This study examined the performance of a novel type of submerged membrane bioreactor (MBR) system which utilizes polyethylene membrane of bigger pore size (mean around 18–28μm) than normal MBRs. The system utilizes plate and frame structure with gravitational effluent
flow. Two such bench scale MBR systems (0.084m3 of mixed liquor volume each) with membrane thickness of 0.48cm and 0.25cm respectively have been operated under an SRT of 6 days for 300 days treating synthetic wastewater containing high molecular weight compounds. Another bench scale
conventional activated sludge (CAS) system (mixed liquor volume = 0.03m3) has been running to make a parallel performance comparison. Biomass in three systems was studied based on their microscopic shape, as well as settling properties. While CAS system biomass grows
well-flocculated bio flocs by settling/flushing-out selection, MBR with 0.48cm-thick membrane maintains most of the fine biomass flocs and forms bigger filamentous-bridging flocs with poor settling quality. Settling property of biomass from 0.25cm-thick MBR stays between the range of the above
two. The difference of microbial species composition between MBR and CAS is also confirmed by Denaturant Gradient Gel Electrophoresis (DGGE) analysis. Discussion of biomass selection linked with system performance helps to understand and improve operation of this novel MBR process in
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