Name different stages involved in sewage treatment.

Explanation

Sewage is the municipal waste water containing large quantities of human excreta
i) Primary 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 selling 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 passed into large tanks called anaerobic sludge digesters.
In the digester.s heteratrophic microbes anaerobically digest bacteria and fungi in sludge producing mixture of gases such as methane, hydrogen sulphide and `CO_(2)` which form the biogas.