Question

Domestic sewage of large cities

• A. Has very high amount of suspended solid and dissolved salts
• B. Is processed by aerobic and then anaerobic bacteria in secondary treatment in sewage treatment plant
• C. Has high BOD as it contains both aerobic and anaerobic bacteria
• D. When treated in sewage treatment plant does not require aeration

Answer 1

The correct option is B Is processed by aerobic and then anaerobic bacteria in secondary treatment in sewage treatment plant

Sewage

Domestic sewage contains a high amount of organic matter compared to suspended and dissolved inorganic salts. Hence, option a is not appropriate.
The presence of higher organic content in the sewage requires the consumption of higher amounts of oxygen by the aerobic microbes during secondary treatment in an aeration tank for oxidation. Hence, option c is incorrect.

Sewage treatment

Sewage treatment is therefore done to reduce the BOD (biochemical oxygen demand) of the sewage so that its pollution potential is reduced. It involves the removal of solid suspended particles as well as the reduction in the amount of organic matter.
The treatment of sewage is usually carried out in two main steps which are the following:-

• Primary treatment: Solid suspended particles are removed using filtration and sedimentation.
• Secondary treatment: Organic matter is oxidised using microbes.

Secondary sewage treatment

During the secondary treatment of sewage, the primary effluent goes through the following stages:-

• Aeration tank: The primary effluent is aerated resulting in the growth of flocs (meshwork of aerobic fungi and bacteria). Hence, option b is incorrect.
• Settling tank: The flocs settle down to form a sediment called activated sludge.
• Anaerobic digesters: The activated sludge is made to flow into anaerobic digesters where anaerobes are used to decompose the sludge.

Hence, option d is the correct alternative.

Final answer: (d)