Effect of zero-valent iron particle size on alleviating acid stress in anaerobic digestion of food waste

This work evaluated the effect of zero-valent iron (ZVI) particle size (150 mu m-100 nm) on the performance of food waste anaerobic digestion (AD) under various acid stress conditions. The results indicated that ZVI significantly improved the AD performance, ensuring successful CH4 production even under high acid stress. However, the extent of this promoting effect was highly dependent on the particle size. Nano-scale ZVI (nZVI) demonstrated superior performance in the AD process in terms of CH4 yield and CH4 production rate compared to micro-scale ZVI (mZVI). Specifically, the CH4 yield in the nZVI reactor was 8.24-8.68% higher than that in the mZVI reactor. This improvement was attributed to nZVI's more effective up-regulatory effect on hydrolase activity, as the activities of protease and alpha-amylase in nZVI reactors were 44.55%-48.54% and 14.65%-77.52% higher, respectively, than those in mZVI reactors. Furthermore, the methanogenic lag phase and maximum CH4 production rate in the nZVI reactor were 41.64%-49.69% shorter and 9.71-37.69% higher than those in the mZVI reactor, respectively. This can be attributed to nZVI's enhanced promoting effect on the activity of coenzyme 420, Syntrophomonas and Methanosarcina, thereby accelerating the CH4 production.