Model-based evaluation of simultaneous nitrification and denitrification in aerobic granular sludge systems

A lab-scale granular sludge sequencing batch reactor (G-SBR) system was operated using synthetic wastewater. The total nitrogen removal efficiency of 85% was obtained together with the achievement of complete total phosphorus removal with average granule diameter of 400 µm. Dual-step nitrification and denitrification model with fixed biofilm thickness was used for performance analysis. The denitrification mode only contributed to TN removal with 25% which can be calculated with process stoichiometry. The remaining nitrogen removal could be explained by simulating simultaneous nitrification and denitrification which was responsible for 75% denitrification during aerobic period. In addition, low NO₃^- concentration at the beginning of the fill period provided advantage for securing a prolonged anaerobic period for enhanced biological phosphorus removal (EBPR). The model parameters of boundary layer thickness (z_BL = 50 µm) and half-saturation of O₂ for nitrite-oxidizing bacteria (K_O2,NOB = 0.5 gO₂/m³) were tuned to fit NO₂ and NO₃ profiles in SBR cycle.