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

(N/A) Due to increasing urbanization,sewage is being produced in much larger quantities than ever before. However,the number of sewage treatment plants has not increased enough to treat such large quantities. Consequently,untreated sewage is often discharged directly into rivers,leading to severe water pollution and an increase in water-borne diseases.
$(b)$ Once the $BOD$ (Biochemical Oxygen Demand) of sewage or wastewater is reduced significantly,the effluent is passed into a settling tank where the bacterial 'flocs' are allowed to sediment. This sediment is called activated sludge. $A$ small part of the activated sludge is pumped back into the aeration tank to serve as an inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here,anaerobic bacteria digest the bacteria and fungi present in the sludge. During this process,these bacteria produce a mixture of gases such as methane $(CH_4)$,hydrogen sulphide $(H_2S)$,and carbon dioxide $(CO_2)$. These gases form biogas,which is inflammable and can be used as a source of energy.