The primary treatment basically involves the physical removal of particles from the sewage through

Primary water treatment removes large pieces of floating debris, oily substances from the sewage. Secondary treatment is called bio-chemical or microbial treatment where many useful aerobic bacteria and fungus grow together to form flocs. These microbes consume major part of the organic matter in the effluent as they grow and reduces the BOD of water. BOD of pure water is 1-2 ppm and BOD of polluted water is 15-18 ppm.