Membrane fouling in anoxic/oxic membrane reactors coupled with carrier-enhanced anaerobic side-stream reactor: Effects of anaerobic hydraulic retention time and mechanism insights

Membrane fouling mechanism of membrane reactors (MBR) coupled with anaerobic side-stream reactor (ASSR) has been extensively studied, while the homologous mechanism in carrier-enhanced ASSR process remained unclear. Anoxic/oxic MBR and three carrier-enhanced ASSR coupled anoxic/oxic MBRs (ASSR-MBRs) were operated in parallel to investigate the effect of hydraulic retention time of ASSR (HRT_SR) on membrane filtration performance and fouling tendency. ASSR-MBR with HRT_SR of 2.5, 5.0 and 6.7 h achieved the sludge reduction efficiency of 24.6%, 32.2%, and 41.0%. The foulant layer resistances were in the order of anoxic/oxic MBR (5.40 ± 0.32 × 10¹² m⁻¹) > ASSR_2.5-MBR (5.39 ± 0.30 × 10¹² m⁻¹) > ASSR_6.7-MBR (3.33 ± 0.42 × 10¹² m⁻¹) > ASSR_5.0-MBR (2.38 ± 0.45 × 10¹² m⁻¹). The improved sludge dewaterability and more stable sludge structure in carrier-enhanced ASSR caused membrane fouling less severe than that in anoxic/oxic MBR. The results of extracellular polymeric substances identified incomplete and over hydrolysis of cell lysate at HRT_SR of 2.5 and 6.7 h, respectively. CLSM results indicated that ASSR-MBR with HRT_SR of 5.0 h achieved the best filtration performance because of the enhanced protein degradation ability of membrane surface microbes. MiSeq sequencing indicated that fouling-eased group bacteria (responsible for the alleviation of membrane fouling) were enriched in ASSR-MBRs, while fouling-caused group bacteria (responsible for the aggravation of membrane fouling), such as filamentous bacteria, were dominant in anoxic/oxic MBR. This study bridged a gap between sludge redcution and membrane fouling in carrier-enhanced ASSR-MBR.