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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.