Biofouling is the undesirable accumulation of microorganisms on the membrane surface and it is one of the major problems affecting the performance and the economy of many membrane installations for the treatment of seawater and wastewater. Unlike inorganic fouling, biological fouling due to microorganisms can be dependent on their growth rate. The paper examines from a theoretical point of view the influence of the membrane recovery ratio on the concentration of biomass in the membrane module. An attempt of validation of the model has been tried using a submerged microfiltration hollow-fibre membrane set-up.