The activated sludge process is an aerobic, suspended growth, biological treatment method. It uses the metabolic reactions of microorganisms to produce a high quality effluent by converting and removing substances that have an oxygen demand.

The activated sludge process was initially developed at a Manchester, England treatment plant in the early 1900s. Several U. S. installations were put into operation in the 1920s, although it was not until the 1940s that the process became widespread.

The activated sludge process is perhaps the most widely used process for reducing the concentration of dissolved, particulate, and colloidal organic pollutants in wastewater.

In the basic activated sludge process, the wastewater enters an aerated tank where previously developed biological floc particles are brought into contact with the organic matter of the wastewater. The organic matter, a carbon and energy source for cell growth, is converted into cell tissue and oxidized end products (mainly carbon dioxide (CO2)), The contents of the aeration tank are called mixed liquor. The biological mass, referred to as the mixed liquor suspended solids (MLSS) or mixed liquor volatile suspended solids (MLVSS), consists mostly of microorganisms, inert suspended matter, and nonbiodegradable suspended matter.

The microorganisms are composed of 70 to 90% organic and 10 to 30% inorganic matter. The types of cells vary depending on the chemical composition of the wastewater and the biological mass.

After the mixed liquor is discharged from the aeration tank, a clarifier separates the suspended solids (SS) from the treated wastewater. The concentrated biological solids are then recycled back to the aeration tank to maintain a concentrated population of microorganisms to treat the wastewater.

The solids that settle to the bottom of the sedimentation tanks or remain after the activated sludge process must be properly treated, managed and, if possible, reused.