Evaluating Oxygen Demand in Mainstream Anammox, A2O and Partial Denitrification/Anammox (PD/A) Processes: A Gradient-Based Sensitivity Analysis

Mainstream anaerobic ammonium oxidation (anammox) offers strong potential to reduce energy-intensive aeration in municipal wastewater treatment. This study evaluated the oxygen demand of mainstream anammox compared to conventional anaerobic-anoxic-oxic (A2O) and partial denitrification/anammox (PD/A) processes. A 5 × 5 × 3 simulation matrix was employed to assess the impacts of sidestream ammonium concentration (SAC), sidestream flow ratio (SFR) and mainstream ammonium concentration (MAC). Results demonstrated that oxygen demand reduction increased significantly with higher SAC and SFR. Notably, the comparative advantage of mainstream anammox expanded up to 18.58% in oxygen savings when evaluated against realistic, suboptimal PD/A scenarios (60% conversion efficiency). Gradient-based sensitivity analysis identified SAC and SFR as the dominant operational factors. Ultimately, this study highlights that mainstream anammox provides superior energy resilience and a robust alternative for nitrogen removal, particularly under fluctuating operational conditions in full-scale applications.