Complete sludge retention MBR appears to be an innovative technology which no sludge will be produced, hence eliminating the sludge disposal cost. However, the understanding of microbial behaviour in the complete sludge retention MBR was still not adequately established. This study was conducted to investigate the biological performances and microbial behaviour of three different SRT MBRs (5 days SRT, 10 days SRT and prolonged SRT). The results revealed that membrane filtration assisted for maintaining the high degree of organic removal (above 97%) for all MBRs. High organic degradation was noticed in the prolonged SRT MBR (89.44%) compared with 5 days SRT (77.64%) and 10 days SRT (85.62%) MBRs. This showed that the prolonged SRT MBR posed a greater capability for further degradation of inert organics and SMP. Extremely low sludge yield (0.0388 g VSS/g COD) was occurred in the prolonged SRT condition compared with 5 and 10 days SRT conditions. The low SOUR did not affect the degradation performance. Whereas, it implied that small amount of oxygen was sufficient to achieve high degree of organic degradation during the prolonged SRT MBR condition.

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