Ammonium partial oxidation to nitrite (i.e., partial nitritation) is required in a two-stage autotrophic nitrogen removal system, to provide effluent suitable for the anammox reaction. This study aims to establish influent (ammonium and bicarbonate concentrations) and operational (dissolved oxygen (DO) concentration and solids retention time (SRT)) conditions that favor partial nitritation. This is achieved through extending the nitritation and nitratation models to predict pH variation as well as the effects of pH, free ammonia (NH3), and free nitrous acid (HNO2) on the two reactions. Experiments were performed on a lab-scale sequencing batch reactor (SBR) operated for over 500 days to provide dynamic data for the calibration of model parameters, particularly those related to the NH3 and HNO2 inhibition on nitrite-oxidizing bacteria (NOB). The influent ammonium (19-84 mM) and bicarbonate (23-72 mM) were varied, which led to dynamic ammonium, nitrite, and nitrate data suitable for model calibration and validation. The model was able to well-describe pH dynamics as well as the inhibitory effects of NH3 and HNO2 on NOB. Model-based scenario analysis was then undertaken to establish the joint regions of influent ammonium and bicarbonate concentrations and the operational DO, temperature, and SRT conditions that favor partial nitritation. The results provide support to the design and optimization of partial nitritation reactors.