Environmental Engineering – IIProf. Rajesh Bhagat

Civil Engineering Department

Y.C.C.E.,Nagpur

B. E. (Civil Engg.) M. Tech. (Enviro. Engg.)GCOE, Amravati VNIT, Nagpur

Experience & Achievement:

➢ Selected Scientist, NEERI-CSIR, Govt. of India.

➢ GATE Qualified Three Times.

➢ Selected Junior Engineer, ZP Washim.

➢ Three Times Selected as UGC Approved Assistant Professor.

➢ Assistant Professor, P.C.E., Nagpur.

➢ Assistant Professor, Cummins College of Engg. For Women (MKSSS, Nagpur)

Mobile No.:- 8483002277 / 8483003474 Email ID :- rajeysh7bhagat@gmail.com

Website:- www.rajeysh7bhagat.wordpress.com

UNIT-I

1) Systems of sanitation: Conservancy and water carriage system. Patterns of

sewage collection systems.

2) Quantity of storm water and sanitary wastewater.

3) Hydraulic Design of sewers - capacity, size, grade, shapes and materials.

2

UNIT-II

1) Sewer Appurtenances – manholes, street inlets, storm water overflows,

inverted siphons, flushing and ventilation.

2) Construction and Maintenance of sewers, equipment’s for maintenance &

safety equipment’s.

3) Sewage pumping.

4) House drainage systems, sanitary fitting and appliances, traps –function and

types, anti-syphonage, inspection chambers, Storm water drainage.

5) Rain water harvesting for individual houses & Different Methods.

3

UNIT-III

1) Characteristics of wastewater.

2) Flow sheet of conventional sewage treatment plant.

3) Preliminary and primary treatment: Screens, Grit chambers, Primary

settling tank. Design of bar screens, grit chambers and primary settling tanks.

4

UNIT-IV

1) Secondary Treatment: Principle of Biological Treatment. Activated

sludge process, Trickling Filter – Concept,

2) SD & SDB: Functioning and Basic Load Calculations. Sludge digestion,

Sludge drying beds.

3) Methods of disposal: Disposal on land and in water stream.

4) Self-purification capacity of stream.

5

UNIT-V

1) Rural sanitation: Pit Privy, Aqua Privy, Bio-gas Recovery, Eco-Sanitation.

Septic tank including soak pit. Imhoff tanks.

2) Industrial Waste Water Treatment: Basic concepts of Industrial Waste Water

Treatment, flow equalization, neutralization. Common treatment alternatives for

industrial waste water.

6

UNIT-VI

1) Introduction to Air Pollution,

2) Meteorological Parameters.

3) Monitoring methods.

4) Techniques of air pollution control.

7

References:-

1. B.C. Punmia, Waste Water Engineering, Laxmi Publication

2. S.K. Garg, Environmental Engineering – Vol – II, Standard Publication

3. G.S. Birdie, Water Supply & Sanitary Engineering, Dhanpat Rai Pub Company

4. M.N. Rao & H.V.N. Rao, Air Pollution, McGraw Hill Publication.

5. M.J. Machghee, Water Supply & Sewage, McGraw Hill Publication.

6. Dr. P N Modi, Sewage Treatment & Disposal & Wastewater Engg., Rajsons

Publications Pvt. Lts

CONCEPT OF TREATMENT OF WASTEWATER:

➢ Means the partial or complete removal of excessive impurities for final disposal

or suitable reuse of wastewater.

NEED OF TREATMENT:

1) If the quantity of sewage is more then receiving water body will be polluted or

land will be sewage sick because of limited self purification capacity of nature.

2) Discharge of large volume high strength sewage affects the environment

adversely.

3) Therefore it is necessary to treat the sewage before disposal.

OBJECTIVE OF TREATMENT OF WASTEWATER:

1) The main objective is to remove nuisance causing excessive impurities so that

sewage can be safely discharged or reused.

2) Should not create unhealthy or unhygienic conditions.

3) Should not pollute the water body.

4) Should be useful for agriculture purpose without any adverse effect.

5) All pathogenic bacteria should be killed.

