Recent studies on shortcut biological nitrogen removal (SBNR), which use the concept of denitrification from nitrite, have reported the key factors affecting nitrite build-up, such as dissolved oxygen (DO) limitation, pH, and free ammonia (FA) and free nitrous acid (FNA) inhibition. This study extends the concept of the traditional minimum substrate concentration (S(min)) to explain the simultaneous effect of those factors. Thus, we introduce the minimum DO concentration (DO(min)) and the maximum substrate concentration (S(max)) that are needed to support a steady-state biological system. We define all three values as the MSC values. The model provides a method to identify good combinations of pH, DO, and total ammonium nitrogen (TAN)to support shortcut nitritation. We use MSC curves to show that the effect of DO-alone and the effect of DO plus direct pH inhibition cannot give strong enough selection against nitrite oxidizing bacteria to work in a practical setting. However, adding the FA and FNA effects gives a strong selection effect that is accentuated near pH 8. Thus, a generalized conclusion is that having pH similar to 8 is favorable in many situations. We defined a specific operational boundary to achieve shortcut nitritation coupled to anaerobic ammonium oxidation (ANAMMOX), in which the effluent concentrations of total nitrite and total ammonium should be approximately equal. Experimental results for alkaline and acidic nitrite-accumulating systems match the trends from the MSC approach. In particular, acidic systems had to maintain higher total ammonium, total nitrite, and DO concentrations. The MSC values are a practical tool to define the operational boundaries for selecting ammonium-oxidizing bacteria while suppressing nitrite-oxidizing bacteria.
