Development of an integrated CFD and biokinetic model of a full-scale oxidation ditch: a case study of La Almunia WWTP

ENG- Wastewater treatment plants are currently facing increasing regulatory and energy pressures. New European discharge limits for nitrogen, phosphorus and micropollutants require higher treatment performance, while also aiming to reduce energy use. These demands expose a long-standing challenge: although biological treatment processes have been modelled for decades, most modelling tools still treat large reactors as if they were perfectly mixed tanks. In reality, full-scale reactors exhibit complex circulation patterns, oxygen gradients and zones with very different biological activity. Without understanding these internal heterogeneities, it becomes difficult to optimise operation or to reduce aeration and mixing costs, which remain the most energy-intensive parts of the process. This thesis addresses that need by applying detailed Computational Fluid Dynamics (CFD) together with Activated Sludge Models (ASM) at full scale, showing how their integration can reveal the hydrodynamic and biological behaviour of operating reactors. The work focuses on a full-scale oxidation ditch at La Almunia WWTP (Zaragoza), and in constructing the model, particular care was taken to achieve a balance between accuracy (capturing key geometrical and operational details) and computational cost, in order to build a practical tool for real-case applications. The validated hydrodynamic model reproduced key flow features observed on site, such as the faster velocities in the outer channel and the presence of recirculation and low-velocity regions. These insights were used to evaluate the effect of different agitation configurations. Results showed that the total mixing input could be reduced by about 30% without exceeding commonly accepted thresholds for low-velocity zones, indicating potential energy savings without compromising process stability. In the final part of the work, the ASM2d biokinetic model was embedded inside the CFD environment to simulate how oxygen and key pollutants evolve throughout the reactor. The model revealed spatial gradients that conventional models cannot capture, including the coexistence of aerobic and anoxic areas during aeration. The study also assessed the representativeness of the plant’s dissolved-oxygen sensor and found that the current position slightly overestimates the average oxygen level of the aerated region. Nearby locations showed differences of up to 1 mg/L, a significant deviation that could influence control performance and reinforces the importance of sensor placement. Overall, the thesis demonstrates how full-scale CFD–ASM2d modelling can improve process understanding, highlight mixing and transfer limitations, and support more energy-efficient operational decisions