Question

Describe secondary treatment of sewage till can be discharged into a natural water body.

Answer 1

Hint: Sewage treatment is a very important procedure as it removes many organic contaminants, toxic chemicals, and biological pathogens which if not treated can lead to adverse effects such as pollution and diseases. Thus sewage treatment is carried out in several steps such as primary, secondary, and tertiary treatments respectively.

Complete answer:
Secondary treatment of sewage is the process by which the sewage quality is made to a certain degree so that it can be released into the environment after its treatment. It generally involves the separation of those solids which can be sedimented.
Further, some biological processes have been involved that lead to the dissolution of organic compounds present in them.
Secondary treatment is applied to sewage after it has undergone primary treatment. In the primary treatment, mainly the floating particles and the solids that are settleable are removed.
Biological processes involved may be aerobic or anaerobic. In the aerobic process, the organic compounds are converted to carbon dioxide, water, and other by-products. In the case of the aerobic process, anaerobic microbes are used. that convert organic compounds to gases like methane, carbon dioxide, and other by-products. Anaerobic bacteria acting on this sewage produces activated sludge. This effluent after treatment can be released into the environment as now it is free from most of the organic contaminants.

Note: Sewage treatment produces biogas that can be used as a source of energy. Moreover, sewage after treatment can be used as liming material as it contains high levels of nitrogen and phosphorus.