Modeling Simultaneous Nitrification–Denitrification Process in an Activated Sludge Continuous Flow Stirred-Tank Reactor: System Optimization and Sensitivity Analysis

Biological nitrogen removal from municipal wastewater is an important issue. Nitrogen removal by simultaneous nitrification-denitrification (SND) process has drawn much attention in the past few years because of potential reduction in capital and operating costs. Performance of SND process treatment in a continuous flow stirred-tank reactor was examined using Activated Sludge Model No. 1. Individual biomass cells were taken to be suspended with dissolved oxygen (DO) equally available to every cell without considering any floe formation. The combination of operating DO concentration and solids retention time were key factors for effective nitrogen removal. Simulation output predicted significant nitrogen reduction at 0.4mg/L operating DO and 15-day solids retention time (SRT). The effect of additional operating conditions on nitrogen removal was evaluated. As sufficient electron donor was necessary for sustaining denitrification, the influent biodegradable COD to total kjeldahl nitrogen ratio had significant effect on overall nitrogen reduction and was effective when such ratio was > 10. Neither hydraulic retention time nor recycle ratio (R) had any notable effect on nitrogen removal. Stochastic simulations considered the natural variability of kinetic and stoichiometric parameters in Activated Sludge Model No. 1 and used Monte Carlo analysis to assess the reliability of SND system operated at optimum conditions. A sensitivity analysis was performed with the stochastic simulation results to identify those model parameters significantly affecting process performance. Predictions from such analysis were found to be in close agreement with previous experimental results and can be used to look into the possibility of converting extended aeration-type activated sludge processes into potential biological nitrogen removal system.