Question

Which of the following in the sewage treatment removes suspended solids?
A. Primary treatment
B. Tertiary treatment
C. Secondary treatment
D. Sludge treatment

Answer 1

Hint: Sewage is the wastewater produced by households and some industrial wastewater. It is necessary to treat the wastewater so that when they are released into the environment they cause less or no harm to the water bodies. It involves multiple stages in which wastes are removed.

Complete answer:
Sewage treatment is the process of removing the wastes and contaminants in the sewage water through physical, chemical and biological approaches. Initially the sewage is subjected to pre-treatment in which plastics, cans, sticks, grit and other trashes are removed. Then fat and grease are skimmed from the surface of the sewage. This is followed by a series of treatments namely: primary, secondary, and tertiary treatment.
In primary treatment, the sewage is allowed to settle after which the sediment and the fats on the surface is further removed. The biological and organic matter are degraded by using water-borne microorganisms in the secondary treatment. The microbes are removed before subjecting the sewage to tertiary treatment. In the final tertiary treatment, the effluent is purified further by oxidising the nitrogen and phosphorus wastes to avoid overgrowth of algae and weed when released into the environment.

Thus, the suspended solids from the sewage is removed during (A) Primary treatment.

Note:
The water at the tertiary level is sometimes used in water golf courses or in agricultural fields depending on the effluent quality. However, pharmaceutical wastes, chemical and pesticide wastes will still be present in the effluent after tertiary treatment. Even though they are present in very less quantity, it will potentially harm the aquatic organisms. So fourth stage treatment is being performed in a few countries by using activated carbon filters.