The portion of the MLSS that is actually eating the incoming food(in terms of COD & BOD) is referred to as the Mixed Liquor Volatile Suspended Solids (MLVSS).  The volatile solids concentration in a sample of mixed liquor will consist mostly of microorganisms and organic matter.  As a result, the volatile solids concentration of mixed liquor is approximately equal to the amount of microorganisms in the water and can be used to determine whether there are enough microorganisms present to purify the water