Zero-valent iron is not always effective in enhancing anaerobic digestion performance

Liquid nitrogen was employed as a low-temperature medium to activate zero-valent iron (ZVI) powder in an attempt to strengthen its enhancement effect on anaerobic digestion (AD) of swine manure (SM). Surprisingly, it was found that both pristine ZVI and liquid nitrogen-pretreated ZVI (LZVI) did not significantly improve the AD performance or change the archaeal community structure. It was hypothesized that ZVI might not be effective at stress-free environment like in these digesters. To confirm this, an additional set of AD experiments were performed at high ammonia stress (about 4000 mg/L), results showed that ZVI and LZVI greatly alleviated ammonia inhibition and increased the CH4 yield by 11.6% and 28.2%, respectively. Apparently, ZVI mainly affected AD systems by changing the metabolism pathways and enhancing the microbial activity to overcome process inhibition, and pretreatment of liquid nitrogen could significantly accelerate the dissolution of ZVI and improve its utilization efficiency, contributing to a greater extend of process recovery and improvement. [Display omitted] •Liquid nitrogen was applied to improve ZVI utilization efficiency during AD of SM.•ZVI/LZVI did not affect CH4 yield and archaeal distribution in healthy digesters.•ZVI could alter metabolisms and activate microbes to overcome AD inhibition.•Liquid nitrogen soaking strengthened ZVI's enhancement effect at high TAN stress.•ZVI showed potentials in process revival.