What is the key difference between primary and secondary sewage treatment ?

Explanation

Primary treatment: it involves the physical removal of large and small particles from the sea was through sedimentation and filtration. First of all floating debris is removed by sedimentation. all the solids that settle from the primary sludge and the supernatant forms are called effluents. The effluent is subjected to secondary treatement.
Secondary treatement: In this, the effluent is passed on to a large areation tanks where it is constantly agitated. The agitation allows vigorous growth of aerobic microbes into flocs. The mass of bacteria associated with fungal filaments to form mesh like structures constitute flocs. The microbes use the major part of hte organic matter from the effluent. The growth of the microbes by consuming the organic matters will probably reduce the biological oxygen demend (BOD). It is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by the microbes. BOD is a measure the rate of uptake of oxygen by microbes in a sample of water. The greater the BOD in sewage, more is its polluting mpotential . So the effluent is treated till the BOD is reduced.