CLASSIFICATION OF TREATMENT METHODS:

Treatment Methods

Unit Operation

Screening

Grit chamber

Detrius tank

Floatation

Sedimentation

Flocculation

Mixing

Heat transfer &

drying

Unit Process

Chemical Unit Process

Neutralization

Coagulation

Precipitation

Oxidation

Disinfection

Biological Unit Process

Suspended Growth process

Ex. ASP

Aerobic & anaerobic digester

Attached Growth Process

Ex. Trickling filter

RBC

Unit operation is the method in which physical forces are predominant

UNIT OPERATIONS:

➢ The removal of contaminates is brought about by application of physical forces or

physical forces predominate.

1) Screening

2) Mixing

3) Grit chamber

4) Detritus tank

5) Sedimentation tank

6) Floatation

7) Flocculation

8) SDB

UNIT PROCESSES:

➢ The removal of contaminates is brought about by chemical or biological reaction

➢ Based on type of agent used they are again classified

1. Chemical Unit Process: reduction or removal is brought about by chemical

reaction by adding chemicals.

2. Biological Unit Process: reduction or removal is brought about by

microorganisms.

A.Suspended Growth Process: The microorganisms are maintained in suspension

by appropriate mixing technique. Ex. ASP, Lagoons, Oxidation ponds, Anaerobic

digester, etc.

B. Attached Growth Process: The microorganisms are attached to an inert packing

materials Ex. Trickling Filter, RBC, etc.

SCHEMATIC FLOW DIAGRAM OF A TYPICAL CONVENTIONAL

SEWAGE TREATMENT PLANT

Raw

Sewage

Screening

Grit or Detritus

tankPST ASPAT SST

ASD ST

SDB

Chlorine Disinfection

Sludge

Sludge

Sludge

Sludge

Liquor

Supernatant

Mannure or Dried Sludge for disposal

ASP

Recirculation

Effluentfor

disposal

ASD = Anaerobic Sludge DigesterSDB = Sludge Drying BedAT = Aeration TankASP = Activated Sludge ProcessSST = Secondary Sedimentation TankPST = Primary Sedimentation TankST = Sludge Thickner

The Wastewater Treatment Process

Functions of Treatment Units:-

➢ Sump & Pump Well:- To collect wastewater in sump well & to pump the sewage to

treatment units from pump wells.

➢ Approach Channel:- To dampen the wastewater flow before it is applied to

subsequent treatment units so that wastewater flow remain as uniform as possible in

the following units.

➢ Screen Chamber:- To remove large size & floating material in wastewater. To

protect the following units getting damaged by abrasion & clogging.

➢ Grit Chamber:- To remove small & heavy inorganic solids ( size 0.2 mm &

specific gravity 2.6 ) from the wastewater.

➢ Skimming Tank:- To collect & remove lighter particles, oil & grease from

wastewater. (Oil & Grease Trap)

➢ Detritus Tank:- To remove heavy organic & inorganic natter from wastewater.

➢ Primary Settling Tank / Sedimentation Tank:- To remove fine suspended

inorganic & settleable organic solids.

Functions of Treatment Units:-

➢ Biological Treatment Units:- To remove colloidal & soluble organic solids by

aerobic & anaerobic process.

➢ Secondary Sedimentation Tank / Settling Tank:- To allow the microorganisms

and other solids to settle & segregate sludge from wastewater.

➢ Sludge Thickener:- To thicken the biological or settled sludge by separation or

removal of liquid or moisture content.

➢ Sludge Digester:- To treat the sludge by stabilizing before final disposal. To reduce

the amounts of sludge to dewater or dispose.

➢ Sludge Drying Bed:- To dry & reduce the volume of the treated sludge by

dewatering before final disposal.

TREATMENT SYSTEM:

➢ The type of combination used from the available unit operation and processes for

treatment of a particular wastewater is known as the treatment system.

Types of treatment system:

1. Preliminary Treatment

2. Primary Treatment

3. Secondary Treatment

4. Tertiary or Advanced or Final Treatment

PRELIMINARY TREATMENT:

➢ To remove floating material, large size particles & heavy inorganic contents of

wastewater that cause operational problems in primary & secondary treatment.

