Computation fluid dynamics model of first-flush runoff through a hydrodynamic separator

Urban rainwater harvesting systems are widely used to solve urban water shortage. However, the initial rainwater collected from urban roads contains numerous particles that severely limit its reuse. This study designed a hydrodynamic separator on a separated sewer systems (between the rainwater collecting main pipe and intercepting main canal) to remove particulate pollutants, avoid rainwater reflux, and realize staggered regulation during flood peak. In this work, a 3D model of the hydrodynamic separator was constructed using SolidWorks software, and state-of-the-art FLUENT computational fluid dynamics was applied in fine-scale simulation of the hydrodynamic separator under complex initial conditions. Numerical tests were performed to validate the capability of the hydrodynamic separator in removing particulate pollutants under different initial conditions. The results of numerical tests showed that particle trajectories are relatively regular spirals in the hydrodynamic separator, whereas large-diameter particles feature high removal rates. Particle removal rate increased with increasing diameter-depth ratio and height difference between the overflow and inlet pipes. Considering the particle removal rate, the optimal order of angles between the overflow and inlet pipes is 90° > 150° > 60° > 120°. The parameters of the hydrodynamic separator in removing initial rainfall particulate pollutants were obtained, and a pilot-scale experiment was conducted, providing theoretical importance in reducing energy consumption of rainwater harvesting pretreatment systems.