(143p) Asm1-Based Modeling and Simulation of a Full-Scale Simultaneous Nitrification and Denitrification Plant

Simultaneous nitrification and denitrification (SND) caused by insufficient oxygen supply into the flocs has been found to take place in many activated sludge processes in wastewater treatment plants (WWTP). Although SND may not be favorable in the WWTP design as it leads to decreasing the reaction rates of both nitrification and denitrification, it can be an option of WWTP operation to avoid the liquid cycle which pumps the nitrate to anoxic tank and thus reduce operating cost. In many publications of modeling and simulation of WWTPs involving nitrification and denitrification, the emphasis has mainly been focused on the process where nitrification and denitrification occur either during different phases or in different tanks. The present contribution aimed to develop a model of activated sludge process which accounts for the nitrification and denitrification occurring simultaneously.

A full-scale WWTP treating municipal wastewater was studied in this work. Although the plant was designed for organics removal only, 30-50% total nitrogen was also removed as it operates at the low oxygen concentration. The aeration tank was composed of 4 compartments in series. The first compartment was used for the activation of return sludge. The wastewater stream was split and fed respectively at the front ends of the rest 3 compartments. The hydraulics was modeled as a series of completely stirred tank reactors. The model of the SND was developed by modifying the ASM1 (activated sludge model No. 1 developed by the international water association task group on Mathematical Modeling and Operation of Biological Wastewater Treatment). The modified model contained 6 processes and 11 components. Instead of using the ?oxygen switch function' to deal with the nitrification and denitrification separately, a factor was introduced to account for the effect of anoxic environment caused by insufficient oxygen supply which resulted in the aerobic and anoxic processes taking place simultaneously. Like ASM3, the organic nitrogen components were not treated as independent variables. They were evaluated as dependent variables of chemical oxygen demand (COD).

The dynamic simulation of the WWTP operating over 90 days was performed. The result showed that the model reflected the patterns observed in the effluent COD, ammonia (NH3-N) and nitrate (NO3-N). The model developed in this work would be helpful for process engineers to investigate different configurations or operating strategies for the existing plant. It also benefits the operators by predicting the plant behavior under various operations and exploring different what-if situations.