A) Primary treatment
B) Secondaiy treatment
C) Tertiary treatment
D) Reverse osmosis stage
Correct Answer: B
Solution :
Sewage treatment involves three stages : (i) Primary treatment stage : It removes most of the suspended wastes and includes fragmentation, sedimentation floatation and filtration. (ii) Secondary treatment stage : The sewage is accumulated in aerated tanks, where micro-organism decompose the organic matter. (iii)Tertiary treatment stage : To remove mineral loads, the sewage undergoes additional filtering and chemical. Polluted water is purified by reverse osmosis technique which does not involves biological process.
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