➢ It is also known as pretreatment in conventional treatment system.

➢ The preliminary Treatment includes:

1. Screen Chamber: to remove large size floating materials.

2. Grit Chamber: to remove grit & sand.

3. Skimming tank: to remove oil & grease.

4. Detritus Tank: to remove heavy organic & inorganic matter

PRIMARY TREATMENT:

➢ To remove settleable suspended solids.

➢ The preliminary treatment unit as well as primary settling tank are included in the

primary treatment system.

➢ Primary settling tank is also known as primary clarifier.

➢ Removal of organic matter in conventional treatment takes place in two steps.

➢ The settleable suspended organic matter is removed by PST while colloidal &

soluble organic fraction is removed in secondary treatment system.

SECONDARY TREATMENT:

➢ To remove colloidal & soluble organic matter.

➢ Biological processes are used to remove remaining contents.

➢ Different treatment units are available:

1. ASP

2. Trickling filter

3. SST

4. RBC

5. Oxidation Pond or Waste Stabilization Pond

6. Oxidation Lagoons or Aerated lagoons

7. Oxidation ditches

TERTIARY TREATMENT:

➢ To remove the concentration of residual impurities after secondary treatment.

➢ Used for treatment for industrial wastewater.

➢ Very expensive.

➢ Different treatment units are available:

1. Ultra and Micro Filtration

2. Ion Exchange

3. Reverse Osmosis

4. Electro dialysis

5. Adsorption

6. Chemical Precipitation

7. Nitrification / Denitrification

BIOLOGICAL OR SECONDARY WASTEWATEWR TREATMENT:-

➢ To remove colloidal & soluble organic matter.

➢ Biological processes are used to remove remaining contents.

➢ Microorganism breakdown complex organic matter into simple and more stable

substances.

➢ Sewage can remain in aerobic or anaerobic condition depending on the availability or

non-availability of Oxygen.

➢ Different treatment units are available:

1. ASP

2. Trickling filter

3. SST

4. RBC

5. Oxidation Pond or Waste Stabilization Pond

6. Oxidation Lagoons or Aerated lagoons

7. Oxidation ditches

24

BIOLOGICAL WASTEWATER TREATMENT:-

To remove organic matter from the wastewater which is present in soluble and colloidal

form or to remove nutrients such as nitrogen and phosphorous from the wastewater.

PRINCIPLES OF BIOLOGICAL WASTEWATER TREATMENT:-

Under proper environmental conditions, the soluble organic substances of the wastewater are

completely destroyed by biological oxidation; part of it is oxidized while rest is converted

into biological mass, in the biological reactors. The end products of the metabolisms are

either gas or liquid; on the other hand, the synthesized biological mass can flocculate easily

and are separated out in clarifiers. Therefore, the biological treatment system usually consists

of a biological reactor, and sedimentation tank, to remove the produced biomass called

as sludge.

ACTIONS BY BACTERIA:-

1) In aerobic condition the bacteria consume oxygen & remain active without causing any

foul smell.

2) The colloidal & dissolved putrescible organic matters present in the sewage are absorbed

within few minutes in the trickling filters and within about one hour in the ASP.

3) Bacteria absorbed the organic matter which is necessary for their life.

4) Bacteria converts the organic matter into stable forms by oxidizing them.

5) Finally nitrates, CO2 & sulphate, etc remain in the sewage. The gas escape in the

atmosphere.

6) In anaerobic condition foul smell is created, due to which only aerobic condition are

prepared.

26

Trickling Filter :-

➢ Filter mostly used in sewage treatment plant can be classified as

1) Intermittent Sand Filters

2) The Contact Beds

3) The Trickling Filter.

1) Intermittent Sand Filters:- Similar in construction to the slow sand filter of water

treatment. Requires large area, due to which are not commonly used now days. Most

suitable small area like hospitals, institutes, etc.

2) The Contact Beds:- In ancient time contact beds were very popular in the treatment of

sewage. Construction is similar to the intermittent sand filter.

3) Trickling Filters:- Used for complete treatment for domestic waste and as roughing filter

for strong industrial waste prior to activated sludge units.

