Question

Passage of effluents into oxidation tank is for?
A) Primary treatment
B) Secondary treatment
C) Tertiary treatment
D) Both A and B

Answer 1

Hint: To make sewage less polluting, it is treated in sewage treatment plants called STPs. Heterotrophic microorganisms are used to treat this waste and these microorganisms are present naturally. This treatment can be done in two parts: primary treatment and secondary treatment.

Complete step-by-step answer:
Primary treatment involves the physical removal of particles- large and small- from the sewage through sedimentation and filtration. These things are separated in parts; firstly, floating debris is separated by sequential filtration. Then by sedimentation, the soil and small pebbles are removed. From the primary sludge, all the solids settle down and the supernatant forms the effluent. From the primary settling tank, the effluent is taken for secondary treatment.
Secondary treatment or biological treatment: The primary liquid waste is moved into big aeration tanks where it is continually agitated and mechanically pumped into it. This allows the vigorous growth of useful aerobic microbes into flocs. So the passage of effluents into oxidation tanks is done for secondary treatment. These microbes gobble up the organic matter in the liquid waste.
Oxidation is done in the secondary treatment and not the primary one.
Therefore the correct answer is option B i.e., secondary treatment.

Note: In the oxidation tank aerobic bacteria are present and these microbes reduce the BOD(biochemical oxidation demand) of the effluent. BOD is the quantity of oxygen that would be gobbled up if all the organic in one litre of water was oxidised by bacteria. Till the BOD lessens, the sewage is treated. The BOD test measures the rate of uptake of oxygen by microorganisms in a sample of water.