Comparing the mechanisms of ZVI and Fe3O4 for promoting waste-activated sludge digestion.

Anaerobic digestion is one of the most promising technologies to stabilize waste-activated sludge (WAS) and recover energy. However, the low efficiency of anaerobic digestion of WAS constrains its application. Supplementing zero valent iron (ZVI) and Fe3O4 in digesters could improve the sludge digestion performance, which has recently been extensively studied. However, the mechanisms behind this improvement remain unclear. In this study, the effects of ZVI and Fe3O4 on the four stages of anaerobic digestion of WAS (solubilization, hydrolysis, acidification and methanogenesis) were investigated. Results showed that ZVI had only a slight effect on the solubilization, hydrolysis and acidification processes, while ZVI significantly promoted the hydrogenotrophic methanogenesis, increasing methane production by 70%. Further investigation indicated that coenzyme F420 activity in the ZVI added reactor was 32.3% higher than in the blank. These results indicate that ZVI promoted anaerobic digestion of WAS through promoting hydrogenotrophic methanogenesis. On the other hand, Fe3O4 obviously promoted the solubilization, hydrolysis and acidification of sludge. Vast Fe2+ was detected in the aqueous phase of the Fe3O4 digester which was a result of dissimilatory iron reduction that can utilize complicated matter as a substrate. This was in agreement with the acceleration of the solubilization, hydrolysis and acidification of the sludge. However, the acetoclastic and hydrogenotrophic methanogenesis with Fe3O4 decreased by 27% and 22% compared to the Fe3O4-free digester, respectively. Further study indicated that Fe3O4 competed with CH3▪S▪CoM for electrons and thus inhibited the methanogenesis process.