Question

List the events that reduce the Biochemical Oxygen Demand (BOD) of a primary effluent during sewage treatment.

Answer 1

Primary effluent is taken in large aeration tanks where it is continuously agitated mechanically and air is pumped into it. This allows the growth of a large number of aerobic bacteria and fungi (flocs). Growing flocs start consuming organic matter present in the effluent. This decreases the biological oxygen demand (BOD) of the effluent. Sewage is treated till the BOD is significantly reduced. 

Answer 2

Events that lead to biogas production from wastewater with reduced BOD are:

1. Wastewater is sent into a settling tank for sedimentation once its BOD has decreased considerably.
2. Most of the sedimented material, activated sludge (bacterial floes), is poured into huge anaerobic sludge digesters from the settling tank. At the same time, a tiny portion is fed back into the aeration tank to act as an inoculum.
3. These tanks produce biogas, a blend of methane, hydrogen sulphide, and carbon dioxide resulting from bacteria and fungi anaerobically digesting the sludge. Because biogas is combustible, it can be used as an energy source.