Response of sludge fermentation liquid and microbial community to nano zero-valent iron exposure in a mesophilic anaerobic digestion system

Abstract

The effects of nano zero-valent iron (NZVI) on sludge anaerobic digestion were investigated from the perspective of the sludge fermentation liquor (SFL) and microbial community structure. Compared with micrometer-scale ZVI, NZVI exhibited a considerable inhibition in methane production during the initial 6 days while the negative effect attenuated subsequently and methane production recovered. Similar to micrometer-scale ZVI, enhanced methane production (108.24 mL per gVS) was obtained with NZVI addition, and increased by 46.1% compared to no-ZVI assay. The results indicated that NZVI could promote hydrolysis-acidification with inhibited conversion of acetic acid to methane in the initial stage, which could be ascribed to the high H₂ partial pressure. The rapid dissolution of NZVI hindered the phosphorus uptake by methanogens, meanwhile the more reductive atmosphere contributed to the degradation of propionic acid. Further investigations on SFL showed that NZVI could facilitate the release of biodegradable compounds without propionic acid accumulation, creating a favorable substrate environment for methanogens. The bacteria and archaea community structure involved in methane production was studied. Results indicated that NZVI could enhance hydrogenotrophic methanogenesis with the highest relative abundances of Clostridia (53.2%) and Methanosarcina (22.6%) among the assays.