New insight into mechanisms of ferroferric oxide enhancing medium-chain fatty acids production from waste activated sludge through anaerobic fermentation

Medium chain fatty acids (MCFAs) production from waste activated sludge (WAS) is restricted by poor biodegradability of WAS and low electron transfer efficiency. Herein, a novel ferroferric oxide (Fe 3 O 4 ) technique was proposed. Results indicated that the MCFAs yield and selectivity were respectively enhanced by 155.4% and 66.7% in the Fe 3 O 4 -mediated WAS. Mechanistic studies disclosed that Fe 3 O 4 promoted substrates degradation through conducting dissimilatory iron reduction (DIR) and stimulating hydrolase activity, providing precursors for chain elongation (CE). Generally, Fe 3 O 4 improved the key processes for MCFA production at different degrees, i.e., hydrolysis, acidification and CE. Interestingly, MCFAs yield enhancement was primarily ascribed to facilitated electron transfer rather than DIR or produced ferrous iron, which could be supported by the analyses of electrochemical properties, electron transfer system activity and morphology. Further, Fe 3 O 4 shifted the key microorganisms in favor of MCFAs production. Overall, this strategy could improve MCFAs production, sludge dewatering and phosphorus removal, concurrently.