Recovery of an acidified food waste anaerobic digestion reactor via NH4+-N and trace metals addition: Performance, microbial community response, and mechanisms

An increased organic loading rate can lead to an imbalance in the production and consumption of volatile fatty acids (VFAs) during anaerobic digestion of food waste, ultimately resulting in reactor acidification. In this study, VFA consumption was enhanced by adding NH4+-N and trace metals (TMs) (Fe, Co, Mo, and Ni). The addition of NH4+-N (330 mg/L) decreased the VFAs concentration in the reactor from 1477.54 to 223.83 mg/L, and remained below 400 mg/L during subsequent operation. The addition of NH4+-N enhanced the acid-base buffering capacity of the reactor, thereby detaching methanogens from the acidified environment and restoring their activity. The addition of TMs gradually decreased the VFAs concentration in the reactor from 3926.23 to 230-330 mg/L. TMs directly enhanced the methanogenic activity and enriched functional methanogens (Methanosarcina) with high acetic acid utilisation efficiency, thereby alleviating reactor acidification.