Q.Discuss how wastewater treatment can be done.

Wastewater treatment purifies sewage in two main biological stages - primary (physical) and secondary (microbial/biological) treatment - lowering its BOD before the treated water is released, while the leftover sludge is anaerobically digested by bacteria to produce biogas.

Municipal wastewater (sewage) contains large amounts of organic matter and microorganisms, many of them pathogenic, and must be treated before being released into a river/water body. Treatment is carried out in a sewage treatment plant (STP) in the following stages:

1. Primary treatment (physical)

• Sewage is passed through bar screens to remove large floating objects.
• It then goes through a grit chamber, where sand and small stones settle out due to gravity.
• In the primary settling/sedimentation tank, solid particles settle as sludge (primary sludge) at the bottom; the remaining liquid is called effluent and is passed on for secondary treatment.

2. Secondary (biological) treatment

• The primary effluent is passed into large aeration tanks, where it is constantly agitated mechanically and air is pumped in to allow vigorous growth of useful aerobic microbes (forming flocs).
• These aerobic microorganisms consume the remaining organic matter in the effluent, which significantly reduces the BOD (Biochemical Oxygen Demand) of the wastewater - BOD is a measure of the organic-pollution load, i.e., the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.
• Once BOD is reduced sufficiently, the effluent is passed into a settling tank where the bacterial flocs (activated sludge) settle down. A small part of this activated sludge is used to re-inoculate the aeration tank (activated sludge process); the rest is transferred to an anaerobic sludge digester.

3. Anaerobic digestion of sludge …