Fe₃O₄@PANI composite improves biotransformation of waste activated sludge into medium-chain fatty acid

Core–shell structured Fe₃O₄@PANI composite was prepared using a chemical oxidation polymerization method and used as the bio-carrier to enhance the biotransformation of waste activated sludge to produce medium-chain fatty acid by anaerobic fermentation. The synthesized composite had stable chemical properties and good biological affinity. The addition of Fe₃O₄@PANI can effectively promote the acidogenesis and chain elongation in the fermentation system. When 1 g/L of Fe₃O₄@PANI was added, the acid production from the fermentation system increased by up to 1.9 times, and the maximum acid production rate increased by 3.4 times. Meanwhile, the production of caproate increased by 2.79 times. It was demonstrated that the added Fe₃O₄@PANI facilitated the release of organic compounds from bacterial cells and improved the electrical activity of the sludge system, thereby accelerating the chain elongation to generate caproate. Moreover, the addition of Fe₃O₄@PANI can effectively enrich the functional microbes related to chain elongation, including Clostridium sensu stricto 12, Dechloromonas, Romboutsia, and IMCC26207.