Calcium hypochlorite pretreatment improves thermophilic digestion of waste activated sludge in an upflow anaerobic sludge blanket reactor

Anaerobic wasted activated sludge (WAS) digestion has been widely applied to reduce sludge volume and generate bioenergy in the form of methane. However, anaerobic WAS digestion performance is often challenged with poor hydrolysis of biomass cellular structures. In the present study, the feasibility of using calcium hypochlorite (Ca(ClO)2) to improve the thermophilic digestion of WAS was studied. Two thermophilic upflow anaerobic sludge bed (UASB) reactors (one with and one without Ca(ClO)2 pretreatment) were operated for 120 days under low and high organic loading rate (OLR) conditions, corresponding hydraulic retention time (HRT) of 10 days and 6 days, respectively. Both reactors achieved satisfied performance during the studied period. Under the low OLR condition, Ca(ClO)2 pretreatment significantly improved WAS total volatile solids (VS) removal efficiency (from 48.06 ± 2.63% to 57.34 ± 3.54%) and methane yield (from 289.2 ± 27.6 to 362.2 ± 36.7 N mL/g VS). However, no significant improvement was observed under the high OLR condition. g_S1 and g_Fervidobacterium were predominant bacteria in the thermophilic UASB reactor fed with Ca(ClO)2 pretreated WAS. Methanosarcina was dominant archaea in both reactors. The treatment mechanism and application potential of using Ca(ClO)2 to enhance the WAS digestibility were further discussed.