The biological or secondary treatment of primary effluent involves passing the effluent into large aeration tanks where constant stirring and agitation is done followed by pumping out of the air. This allows a magnified growth of aerobic microorganisms into masses called as flows. These microorganisms then utilize a lot of oxygen in order to break down the organic matter present in the effluent. This results in a significant decrease in the BOD or biochemical oxygen demand of the effluent. The sewage water is further treated until the BOD is reduced.