Fig. 4.2 TF (16)

27Fig. 4.2 TF (16)

.

Trickling Filter:-

1) Used for biological treatment of wastewater, which are amenable to aerobic biological

processes.

2) Used for complete treatment or moderately strong wastewater and as roughing filter for

strong wastewater prior to ASP.

3) Posses a unique capacity to handle shock loads and provide dependable performance with

minimum supervision.

4) The primary sedimentation tank is provided prior to trickling filter so that the settleable

solids in the sewage may not clog the filter.

5) The trickling filter is followed by secondary settling tank for removal of settleable bio-

solids produced in filtration process.

Fig. 4.3 TF (16)

.

Working of Trickling Filter:-

1) Trickling filter is an attached growth process for treatment of wastewater.

2) In trickling filter, the sewage is allowed to sprinkle or to trickle over a bed of course,

rough, hard material and it is then collected through the under drainage system.

3) After getting trickled over the filter filtering media sewage starts percolating downward

through the contact media, while moving down it is acted upon by the bacteria.

4) The dissolved & colloidal organic matter are precipitated & flocculated by the action of the

bacteria & forms a gelatinous layers at the surface of the contact media.

5) The aerobic bacteria eat up this gelatinous layer & oxidize it & convert it to sulphate,

phosphete, nitrate, etc.

6) The oxidation of the organic matter is carried out under aerobic conditions.

7) With the time, the gelatinous slime become thicken & heavier and move towards the

bottom of the trickling filter & pass out with the effluent & settle in the secondary settling

tank.

Fig. 4.3 TF (16)

.

Construction of Trickling Filter:-

1) Essentially consists of a masonry or RCC tank which circular or rectangular in plan.

2) Coarse aggregates of impervious nature is filled in this tank which acts as filtering media.

3) The under drainage system is provided in the bed to collect the effluent.

4) In high rate trickling filter, the depth of tank varies from 0.9 to 1.8 m.

5) In low rate trickling filter, 1.8 to 3m.

6) In filter sufficient natural ventilation should be there to increase efficiency

7) The most common filter media crushed stones, blast furnace slag, anthracite coal, clinker,

etc are used.

8) The size of media should be between 30 mm to 100 mm.

(Biological slime consisting of aerobic bacteria and other biota builds up around the media

surface. Organic material in the sewage is absorbed on the biological slime, where they are

partly degraded by the biota, thus increasing the thickness of the slime. Eventually there is

a scouring of the slime and fresh slime layer begins to grow on the media. This

phenomenon of scouring of the slime is called sloughing of the filter.)

.

Types of Trickling Filter:-

1) High Rate Trickling Filter and Standard or Ordinary or Low Rate Trickling filter.

2) Construction details are same except that provision is made in high rate trickling filter for

recirculation of sewage through the filter by pumping a part of the filter effluent to the PST

and re-passing through the filter.

Recirculation in High Rate Trickling Filter:-

1) Recirculation is returning portion of the treated or partly treated sewage to the treatment

process.

2) It absorbs the fluctuating in sewage flow & thus keeps the bed working.

3) It dilutes the influent with better quality waste and this making it fresh and reduces odor.

4) It maintains a uniform rate of organic and hydraulic loading.

5) It provides longer contact of the applied sewage with the bacterial film on the contact

media and accelerating the biological oxidation process.

6) It increases the efficiency by reducing the BOD load generally.

.

Bio-filter:-

1) Bio-filter are essentially high rate filter but comparatively shallower than trickling filter.

2) Two stage treatment.

3) Two high rate trickling filter are connected in series.

4) Efficiency of filtration can be increased by passing same effluent in another trickling filter

connected in series.

5) Depth of tanks are reduced to 0.9 m to 1.2 m..

6) More flexibility in operation. ( Size also reduced due to two units in place of one)

7) Concentrated sewage – works in series & weak sewage – works in parallel.

8) As the depth have been reduced they are economical

9) Effluents are getting uniformly loaded.

.

Troubles at Filter Site & Their remedies:-

Odour Nuisance:-

➢ Due to anaerobic decomposition, sludging & undesirable growth.

