Bioelectrochemical system for nitrogen removal: Fundamentals, current status, trends, and challenges

Biological nitrogen removal (BNR) is essential for the treatment of nitrogen-containing wastewater. However, the requirement for aeration and the addition of external carbon sources, resulting in greenhouse gas emissions and additional costs, are disadvantages of the traditional BNR process. Alternative technologies have been devised to overcome these drawbacks. Bioelectrochemical nitrogen removal (BENR) has been proposed for efficient nitrogen removal, demonstrating flexibility and versatility. BENR can be performed by combining nitrification, denitrification, anaerobic ammonium oxidation (ANAMMOX), or organic carbon oxidation. Bioelectrochemical-ANAMMOX (BE-ANAMMOX) is the most promising method for nitrogen removal, as it can directly convert NH₄⁺ to N₂ and H₂ in one step when the electrode is arranged as an electron acceptor. High-value-added hydrogen can potentially be recovered with efficient nitrogen removal using this concept, maximizing the benefits of BENR. Using alternative electron acceptors, such as electrodes and metal ions, for complete total nitrogen removal is a promising technology to substitute NO₂^- production from NH₄⁺ oxidation by aeration. However, the requirement of electron donors for NO₃^- reduction, low NH₄⁺ removal efficiency, and low competitiveness of exoelectrogenic bacteria still remain the main obstacles. The future direction for successful BENR should aim to achieve complete anaerobic NH₄⁺ oxidation without any electron acceptor and to maximize selectivity in H₂ production. Therefore, the bioelectrochemical pathways and balances between efficient nitrogen removal and high-value-added chemical production should be further studied for carbon and energy neutralities.