Practical Indices for Minimizing External Carbon Demand in Mainstream ANAMMOX: A Sensitivity‐ and Regression‐Based Approach

ABSTRACT Mainstream anaerobic ammonium oxidation (ANAMMOX) offers a promising alternative to conventional nitrogen removal processes in municipal wastewater treatment. However, research has largely focused on its potential to reduce oxygen demand rather than external carbon requirements. This study compared the external carbon demand of mainstream ANAMMOX with that of anaerobic–anoxic–oxic (A2O) and partial denitrification/ANAMMOX (PD/A) processes. Stoichiometric ratios, validated in a laboratory‐scale packed‐bed biofilm reactor, were implemented in a simulation framework reflecting representative conditions in Korean wastewater treatment plants. A 5 × 5 × 3 × 3 simulation matrix evaluated external carbon demand and its sensitivity to sidestream ammonium concentration (SAC), mainstream ammonium concentration (MAC), mainstream total organic carbon concentration (MTC), and sidestream flow rate (SFR) through gradient‐based analysis and regression modeling. Mainstream ANAMMOX reduced external carbon demand by up to ~10.8%–72.2% compared with A2O and ~3.7%–41.2% compared with PD/A, depending on influent loading conditions. SAC and MTC were identified as the dominant drivers of carbon reduction. Notably, higher MAC levels slightly diminished the relative carbon savings due to the increased external carbon requirement for polishing the stoichiometric nitrate byproduct. Three composite indices (sidestream effective nitrogen load index, total effective nitrogen load index, and sidestream contribution ratio SCR) were proposed, with SCR exhibiting the highest predictive accuracy. Although the absolute savings are site‐specific, these findings suggest that mainstream ANAMMOX can substantially decrease reliance on external carbon sources, and the proposed indices provide practical tools for process evaluation and design optimization.