The self purification of natural water systems is a complex process involving physical, chemical, and biological factors. Dissolved oxygen levels below 4-5 mg/L can reduce the forms of life that can survive. Several factors affect dissolved oxygen availability including dilution, dispersion, temperature, sunlight, oxidation rates, and reduction processes. When wastewater is discharged into a stream, it creates four zones: degradation, active decomposition, recovery, and clearer water as the stream restores to its natural condition.

1. Self Purification

2. SELF PURIFICATION
• The self purification of natural water systems is a complex
process that often involves physical, chemical, and biological
processes working simultaneously.
• The amount of dissolved Oxygen (DO) in water is one of the
most commonly used indicators of a river health.
• As DO drops below 4 or 5 mg/L the forms of life that can survive
begin to be reduced.
• A minimum of about 2.0 mg/L of dissolved oxygen is required to
maintain higher life forms.
• A number of factors affect the amount of DO available in a river.

3. ACTIONS INVOLVED IN OR FACTORS
AFFECTING SELF PURIFICATION
DILUTION : Due to dilution, concentration of organic matter is
reduced and the potential nuisance of sewage is also reduced.
When sufficient dilution water is available in the receiving water
body, the DO level in the receiving stream may not reach to zero due
to availability of sufficient DO initially in the river water.
DISPERSION DUE TO 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.

4. ACTIONS INVOLVED IN OR FACTORS
AFFECTING SELF PURIFICATION
TEMPERATURE :
The quantity of DO available in stream water is more in cold
temperature than in hot temperature. In summer, rising temperature
reduces solubility of oxygen.
At low temperature, the activities of bacteria is low and hence the
rate of decomposition will be slow and DO will be more because of
increased solubility of oxygen in water.
SUNLIGHT : Algae produces oxygen in presence of sunlight due to
photosynthesis.
Therefore, sunlight helps in purification of stream by adding oxygen
through photosynthesis.

5. ACTIONS INVOLVED IN OR FACTORS
AFFECTING SELF PURIFICATION
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.
REDUCTION : Reduction occurs in the streams due to hydrolysis of
organic matter biologically or chemically.

6. ZONES OF POLLUTION IN THE
STREAM
1st
ZONE OF DEGRADATION: This zone is situated just below the
outfall sewer when discharging its contents into the stream.
2nd
ZONE OF ACTIVE DECOMPOSITION: It is situated just after the
degradation zone and marked by heavy pollution.
3rd
ZONE OF RECOVERY: The process of recovery starts, from its
degraded condition to former condition.
4th
ZONE OF CLEARER WATER: The natural condition of stream is
restored.

7. ZONES OF POLLUTION IN THE
STREAM
1st
ZONE OF DEGRADATION: This zone is situated just below the
outfall sewer when discharging its contents into the stream.
2nd
ZONE OF ACTIVE DECOMPOSITION: It is situated just after the
degradation zone and marked by heavy pollution.
3rd
ZONE OF RECOVERY: The process of recovery starts, from its
degraded condition to former condition.
4th
ZONE OF CLEARER WATER: The natural condition of stream is
restored.