Methane production from waste activated sludge by combining calcium peroxide pretreatment with zero valent iron bio-enhancement: Performance and mechanisms

Calcium peroxide (CaO2) pretreatment has been confirmed to be an effective method for the promotion of waste activated sludge (WAS) digestion. However, the CaO2 reaction residues (i.e., free radicals and ammonia) are unfavorable to the subsequent anaerobic digestion process. This study proposes a new strategy to minimize the inhibition of the residues by adding zero-valent iron (ZVI) to the CaO2 pretreated sludge. The results indicated that the combination of CaO2 pretreatment and ZVI addition exhibited better performance. Methane production was improved by 9.7% and 13.8%, and sludge reduction was enhanced by 4.0% and 6.0% compared with sole CaO2 pretreatment or ZVI addition reactors, respectively. Mechanism analysis indicated that ZVI could enhance the activities of the electron transport system (ETS) and coenzyme F420, which could reduce the inhibition to methanogens caused by high concentrations of ammonia nitrogen. Microbial community analysis indicated that ZVI could effectively improve the abundance of hydrogenotrophic methanogens.