Fouling in membrane bioreactors (MBRs) represents one of the most significant barriers to their more widespread implementation for both municipal and industrial wastewater treatment. It exerts a limit on the membrane permeability, i.e. the flux through the membrane per unit transmembrane pressure, and thus the productivity of the process per unit membrane area installed. As with all membrane processes, extensive investigation of factors contributing to fouling in MBRs, and the subsequent identification of ameliorative measures that may be taken to control it, has taken place since the process was first commercialised 30 years ago. Key findings of pertinent research in this area and operational experience in full-scale plants are summarised, along with the primary facets of the MBR process itself. The most recent evidence suggests that permanent fouling, i.e. fouling not substantially removed by physical cleaning (backflushing), results mainly from certain dissolved or colloidal organic materials, and such adsoptive fouling takes place at even the lowest operational fluxes. Fouling by suspended solids, on the other hand, may be largely controlled by operation below the so-called “critical” flux, which may be increased by more vigorous aeration, and/or by periodic backflushing. It is concluded that more work is required on characterisation of species responsible for permanent fouling.

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