Sludge reduction and microbial structures in MBRs: Features influencing the sustainable adoption of MBRs

This chapter reviewed recent advances of sludge reduction and microbial population in membrane bioreactors (MBRs). The fundamental mechanisms of sludge reduction, cell lysis-cryptic growth, uncoupling, predation, and maintenance metabolism were introduced. Maintenance metabolism was the major mechanism of sludge reduction in conventional MBR. Uncoupling metabolism through side-stream reactor (SSR) configuration and predation are promising methods that can be combined with MBRs for sludge minimization and applied to full-scale applications. SSR coupled MBR (SSR-MBR) obtained sludge reduction efficiency that ranged from 18.6% to 49.7%, and critical factors influencing sludge reduction include hydraulic retention time (HRT) of SSR, side-stream ratio, and redox state of SSR. Compared with conventional MBRs, inserting SSR brought up sludge floc structure and accelerated membrane fouling process, while carriers packed in SSR moderated membrane fouling. Hydrolysis of particulate organic matters, lysis, and cryptic growth of biomass are the three primary mechanisms for sludge in situ reduction, which were brought out by the enrichment of functional bacteria response for fermentative acidogenic and hydrolytic, predation, and slow growth under microaerobic or anaerobic condition in SSR.