➢ Effluent obtained from the filter can be re-circulated to continue aerobic condition.

➢ Sewage can be kept fresh by chlorination.

Ponding Nuisance:-

➢ When all voids are filled up due to choking, sewage can not pass through filter &

accumulates at surface in the shape of pond.

➢ By drying the media by exposing to the sun & loosening the aggregates with steel bars.

➢ By washing & flushing the filter media with high velocity jet.

➢ By applying heavy dose of chlorine after every 2 – 3 days.

Fly Nuisance:-

➢ flies breed & developed at the sites.

➢ By chlorinating the sewage or by insecticides.

SN Parameter Standard Rate

Trickling Filter

High Rate

Trickling Filter

1

2

3

4

5

6

7

8

9

10

11

Operation Cost

Space Required

Flexible

Skilled Supervision Required

Organic Loading Rate, Kg of BOD 5 per

ha-m per day

Depth of Filter

Size of Filter Media

Hydraulic Loading Rate, ML/ha/day

Recirculation

Effluent

Interval Dosing

More

More

Less

Less

900 to 2200

1.8 to 2.4

30 to 80 mm

22 to 44

Not Provided

Highly Stabilized

Intermittent

5 minutes

Less

Less

More

More

6000 to 18000

0.9 to 1.8 m

30 to 60 mm

110 to 330

Provided

Not Highly Stabilized

Continuous

15 Seconds

35

Activated Sludge Process:-

1) Aerobic biological treatment system.

2) Units of process are aeration tank, secondary settling tank, sludge return, etc.

3) Wastewater is aerated in an aeration tank for a period of few hours. Microorganisms

stabilize the organic matter.

4) After aeration, sewage is taken to the secondary settling tank where separation of

biological mass (solids)

5) Oxidation reaction is the main mechanism of BOD removal in the activated sludge

process.

6) Activated sludge is the sludge obtained by settling sewage (Solids) in presence of an

abundant oxygen. Biologically active & contains great numbers of aerobic bacteria &

other microorganism which have an unusual property to oxidize the organic matter. (96

% moisture content)

7) Activated sludge is re-circulated before aeration tank.

Fig. 4.1 ASP (16)

36

Figure of Activated Sludge ProcessFig. 4.1 ASP (16)

1) The biomass generated in the aeration tank is generally flocculent.

2) Biomass is separated in a SST & is recycled partially to the aeration tank.

3) The mixture of recycled sludge & sewage in the aeration tank is referred as mixed liquor.

4) Aeration units are main units of ASP, to supply oxygen to the sewage to keep the reactor

content aerobic & to mix up the return sludge & wastewater thoroughly.

5) Once the required concentration of microorganism in the mixed liquor has been reached its

further increase is prevented by the regulating quantity of sludge recycled.

6) Detention period between 6 to 8 hours.

7) The volume of aeration tank is also decided by considering the return sludge, which is

about 25 to 50% of the wastewater volume.

8) Normally liquid depth should be between 3 and 4.5 m. (F.B. = 0.3 to 0.6 m)

9) The mode of air supply in aeration tank can be either diffused air aeration, by

supplementing compressed air from tank bottom, or by mechanical aerators provided at

surface.

Modifications in ASP:-

Several modifications to the conventional system have been developed to meet specific

treatment objectives. Major modification are follows:-

1) Tapered aeration or Controlled aeration: BOD of mixed liquor is not uniform throughout,

it is max. near inlet & minimum near the outlet. Therefore attempts made to supply air to

match oxygen demand along the length of the tank.

2) Step aeration: Settled sewage is introduced at several points along the tank length which

produces more uniform oxygen demand throughout.

3) Contact stabilization: provides for re-aeration of return activated sludge separately from

the final clarifier, which allows a smaller aeration or contact tank.

4) Completely mixed: It is achieved by distributing the sewage and the return sludge

uniformly along one side & withdrawing the aerated sewage uniformly along the opposite

side of tank.

5) Extended Aeration process operates at a low organic load producing lesser quantity of well

stabilized sludge.

Modifications in ASP:-

6) Two stage aeration: In two stage aeration the sewage is allowed to flow in a pair of

aeration & sedimentation units.

