Question

Consider the following statements about secondary sewage treatment. Which of the statements given is correct?

(i) In secondary treatment useful aerobic microbes grow rapidly and form floes of bacterial and fungal filaments in a mesh-like structure.

(ii) The growing microbes consume organic matter reducing the BOD after which the effluent is passed into the settling tank.

(iii) In the settling tank, the bacterial floes settle and the sediment is called activated sludge.

(iv) A small part of the sludge is used as an inoculum. The remaining part is passed into anaerobic sludge digesters.

(v) On the anaerobic digestion of bacteria and fungi, a mixture of gases such as carbon dioxide, nitrogen, and carbon monoxide is produced.

A. (i), (ii), (iii) and (iv)
B. (i), (iii), (iv) and (v)
C. (ii), (iii), (iv) and (v)
D. (i), (ii), (iii), (iv) and (v)

Answer 1

Hint:
Sewage treatment is the method of removing waste and contaminants from wastewater of the city, society, colony etc. This wastewater primarily contains household sewage along with certain industrial wastewater. In sewage treatment plants various approaches are used like physical, chemical, and biological approaches to eliminate contaminants and produce treated wastewater that is safe to discharge into the environment.

Complete step-by-step answer:
 Secondary sewage treatment is a biological process that removes the organic matter of sewage through the action of microbes. Secondary treatment of wastewater works on a deeper level than the primary level. It is known as biological treatment because it is designed to substantially degrade the biological content of the waste through aerobic biological processes. This step eliminates the dissolved organic matter by the use of biological agents. This is performed with the help of microbes that can consume organic matter converting it into water, carbon dioxide, and energy for their own growth and reproduction.
In secondary sewage treatment, primary effluent is passed into the big aeration tanks with constant mechanical agitation and air supply. Aerobic microbes grow quickly and form flocs. Flocs are masses of bacteria that consume organic substances and thus decrease the biochemical oxygen demand(BOD). When the BOD of sewage or waste is significantly reduced, the effluent is then passed into a settling tank in which the bacterial 'flocs' are allowed to sediment. This sediment is known as activated sludge. A small portion of the activated sludge is pumped back into aeration tanks to act as inoculum. The remaining most important part of the sludge is pumped into large tanks known as an anaerobic sludge digester. Other types of bacteria, which grow anaerobically, digest the fungi and the bacteria in the sludge. Throughout this digestion, bacteria produce a mixture of gases like hydrogen sulphide, methane, and carbon dioxide. These gases form biogas and can be used as a source of energy as it is combustible.

Therefore the correct answer is Option A.
Note: The secondary stage of treatment eliminates around 85 per cent of the organic matter in sewage by making use of the bacteria in it. The principal secondary treatment methods used in the secondary treatment are the activated sludge process and the trickling filter.