Sludge reduction and microbial structures of aerobic, micro-aerobic and anaerobic side-stream reactor coupled membrane bioreactors.

Graphical abstract Highlights • DO effect on sludge reduction was studied by side-stream reactor (SSR) coupled MBR. • Anaerobic and micro-aerobic SSR-MBR obtained higher nitrogen removal than aerobic. • Micro-aerobic SSR-MBR had higher sludge reduction (61.1%) than anaerobic and aerobic. • Anaerobic SSR favored hydrolysis and fermentation, and aerobic enriched slow growers. • Micro-aerobic SSR benefited hydrolytic, fermentative and slow-growing bacteria. Abstract An anoxic/oxic membrane bioreactor (MBR) and three side-stream reactor (SSR) coupled membrane bioreactors were operated in parallel to investigate effects of dissolved oxygen (DO) level in SSR on sludge reduction and microbial community structure of SSR-MBRs. The four MBRs were equally efficient in COD and ammonium nitrogen removal. The anaerobic and micro-aerobic SSR favored nitrogen removal through denitrification, simultaneous nitrification and denitrification and autochthonous substrate release as carbon source. The micro-aerobic SSR achieved greatly higher sludge reduction efficiency (61.1%) than anaerobic (37.3%) and aerobic SSR (7.9%). Micro-aerobic SSR obtained the highest endogenous decay constant (0.035 d−1) compared to anaerobic (0.023 d−1) and aerobic SSR (0.015 d−1). High-throughput sequencing results revealed that anaerobic SSR enriched hydrolytic and fermentative bacteria, aerobic environment favored the growth of slow-growing bacteria, and micro-aerobic SSR stimulated biological activities of both anaerobic and aerobic bacteria. [ABSTRACT FROM AUTHOR]