Effect of Fe addition on volatile fatty acids evolution on anaerobic digestion at high organic loading rates.

Excessive acidification frequently occurs in the anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) at high organic loading rates (OLR), due to the accumulation of non-acetic volatile fatty acids (VFA). In this study, the performance of Fe in enhancing various VFA production and metabolism was investigated. The butyric acid concentration in a high OLR reactor with Fe addition decreased from 7200 to 0mg/L after a short lag phase, and the total VFA (TVFA) concentration also decreased substantially. The corresponding dominant acidogenesis type also changed from butyric type to propionic type fermentation. Furthermore, the CH yield of the reactor with added Fe was approximately 595ml CH/g VS, which was an increase of 41.7% compared with the biochemical methane potential (BMP) test results in controls without added ZVI. A microbial diversity analysis, using high throughput sequencing, showed that Methanofollis and Methanosarcina were dominant in terms of the archaeal structures of the Fe reactor at the butyric converting stage; however, Methanosaeta was predominant in the reactor during the control BMP test. These results suggested that Fe can convert non-acetic VFA to acetic VFA and improve the CH yield by enhancing the activity of methanogens.