Question

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

Answer 1

Hint:Large quantities of waste water is generated in cities and towns. The major component of this waste water is human excreta. This municipal wastewater is known as sewage. It contains large amounts of organic matter and microbes. Many of which are pathogenic.

Complete answer:
Sewage which is carried out through sewerage systems, cannot be discharged directly into water bodies like rivers and streams. Before disposal, hence, sewage is treated in sewage treatment plants [STPs] to make it less polluting. Treatment of waste water is done by heterotrophic microbes naturally present in sewage.
This treatment is carried out in following stages: (a) Primary treatment, (b) Secondary treatment, (c) Tertiary treatment
a) Primary treatment :- It is a physical process which involves removal of large and small particles from sewage through filtration and sedimentation
b) Secondary treatment includes:
> Cyclic treatment ;- it involves both treating the waste and sewage . The waste can come from industries. It is also called cyclic activated sludge treatment. It is generally used for Potsdam water treatment
>Activated sludge treatment :- The sewage water is treated till the BOD is reduced. Once the BOD of sewage or wastewater is reduced significantly, the effluent of aeration tank is then passed into a settling tank where bacterial flocs are allowed to sediment. This sediment is called activated sludge.
c) Tertiary treatment :- If the waste water is to be used for domestic purposes, then tertiary treatment is done. It is a physico-chemical process in which chlorine gas, ozone gas, zirconium, and perchlorate salts. UV rays or reverse osmosis etc., are used to remove DDT, pesticides, pathogens and turbidity in wastewater.

Hence the correct answer is option ‘D’ i.e, processed by aerobic and then anaerobic bacteria in secondary treatment in sewage treatment plants.

 Note:Sewage treatment is done so that water may not get polluted. As when sewage will be directly transferred to water bodies it may cause death of aquatic animals. It also pollutes water and makes it unfit for consumption.