several atmosphere when air is

dissolved.

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Role of Microorganisms

Microorganisms (mainly bacteria), is used in the stabilization of organic ma

Converts the colloidal and dissolved carbonaceous organic matter in to and into cell tissue known as protoplasm and can be removed from the tre

gravity settling.

Removal of the cell tissue from the solution is important, otherwise, the

itself is organic in nature will be measured as BOD in the effluent.

Decomposition of Organic WasteTwo methods

1. Aerobic process: Oxygen is used by the microorganisms for the decom

2. Anaerobic process: Oxygen is not used by the microorganisms for the

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Aerobic Processes

presence of oxygen rapid conversion

release lots of energy

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Anaerobic Processes

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Bacterial Population Dynamics

The bacterial growth pattern has four distinct phases

1. The lag phase- The bacteria initially acclimatize to their new surroundisynthesizing new cells.

2. The log-growth phase The bacterial cells divide exponentially.

3. The stationary phaseThe population remains constant as a result of

between the growth of new cells and the death of old cells.

4. The death phase As the limitations in the food supply increase, the b

rate exceeds the production of new cells.

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How do microorganisms grow when inoculated

into a batch of growth medium?

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Growth Kinetics : The Monod Equation

The rate of increase of cell biomass in the log growth phase is given by the relationship

rg=X

X - conc, of microbial mass, mg/l

- specific growth rate, s-1

rg - rate of growth of cell biomass, mg/l.s

Specific growth rate is a function of environmental conditions for the organism, includin

concentration. There is a maximum rate at which organisms can grow even with plenty of nutrients

substrate becomes limited, growth slows down a simple equation describing this behMonod model.

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(mg/L)ionconcentratsubstrateis

(mg/L)constantsaturationis

growthformodelMonod

s

s

max

s

K

sK

s

Sp

.growthrate

Limiting substrate conc., S

Specific growth rate as a function of limiting substrate concentration

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Aerobic biological Treatment

Oxygen supplied to the bacteria is consumed under controlled

and most of the BOD is removed in the treatment plant. The principal requirements of a biological waste treatment pro

adequate amount of bacteria that feed on the organic materia

waste water, oxygen, and some means of achieving contact b

bacteria and the organics.

The most commonly used systems for biological waste treactivated sludge system and biological-film system.

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Activated Sludge Process

The waste water is brought into contact

with a diverse group of microorganismsin the form of a flocculent suspension in

an aerated tank.

The essential features of this process

are: an aeration stage, solid-liquid

separation following aeration, and a

sludge recycle system.

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Air Supply and Process Modifications

In the initial stages of contact between microorganisms and the organic matter in the w

process requires a great deal of oxygen.

Conventional system

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Aeration is intensified at the inlet end and reduced along the length

of the aeration tank. Major improvement in reducing the cost of

pumping air as air is used more effectively.

Fresh feed is introduced at several points along the aeration tank.

Even distribution of oxygen demand is achieved through out the

aeration stage.

Fresh feed and recycled sludge are combined and introduced at several

points in the aeration tank from a central channel, and the effluent leaves

the tank from both sides. This ensures a uniform supply and demand of

oxygen along the tank.

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Contact Stabilization

Stabilization is carried out on the return sludge stream which h

concentration than that of the mixed liquid in the aeration tank, the

reduced. The stabilized sludge is then recycled to the aeration tank.

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Pure oxygen system

Use of pure O2promote increased bacterial activity, decreased sludge volumsludge settleability.

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Sludge volume Index (SVI)

Measure of the settling ability of the sludge: defined as the volume in m

by one g of sludge after it has settled in one lit. cylinder for 30 minutes.

SVI unit ml/g

SVI = Sludge volume after settling for 30min, ml/l X 1000

MLSS concentration, mg/l

SVI varies from 40 to 100 for a good sludge.

The poor settling ( high SVI), may be due to high concentration of he

temp. variations.

Bulking can be prevented by adequate pH control, sufficient aeration

chemicals like hydrogen peroxide to the aeration basin.

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Trickling Filters

Commonly used biological waste treatment process.

Good adaptability to handle peak shock loads and the ability to function sa short period of time.

They are affected by temperature and therefore cold weather slows

activity in the filter.

Milk processing, paper mill and pharmaceutical wastes are among t

trickling filters.

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Conventional rock filters have been designed based on hydraulic a

loadings and are classified as standard or low rate or high rate trickling filt Low rate filters have low hydraulic and organic loadings, while these are

high rate filters.

Low rate filters are suitable for treating weak waste waters whereas high r

are used for partial treatment of wastes before the waste is sent for further

Synthetic plastic materials can be used as packing media in trickling filter

filters are known as super rate filters. It has a higher degree of treatment capability because of the increase in th

surface area and is used for treating high strength wastes.

