The summary of crucial issues in current anammox application.

Anaerobic ammonium oxidation (anammox) is a promising low-carbon pathway for nitrogen removal, but its actual application is still limited in some scenes due to unstable nitrite supply, nitrite-oxidizing bacteria (NOB) proliferation, biomass washout, and low nitrogen removal rate. This review summarizes the crucial issues governing practical anammox application. The kinetic mismatch between partial nitritation-derived nitrite production and anammox consumption is discussed together with the operational differences between one-stage and two-stage partial nitritation/anammox configurations, including nitrite accumulation, pH/alkalinity balance, free ammonia (FA)/free nitrous acid (FNA) dynamics, NOB control, and nitrous oxide emission risk. For reactor management, real-time control guided by monitoring of nitrogen species is proposed as a more reliable strategy than control based on fixed dissolved oxygen setpoints, whereas FA/FNA inhibition alone is insufficient under mainstream conditions. Inhibition diagnosis, staged startup, phosphorus recovery, engineered third sludge, and high-rate mainstream anammox are further evaluated. This review provides practical perspectives for developing mainstream anammox into a stable and reliable low-carbon wastewater treatment process.