The relative nitritation rate (q*(AOB)), nitratation rate (q*(NOB)), and nitrite accumulation potential (NAP,q*(AOB)/q*(NOB)) were investigated to assess the operational conditions for stable and maximized nitritation in a continuous-stirred tank reactor (CSTR) with activated (flocculent) sludge. Novel stability ridge and pH optima curves were identified on a STAN-pH plane. The stability ridge divides the stable and unstable reaction regions, while the pH optima curve identifies optimal pH values for fastest nitritation at any desired STAN. Thus, the best operational pH for a desired STAN is found on the pH optima curve when it resides in the stable region; otherwise, the stability ridge itself indicates the best operational pH value. The locations of the stability ridge and pH optima are subject to the structure of the kinetic equation and parameter values. While the NAP decreases with an increase in DO, a necessary NAP for sufficient nitrite accumulation (e.g., NAP >= 2) is still ensured unless STAN becomes very low (e.g., <= 40 mgN/L). Consequently, desired nitritation performance can be achieved by optimizing both the sludge retention time and pH; a lower pH is essential to generate an anammox-suited feed. Modelling results were verified by treating a semi-synthetic sewage sludge digester effluent having similar to 600 mgTAN/L at 30 degrees C and varying pH. (C) 2016 Elsevier Ltd. All rights reserved.