Sub-critical flux fouling in membrane bioreactors - a review of recent literature

Membrane fouling is one of the main factors affecting the diffusion of membrane bioreactors to wastewater treatments. Fouling is generally held to persist above a so-called “critical flux”, below which a steady-state membrane permeability is assumed to be attainable. However, a number of recent studies have demonstrated that low-level fouling can take place at very low fluxes which are substantially below the critical value. This short survey is focused on reported evidences of membrane fouling under sub-critical conditions and examines possible links between operational paramenters, mixed liquor characteristics and fouling propensity. The reported experimental evidences suggest no unequivocal connection between parameters such as permeability decline (dK/dt) and fouling rate (dP/dt) and sustainable long term sub-critical operation. In this respect, test size and plant configuration also play a relevant role. Step-flux experiments may provide information on fouling propensity under sub-critical flux, but the fouling rates determined with this method are generally not applicable to long term operation. Mixed liquor quality determinants such as EPS and SMP and their relative protein/carbohydrate ratios are likely to contribute to sub-critical fouling in MBRs. Possible relationships between these parameters and plant operation (sludge age, biomass concentration, hydraulic retention time, etc.) are briefly discussed.