Activated sludge refers to a mass of microorganisms cultivated in the treatment process to break down organic matter into carbon dioxide, water, and other inorganic compounds. The activated sludge process has three basic components:

1. A reactor in which the microorganisms are kept in suspension, aerated, and in contact with the waste they are treating;
2. Liquid-solid separation; and

A sludge recycling system for returning activated sludge back to the beginning of the process. There are many variants of activated sludge processes, including variations in the aeration method and the way the sludge is returned to the process.

Activated Sludge Process

Activated sludge refers to biological treatment processes that use a suspended growth of organisms to remove Biological Oxygen Demand (BOD) and suspended solids. As shown below, the process requires an aeration tank and a settling tank.

In addition, support equipment, including return pumps, waste pumps, flow measurement devices for return and waste, as well as equipment to provide aeration (mixers and/or blowers) is also required.

Note: Activated sludge processes may or may not follow primary treatment. The need for primary treatment is determined by the process modification selected for use. All activated sludge systems include a settling tank following the aeration tank.