Abstract. Recirculating biofloc technology (RBFT) has been gradually acknowledged for its positive effect on the control of biofloc concentration using a hydrodynamic vortex separator (HDVS). To operate an RBFT system at maximum performance, the removal efficiency of an HDVS at different hydraulic retention times (HRTs) must be fully predictable. Hence, a numerical study of the fluid flow and particle dynamics was performed to characterize the performance of an HDVS at varying HRTs. First, flow simulation was conducted to determine an economical mesh size at an HRT of 248 s. Then, with respect to the total suspended solids (TSS) in the RBFT system and the physical properties of the flocs, two-way coupling of the dense discrete phase model (DDPM) and discrete element model (DEM) methods was used to predict floc tracking in an HDVS. Additionally, the Reynolds averaged Navier-Stokes (RANS) equations with the Reynolds stress turbulence model (RSM) were solved using the finite volume method based on the semi-implicit method pressure-linked equations (SIMPLE) pressure correction algorithm in the computational domain. Finally, pilot-scale studies were conducted to verify the simulation models. Based on the simulation results, floc management in an RBFT system is briefly discussed. Due to limited research on the numerical simulation and operating conditions of an HDVS in an RBFT system, this article describes an original investigation of the modeling approach. Keywords: Computational fluid dynamics, Dense discrete phase model, Discrete element model, Floc management, Flow field, Removal efficiency, Total suspend solids.