The number of membrane bioreactor (MBR) installations is increasing worldwide, not only for small-scale industrial WWTPs but also for larger-scale municipal WWTPs. In Europe, MBR has been installed in municipal WWTPs since late 1990s, and more than 100 full-scale plants are operated at the moment. In this paper, present state of European municipal MBRs is described in terms of design and operating conditions, as well as operating problems and their solutions, based on the information collected from 17 full-scale WWTPs by interview and questionnaire survey. Decisive factors of MBR installation at these plants were footprint and effluent quality. Full-aerobic and pre-denitrification were the most common reactor configurations, nearly half of them being equipped with independent filtration tanks. Operating conditions of bioreactor and filtration, including membrane flux and cleaning strategy, were different from plant to plant, as a result of plant-specific optimization experiences, even among the similar type of membrane. Operating problems specific for MBR were reported, including blocking/failure of pre-screen, sludging/hair-clogging of membrane, damage on membrane unit, air in permeate pipes, as well as conventional troubles including occurrence of scum and initial trouble in instrumentation and control systems. Aspects for further optimization of MBR design were also pointed out by the operators.

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