MLSS is defined as what measurement in the aeration tank's mixed liquor?

• A. The temperature of the mixed liquor
• B. The concentration of chlorine in the effluent
• C. The amount (mg/L) of suspended solids in the mixed liquor of an aeration tank
• D. The total dissolved solids in the water

Answer: (C)

MLSS stands for mixed liquor suspended solids, and it is the concentration of suspended solids in the aeration tank’s mixed liquor, expressed in mg/L. This measure reflects the amount of biomass and other solids present in the reactor, which directly affects oxygen transfer, mixing, and the biological treatment that happens in the tank. It’s different from dissolved solids (TDS), which are not suspended, and it’s not a measure of temperature or chlorine in the effluent. Keeping the right MLSS helps maintain proper sludge age and treatment performance: too high MLSS can hinder oxygen transfer and mixing, while too low MLSS may not provide enough biomass to effectively treat the wastewater.