The activated sludge system relies on biology to remove pollutants.  Due to biological growth behavior, every pound of pollutant (e.g. BOD or ammonia) we remove will produce a fixed amount of new biomass. This new amount of biomass is called the yield.  Yield refers to biological growth only, and we can measure that as mixed liquor volatile suspended solids (MLVSS).  But as you may know, over time things other than microorganisms accumulate in the activated sludge system too. The total amount of material in the aeration basin is measured as mixed liquor suspended solids (MLSS). This accumulation effect (biological and other) gives rise to the need to regularly remove material from the aeration basins.  The practice of removing material is calling wasting. The material removed is called Waste Activated Sludge (WAS).  The mass of WAS removed is called solids production and is measured as lbs TSS/day.  We must waste to maintain treatment performance. How do you make wasting decisions, i.e. how much do you waste and when?