The study was based on a full scale activated sludge plant (AS) compared to a parallel operated pilot membrane bioreactor (MBR) with flat sheets membranes. Both systems received their influent from an anaerobic bioreactor treating paper mill wastewater. MBR produced an effluent of much better quality than AS in terms of suspended solids, containing 1 mg/L or less in 80% of the monitoring time, while the AS effluent contained 12 mg/L. This could save the necessity of further treatment by filtration in the case of MBR. Other effluent quality parameters, such as organic matter (COD and BOD), phosphorus and ammonia nitrogen, did not indicate substantial differences between AS and MBR. Calcium carbonate scaling and formation of a bacterial layer on the membrane caused severe flux reduction. The membrane blockage because of scaling and biofouling proved to be very serious, therefore, it required proper and more complicated maintenance than the AS system. This study leads to the conclusion that in the case of paper mill wastewater, after anaerobic biotreatment, if there is no need for excellent effluent quality in terms of suspended solids, the replacement of the AS by the MBR would not be strongly justified, mainly because of maintenance cost.

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