Factors affecting self-purification

Self-purification is the natural recovery of a polluted stream. Its rate depends on the following factors.

1. Dilution: the greater the flow and volume of the river compared to the sewage, the more the pollutants are diluted and the more oxygen is available. A high dilution factor (above 500) means a fast recovery.
2. Temperature: high temperature increases the rate of bacterial decomposition ($K_T=K_{20}\,1.047^{T-20}$) and so the self-purification is quicker, but oxygen solubility falls, which reduces the DO. In cold weather the process is slow.
3. Sunlight: kills bacteria by UV rays and allows algae to produce oxygen by photosynthesis. Turbid water lowers this effect.
4. Velocity and turbulence of the stream: turbulence mixes the sewage and increases reaeration by exposing the water to the air. Fast, shallow, rocky streams recover faster than slow, deep ones.
5. Rate of reaeration: depends on the surface area, depth, wind and temperature; the constant $K_2$ is high in shallow, fast streams.
6. Dissolved oxygen in the river water: a higher initial DO gives more capacity for oxidising the pollutants without going anaerobic.
7. Sedimentation: the settleable solids settle in a quiet stream, which clarifies the water (but the deposited sludge then decomposes and may use oxygen).
8. Nature and quantity of the pollutant: a strong waste with high BOD, toxic industrial wastes, oil, and non-biodegradable matter reduce the rate. The more biodegradable the waste, the faster is the purification.
9. Presence of bacteria, algae, protozoa and plants: microorganisms decompose waste; algae give oxygen by photosynthesis; protozoa feed on bacteria.
10. Chemical actions: oxidation, reduction, coagulation by dissolved chemicals, and pH.
11. Time of travel / length of stream: self-purification needs time; a long stream with no new pollution allows full recovery.