Mechanistic insights into the roles of ferric chloride on methane production in anaerobic digestion of waste activated sludge

Ferric chloride (FC) is widely used in sewage treatment in practice and would influence anaerobic digestion by remaining in waste-activated sludge (WAS). However, the effects and mechanisms of FC involved in the WAS anaerobic digestion system have yet to be thoroughly elucidated. This study revealed the different effects and underlying mechanism of FC addition on each key stage of WAS anaerobic digestion. It was found that FC enhanced methane production by 114.7–197.2%, with the maximum obtained at the dosage of 234 mg Fe(III)/L. Further study revealed that the dissimilatory iron reduction (DIR) induced by FC was the critical route that impacted the whole process of WAS anaerobic digestion. FC promoted the WAS solubilization, hydrolysis, and acidification via DIR process, since FC could serve as electron acceptors to accelerate the decomposition and degradation of WAS complex organics, and accept the Intermediate electrons to stimulate the bioconversion of acetic acid from amino acids and monosaccharides. However, FC inhibited methane production from acetoclastic and hydrogenotrophic methanogenesis by 29.2% and 28.4%, which was attributed to the DIR process competed with methyl-CoM for electrons from [CO] and HS-HTP, and inhibited the bioconversion from methyl-CoM to methane. Microbial community analysis confirmed that FC enriched Fe(III)-reducing genera and the bacterial microorganisms related to hydrolysis and acidification, but decreased the richness of methanogens. Overall, this study contributes to a better understanding of the mechanisms of FC integrated into WAS anaerobic digestion, and laid the foundation to optimize the routes for WAS energy/carbon recovery.