7) Modified Aeration: In this method less quantity of returned sludge, shorter detention

period & lesser amounts of compressed air is used therefore it is called high rate treatment.

Classification of Aeration Units:-

➢ Diffuser Air Units

➢ Mechanical Aeration Units

➢ Combined Mechanical & Diffuser Air Units

Sludge Digestion:-

1) Decomposition of complex organic substances present in sludge into simpler stable

compounds by bio-chemical reaction brought about by the anaerobic bacteria.

2) Volume of sludge is reduced by 60 to 75% due to solid are converted into liquid & gases.

3) Gases can be used as fuel & digested sludge has very good fertilizing value.

4) Sludge is broken up into 3 parts: digested sludge, gases & Supernatant liquor.

5) The tank in which sludge digestion is carried out is a called sludge digestion tank.

6) Three stages are occurred in the biological process of sludge digestion.

1) Acidification: Sludge begins to decompose anaerobically, bacteria attacks easily on

available food substances such as carbohydrates & soluble nitrogenous compounds.

Intensive acids are produced.

2) Liquification: Organic acids & nitrogenous compounds are liquified ie transformed

from large solids particles to either a soluble or finely or finely dissolved form. pH

rises a little hence this process is also called acid regression.

3) Gasification: Due to breaking proteins & organic acids, large volume of methane &

small of volume of CO2 are evolved.

.

Anaerobic Digestion Process:- (Sludge Digestion Tank)

➢ The purpose of the anaerobic process is to convert sludge to end products of liquid and

gases while producing as little biomass as possible.

➢ The process is much more economical than aerobic digestion.

The process has now been described by the following four steps:

1. Hydrolysis: large polymers are broken down by enzymes.

2. Fermentation: Acidogenic fermentations are most important, acetate is the main end

product. Volatile fatty acids are also produced along with carbon dioxide and hydrogen.

3. Acetogenesis: Breakdown of volatile acids to acetate and hydrogen.

4. Methanogenesis: Acetate, formaldehyde, hydrogen and carbon dioxide are converted to

methane and water.

Stages of Sludge Digestion Process

Sludge Drying Bed:-

1) Sludge drying beds are commonly used in small wastewater treatment plants to dewater the

sludge prior to final disposal.

2) Drying of the digested sludge is made on open beds of land, these open beds are called as

SDB.

3) Two mechanisms are involved in the process, such as filtration of water through the sand,

and evaporation of water from sludge surface.

4) A typical sludge drying bed consist of 15 to 30 cm of coarse sand layer underlain by

approximately 20 to 45 cm of grade gravel ranging in size from 0.6 to 4 cm.

5) Open jointed tubes of 10 to 15 cm diameter spaced at 2.5 to 6 cm are laid in the gravel to

provide drainage for liquid passing through the bed.

6) Sludge is applied to the drying bed in layer of 20 to 30 cm, depending upon local climatic

conditions the sludge is allowed to dry for two to four weeks.

7) Enclosing drying beds with glass can improve the performance of the dewatering process,

particularly in cold or wet climates.

.

Factors Affecting Self Purification of Stream:-

1) Dilution: When sufficient dilution water is available in the receiving water body, where the

wastewater is discharged, the DO level in the receiving stream may not reach to zero or critical

DO due to availability of sufficient DO initially in the river water before receiving discharge of

wastewater.

2) Current: When strong water current is available, the discharged wastewater will be thoroughly

mixed with stream water preventing deposition of solids. In small current, the solid matter from

the wastewater will get deposited at the bed following decomposition and reduction in DO.

3) Temperature: The quantity of DO available in stream water is more in cold temperature than in

hot temperature. Also, as the activity of microorganisms is more at the higher temperature, hence,

the self-purification will take less time at hot temperature than in winter.

4) Sunlight: Algae produces oxygen in presence of sunlight due to photosynthesis. Therefore,

sunlight helps in purification of stream by adding oxygen through photosynthesis.

5) Rate of Oxidation: Due to oxidation of organic matter discharged in the river DO depletion

occurs. This rate is faster at higher temperature and low at lower temperature. The rate of

oxidation of organic matter depends on the chemical composition of organic matter.

