The combination of primary effluent and active biological solids (return sludge) in the activated sludge process that is fed into the aeration tank.

When the activated sludge in an aeration tank is mixed with primary effluent or the raw wastewater and return sludge, this mixture is then referred to as mixed liquor as long as it is in the aeration tank. Mixed liquor also may refer to the contents of mixed aerobic or anaerobic digesters.

The mixture of activated sludge, wastewater, and oxygen, wherein biological assimilation occurs.