(a) What would happen if a large volume of untreated sewage is discharged into a river?

(b) In what way anaerobic sludge digestion is important in sewage treatments?

a. Effects of discharge of untreated sewage into rivers:

● Reduced potability: The water will become unfit for drinking

● Increase in BOD (Biochemical Oxygen Demand): Aquatic animals will be affected as dissolved oxygen will be reduced.

● Incidence of water borne disease: Diseases like cholera, typhoid etc. will spread uncontrollably due to the presence of plenty of harmful microorganisms.

● Increase in offensive smell: Foul odour will be produced due to the decomposition of the organic matter present in the sewage discharge. This is mainly due to the evolution of hydrogen sulphide.

● Enhanced algal bloom: The eutrophication of water bodies (enriched with nutrients) will cause an overgrowth of algae resulting in reduction of dissolved oxygen. This will further result in the death of fish.

● Decreased soil fertility: These untreated sewage water may come in contact with the soil leading to soil pollution and thereby affecting soil fertility.

b. Anaerobic digestion:

The process of anaerobic digestion happens in the digester during the secondary treatment of sewage. Here, the microbes which grow anaerobically like Methanobacteria, digest the bacteria and fungi in the sludge and produce gases like methane, hydrogen sulphide and carbon dioxide.

Biogas:

The gases produced, collectively called biogas, can be used as a source of energy.

Fertiliser:

The residue left after the anaerobic digestion contains a lot of nutrients, therefore is used as a fertiliser for the crops. The anaerobic digestion prevents the soil and water pollution caused by direct discharge of sludge into the environment.