A long aging time had severely inhibited methane production from Fe-chemically enhanced primary sedimentation sludge during anaerobic digestion

Fe-chemically enhanced primary sedimentation (CEPS) process is a cost-effective method to capture organics from municipal wastewater into Fe-CEPS sludge. Anaerobic digestion (AD) has been proven to be an efficient and economical method to treat Fe-CEPS sludge. Meanwhile, the CEPS sludge produced by sedimentation is generally stored on-site for days to months. During this period, different iron species in Fe-CEPS sludge can form, potentially influencing subsequent methane production. Thus, this study investigated the biodegradability of Fe-CEPS sludges after different aging times, ranging from 0 to 35 days, during the AD process. The results indicated that iron-bearing species gradually transformed into more crystalline phases, with the content increasing from 6.17 to 21.72 mg/g as the aging time extended. Furthermore, the highest methane yield was obtained from the fresh Fe-CEPS sludge, reaching a value of 186.18 mL CH4/g TS. However, with increasing aging time, methane yields significantly decreased, reaching the lowest yield of 35.73 mL CH4/g TS in the group with an aging time of 35 days. This suggests that prolonged aging time severely inhibits methane production from Fe-CEPS sludge. Significant negative relationships were observed between methane yield and vivianite-P, crystalline iron (Cry-Fe) (r = −0.97, p < 0.01; r = −0.94, p < 0.01). Conversely, the TOC content of the aged sludges exhibited a strongly positive correlation with methane yield (r = 0.88, p < 0.05). These findings suggest that the formation of vivianite-P, Cry-Fe, and microbiological oxidation resulting from prolonged aging times are the main contributors to inhibiting subsequent methane production.