What Is Mixed Liquor Suspended Solids (MLSS) Concentration?

Mixed Liquor Suspended Solids (MLSS) concentration, also known as mixed-liquor sludge concentration, indicates the concentration of activated sludge in the mixed liquor—that is, the total mass of activated-sludge solids contained per unit volume of mixed liquor, expressed in mg/L. Because it is easy to measure, it is commonly used in engineering as a relative indicator of the amount of activated-sludge microorganisms. [2]

Quantitative filter paper, muffle furnace, oven, desiccator, color-indicator desiccant, analytical balance with 0.1 mg readability. [3]

① Dry quantitative filter paper at 103–105°C, cool in a desiccator, and weigh; repeat until a constant weight is obtained or the weight loss is less than 4% of the previous weighing. The weight is m0 (the final value is obtained by cooling in a desiccator after 8 h of drying, or 1 g may be taken directly for 12.5-cm filter paper).

③ Place the clean crucible in the oven and dry for 1 h. Remove, cool in a desiccator to equilibrium temperature, and weigh; the weight is m2.

② Filter a 100 mL sample through the filter paper obtained in step 1, place in an oven at 103–105°C to dry, remove, cool in a desiccator to equilibrium temperature, and weigh; repeat drying until a constant weight is reached or the weight loss is less than 5% of the previous weighing or 0.5 mg (whichever is smaller). The weight is m1 (the final value is obtained by cooling in a desiccator after 8 h of drying and then weighing).

④ MLSS = (m1 − m0) / 0.1 (the final value is obtained by cooling in a desiccator after 8 h of drying and then weighing). [4]