Abstract: A process and apparatus for aerating waste water in order to induce the separation of fat and grease and other substances from waste water comprises the steps of: pumping the water through a diffuser, wherein the air is introduced in small bubbles into the water through a permeable interface positioned between a water chamber and an air chamber; and controlling the pressure of the air across the permeable interface between the air chamber and water chamber, such that a predetermined, relatively low pressure differential pressure exists across the permeable interface. The pressure differential is maintained by a piloted pressure regulator so as to be high enough to cause an adequately large quantity of air to be introduced into the water. The pressure differential is low enough that shear forces tending to dislodge solid particles from air bubbles are minimized. A concentric tubular diffuser maximizes air diffusion into the liquid.