Mixed liquor is a mixture of raw or settled wastewater and activated sludge contained in an aeration basin in the activated sludge process. Mixed liquor suspended solids (MLSS) is the concentration of suspended solids in mixed liquor, usually expressed in milligrams per liter (mg/l).
Within the aeration basin, dissolved oxygen (DO), pH, active biological mass, mixing, rate of oxygen utilization, temperature, MLSS concentration and retention time are critical factors that must be closely monitored.
Effluent from a primary clarifier and return activated sludge from a secondary clarifier are mixed together and pumped to an aeration basin. In the basin, aerobic microbes decompose organic matter in the mixed liquor. The MLSS concentration is typically 2500 mg/l with a calibrated range of 0–5000 mg/l.