The mass of microorganisms, mainly bacteria, that develops when sewage or waste water is aerated. The term also refers to the type of sewage treatment process that uses bacteria that feed on organic wastes to speed up their rate of decomposition in waste water. The combination of wastewater and biological mass is commonly known as mixed liquor. In all activated sludge plants, once the wastewater has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated supernatant is run off to undergo further treatment before discharge.