A membrane bioreactor (MBR), comprising an anaerobic reactor followed by crossflow MF, was operated at a fixed flux substantially lower than the nominal “critical flux” of the feed. A slow rise in transmembrane pressure (TMP) was observed which increased with imposed flux and organic loading to the bioreactor. After extended operation there was a sudden transition to a rapid TMP rise. This TMP profile was reproducible and depended on the imposed flux. The initial and gradual TMP rise is believed to be caused by the deposition of extracellular polymeric substances (EPS). The sudden rise in TMP is believed to be caused by local flux in regions of the membrane increasing (to maintain the fixed permeate flow) and exceeding the critical flux of the dominant foulant (biomass). Membrane autopsy revealed a significant and uneven distribution of fouling by EPS.

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