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

Q. Select the correct statement regarding activated sludge formed during secondary sewage treatment.

Q.
A conventional activated sludge plant treating $1000m^3/d$ of municipal wastewater disposes of its anaerobically digested sludge on relatively impervious farmland. Use the following data:

1. Raw Sewage: SS=225mg/L; 70% volatile; BOD=190 mg/L
(Excess activated sludge returned to primary)

2. Primary Settling: SS=50% removal; BOD=30% removal

3. Excess Activated Sludge: 0.4gm VSS produced per gram BOD applied
80% volatile of total

4. Anaerobic Digester: VSS reduced 50%; Digested sludge concentration =6%
Sludge specific gravity =1

5. Application on farmland: $2m^3/ha-d$

Area requirements (hectare) for disposal of the sludge on farmland shall be

Q. Study the given differences between primary sludge and activated sludge and select the incorrect ones.

	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.

Q. A conventional activated sludge plant treating $1000m^3/d$ of municipal waste water disposes off its anaerobically digested sludge on relatively impervious farmland. Use the following data.

1. Raw Sewage: SS=225 mg/L ; 70 % volatile; BOD=190 mg/L
(Excess activated sludge returned to primary)

2. Primary Settling: SS=50 % removal; BOD=30 % removal

3. Excess Activated Sludge: 0.4gm VSS produced per gram BOD applied
80 % volatile of total

4. Anaerobic Digester: VSS reduced 50%; Digested sludge concentration =6%
Sludge specific gravity =1

5. Application on farmland: $2m^3/ha-day$

Total volatile suspended solids to be anaerobically digested (kg/d VSS) shall be

Q. Describe the steps that are followed during secondary treatment of sewage. [3 Marks]