In the waste water treatment process, aeration, to be effective, must transfer oxygen to the liquid for use by microorganisms. To produce a flock and mixing the liquor, diffused aeration is effective method for aeration by means of porous diffusers installed at the bottom of the tank. In the aeration process, oxygen transfer takes place by molecular diffusion through the interface between air and liquid and it increases in proportion to the interface area. For a given air flow, as the number of bubbles increases, the surface area increases and the rise velocity of bubbles decreases. The diffused aeration transfer more oxygen to the liquid than mechanical aeration. i. e. diffuser emerging 2mm bubbles presents 6 times more surface area to the liquid than coarse bubble aerator emitting 10 to 12 mm bubbles. Also, rise velocity is lower. Surface area and rise velocity of bubbles contribute to the higher oxygen transfer capacity of diffused aeration system.