Lab-scale and pilot-scale activated sludge bioreactors with integrated microfiltration membranes were operated over a period of up to three years. During the entire operation period no excess sludge was removed from the bioreactors apart from sampling, resulting in highly concentrated biomass in the reactors. The dry weight of the sludge ranged from 15 to 23 g MLSS l–1 for a plant fed with municipal wastewater and up to 60 g ll–1 for a lab-scale plant fed with high strength molasses. Stable biomass concentrations were reached at F/M ratios as low as approximately 0.07 kg COD (kg MLSS)–1 d–1. The degradation performance of the analyzed reactors was high and stable. Direct microscopical studies revealed high amounts of free suspended cells and at various times also high numbers of filamentous bacteria. Surprisingly only low numbers of protozoa were observed during most of the time. By use of fluorescent in situ hybridization (FISH) only about 40% to 50% of all bacteria emitted probe conferred fluorescence signals sufficient for detection, compared to around 80% cells detectable in conventional activated sludge. Studies on oxygen consumption rates indicated that the biomass in the bioreactor was substrate limited. These data suggest that substrate is mainly oxidized and not used for growth purposes which offers the possibility to operate membrane bioreactors with significantly reduced secondary sludge production.

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