(a) Write the first step the primary effluent undergoes when it enters the secondary treatment plant and state the purpose.

(b) What is the level of B.O.D indicative of in the secondary treatment plant ? Mention its significance.

Answer :

(a) Primary effluent passes to aeration tank/ it is constantly agitated mechanically and air is pumped into it, this allows vigorous growth of flocs (masses of bacteria associated with fungal filaments to form mesh like structure)/ to consume organic matter/to reduce BOD level of primary effluents.
(b) It is a measure of organic matter /polluting potential in the water, greater BOD more is polluting potential.