Activated Sludge · Solids Inventory and Return Control
What Is MLSS?
Mixed liquor suspended solids, or MLSS, is the concentration of suspended material in mixed liquor. It includes living organisms, inactive biological material, trapped organic particles, and inorganic solids. MLSS is usually reported in milligrams per liter, or mg/L.
MLSS is measured by filtering a known sample volume, drying the retained material, and determining its mass under an approved laboratory method. Representative sampling and correct laboratory technique are essential.
A higher MLSS concentration does not always mean more active treatment capacity. The added material may include inert solids, and basin volume or stored solids may also change.
Operators trend MLSS with MLVSS, sludge age, loading, wasting, settleability, and effluent solids. Use the sampling location and frequency in the plant's approved control plan.
Key point: MLSS counts all suspended solids in the mixed liquor, living and nonliving, organic and inorganic.
Keep learning
Activated Sludge Process Control for Wastewater Operators
This page comes from the course. It covers aeration, settling tests, solids inventory, return and waste flows, sludge age, microscopy, process upsets and daily records, one step at a time, with short knowledge checks and a certificate of completion.
Take the course