Standard sumps are an important component of our stormwater drainage system. Recently they have received significant attention as a stormwater pre-treatment device to remove sediment from stormwater runoff. The objective of this research is to explore some simple structures to be installed inside standard sumps to improve sediment removal efficiency. A number of structures were tested and two structures were found to be most effective in sediment removal. Both structures can increase the sediment removal rate by around 20–25% for sediment sizes of 80–140 μm and 110–170 μm under all tested flowrates, and 10–20% for sediment of 160–240 μm. The flow patterns in these structures were simulated using a numerical model, and the energy loss was also examined. The results of this study offer a new direction for the development of stormwater treatment devices.