Beyond Nitrification–Denitrification and Anammox: Can Dirammox Reshape Nitrogen Removal in Wastewater Treatment?

Abstract Distinct from conventional aerobic nitrification–denitrification, which remains the dominant biological nitrogen removal (BNR) strategy, and the anaerobic ammonium oxidation (anammox) process, the recently identified direct aerobic ammonia oxidation (Dirammox) pathway introduces a fundamentally new paradigm. Dirammox enables direct conversion of ammonia to dinitrogen under fully oxic conditions, with hydroxylamine (NH2OH) serving as a key intermediate. This pathway is mediated by a novel group of heterotrophic ammonia-oxidizing bacteria and, in theory, offers substantial advantages, including reduced aeration demand, simplified process configurations, and lowered greenhouse gas emissions. In this perspective, we trace the historical evolution of BNR technologies, critically re-evaluate the intrinsic limitations of established pathways, and synthesize emerging evidence for Dirammox, spanning microbial discovery, metabolic mechanisms, and early-stage engineering exploration. Rather than representing an incremental optimization of existing processes, Dirammox constitutes a conceptual shift in BNR. Its development holds the potential of reshaping future wastewater treatment paradigms toward low-carbon, energy-efficient, and climate-resilient systems, which calls for intensified mechanistic investigation and rigorous engineering validation.