Comparison of Biomass Selection Between a Novel Membrane Bioreactor and Activated Sludge Process

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ABSTRACT
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 0.48cmthick 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 the future.

INTRODUCTION
Conventional activated sludge systems have been widely used in both the industrial and municipal wastewater treatment industry for years. With the treated wastewater discharge criteria becoming ever more stringent, the Membrane Bioreactor approach, which combines a biological stage and a membrane module, has become of increasing interest and has received extensive analysis.

MBR typically has two types of configurations: external membrane and submerged membrane (Figure 1), which mostly depends on suction pump to create trans-membrane pressure (TMP, 1~ 4bar for external; around 0.5bar for submerged) to collect effluent through the membrane (B. Marrot et al., 2004).

Compared with conventional activated sludge processes, MBRs raised people’s interest with the advantages of system compactness, higher effluent quality (much less TSS), longer and controllable SRT, and normally higher MLSS resistant to shock load. However, the application of MBRs in the wastewater treatment industry is still limited by factors such as membrane fouling, high cost of membrane material and energy needed for overcoming trans-membrane resistance.

Considering its lower cost advantage, a submerged MBR with gravitational flow has been tested by Tatsuki Ueda for municipal wastewater treatment (Tatsuki Ueda, et al., 1999). With 0.3m of water head and 0.4 m/d of flux, satisfactory effluent quality has been reached using 0.4μm polyethylene membrane.

This study tried to test another submerged MBR with gravitational flow by comparing it with an activated sludge system. This novel MBR has bigger membrane pore sizes (18~28μm) and requires much less water head (5cm), which has been proved successful in groundwater contamination treatment (Maher M. Zein, et al., 2004).

Both biological reaction zone and membrane filtration play a significant role in this study of MBR performance. Biomass selection analysis has been carried out simultaneously in this study of MBR and CAS systems.

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