Question

After significant reduction of BOD during secondary treatment of sewage, what further processes are carried out ? Mention any one important outcome of these processes.

Hint:

Think of the settling tank, activated sludge, anaerobic sludge digesters and the gas they produce.

Answer 1

Step 1: Understand where the process stands:
Secondary (biological) treatment keeps the sewage effluent in aeration tanks. Useful aerobic microbes form flocs and digest the organic matter. This lowers the BOD (biochemical oxygen demand) of the effluent a great deal.

Step 2: Settling of the flocs:
Once the BOD has fallen enough, the effluent goes into a settling tank. Here the bacterial flocs are allowed to sediment.
The sediment is called activated sludge.

Step 3: Fate of the activated sludge:
A small part of the activated sludge is pumped back into the aeration tank. It works as inoculum, so fresh effluent gets a ready supply of microbes.
The remaining large part goes into big tanks called anaerobic sludge digesters. There anaerobic bacteria digest the bacteria and fungi present in the sludge.

Step 4: Release of the treated effluent:
The clarified effluent from the settling tank is released into natural water bodies such as rivers and streams.

Step 5: Outcome:
During digestion in the digesters, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. This mixture is biogas, which can be used as a source of energy.

Final Answer:
Further processes are settling of flocs to get activated sludge, recycling a part of it as inoculum, anaerobic digestion of the rest in sludge digesters, and release of the effluent into water bodies. One outcome is production of biogas, a source of energy.