BACKGROUND The completely autotrophic nitrogen removal over nitrite (CANON) system has excellent performance in treating ammonium-rich wastewater and it depends on ammonia oxidizing bacteria (AOB) working together with anammox bacteria in a reactor. Higher ammonium nitrogen loading requires higher aeration to provide sufficient dissolved oxygen (DO) for nitritation, whereas high-strength aeration in theCANON will destroy the sludge structure and inhibit the activity of anammox bacteria. The present study uses a stable pre-nitritation system to reduce a part of nitritation and also decrease aeration intensity in the CANON. RESULTS Continuous aeration to maintain the DO concentration between 1.5 and 4.3 mg L(-1)in the CANON achieved a stable nitrogen removal rate (1.22 kg N m(-3)d(-1)) from iron oxide red wastewater with 400-1016 mg N L(-1)after 183 days of operation. Using a mixed alkalinity donor [sodium carbonate (Na2CO3) and bicarbonate (NaHCO3)] in the combined system can save approximate to 14.1-28.2% of the alkalinity cost compared with using NaHCO(3)only. CONCLUSIONS A zeolite biofilm aeration filter (ZBAF) used in a pre-nitritation process can tolerate high DO concentration of <= 6.5 mg L(-1)for nitritation and avoid all of the nitritation taking place in the CANON system. This can protect the sludge structure and anammox bacteria from high-strength aeration erosion. (c) 2020 Society of Chemical Industry