Effects of the C/N ratio and bacterial populations on nitrogen removal in the simultaneous anammox and heterotrophic denitrification process: Mathematic modeling and batch experiments

A mathematic model was developed for describing the nitrogen and organic carbon removal via the simultaneous anaerobic ammonium oxidation (anammox) and heterotrophic denitrification (SAD) process. The model considered anammox bacteria, heterotrophic denitrifying bacteria, ammonium, nitrite, nitrate, and readily biodegradable carbon. The model parameters were calibrated by using the experimental data in the single batch reactors inoculated with anammox sludge of 190 mg-VSS/L and activated sludge of 110 or 220 mg-VSS/L for treating wastewater containing ammonium of 30 mg-N/L and nitrate of 40 mg-N/L. The rapid depletion of nitrate and acetate as the sole carbon source were observed in the reactor. Subsequently, the temporarily accumulated nitrite was removed with ammonium. The initial C/N ratio of 1.5-2.0 and the population ratio of heterotrophic denitrifying bacteria to anammox bacteria within 0.3-0.4 were suitable conditions for the batch SAD process. The sensitivity analysis of kinetic parameters revealed that the most influential parameters for the nitrogen and organic carbon removal were the half-saturation constants for nitrite of heterotrophic bacteria and anammox bacteria and the anoxic reduction factors of heterotrophic bacteria on the nitrate-reducing rate and the nitrite-reducing rate, which closely associated with nitrite accumulation in the SAD process.