Different zones of pollution in river stream:- (Oxygen Sag Curve)

A river stream undergoing self purification can be divided into following four zones of pollution.

1) Zone of degradation: This zone exists upto a certain length beyond the point just below the sewage

out fall. Water becomes dark & turbid along with foundation of sludge deposits at the bottom.

2) Zone of active decomposition: This highly polluted zone. Water becomes grayish & much darker.

DO falls down to almost zero.

3) Zone of recovery: stream rover from its degradation condition. Water becomes clear & algae

reappears. DO level increases.

4) Zone of cleaner water: River comes to original level. DO level reaches to saturation level ^ usual

aquatic life prevails.

Deoxygenation and Reoxygenation Curve:

1) When wastewater is discharged in to the stream, the DO level in the stream goes on

depleting. This depletion of DO content is known as deoxygenation.

2) The rate of deoxygenation depends upon the amount of organic matter remaining (Lt), to

be oxidized at any time t, as well as temperature (T) at which reaction occurs.

3) The ordinates below the deoxygenation curve (Figure) indicate the oxygen remaining in

the natural stream after satisfying the bio-chemical demand of oxygen.

4) When the DO content of the stream is gradually consumed due to BOD load, atmosphere

supplies oxygen continuously to the water, through the process of re-aeration or

reoxygenation, i.e., along with deoxygenation, re-aeration is continuous process.

The oxygen sag or oxygen deficit in the stream at any point of time during self purification

process is the difference between the saturation DO content and actual DO content at that

time.

Oxygen deficit, D = Saturation DO –Actual DO

52

The rate of reoxygenation depends upon:

1) Depth of water in the stream: more for shallow depth.

2) Velocity of flow in the stream: less for stagnant water.

3) Oxygen deficit below saturation DO: since solubility rate depends on difference

between saturation concentration and existing concentration of DO.

4) Temperature of water: solubility is lower at higher temperature and also saturation

concentration is less at higher temperature.

53

Disposal Methods of Wastewater:

1) Into Surface water.

2) On Land.

3) Others: Reuse for farming, gardening, etc.

PCB EPA WHO

Parameter Water body Land Water body Land Land

COD - - - - 250

BOD 30 100 20 100 100

TS - 250 400 1000 1000

SS 40 100 25 50 100

pH 6-9 6-9 6-9 6-9 6.5-8

EFFLUENT STANDARDS for DISPOSAL

Need of study of characteristics of wastewater ?

➢ Information about strength, composition & characteristics of wastewater is important in the

design of treatment system & the amount of pollutants to be removed up to prescribed level

set by the local authority.

TERTIARY TREATMENT:

➢ To remove the concentration of residual impurities after secondary treatment.

➢ Used for treatment for industrial wastewater.

➢ Very expensive

➢ Different treatment units are available:

1. Ultra and Micro Filtration

2. Ion Exchange

3. Reverse Osmosis

4. Electro dialysis

5. Adsorption

6. Chemical Precipitation

7. Nitrification / Denitrification

Question Bank:-

1) What is activated sludge? Explain with neat sketch ‘Activated Sludge process’?

2) What do you understand by ‘High rate trickling filter’? Discuss with neat sketch?

3) Draw a neat sketch of Trickling Filter. Explain its working and operational troubles.

4) State why COD values are more than BOD values?

5) What is sludge digestion? Why it is necessary? Explain sludge digestion process with neat sketch

of ‘Sludge Digester’?

6) Describe anaerobic, aerobic and facultative process?

7) What do you understand by ‘Biological treatment of wastewater’? Explain in brief various

processes involved in the treatment?

8) Explain a Waste stabilization ponds or Oxidation ditch or Aerated lagoon.

9) What do you mean by ‘sewage farming’? State and explain various methods of application

sewage to form?

10) Discuss in brief the various methods adopted for disposal of sewage.

11) Comment on wastewater disposal on land and into water body?

12) Write a short notes on

a) Sewage farming b) Sewage sickness c) Wastewater disposald) Trickling filter e) Sludge drying bed f) Waste stabilization ponds