Effect of zero-valent iron addition on ammonia inhibition alleviation and fecal indicators reduction in anaerobic digestion of pig manure

Anaerobic digestion (AD) is widely used for the treatment of pig manure, but is easily suffered from ammonia inhibition. The residual pathogenic bacteria and antibiotic resistance genes in the digestate pose potential risks to human health and the environment. Zero-valent iron (ZVI) addition in AD has been extensively investigated to enhance AD performance, but little is known about the impacts of ZVI addition on ammonia inhibition and the digestate biosafety. In this study, the effect of ZVI addition on ammonia inhibition alleviation and reduction of fecal indicator bacteria and antibiotic resistance genes in AD of pig manure and its mechanisms were investigated. ZVI addition at 10 g/L effectively alleviated ammonia inhibition and promoted a shift in dominant methanogens, improving the tolerance of AD systems to high concentration of ammonia. Interestingly, ZVI addition within 5–20 g/L significantly enhanced the reduction of total coliforms, Enterococci and tetracycline resistance genes in the digestate, which was due to the generation of ferrous ions, causing the co-precipitation of suspended solids and indicator bacteria. These results provided valuable insights for the potential application of ZVI addition in the AD of pig manure.