Understanding mechanisms of sludge in situ reduction in anaerobic side-stream reactor coupled membrane bioreactors packed with carriers at different filling fractions.

• Roles of filling fraction of carriers in anaerobic side-stream reactor was studied. • Carriers with filling fraction of 25% achieved the highest sludge reduction of 50.5%. • Packing carriers accelerated enzyme secretion and enhanced hydrolysis and cell lysis. • Enhancement by carriers themselves and to the bulk sludge accounted for 25% and 75%. • Excessive carriers had negative effect on biomass stability and movement of carriers. An anoxic/oxic membrane bioreactor (AO) and three pilot-scale anaerobic side stream reactors (ASSR) coupled MBRs (ASSR-MBRs), packed with 0%, 25% and 50% carriers in ASSRs, were continuously operated to study the mechanisms for sludge reduction. Four systems showed efficient COD and NH 4 +-N removal, while packing carriers significantly enhanced nitrogen removal. 25% filling fraction (AP 25) achieved the highest sludge reduction efficiency of 50.5% compared to 0% (21.7%) and 50% (39.7%). Compared to ASSR-MBR, carriers enhanced the release of dissolved organic matters, and accelerated the secretion of enzyme for cell lysis and hydrolysis. In AP 25 , the presence of carriers prompted the formation of environment propitious to sludge reduction in bulk sludge. AP 25 tended to enrich hydrolytic, fermentative and denitrifying bacteria to accelerate hydrolysis process, while excessive carriers had negative effect on biomass stability and movement of carriers. [ABSTRACT FROM AUTHOR]