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 Fe 3 O 4 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 Fe 3 O 4 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, Fe 3 O 4 obviously promoted the solubilization, hydrolysis and acidification of sludge. Vast Fe 2+ was detected in the aqueous phase of the Fe 3 O 4 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 Fe 3 O 4 decreased by 27% and 22% compared to the Fe 3 O 4 -free digester, respectively. Further study indicated that Fe 3 O 4 competed with CH 3 ▪S▪CoM for electrons and thus inhibited the methanogenesis process.