L-17Self Purification of Streams and

DO Sag Curve DO Sag Curve

Industrial Waste Treatment

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Can you identify the river (from India)?

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Water Pollution

Causes of Water Pollution

� Factors that contribute to water pollution can be categorized into two different groups� Point sources� Non-point sources

� Point sources are the easiest to identify and control

� Non point sources are ambiguously defined and harder to control

Point Sources

� Some point sources of water pollution include� Waste products from factories� Waste from sewage system� Waste from power plants� Waste from power plants� Waste from underground coalmines� Waste from oil wells

� They are called point sources because they are direct sources of water pollution and can be reduced and monitored

Example of a point source

Non-point Sources

� The term non-point source comprises of large range of sources such as:� when rain or snow moves through the ground and

picks up pollutants as it moves towards a major body of waterbody of water

� the runoff of fertilizers from farm animals and crop land

� air pollutants getting washed or deposited to earth � storm water drainage from lawns, parking lots, and

streets

Non-point source: Agricultural runoff

Point and Non-point Source Pollutions

Organic Pollution and Oxygen Demand

Bacteria

C6H12O6 +6O2 ���� 6CO2 + 6H2O

180 g 192 g

1g glucose = 192/180g O2

1g glucose consumes 1.07g O21g glucose consumes 1.07g O2

1g glucose has BOD of 1.07g

Organic matter in waste + O2 in stream ���� CO2 + H2O

DO level ≤ 2.0 mg/L, some fish and other aquatic animal Species may be distressed and some species may even die due to suffocation.As a rule of thumb DO level of 4 mg/L should be maintained

Organic Pollution and Fish Kill

Disposal by dilution

� Disposal by dilution is a process in which the treated wastewater from ETPs is discharged in a large static body of water or in moving water bodies such as rivers or streams.

� The discharged wastewater is purified in due � The discharged wastewater is purified in due course of time, by the self purification process of natural waters.

� The effluent discharge and degree of treatment of wastewater depends upon the self purification capacity of the river and its intended water use

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Conditions favouring dilution without

treatment1. Where wastewater is quite fresh.2. SS have been removed from wastewater3. Volume of receiving water body is more than

the wastewater dischargeDilution water having high DO, to satisfy the 4. Dilution water having high DO, to satisfy the BOD of wastewater

5. Where swift forward currents are available6. Wastewater does not contain toxic substances7. Water is not used for drinking immediately

after point of discharge.13

Standards of dilutionDilution factor Standards of purification required

Above 500 No treatment required. Raw sewage can be directly discharged into river

Between 300 to 500 Primary treatment such as PST is required so that SS concentration is less than 150 mg/litthan 150 mg/lit

Between 150 to 300 Treatment such as screening, sedimentation and chemical precipitation are required so that SS concentration is less than 50 mg/lit

Less than 150 Thorough treatment is required SS should be less than 50 mg/lit and BOD 5 should be less than 20 mg/lit

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ACTIONS INVOLVED IN SELF

PURIFICATION OF STREAMS

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Qw, BODw, DOw

Qr, BODr, DOr

1.Dilution :-2. Combined BOD and DO determination for a stream

Residents• Combined waste flow

Qmix = Qr + Qw

• Combined BOD

Qmix, BODmix, DOmixQrBODr + QwBODw

Q1 + Q2

BODmix =

• Combined DO

Qr.DOr + Qw.DOw

Q1 + Q2DOmix =

2. Dispersion due to currents

� Self purification largely depends upon currents, which readily disperses wastewater in the stream, preventing locally high concentration of pollutants.

� High velocity improves aeration which � High velocity improves aeration which reduces the concentration of pollutants.

� High velocity improves raeration which reduces the time of recovery, though length of stream affected by the wastewater is increased.

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3. Sedimentation

� If stream velocity is lesser than the scour velocity of particles then sedimentation will takes place, which has two effects

1. SS contribute largely to BOD will be removed by settling and hence removed by settling and hence downstream water quality will be improved.

2. Due to settled solids anaerobic decomposition may take place

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4. Oxidation

� The organic matter present in the wastewater is oxidized by aerobic bacteria utilizing dissolved oxygen of the natural waters.

� This process continues till complete oxidation of organic matter takes place.organic matter takes place.

� The stream which is capable of absorbing more oxygen through reaeration etc can purify heavily polluted water in short time

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5. reduction

� Reduction occurs in the stream due to hydrolysis of organic matter biologically or chemically.

� Anaerobic bacteria will split the organic matter into liquids and gases, thus paving matter into liquids and gases, thus paving the way for stabilization by oxidation

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6. temperature

� At low temp activity of bacteria is low., and hence decomposition is slow., though DO will be more because increased solubility of oxygen in water.

� At higher temperature purification will � At higher temperature purification will take lesser time though amount of DO is less in the water.

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7. Sunlight

� Sunlight helps certain micro-organisms to absorb CO2 and give out oxygen, thus resulting in self purification.

� Sunlight acts as disinfectant and stimulates growth of algae which produces stimulates growth of algae which produces oxygen during photosynthesis.

� Hence wherever there is algal growth water contains more DO during daytime.

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ZONES OF POLLUTION IN

THE STREAM

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Biota vs. D.O.

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DO SAG CURVE

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DO Sag Curve

Residents

Q1, BOD1

SS1

Industry

Q2, BOD2

SS2

Dis

solv

ed o

xygen, m

g/L Saturation DO (Cs)

Travel time, days

Initial

deficit D0

Oxygen addition(reaertaion)

Oxygen depletion

Criticaldeficit (Dc)

tc

Self purification of streams:Contributors

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Initial Initial Deficit (DDeficit (Daa)) Saturation DO (DoSaturation DO (Doss))

DeficitDeficit

CriticalCritical

1010

88

66

44

Dis

solv

ed O

xyge

n (m

g/L

)D

isso

lved

Oxy

gen

(mg/

L)

Typical DO sag curve

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DO Concentration (DO)DO Concentration (DO)

CriticalCriticalPointPoint

ttcc

2 4 6 8 102 4 6 8 10

Travel Time (d)Travel Time (d)

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22

Dis

solv

ed O

xyge

n (m

g/L

)D

isso

lved

Oxy

gen

(mg/

L)

Theory questions

Q1. Explain self purification of stream process.Q2. Discuss- What are point and non point

sources?Q3. Write short notes on

Actions involved in self purification of stream.1. Actions involved in self purification of stream.2. Zones of pollution in the stream.3. Streeter Phelps equation4. DO sag curve

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Objective questions

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