A baffled reactor with repeated coupling of aerobic and anaerobic conditions (rCAA) was constructed and the mechanism of the on-site sludge reduction was analyzed systematically. During the long-term operation of 3 years, the chemical oxygen demand (COD) removal efficiency reached over 90% on average and the sludge yield in the rCAA system was 0.098 kg suspended sludge (SS) kg(-1) COD, which was approximately 36.3% of that in a conventional activated sludge process at an influent COD of 700-800 mg L-1 and hydraulic retention time (HRT) of 15 h. Analysis of soluble components, sludge characteristics, microorganism community and metabolic activities in the rCAA system showed that, when the sludge in the aerobic zone flowed into the anaerobic zone, sludge destruction and degradation occurred. Intracellular organic substances were then released and further hydrolyzed to small molecular compounds. These intermediate substances were degraded in subsequent aerobic compartments. In this way, cryptic growth of the sludge was realized that could contribute to sludge reduction. Microbial ecological analysis by denaturing gradient gel electrophoresis (DGGE) further showed that the rCAA process spatially domesticated the different microbial communities along the flow direction of the bioreactor, which enhanced excess sludge lysis and transformation, cryptic growth and bacterial predation. (C) 2012 Elsevier B.V. All rights reserved.