A Review on Electron Acceptors Involved in Anaerobic Ammonium Oxidation: Pathways, Electron Transfer Enhancement, and Microorganisms.

The global carbon emission agenda is driving the demand for anaerobic ammonium oxidation (Anammox)-based carbon-efficient nitrogen removal, which is closely dependent on different electron acceptors. Here, the recent advancements in Anammox electron acceptors were reviewed in terms of pathways, electron transfer enhancement, and microorganisms. The metabolic mechanisms mediated by six confirmed electron acceptors (NO2 -, NO, Fe3+, SO4 2-, MnO2, and graphene oxide) and two candidate ones (CO3 2-/HCO3 -, and V5+) for Anammox were discussed with electron transfer pathways and functional genes/enzymes clarified. Two extracellular electron transfer enhancement methods, i.e., conductive material addition and voltage application, were outlined with reinforced nitrogen removal efficiency and optimal implementation conditions included. The key factors affecting different Anammox processes, i.e., organic matter, temperature, pH, and DO, were reviewed with favorable ranges summarized. The functional microbes involved in Anammox were discussed. Regarding the advancement in the Anammox electron acceptors, the relevant knowledge gaps for future investigations were proposed. This review helps better understand the Anammox mechanisms under different electron acceptor conditions and provides a theoretical basis for the development of highly efficient nitrogen removal techniques based on Anammox.