The method depends on establishing and maintaining a population of degrading micro-organisms and providing close contact with the degrading microorganisms and supply of dissolved oxygen. The microorganisms feed & grow upon the oxidizable material in the wastewater and form a suspended floc of “activated sludge” in the water. Air bubbled through the water by means of diffused or mechanical aeration that replenishes the oxygen needed for the biological oxidation. After a period of time the mixture of wastewater and activated sludge, known as “mixed liquor”, flows to a clarifier where it is allowed to settle. Part of the settled activated sludge is usually mechanically returned (by pump) to the aeration tank or vessel to start the process again.