(i) Primary treatment or physical treatment
• It is the physical removal of large and small particles from sewage.
• First, the floating debris is removed by sequential filtration by passing through wire mesh screens.
• Then, the grit (soil and small pebbles) are removed by sedimentation in settling tanks. The sediment is called primary sludge and the supernatant is the effluent.
• The effluent is taken for secondary treatment.
(ii) Secondary treatment or biological treatment
• Primary effluent is passed into large aeration tanks with constant mechanical agitation and air supply.
• Useful aerobic microbes grow rapidly and form flocs.
• Flocs are masses of bacteria associated with fungal filaments to form mesh-like structures.
• The growing microbes consume organic matter and thus reduce the biochemical oxygen demand (BOD).
• When BOD of sewage has reduced, the effluent is passed into settling tank.
• Here, the bacterial flocs settle and the sediment is called activated sludge.
• A small part of the sludge is used as an inoculum in the aeration tank and the remaining part is passed into large tanks called anaerobic sludge digesters.
• In the digesters, heterotrophic microbes anaerobically digest bacteria and fungi in sludge producing mixture of gases such as methane, hydrogen sulphide and CO2 which form the biogas.
The sewage treatment is essential before being released into water bodies so as to check water borne diseases or pathogenic organisms due to water pollution.
