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 (FeCl₃) was 48 h faster than that of R2 (zero-valent iron (ZVI)). It was because the FeCl₃ could rapidly promote granulation of sludge as a flocculant. However, ZVI released Fe²⁺ through corrosion slowly, and then the Fe²⁺ increased start-up speed by enhancing enzyme activity and enriching methanogens. In addition, the ZVI and FeCl₃ 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 Fe²⁺ could inhibit anaerobic bacteria activity, excess Fe³⁺ could be changed into iron hydroxide precipitation to hinder the mass transfer process of anaerobic bacteria under the alkaline condition. The FeCl₃ 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.