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Comparison of Trickling filters with Activated sludge systems

Trickling filters Activated Sludge Systems

1. Bacterial growth is fixed on the media 1. Bacterial growth is suspended as a

dispersed flow.

2. All solids from the settler are wasted 2. Solids from the settler are partially

recycled.

3. Less sensitive to shock loading, more

stable.

3. More sensitive to shock loadings,

require closer process control.

4. Produce insects and odours. 4. Produce spray clouds.

5. Less effective in removing disease

causing organisms.

5. More effective in removing pathogens

than trickling filters.

6. Low operating costs. 6. High operating costs.

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Trickling filter : The performance of trickling filters can be predicted by the E

Se/Si = exp [ -KLAsm( A/Q) n]

Where Se= Concentration of settled effluent from the filter, mg/l

Si= Concentration of the influent to the filter, mg/l

K = empirical rate constant , m/d

As= specific surface area of the filter= surface area Afof the filter, m2/u

A = cross sectional area of filter, m2

Q = waste water flow rate, m3/s

m,n = empirical constants, L= filter depth, m

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This is similar to the one developed by Schulze

Se/Si = exp [ -K1L ( A/Q) n] K1 = empirical rate constant with units

If m=n=1 both equations are same.

The temperture correction for K may be computed from KT= K25(T-25)

-temp coefficient, taken as equal to 1.08

The sp.surface area Ascan be calculated as As= 6 (1-)/ d

- the porosity

Sphericity of the packing media

d - geometric mean size of the packing media-

-

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Biosolids Treatment and Disposal

Types of Biosolids

Primary sludge : Solids that settle out in the primary settling basin

Biological or Secondary sludge: Solids that have grown in a secondary treatment pro

suspended growth)

Composition of sludge

Water

Micro organisms

Viruses, pathogens and germs in general

Organic particles, heavily biodegradable

Organic compounds, Heavy metals

Micropollutants, Pharmaceuticals,

All non-degraded compounds extracted from wastewater are found in the sludge

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The common operations of sludge treatment and disposal involve the follo

1. Concentration (thickening)2. Digestion

3. Conditioning

4. Dewatering

5. Oxidation

6. Ultimate sludge disposal

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1. Concentration : Removal of water from the sludge and reduce its volum

possible so that the sludge can be handled more efficiently.

Common methods used are gravity settling and flotation.

Gravity thickening : The sludge is subjected to gentle agitation by means o

which enhances settling. The thickening of activated sludge is complicated

action, especially under warm conditions when the bacteria in the sludg

organic matter and release gases.

Flotation: The sludge can be thickened by air flotation and this technique cathe sludge to bring its solid content to 4 %.

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2. Digestion : The sludge is stabilized by digesting it underaerobic or anaerobic conditions. In Anaerobic digestion

(common), the organic content of the sludge decomposes to give

mainly methane and carbon dioxide and at the same time the

bound water is released from the sludge. In aerobic digestion, the

sludge is aerated in a tank for about 20 days, at ambient

temperatures. During the process the bacterial cells are destroyed

and a substantial portion of the sludge is oxidized resulting in the

reduction of the solid content by about 30 %. Shallow lagoons are

employed as digesters. Large land areas are required for sludge

lagoons and odour problems may occur frequently.

Anaerobic S

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3. Conditioning ( to improve its dewatering characteristics) : Done by adding chemicals like Fpolyelectrolytes. These chemical bind the sludge particles together and encourage the relea

Heat treatment is a physical conditioning method. The sludge is heated under pressure andthe gel structure of the sludge breaks down so that the water is released. Heat treatment

sterilizing the sludge; at the same time the sludge is partially oxidized and completely stabiliz

4. Dewatering: The thickened sludge is dewatered for efficient handling and disposal.

mechanical methods, the most common being centrifugation and filtration which includes

vacuum filtration. In centrifugation, the conditioned sludge is added to a rotating bowl tha

into a cake and a dilute stream. The solid cake is transported within the bowl and is removeat one end of the bowl; the liquid is removed at the opposite end. Centrifugation is a co

requires careful control of process variables.

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Solid bowl centrifuge

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Drying beds are commonly used for dewatering. The bed consist of a filte

which the sludge is applied to a depth of up to 250 mm, depending on th

Dewatering takes place by a combination of drainage and evaporation.

Removal of dried sludge is carried out mechanically.

Heat drying may be utilized in applications where the sludge is to be inci

a saleable commodity can be produced.

Major problem associated with this process is the control of gases and as

may be generated in drying.

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5. Oxidation : Sludge may be oxidized to reduce the organic content with the consequent

and a significant reduction in their volumes .Incineration and wet oxidation are the comm

oxidation.Incineration is performed in a multiple hearth furnace ( fluidized beds or flash dryers may

multiple hearth furnace, the sludge passes downwards through a series of hearths. Vap

occurs in the upper hearths, followed by incineration in the lower ones.

