Enhancing anaerobic digestion efficiency using CFD modeling: comparison between three mixing methods

Anaerobic digestion plays a crucial role in wastewater treatment by stabilizing sludge and producing biogas. Inefficient mixing, however, can lead to poor fermentation, reduced biogas yield, and sludge stratification. This study applies Computational Fluid Dynamics (CFD) modeling to optimize the mixing process in the large-scale anaerobic digester of the Fkih Ben Saleh wastewater treatment plant (WWTP), characterized by its taller reactor and higher volumetric load compared to conventional digesters. Fluid flow was simulated using the Navier–Stokes equations under turbulent conditions, with the Shear Stress Transport (SST) turbulence model to accurately capture recirculation zones and mixing effects. Various mixing configurations, including nozzle mixing, mechanical agitation, and draft tube mixing, both with and without recirculation, were evaluated and compared in terms of velocity profiles, particle trajectories, dead zone reduction, sludge homogenization, and biogas production. Results indicate that nozzle mixing with recirculation reduces dead zones by 70% and achieves the highest mixing efficiency (85%) among the investigated configurations, resulting in the highest Biogas-Yield Improvement Indicator (76%). The predicted velocity fields and flow structures show good agreement with experimental trends reported in the literature. This study highlights CFD’s potential as a quantitative tool for comparing mixing strategies and optimizing digesters with atypical geometries and operating conditions, which conventional design methods may not fully address.