The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it which allows vigorous growth of useful aerobic microbes. A, C, E and G are correct

Aerobic microbes consume the major part of the organic matter in the effluent which significantly reduces the BOD (Biochemical Oxygen Demand) of the effluent. A, B, C, F and G are correct

The greater the BOD of waste water more is its polluting potential so sewage water is treated till the BOD is reduced. A, C, D, E and F are incorrect

In settling tank bacterial ‘floes’ are allowed to sediment and this sediment is called primary sludge. (E) A small part of the primary sludge is pumped back into the aeration tank to serve as the inoculum. (F) In anaerobic sludge digesters bacteria which grow anaerobically, digest the bacteria and the fungi in the sludge. (G) During anaerobic sludge digestion, bacteria produce biogas such as methane, hydrogen sulphide and carbon dioxide. All statements are correct