(a) What would happen if a large volume of untreated sewage is discharged into a river?
(b) In what way anaerobic sludge diagestion is important in sewage treatments?

Explanation

(a) Untreated sewage contains large amounts of organic matter and microbes, many of which are pathogenic. So the consumption of such water by humans and useful animals increases the chances of spread of a number of water-borne diseases like cholera, diarrhoeal diseases, typhoid etc.
Presence of high amounts of organic wastes in water increases the Biochemical Oxygen Demand (BOD) while decreases the Dissolved Oxygen (DO) which causes death of aquatic animals like fishes.
(b) Major part of activated sludge is pumped into anaerobic sludge digesters where anaerobic bacteria digest the bacteria and fungi during which a mixture of gases like methane, `CO_2` and `H_2S` is produced which can be used as biogas as source of energy. It also produces manure.