Engineering microbial partnerships for stable anammox nitrogen removal

The anaerobic oxidation of ammonium (anammox) is a promising low-carbon process for nitrogen removal in wastewater treatment. Still, reactor behavior is affected by microbial symbiosis (rather than by anammox bacteria acting independently). Current reviews tend to present general nitrogen conversion patterns, but they fail to effectively capture the cooperative and competitive relationships that govern substrate supply, community stability, and resilience to processes. This review critically examines the interaction of anammox bacteria with other key functional partners, e.g., ammonia-oxidizing bacteria, dissimilatory nitrate reduction to ammonium (DNRA) bacteria, comammox organisms, and denitrifying anaerobic methane oxidation (DAMO) related microorganisms. Nitrite provisioning, ammonium regeneration, extracellular electron transfer, and spatial organization in biofilms and granules are examined in detail. This review presents three preliminary decision-support frameworks to organize reactor-level evidence, as well as to synthesize microbial symbiosis and route regulation in anammox systems. These include a decision-making framework that links influent nitrogen features to candidate anammox configurations, a C/N-pH working map for summarizing reported operational patterns, and a qualitative dissolved oxygen (DO) evidence spectrum for evaluating reported microbial interactions. This review integrates microbial partnerships, metabolic interactions, and operational control into unified engineering-oriented guidance tools for nitrogen elimination systems, in contrast to previous evaluations that address individual stressors, reactor types, or process configurations in isolation, without linking inter-guild interaction stability to multi-variable, parameter-anchored engineering guidance.