Explain the stages included in sewage treatment.

(N/A) Sewage treatment is carried out in two main stages:
$1$. Primary Treatment: This stage involves the physical removal of large and small particles from sewage through filtration and sedimentation.
- Initially,floating debris is removed by sequential filtration.
- Then,grit (soil and small pebbles) is removed by sedimentation.
- All solids that settle form the 'primary sludge',and the supernatant forms the 'effluent'.
$2$. Secondary Treatment or Biological Treatment: The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it.
- This allows for the vigorous growth of useful aerobic microbes into 'flocs' (masses of bacteria associated with fungal filaments to form mesh-like structures).
- While growing,these microbes consume the major part of the organic matter in the effluent,which significantly reduces the $BOD$ (Biochemical Oxygen Demand) of the effluent.
- $BOD$ refers to the amount of oxygen that would be consumed if all the organic matter in one liter of water were oxidized by bacteria.
- The greater the $BOD$ of wastewater,the more is its polluting potential.
- Once the $BOD$ is reduced significantly,the effluent is passed into a settling tank where the bacterial flocs are allowed to sediment. This sediment is called 'activated sludge'.
- $A$ small part of the activated sludge is pumped back into the aeration tank to serve as an inoculum.
- The remaining major part of the sludge is pumped into large tanks called 'anaerobic sludge digesters'.
- Here,anaerobic bacteria digest the bacteria and fungi in the sludge,producing gases such as methane,hydrogen sulfide,and carbon dioxide,which form 'biogas'.
- The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.