Alleviating acid stress in food waste anaerobic digestion by zero-valent iron and magnetite

This investigation compared the roles of zero-valent iron (ZVI) and magnetite (Fe₃O₄) in food waste anaerobic digestion. The results showed that both additives enhanced greatly the CH₄ production rate and CH₄ yield, and ZVI had a better promoting effect. The addition of ZVI or Fe₃O₄ reduced the lag phase of methanogenesis by 73.51 % and 54.22 %, respectively, and increased the maximum CH₄ production rate and CH₄ yield by 67.43 % and 24.39 %, 37.91 % and 12.86 %, respectively. The ZVI chemical corrosion and Fe₃O₄ biochemical reduction played a key role in promoting AD performance. The additives promoted the activity of co-enzyme 420, thereby accelerating the CH₄ production. Additionally, the additives facilitate hydrolysis by improving the activities of the protease and α-amylase, thereby increasing the CH₄ yield. Compared with Fe₃O₄, ZVI had a better up-regulatory effect on key enzyme activities, resulting in higher CH₄ production rate and CH₄ yield. Fe₃O₄ favoured the growth of Syntrophomonadaceae and Methanosaeta, whereas ZVI favoured the growth of more syntrophic bacteria (i.e., Syntrophomonadaceae, Syntrophobacter, and Syntrophorhabdus) and Methanosarcina. This study suggests that the ZVI is more attractive for enhancing industrial scale AD processes due to its superior performance, and greater abundance compared to Fe₃O₄.