The effects and mechanisms of zero-valent iron on anaerobic digestion of solid waste: A mini-review

For a long time, anaerobic digestion (AD) is considered to be one of the most promising methods to solve the problem of solid waste recycling. The addition of zero-valent iron (ZVI) in AD system is proved to be of great improvement. In this review, we reviewed the principle of ZVI in AD process and the mechanism studied at present. Then, the applications of ZVI in various solid waste AD system, such as sludge, manure, and food waste (FW), were thoroughly reviewed. In particular, some important factors for ZVI performances were carefully addressed, including pretreatment method, particle size and addition amount. Most studies reported that the proper amount of ZVI has a good effect on microbial activity, that is, the control of a certain dosage of ZVI has a positive effect, mainly manifested in better long-term stability of the digester, such as greater substrate degradation, lower volatile fatty acids (VFAs) concentration and higher microbial abundance and methane production. However, due to the great variance of the AD operation and digestion substrates, there are still many challenges when designing an efficient and cost-effective AD system for practical solid waste recycling. Therefore, further studies are acquired for better utilize ZVI to enhance AD performances according to different conditions. [Display omitted] •The mechanism of ZVI in AD includes reduction, adsorption, electrolysis and precipitation.•ZVI can alleviate the inhibition of ammonia nitrogen in AD.•ZVI improves methane production by promoting enzyme activity and acting as an electron donor.•Real-field implementation problems and scope for further studies are described.•The cost analysis and recovery of ZVI are worthy of further study.