Identifying the Effect of Non-Ideal Mixing on a Pre-Denitrification Activated Sludge System Performance through Model-Based Simulations

Abstract

Effectiveness of a pre-denitrification activated sludge treatment system is governed by the kinetics of the biological reactions, and the hydrodynamic mixing behavior in the reactors. Achieving good mixing conditions within a reactor not only enhances the transfer of reactants but also ensures homogeneous environmental conditions throughout the vessel when required, allowing for an effective usage of the reactor’s total volume, leading to optimized, low-cost operation. In this work, a pre-denitrification activated sludge system performance with regards to the biological treatment of organic carbon and nitrogen was investigated, under two scenarios for non-ideal mixing in the anoxic reactor. The system performance is simulated based upon the Activated Sludge Model 1 model’s biological reactions, and combining two non-ideal mixing two-parameter models: CSTR with bypass and dead volume, and two CSTRs with exchange. Performance discrepancies were then identified in the presence of non-ideal mixing. The system’s performance was found to be more susceptible to the presence of a dead volume/bypass scenario compared to the two CSTRs with material exchange scenario. Under non-ideal mixing conditions, effluent concentrations of Total Kjeldahl Nitrogen, organic carbon increased marginally, while effluent concentration of nitrate increased significantly. Similarly, the waste stream concentrations of Total Kjeldahl Nitrogen and organic carbon increased significantly as a result of an increase in the concentration of the heterotrophic biomass. The outcome of this study provides an insight when troubleshooting the operation of pre-denitrification activated sludge systems for non-ideal mixing conditions.