Mixed Liquor Suspended Solids (MLSS) are the concentration of suspended solids in the mixed liquor of an aeration tank, indicating the amount of biomass, typically measured in mg/L. And is denoted by X. Mixed Liquor Suspended Solids is usually measured using the Milligram per Liter for Density. Note that the value of Mixed Liquor Suspended Solids is always positive. Typically, the value of Mixed Liquor Suspended Solids lies in the range from 100 to 12000.