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.
- At a high f/m ratio - micro-organisms are in the log-growth phase which is characterised by excess substrate and maximum rate metabolism such that microbial growth is unrestricted. When the f/m ratio is very high - Micro-organisms do not form flocs and are generally dispersed making it difficult to settle and form a recyclable sludge. Also since food is in excess, not all the organic material will be utilised and the remainder may pass out in the final effluent.
- At low f/m ratios - the substrate becomes limiting and the overall metabolic action in the aeration tank is endogenous such that cell lysis occurs. With a consequent decrease in growth rate, there is a decrease in the rate of oxygen uptake.
- In contrast, very low f/m ratios - put the micro-organisms into a food-limited environment. Under low sludge loadings there is almost complete oxidation of organics resulting in a high quality effluent with the micro-organisms flocculating and settling rapidly. Very low f/m ratios however promote the growth of filamentous bacteriawhich may result in bulking conditions at the plant.