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.