Application of Computational Fluid Dynamics (CFD) to Stormwater Clarification Systems

Particulate matter transported by the urban stormwater is a hetero-disperse mixture of particles. These particles are a potential concern not only because of the environmental and ecological issues related to particles, but also since many contaminants are bound to the surface of particles and can be transported through the aquatic environment. Recent studies have concluded that traditional analytical and semi-empirical methods are defensible approaches to modeling UOP (Unit Operations and Processes) behavior when combined with monitoring to stormwater loadings. However such methods are challenged under common non-ideal flow conditions and can misrepresent actual behavior subject to complex loadings. One alternative is application of the Navier-Stokes (N-S) equations; the fundamental equations of fluid flow. For complex flows, heterogeneous loadings and complex treatment geometries, solving these equations analytically is simply not a feasible task without simplifications that can compromise the value of such solutions. This has led to the development of various numerical schemes to solve the N-S equations. Advances in computational power have facilitated the use of the Finite Element Method (FEM) to investigate the multiphase dynamics. The CFD (Computational Fluid Dynamics) approach to examining and modeling the behavior of a stormwater UOP provides a complete picture of transport phenomena within the geometry of the UOP considered. An examination of particle treatment in stormwater is presented. This study examines the hydrodynamic and particulate response of a continuous deflective separator (screened hydrodynamic separator) with a set of algorithms incorporated into a CFD model. A hydrodynamic model was used to model flow and a Discrete Phase Model (DPM) determined particle trajectories and predict particle separation. Measured and modeled results indicate that for an influent suspended sediment concentration (SSC) of 200 mg/L, effluent SSC was 90 mg/L; and for an influent SSC of 0 mg/L a 50% sump depth of sediment yielded an effluent SSC of 100 mg/L. In order to ensure the validity of the model, CFD simulation results were calibrated and validated with monitored data and material balances to ensure results represented reality. The calibrated and validated CFD model demonstrated success in predicting particle separation behavior of the screened hydrodynamic separator.