CFD modelling of N₂O emission from surface aerated activated sludge reactors – Systematic mesh refinement and sensitivity analysis

The effective numerical prediction of nitrous oxide (N2O) emission – a potent greenhouse gas – from biological wastewater treatment plants (WWTPs) is crucial for developing effective climate change mitigation measures for urban water systems. Based on previous research (Qiu et al., 2019), the novel method for estimating the alpha factor and the stripping gas mass transfer coefficient for N2O (kLaN2O) was developed using experimental and CFD simulation data.

By combining hydrodynamics, gas mass transfer and the NDHA biokinetic model, a threedimensional, single-phase, CFD simulation model of a surface-aerated activated sludge oxidation ditch has been developed. The main aims of this study include (i) mesh refinement of the current three-dimensional, singlephase, CFD model by employing the grid convergence index (GCI); (ii) assessing the impact of mesh refinement on predicting N2O emission from activated sludge reactor; and (iii) quantifying the relative sensitivity of model outputs associated with the prediction of N2O emission to selected design and flow boundary conditions, thereby identifying areas for improving reactor design and operation.