Why is air added to an activated-sludge aeration tank?

In the activated-sludge method, air is added to the aeration tank primarily to provide the oxygen necessary for microorganisms to thrive and metabolize organic matter.1

Under these oxygenated conditions, a healthy suspension of biological solids—composed mostly of bacteria—is maintained. These microbes require oxygen to decompose dissolved organic substances, a process that effectively reduces the biochemical oxygen demand (BOD) of the wastewater.12

Beyond supplying oxygen, the injection of air—whether through porous diffusers or mechanical mixers—provides a rapid mixing action that ensures the microorganisms remain in contact with the organic materials they are designed to remove. By maintaining these aerobic conditions, the system prevents the water from becoming septic, which would otherwise lead to the development of undesirable odors and the potential destruction of aquatic life if discharged prematurely.12