A Model-Based Framework for Identifying and Classifying Feasible Operating Points for Partial Nitrification in Continuous-Flow Activated Sludge Reactors

Partial nitrification (PN) is a promising strategy for reducing aeration demand and improving the energy efficiency of biological nitrogen removal in wastewater treatment. However, maintaining stable PN in continuous-flow activated sludge reactors remains challenging due to the recovery of nitrite-oxidizing bacteria (NOB) and the absence of cyclic operational phases that naturally promote microbial selectivity in sequencing batch reactors. This study proposes a model-based multi-criteria optimization framework to identify and classify feasible operating conditions for stable PN in continuous-flow activated sludge reactors. A modified Activated Sludge Model No. 1 (ASM1) was used to describe the dynamics of ammonia-oxidizing bacteria, nitrite-oxidizing bacteria, and heterotrophic biomass, while equilibrium points were determined through steady-state optimization and evaluated using a multi-criteria feasibility analysis based on nitrite accumulation (β), ammonium oxidation efficiency (α), oxygen uptake rate (OUR), hydraulic retention time (HRT), and sludge retention time (SRT). Seasonal variability was incorporated through summer and winter operating scenarios. Results indicate that stable PN can be achieved under operating conditions of pH 7.5–8.5, dissolved oxygen concentrations between 0.3 and 2.5 mg/L, HRT values of approximately 2–3 h, and SRT values between 10 and 20 d. Under these conditions, high nitrite accumulation (β>0.8) and ammonium oxidation efficiency (α>0.8) were maintained with moderate oxygen demand, although seasonal differences revealed greater operational flexibility in summer and tighter constraints in winter. The proposed framework provides a systematic approach for identifying robust and energy-efficient operating regions in continuous-flow PN systems and establishes a foundation for future supervisory control implementation in full-scale wastewater treatment applications. The study also shows that over 40% energy savings could be achieved at optimal equilibrium points for partial nitrification compared to full nitrification.