2 shows the production of methane in the ZVI-anaerobic
sludge combined systems at rZVI/VSS ranging from 0 to 5.26. The
result indicated that the methane production was significantly
affected by the ZVI adding. The methane generation increased with
the amount of ZVI addition initially, but higher doses would make
the ZVI-facilitated effect weaker. After 30 h, the most accumulation
of methane was observed at the rZVI/VSS of 2.63, which was
a little more than that produced at the rZVI/VSS of 5.26, suggesting
that excessive ZVI doses did not stimulate the methane production
further and probably exerted negative effects on microbial
activity. In view of the contribution efficiency of ZVI adding, the
maximum value was 48.6 mLCH4/gFe obtained at the rZVI/VSS of 0.74
(see Table 2 in Supplementary material), further increasing the ZVI
doses resulted in a decrease in the contribution efficiency of per
ZVI. From Fig. 2, results indicated that the combined system was
more economical at the rZVI/VSS of 0.32 and 0.74, while it showed
more potential enhancement in methane generation at the rZVI/VSS
of 1.58 and 2.63. Considering no more methane was produced with
the increased ZVI doses at the rZVI/VSS of 5.26, the ZVI concentration
of 50 g/L was thought to be excessive for the microbial biomass in
this test