Effect of zero-valent iron and trivalent iron on UASB rapid start-up.

In order to realize the rapid start-up of upflow anaerobic sludge blanket (UASB) reactor, the iron ion in different valence state was added to UASB. The results indicated that the start-up time of R3 (FeCl) was 48 h faster than that of R2 (zero-valent iron (ZVI)). It was because the FeCl could rapidly promote granulation of sludge as a flocculant. However, ZVI released Fe through corrosion slowly, and then the Fe increased start-up speed by enhancing enzyme activity and enriching methanogens. In addition, the ZVI and FeCl could promote hydrolysis acidification and strengthen the decomposition of long-chain fatty acids. The detection of iron ions showed that iron ions mainly existed in the sludge. Because the high concentration of Fe could inhibit anaerobic bacteria activity, excess Fe could be changed into iron hydroxide precipitation to hinder the mass transfer process of anaerobic bacteria under the alkaline condition. The FeCl was suitable to be added at the initial stage of UASB start-up, and the ZVI was more fitted to be used in the middle stage of reactor start-up to improve the redox ability.