The document discusses the self-purification process of streams. It defines self-purification as the automatic purification of polluted water over time by the stream itself. The key factors that support self-purification are dilution, current, sedimentation, temperature, sunlight, oxidation, and reduction. As the polluted water travels downstream, it progresses through four zones: the degradation zone near the discharge, the active decomposition zone, the recovery zone where water quality improves, and finally the clear water zone where the stream returns to its natural state.

1. Presentation on
Self-Purification of Streams
Under guidance of
Madan Mohan Malaviya University of Technology,
Gorakhpur
Submitted by-
Rishi Singh (2020013215)
Rachit Chaturvedi (2020013214)
Submitted to-
Dr Vinay Bhushan Chauhan
introduction
definition
factors
factors
oxygensag
process

2. introduction
definition
factors
factors
oxygensag
process
Definition- The process of automatic purification of polluted water
in due course of time by the stream itself which has been polluted
is called as self-purification process.
o When sewage is discharged into the river, the receiving water gets polluted due to
waste products present in sewage effluent.
o The organic matter present in the sewage gets oxidized by bacteria and converted to
simple, inoffensive, stable substances.
o In this process of oxidation, dissolved oxygen (DO) content of stream is utilized. So
DO of stream starts to decrease and BOD (biochemical oxygen demand) increases.
o A minimum of about 2.0 mg/L of dissolved oxygen is required to maintain life.
o The deficiency of DO thus created in stream water is filled up by the absorption of
atmospheric oxygen.
o As stream water travels further, BOD gradually reduces and DO increases and
reaches its saturation level. The stream water becomes free of sewage pollution and
comes in its natural purified state.

3. introduction
definition
factors
factors
oxygensag
process The various actions involved in self-purification process are physical,
chemical and biological in nature, and are as listed below:
1. Dilution 2. Current 3. Sedimentation
4. Temperature 5. Sunlight 6. Oxidation 7. Reduction
1. Dilution:
When the sewage is mixed up with large volume of water or it is much diluted, the
sewage always remains in aerobic condition and anaerobic condition never comes
because always dissolved oxygen remains present in the water.
(Qr.Cr + Qw.Cw)
(Qr + Qw)
2. Current:
It directly depends on the current. When there is no current the sewage matters deposit
near the outfall causing formation of sludge bank and foul odours. In heavy current, the
sewage is thoroughly mixed up with the stream water preventing all nuisances. In slow
current, the sedimentation takes place causing growth of algae resulting in the
production of oxygen. Therefore, slow current is better for the self-purification.
3. Sedimentation:
With slow current, the heavier solids settle in the stream bed and start anaerobic
decomposition. The products of decomposition are again mixed with water by the
current. If the dilution is sufficient, the anaerobic condition will not develop and the
scouring tendency of streams during flood will wash the deposits.
4. Temperature:
As the activities of organisms depend on the temperature, the self- purification will also
depend on temperature. At low temperature the organism’s activities are slow due to
Where C˳= initial concentration of river after mixing
Cr= concentration of river before mixing
Cw= concentration of constituents in wastewater
Qr= flow rate of river ,Qw= flow rate of sewage

4. introduction
definition
factors
factors
oxygensag
process which the rate of decomposition will be slow and in warm temperature this action is
reverse. Therefore, in summer, the stream will get self-purification is less than in winter.
But the quantity of dissolved oxygen is more in cold water than in hot water.
5. Sunlight:
The pathogens are killed if they are exposed to sunlight, therefore, sunlight helps in
self-purification. Algae also grow in sunlight causing production of oxygen. Therefore,
indirectly sunlight helps in the oxidation of unstable matter.
6. Oxidation:
The organic matter immediate
after mixing with the stream or
large quantity of water starts
getting oxidized due to the
development of the oxidizing
organisms present in the water.
This process prevails till the
complete oxidation of the
organic matters. The oxygen
demand is satisfied & stream becomes purified, due to this phenomenon The streams
get their oxygen demand by its aeration by wind or by the microscopic organisms.
7. Reduction:
The reduction occurs in the streams due to the hydrolysis of the organic matters
biologically or chemically, Anaerobic organisms start the splitting complex organic
matter present in the sewage. This action produces odors and gases and the
stabilization starts.

5. introduction
definition
factors
factors
oxygensag
process Oxygen Sag Analysis:
The oxygen sag or oxygen deficit in the
stream at any point of time during self
purification process is the difference b/w
the saturation DO content and actual DO
content at that time.
The saturation DO value for fresh water
depends upon the temperature and total
dissolved salts present in it. Oxygen sag
is a key indicator of water quality and the effects of pollution.
Oxygen deficit (D) = Saturation DO – Actual DO
Deoxygenation and Reoxygenation Curves:
When wastewater is discharged into the stream, the DO level in the stream goes on
depleting. This depletion of DO content is known as deoxygenation. The rate of
deoxygenation depends upon the amount of organic matter remaining to be oxidized,
as well as temperature.
When the DO content of the stream is gradually consumed due to BOD load,
atmosphere supplies oxygen continuously t water, through the process of re-aeration or
reoxygenation, i.e., along with deoxygenation, re-aeration is continuous process.

6. introduction
definition
factors
factors
oxygensag
process 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 of oxygen is lower at higher temperature and also
saturation concentration is less at higher temperature.
Process of Self-Purification of Stream:
(a) Degradation Zone: (b) Active Decomposition:
(c) Recovery Zone: (d) Clear Water Zone:

7. introduction
definition
factors
factors
oxygensag
process
(a) Degradation Zone:
This zone is situated just near the outfall of sewage. The water is turbid with dark
color. The decomposition of solid matters take place in this zone and the anaerobic
decomposition prevail.
(b) Active Decomposition Zone:
The water is greyish and darker than the previous zone. The objectionable odor's of
hydrogen sulphone and other sulphur compounds prevail and scum may also be seen
on the surface in this zone. It has least amount of DO.
(c) Recovery Zone:
In this zone, the stabilization of organic matters takes place and BOD of water is
reduced. The contents of DO start rising up above 40% of the saturation value. The
bacterial load decreases as the food supply of bacteria diminishes.
(d) Clear Water Zone:
In this zone, stream attains normal conditions as were prevailing before sewage was
discharged into it.
REFERENCES- NPTEL IIT Kharagpur Web Courses