Inhibitory effect of released phosphate on the ability of nano zero valent iron to boost anaerobic digestion of waste-activated sludge and the remediation method

The addition of nano zero valent iron (NZVI) to anaerobic systems is an attractive strategy to enhance the anaerobic digestion (AD) of waste-activated sludge (WAS). However, high minerals contents within the WAS may release inorganic salts and regenerate precipitates in AD, leading to changes in the morphology of the sludge and affecting mass transfer between syntrophic bacteria and methanogens. In this study, the addition of NZVI was found to significantly accelerate CH4 production early in the AD process of WAS. With continuous AD, the internal channels of the anaerobic granular sludge (AGS), which are crucial for mass transfer, were clogged by phosphate precipitates that were formed due to the reaction between Fe2+ and PO43− released from phosphorus-containing minerals in WAS. This caused a decrease in CH4 production. The combination of NZVI and granular activated carbon (GAC) reduced the production of phosphate precipitates due to the adsorption on Fe2+ by GAC; this mitigated the inhibitory effect of phosphate precipitates on CH4 production. Additionally, there was a sharp increase in genes involved in hydrogenotrophic methanogenesis in the NZVI and GAC + NZVI groups, while there was no significant change in the genes involved in acetoclastic methanogenesis in the NZVI group compared to the control group. The formation of H2 and the direct release of electrons from NZVI corrosion, which boost CH4 production, occurred simultaneously. This study is the first detailed investigation into the mechanisms underlying the inhibition and bioaugmentation effects of NZVI on AD of WAS in a complex system.