Efficiency of evaporation and incineration 55 %

The fluidized bed system consists of a bed of sand fluidized by air. When the sludge is i

particles are dried instantly as they are dispersed and are oxidized.

Wet oxidation : Sludge is ground, mixed with air in stoichiometric proportions and th

temperature and pressure in a reactor.

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The sludge is oxidized to produce a mixture of gas, liquid and ash.

The products from the top of the reactor are passed through heatexchangers to recover heat .

Wet oxidation pro

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Ultimate sludge disposal : Wet digested sludge may be sprayed on to cropland where

a fertilizer or fertilizer base. Dried sludge may be used as a landfill or a solid conditioner. Wdewatered sludge or ash from incineration may be transported from the shore to dumping g

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Goals of sludge treatment

Volume reduction Thickening

Dewatering

Elimination of pathogenic

germs If used in agriculture as fertiliser or compost

Stabilisation of organic

substances

Gas production

Reduction of dry content

Improvement of dewatering

Reduction of odour

Recycling of substances Nutrients, fertiliser

Humus

Biogas

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Sequence of operations for sludge treatment

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Advanced Wastewater treatment

Wide variety of methods are used to remove suspended solids, BOD, plant nutrients dissolved solid

substances.

Removal of suspended solids

Two methods used : Micro straining, Chemical coagulation followed by settling and mixed media fil

Wash water

Microstaining

Microstainers can handle hig

influent, But cannot handle

content and shock loads.

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Coagulation & Filtration

To improve or make possible the removal of negatively charged colloidal suspensionnormally settle out and cannot be removed by conventional physical treatment.

Coagulation is done by the addition of positive ions, e.g.,Al3+

In industrial waste water treatment, coagulation is frequently used for oily emulsions, an

and nonsettelable solids such as pigments, paper fibre, meat and tannery effluen

beneficiation slimes.

The most widely used coagulants for waste water treatment are aluminium and iron salts

At high pH, these salts produce insoluble aluminium hydroxide or ferric hydroxide flocs.

Al2(SO4)3+ 6 H2O 2Al(OH)3+ 3 H2SO43 H2SO4+ 3 Ca(HCO3)2 3CaSO4+ 6 H2CO36H2CO3 6CO2+ 6 H2O

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The overall reaction can be represented as :

Al2(SO4)3+ 3Ca(HCO3)2 2 Al(OH)3+ 3 CaSO4+ 6 CO2

The aluminium hydroxide flocs entrap the solid particles. The ppt is the

produce large , dense settleable solids.

Sometimes with very low concentration of colloidal matter, floc form

therefore coagulant aids like polyelectrolytes are added to promote tflocculation process.

The addition of chemicals for promoting the coagulation- flocculation ca

suspended and colloidal solids , if properly performed.

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After the formation , the solution is led to a settling tank and it is allo

Most of the flocculated material is removed in the settling tank. Some flocs particles do not settle and these can be removed by filtr

carried out using beds of porous media (sand or coal).

Mixed media filter can be used as an alternative .

It consist of fine garnet in the bottom layer, silica sand in the middle lay

coal in the top layer.

It allows both surface straining and in-depth flitration, without caussurface plugging or solids break-through.

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Removal of Dissolved Solids

Both organic and inorganic types

Soluble organics can be removed from wastewaters by adsorption on activ

Organic chemicals like phenol and amines can be recovered by solvent ext

Removal inorganic constituents can be achieved by ion exchange, ele

reverse osmosis.

Adsorption on activated C

The dissolved substances in waste water are attracted to and adhere to t

adsorbent ( activated C).

Attractive method for removing soluble organics such as phe

hydrocarbons, surfactants, colour and odour producing substances from w

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Adsorption based on granular activated C can either be fixed bed type or ex

Adsorber configs. For granular C waste treatment

In the series operation, the fi

removed from service when the be

fresh column is placed at the end

second column becomes the lead

operations, the beds become satu

and are replaced by a fresh batch.

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Regeneration of activated C

Thermal regeneration can be done by dewatering the activated C and a multiple hearth furnace where the adsorbed organics are oxidized at

925oC.

The regenerated C is then water quenched and stored for return to

system.

In solvent regeneration, solvents such as methanol, acetone and benz

organic matter from the adsorbent and the solvent is then removed by s

With this, the cycle of absorption-desorption can be repeated many tim

has to be replaced.

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DuPonts powered activated c

Activated C can be added directly to waste waters which

eliminates the need of C adsorption beds or columns.

In DuPonts process, the powdered activated C is added

directly to the aeration tanks.

The simpler organic molecules which are difficult to adsorb

on activated C are easily removed by the biological system

where as the high molecular wt. compounds are removed by

the activated C.

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