Anticipating on potential electron acceptors for Anammox

Anammox is a highly efficient, environmentally friendly, and promising form of biological denitrification. Since nitrite is not present in the majority of wastewater, the lack of electron acceptors (nitrite) greatly limits the practical application of Anammox. However, many researchers have overlooked the option of employing other electron acceptors. Much evidence suggests that Anammox is an electroactive microorganism that can deliver electrons to various extracellular electron acceptors. This information informs researchers that Anammox cells are not limited to accepting electrons only from electron acceptors within their interior. It also provides the possibility that Anammox bacteria can utilize other electron acceptors. Many studies have shown the possibility that Anammox bacteria can use other electron acceptors. However, the implementation of the Anammox process for various electron acceptors faces some challenges in actual applications. We review several electron acceptors (nitrite, nitric oxide, ferric iron, sulfate, and electrode) that have been shown to be utilized by Anammox as electron acceptors. This paper provides a comprehensive review of the physicochemical properties of these electron acceptors, elucidates the intricate transformation pathways of ammonia nitrogen catalyzed by distinct electron acceptors, and highlights the pivotal role of Anammox bacteria in the nitrogen cycle. The electron transfer theory is used to explain the reasons for the Anammox nitrogen removal efficiency differences under different electron acceptors. It also discusses Anammox's extracellular and intracellular electron transfer mechanisms in the presence of several electron acceptor types. The aim is to enhance researchers' understanding of the metabolic variety of Anammox bacteria and to realize a practical application. Thus, the study's information gaps, shortcomings of different electron acceptors Anammox process, and the required further improvements are all identified and discussed.