Numerical modelling of hydraulic efficiency and pollution transport in waste stabilization ponds

Abstract

This paper investigates the hydraulic performance and solute transport processes in waste stabilization ponds (WSPs). A numerical model comprised of Reynolds-averaged Navier–Stokes (RANS) flow hydrodynamic model with the standard k − ε turbulence closure coupled with the advection-diffusion solute transport model, is developed in a three-dimensional Cartesian coordinate system. The proposed numerical model is successfully validated against laboratory-scale physical modelling measurements of flow hydrodynamics and solute characteristics across trapezoidal pond geometry. The developed numerical model is adopted to run series of scenario-based simulations to investigate the effects of WSP's geometrical features and implementation of an island retrofitting on the hydraulic performance and treatment efficiency of the WSPs. Fifteen pond configurations with varying side-walls slope and island configurations are simulated. Vertical and horizontal structures of flow hydrodynamics across the pond are investigated. Solute transport processes are studied through determining residence time distribution (RTDs) curves based on numerical tracer simulations. Two deflector island configurations (parallel and rotated) are simulated to investigate their influence on enhancing the hydraulic performance of the WSP. The analysis of the numerical results indicates an overall positive impact of deflector island retrofitting on the hydraulic performance of the WSP. The side-walls slope are shown to play a key role in determining the overall performance of the WSP. For the cases with side-walls slope of 1:1, 0.5:1 and 0:1, the hydraulic efficiency of the WSP was enhanced by adding both parallel and rotated islands. However, for the cases with side-walls slope of 2:1 and 1.5:1, addition of island deflector is shown to have negative impacts on the hydraulic performance of the waste stabilization pond.