AbstractConventional activated sludge processes, as the main biological treatment approach for municipal wastewater treatment in centralized wastewater treatment plants (WWTP), generate a large amount of waste-activated sludge (WAS) that needs to be managed before disposal. However, the degradation of WAS is usually limited by the slow hydrolysis rate and low biodegradable rate. Here, an effective pretreatment strategy using calcium hypochlorite treating waste-activated sludge before aerobic digestion was explored. Performance results showed that after the pretreatment with 0.1 g Ca(ClO)2 per gram of total solids (TS), volatile solids (VS) in WAS were reduced by 65.05%±2.68% after 20 days of aerobic digestion, nearly twice the reduction of VS in un-pretreated WAS. The best treatment conditions under tested experiment conditions were 12 h of WAS pretreatment process with 0.1 g Ca(ClO)2 per 1 g TS and 12 days hydraulic retention time (HRT) of aerobic digestion. Kinetic model analysis revealed that the VS reduction of WAS during aerobic digestion was enhanced by the Ca(ClO)2 pretreatment, likely through cell lysis and extracellular polymeric substances (EPS) decomposition. Moreover, microbial community analysis revealed that the relative abundance of certain bacteria (e.g., Xanthomonadaceae, Saprospirales, and Cytophagaceae) increased, which can be attributed to the change of the chemical and biological environment in the aerobic digesters after pretreatment. This study demonstrated the feasibility of applying calcium hypochlorite pretreatment in WAS aerobic digestion to improve sludge digestibility and volume reduction.