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Causes of Water Pollution
Factors that contribute to water pollution
can be categorized into two different
Point sources are the easiest to identify
Non point sources are ambiguously
defined and harder to control
Some point sources of water pollution
Waste products from factories
Waste from sewage system
Waste from power plantsWaste 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
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
air pollutants getting washed or deposited to earth
storm water drainage from lawns, parking lots, and
Organic Pollution and Oxygen Demand
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
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 dueThe 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 12
Conditions favouring dilution without
1. Where wastewater is quite fresh.
2. SS have been removed from wastewater
3. Volume of receiving water body is more than
the wastewater discharge
Dilution water having high DO, to satisfy the4. Dilution water having high DO, to satisfy the
BOD of wastewater
5. Where swift forward currents are available
6. Wastewater does not contain toxic substances
7. Water is not used for drinking immediately
after point of discharge. 13
Standards of dilution
Dilution 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
ACTIONS INVOLVED IN SELF
PURIFICATION OF STREAMS
Qw, BODw, DOw
Qr, BODr, DOr
2. Combined BOD and DO determination for a stream
• Combined waste flow
Qmix = Qr + Qw
• Combined BOD
Qmix, BODmix, DOmixQrBODr + QwBODw
Q1 + Q2
• Combined DO
Qr.DOr + Qw.DOw
Q1 + Q2
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 whichHigh 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. 17
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 henceremoved by settling and hence
downstream water quality will be
2. Due to settled solids anaerobic
decomposition may take place
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
Reduction occurs in the stream due to
hydrolysis of organic matter biologically or
Anaerobic bacteria will split the organic
matter into liquids and gases, thus pavingmatter into liquids and gases, thus paving
the way for stabilization by oxidation
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 willAt higher temperature purification will
take lesser time though amount of DO is
less in the water.
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 producesstimulates growth of algae which produces
oxygen during photosynthesis.
Hence wherever there is algal growth
water contains more DO during daytime.
Deficit (DDeficit (Daa)) Saturation DO (DoSaturation DO (Doss))
Typical DO sag curve
DO Concentration (DO)DO Concentration (DO)
2 4 6 8 102 4 6 8 10
Travel Time (d)Travel Time (d)
Q1. Explain self purification of stream process.
Q2. Discuss- What are point and non point
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 equation
4. DO sag curve