The identification and quantification of foulants in membrane bioreactors present a major challenge due to their complexity resulting from biomass heterogeneity. Fouling is normally characterised with respect to the critical flux, this being conventionally viewed as being the flux below which a reduction in membrane permeability does not take place. However, recent studies have revealed that such fouling arises even at very low fluxes. Moreover, fouling rates can differ substantially between different experiments, trials and installations even when operated under apparently similar conditions of biomass and soluble organic carbon concentrations. The methods available for quantifying and analysing fouling are reviewed with specific reference to recent data on sub-critical flux behaviour. It is concluded that HPSEC analysis of extracted biomass fractions may provide the most valuable data towards the determination of differences in fouling propensity between different biomasses, as inferred in conventional flux step analysis.

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