Food-to-Microorganisms (F/M) Ratio


The food-to-microorganisms (F/M) ratio indicates the organic load into the activated sludge system and is expressed in kilogram BOD per kilogram of MLSS per day.

The food to microorganism (F/M) ratio is one of the significant design and operational parameters of activated sludge systems. A balance between substrate consumption and biomass generation helps in achieving system equilibrium, thus the F/M ratio is a key parameter responsible for the decomposition of organic matter.

Q       =   flow rate of sewage in MegaLitres/day;

BOD   =  5-day biochemical oxygen demand (mg/L);

MLSS =  mixed liquor suspended solids (mg/L) &&

V        =   volume of aeration tank (in MegaLitres).

The f/m ratio maintained in the aeration tank in an activated sludge system controls the rate of biological oxidation as well as the volume of microbial biomass produced by maintaining microbial growth in a particular growth phase.