Aquatic Pollution and Coastal Management

Unit 2 : Sewage and domestic wastes, treatment and reuse

2.5.3. Secondary Treatment
It is a biological process, which relies on naturally occurring microorganisms acting to break down organic material and purify the liquid. The microorganisms are encouraged to grow on stones over which the sewage is trickled. A film of microbial growth develops on the filtering medium, which may be of 0.2-2.0 mm thick. The film consists of bacteria, fungi, protozoa and algae. The process can be speeded up by blowing air into the sewage tanks.