Abstract: Mass reduction rates of particles and metals were simulated for a two-compartment settling tank composed of a storage
compartment and a continuous flow compartment. Particle-size distribution, rainfall, and flow data from 16 storm events measured at three
highway sites were used. The volume ratio i.e., ratio of surface areas for a given depth between storage and continuous flow compartment
was optimized for a given design storm size to maximize total mass reduction rates of particles and heavy metals. Measured settling
velocity profiles of runoff samples were used in the simulation. Simulation results showed that in a given total design storm, larger storage
compartment fractions 0.95 enhanced the removal of smaller particles 2–104 m and particulate phase metals, and even a small
fraction 0.05 of continuous flow compartment effectively removed larger particles 104–1,000 m. A volume fraction of 0.75 for the
storage compartment is suggested to optimize annual reductions of particles and associated heavy metals.