F/M ratio is a relationship used in wastewater treatment plants viz Sewage Treatment Plants Effluent Treatment Plants. F/M is also called sludge load. This relationship is based on the concept that the quantity of food available per unit mass of microorganisms is related to the efficiency of the system.

Hence, it is understood that, the higher the BOD load supplied per unit value of the biomass (high F/M ratio), the lower is the substrate assimilation efficiency (the absorption and digestion of food / nutrients by biological system), but, on the other hand, the lower is the required reactor volume.

Conversely, when less BOD is supplied to the bacteria (low F/M ratio), the demand for food is higher, which implies a greater BOD removal efficiency and a larger reactor volume requirement. In a situation in which the quantity of food supplied is very low, the mechanism of endogenous respiration becomes common, which is a characteristic of low-rate (e.g. extended aeration) systems.