Computational fluid dynamics modelling of velocity profiles within a hydrodynamic separator

To help improve the understanding of the process of hydrodynamic separation, a computational fluid dynamic model of a 1600 mm diameter prototype hydrodynamic separator has been developed. The separator was modelled without a baseflow and configured for grit removal in a manner typical of operation within a wastewater treatment works. Some simplifications were made to the complex geometry to reduce the number of elements in the mesh and the results discussed are based on a renormalisation group k-epsilon (RNG) turbulence model. Experimental 3-components velocity measurements were made in a physical prototype separator operating under similar conditions, and these were found to be of a similar order to those predicted in the CFD model thus giving confidence in the results. The CFD model showed that the flow pattern in the separator is helical with secondary recirculation patterns. There is an asymmetrical flow pattern within the device due to the high velocities at the inlet causing skewing of the flow around the central shaft and cone. The paper illustrates that the high vertical velocities within the device permit only particles with a high settling velocity to be removed, with the fine suspended solids (mostly organic) being passed forward for treatment in the wastewater treatment works.