Write a note on municipal sewage treatment.

Explanation

The sewage in sewage treatment plants is carried out in two stages :
1. Primary treatment. It involves physical removal of particles-large and small from the sewage through filtration and sedimentation. Floating debris is removed by filtration and grit are removed by sedimentation. Thus, all solids that settle form the primary sludge and the supernatant forms the effluent.
2. Secondary treatment. The effluent from primary treatment is passed to aeration tanks where air is pumped into it. This allows the growth of useful aerobic microbes into floes (masses of bacteria associated with fungai filaments). During this, microbes consume the major part of the organic matter in the effluent: This reduces the BOD of the effluent.
The effluent is then passed into a settling tank where bacterial floes one allowed to sediment. This sediment is called activated sludge. The small portion of this activated sludge is again passed b aeration tank to serve as the inoculum. The remaining major part of this sludge is pumped into arge anaerobic sludge digesters. Here, anaerobic bacteria digest the bacteria and the fungi in the ludge. During this digestion, bacteria produce mixture of gases such as methane, `H_2S` and `CO_2`. These gases form biogas.