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.

Recently available data from different experiences on sub-critical flux operation of MBRs allow an assessment of relevant parameters affecting the sustainability of long-term operation under these conditions. In terms of operational parameters, the following observations can be made: • Information on sub-critical behaviour obtained at the laboratory or pilot scale is difficult to transfer to full-scale plants, since the plant size probably impacts upon the hydrodynamics in a submerged system. • Values of permeability decline (dK/dt) and fouling rate (d P/d t) vary widely between studies, with no observable trend other than the tendency towards increased fouling at smaller scales of operation and for analogues. • Critical filtration time (tcrit) has been observed only in small-scale experiments, with the limited data available suggesting that it is a function of flux. • Short-term fouling experiments (step-flux) may be helpful in detecting the occurrence of fouling even at sub-critical fluxes and quantify relative fouling propensity of factors such as feedwater characteristics, but the absolute measured fouling rates do not appear to be applicable to long-term operation. In terms of mixed liquor characteristics, the main conclusions are the following: • Sludge concentration is more significant in super-critical than sub-critical operation. • HRT and SRT impact upon the soluble and colloidal fractions in the MLSS. • Extractable EPS are the dominant microbial product in the MBR mixed liquor independent of the feed and operating conditions, and are mainly composed of proteins and lower concentrations of carbohydrates.