Stormwater treatment： examples of computational fluid dynamics modeling

Control of rainfall-runoff particulate matter （PM） and PM-bound chemical loads is challenging; in part due to the wide gradation of PM complex geometries of many unit operations and variable flow rates. Such challenges and the expense associated with resolving such challenges have led to the relatively common examination of a spectrum of unit operations and processes. This study applies the principles of computa- tional fluid dynamics （CFD） to predict the particle and pollutant clarification behavior of these systems subject to dilute multiphase flows, typical of rainfall-runoff, within computationally reasonable limits, to a scientifically acceptable degree of accuracy. The Navier-Stokes （NS） system of nonlinear partial differential equations for multi- phase hydrodynamics and separation of entrained particles are solved numerically over the unit operation control volume with the boundary and initial conditions defined and then solved numerically until the desired convergence criteria are met. Flow rates examined are scaled based on sizing of common unit operations such as hydrodynamic separators （HS）, wet basins, or filters, and are examined from 1 to 100 percent of the system maximum hydraulic operating flow rate. A standard turbulence model is used to resolve flow, and a discrete phase model （DPM） is utilized to examine the particle clarification response. CFD results closely follow physical model results across the entire range of flow rates. Post-processing the CFD predictions provides an in-depth insight into the mechanistic behavior of unit operations by means of three dimensional （3-D） hydraulic profiles and particle trajectories. Results demon- strate the role of scour in the rapid degradation of unit operations that are not maintained. Comparisons are provided between measured and CFD modeled results and a mass balance error is identified. CFD is arguably the most powerful tool available for our profession since continuous simulation modeling.

Collector efficiency for hydrosol deep-bed filtration of non-Brownian particles at low and finite Reynolds numbers

Aqueous filtration systems with granular media are increasingly implemented as a unit operation for the treatment of urban waters.Many of these aqueous filtration systems are designed with coarse granular media and are therefore subject to finite granular Reynolds numbers(Reg).In contrast to the Reg conditions generated by such designs,current hydrosol filtration models,such as the Yao and RT models,rely on a flow solution that is derived within the Stokes limit at low Reg.In systems that are subject to these finite and higher Reg regimes,the collector efficiency has not been examined.Therefore,in this study,we develop a 3D periodic porosity-compensated face-centered cubic sphere(PCFCC)computational fluid dynamics(CFD)model,with the surface interactions incorporated,to investigate the collector efficiency for Reg ranging from 0.01 to 20.Particle filtration induced by interception and sedimentation is examined for non-Brownian particlesfanging from 1 to 100 μm under favorable surface interactions for particle adhesion.The results from the CFD-based PCFCC model agreed well with those of the classical RT and Yao models for Reg<1.Based on 3150 simulations from the PCFCC model,we developed a new correlation for vertical aqueous filtration based on a modified gravitation number,NG^*,for the initial deep-bed filtration efficiency at lower yet finite(0.01 to 20)Reg.The proposed PCFCC model has low computational cost and is extensibile from vertical to horizontal filtration at low and finite Reg.