Computational fluid dynamics (CFD) modeling of removal of contaminants of emerging concern in solar photo-Fenton raceway pond reactors

The impact of mixing and hydrodynamics on the removal of contaminants of emerging concern (CECs) detected in a secondary WWTP effluent by the solar photo-Fenton process in a pilot scale Raceway Pond Reactor (RPR) was investigated by computational fluid dynamics (CFD). The CFD model incorporated the solar photo-Fenton CECs oxidation kinetics at neutral pH with Fe³⁺-EDDS, the radiation transport through the water, and the turbulent flow field produced by a paddle wheel mixer. The fluid dynamics was solved by a transient-multiphase flow model (Volume of Fluid with Sliding Mesh Model) and by a steady-state momentum source domain (SD) model. Experimental RPR mixing time and CECs removal under transient conditions validated the models. The computationally faster SD model predicted the CECs removal varying the paddle wheel rotational speed, the solar irradiance, and fluid residence time. Deviations from an ideal CSTR were significative (>10%) when the CECs half-life/mixing time ratios were >1, while residence time had minor influence. The mixing effects were amplified in a scaled-up RPR and treatment capacity decreased 10% compared with a CSTR. Overall, this study reveals that the design of hydrodynamics in large-scale RPRs must be carefully examined to reduce power consumption while increasing mixing performance.