CFD and Biokinetic Model Integration Applied to a Full Scale WWTP

Current simplified wastewater treatment plant (WWTP) model efforts do not take into account detailed local variations in bioreactors, despite the evidence pointing to their importance for detailed process design and evaluation. Non-uniform distribution of components such as dissolved oxygen can influence the process rates and hence the overall performance of wastewater treatment systems. Consequently, a more detailed knowledge of the system with respect to local substrate concentrations could be of great use for process optimization and control in many applications. This contribution presents the integration of Computational Fluid Dynamics (CFD) modelling with a biokinetic model for a full scale activated sludge tank of a municipal wastewater treatment plant. Firstly, the hydrodynamic model was constructed, simulated and validated. Secondly, the Activated Sludge Model 1 (ASM1) was integrated by means of scalars and source/sink terms in order to simulate the biological processes. Hereby, a detailed description of oxygen mass transfer from the gas to the liquid phases was included using a constant bubble size and local gas holdup. Additionally, the impact of wet weather and high loading conditions was evaluated by simulating different scenarios with varying gas and liquid flow rates. Results of the combined CFD-biokinetic model were evaluated based on expert judgement as well as available process data. Through the integrated modelling approach developed, regions of bad mixing are observed and their potential impact on process rates is evident. This can now form the basis for improved simplified WWTP models.