Insight into the roles of ferric chloride on short-chain fatty acids production in anaerobic fermentation of waste activated sludge: Performance and mechanism

• The effect of ferric chloride on short chain fatty acids production was evaluated. • Solubilization, hydrolysis, and acidogenesis were promoted by ferric chloride. • Ferric chloride promoted acetogenesis and high molecular organic acids conversion. • Ferric chloride changed the acid-fermentation to propionic acid-type fermentation. • Excessive ferric chloride inhibited acid-fermentation by strong aggregation. Ferric chloride (FC) is widely used in sewage treatment and sludge conditioning, which could be inevitably accumulated in waste-activated sludge (WAS). However, its effect on short-chain fatty acids (SCFAs) production from WAS anaerobic fermentation has yet to be thoroughly investigated. This study aims to reveal how different dosages of FC affect the production of SCFAs and elucidate the corresponding mechanism. Experimental results showed that 16 mg Fe/g total suspended solids (TSS) of FC was favorable for SCFAs production, with the maximum production was 3.3 times of the control. Mechanistic study revealed that FC induced dissimilatory iron reduction (DIR) and enhanced hydrolase activities, contributing to the disintegration of WAS flocs and the hydrolysis of complex organics. Moreover, FC stimulated the productivity of HAc by promoting the release of carbon-rich substrates and accepting the intermediate electrons generated in acetogenesis process. Further study indicated FC inhibited the methanogenesis and changed the acid-fermentation type by affecting pH and ORP, which was conducive to SCFAs accumulation. Microbial community analysis confirmed that FC enriched the bacterial microorganisms related to hydrolysis and acidogenesis, and increased the abundance of Fe(III)-reducing genera. However, excessive FC (>16 mg Fe/g TSS) inhibited SCFAs production, due to the strong aggregation increased mass transfer resistance and the “over-acidification” damaged the microbial community.