Suspended solids (SS) in the storm-water makes up a significant source of total suspended solids in wet weather flow. With appropriate modification and maintenance, the standard sumps in the drainage system can remove SS from storm-water runoff as a best management practice device. To increase the removal efficiency, especially in the condition of high flow rate, inclined plates, based on the shallow pool sedimentation theory, have been designed and refitted to the sump. Its performance under the different surface load and flow rate were evaluated through scale models. The results show that the preliminary design, Model A, had limited removal efficiency, and even played a negative role sometimes due to the concentrated flow in the axis. The optimizations through installing a non-uniform porous baffle (Model B) and adopting inverted V-shaped plates (Model C) were improved, and results show that removal efficiency rate can be increased by around 15–20%, even at high flow rates. Moreover, too many plates cannot improve the removal rate further, because they make the cross-section decline and lead to higher velocity between plates.