Study the following statements and select the incorrect ones.
(i) Physical removal of large and small particles through filtration and sedimentation is called primary sewage treatment.
(ii) Secondary sewage treatment is mainly a mechanical process.
(iii) Activated sludge sediment in a sewage treatment plant is a rich source of aeroic bacteria.
(iv) Biogas, commonly called as gobar gas, is pure methane.

Options

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

Answer

B

Explanation

Secondary treatment is also called biological treatment or microbial degradation. It is mainly a biological process. Biogas is a mixture of gases, containing predominantly methane `(50 - 70%), CO_(2) (30- 40%)` and traces of hydrogen `H_(2)S` and nitrogen.