Aerobic MBRs for domestic wastewater treatment: a review with cost considerations

Membrane bioreactors (MBRs) present a means of intensively biologically treating high COD or BOD wastewaters but, like other membrane processes, are constrained by their tendency to foul. Fouling is the general term given to those phenomena responsible for increasing membrane hydraulic resistance. It can be reduced by maintaining turbulent conditions, operating at sub-critical flux and/or by the selection of a suitable fouling-resistant membrane material. The performance of various MBRs is appraised with reference to (i) fouling propensity, and (ii) removal of organics and microorganisms. Energy costs for the two process configurations for MBRs, submerged and side-stream, are reported with particular attention paid to aeration and recycle pumping costs. A number of commercial plants treating domestic wastewater are described, with further details of the most recent full-scale MBR for sewage treatment tabulated. It is shown that the side-stream configuration has a higher total energy cost, by up to two orders of magnitude, compared with the submerged system due to the recycle component. The submerged configuration operates more cost effectively than the side-stream configuration with respect to both energy consumption and cleaning requirements, with aeration providing the main operating cost component as it is required for both mixing and oxygen transfer. On the other hand, the lower flux under which the submerged system operates implies a higher membrane area and thus a higher associated capital cost. It is concluded that the MBR is a highly effective treatment process for wastewater treatment in areas requiring a high quality effluent (such as discharge to bathing waters or water reuse) or specialisation in the microbial community (e.g, high strength liquors, effective nitrification). (C) 2000 Elsevier Science B.V. All rights reserved.