Because the activated sludge process is highly controllable, it is the most commonly used biological treatment for municipal and industrial wastewater. The rate and extent of biochemical conversion of influent organic matter to biomass during the process is dictated by the mass of active microorganisms present. Maintaining the proper mass of active microorganisms in the aeration basin is a function of return activated sludge (RAS) concentration, which, in turn, depends on mixed liquor settleabliity and the degree of thickening and compaction achievable in the secondary clarifier.