Answer within 200 words : Discuss how waste water treatment can be done .

Explanation

The treatment of sewage or waste water is carried out in following stages
(i) Primary treatment It is a physical process that involves physical removal of large as well as small particles from the sewage through filtration and sedimentation in primary settling tanks.
All solids undergo sedimentation and settle down to constitute the primary sludge. The supernatant left after removing primary sludge, constitutes the effluent. This effluent is taken out for secondary treatment. The sewage sludge is treated wia sludge digestion. This sludge can also be disposed by several ways. It can be used to produce biogas also.
(ii) Secondary treatment it involves microbial degradation of organic matter. The primary effluent is passed into large aeration tanks, where it is constantly agitated mechanically. Air containing heterotrophic microorganisms is constantly pumped into the effluent. This allows a vigorous growth of aerobic microbes into flocs.
While growing, these microbes consume the major part of the organic matter, present in the effluent which significantly reduces the BOD (Biochemical Oxygen Demand).
The sewage water is further treated till the BOD reduces significantly. Then, the effluent of aeration tank is passed into a settling tank, where the bacterial flocs" are allowed to sediment. This sediment is called activated sludge.
The major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, methanogenic bacteria, which grow anaerobically, digest the bacteria and fungi present in the sludge.
During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide (biogas) which can be used as the source of energy as it is inflammable, The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.
(iii) Tertiary treatment It is a physico-chemical process. It involves disinfecting the water, if it is to be used for domestic purposes. It is done by various methods such as reverse osmosis, treating with chlorine, ozone gas or perchlorate salts.