(a) Describe the process in which BOD is reduced during sewage treatment.
(b) What is activated sludge?
[3 marks]

Open in App

Solution

a) BOD or biological oxygen demand is the amount of oxygen required by the bacteria to break down the organic matter. [1 mark]

BOD is reduced during the secondary treatment of the sewage by the aerobic bacteria which grow into flocs. These
microbes use up the major part of the organic matter in the effluent. This reduces the BOD. [1 mark]

b) After the secondary treatment is completed, the effluent is passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment. This sediment is
called activated sludge. [1 mark]

Suggest Corrections

	Primary Sludge	Activated sludge
(i)	It is sludge formed during primary sewage treatment.	It is sludge formed during secondary sewage treatment.
(ii)	It possesses flocs of decomposer microbes.	It does not possess flocs of decomposer microbes.
(iii)	It does not require aeration.	Formation of activated sludge requires aeration.
(iv)	A lot of decomposition occurs during formation of primary sludge.	Very little decomposition occurs during formation of activated sludge.