Application Note

Mixed Liquor Suspended Solids In Wastewater

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).

Critical factors

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.

access the Application Note!

Get unlimited access to:

Trend and Thought Leadership Articles
Case Studies & White Papers
Extensive Product Database
Members-Only Premium Content
Welcome Back! Please Log In to Continue. X

Enter your credentials below to log in. Not yet a member of Water Online? Subscribe today.

Subscribe to Water Online X

Please enter your email address and create a password to access the full content, Or log in to your account to continue.


Subscribe to Water Online