There are many standard sumps in urban stormsewer systems that may qualify as a best management practice to pre-treat stormwater runoff by removing suspended sediment from the water. However, no data on the effectiveness of sediment removal and required maintenance schedules of standard sumps exist. Such data could justify providing pollution prevention credits for the use of standard sumps. The goals of this study are to (1) evaluate several standard sump configurations for sediment capture and washout; (2) develop sediment capture and washout functions for the assessment and design of standard sumps as stormwater treatment devices; and (3) design a retrofit for standard sumps to increase sediment capture and decrease washout. To determine how much suspended sediment they remove from stormwater runoff, two standard sumps of different size and a scale model sump were tested in a laboratory setting. Removal efficiency under low flow conditions as well as sediment washout rate under high flow conditions was measured. The sumps did remove suspended sediment at low flows, but at high flows the washout was substantial. A porous baffle, named SAFL Baffle, was designed and tested as a possible retrofit to the standard sump. Results indicate that, with the correct baffle configuration, the washout of sediments accumulated in the sump can be nearly eliminated for flows up to approximately the 10-year design storm runoff, and removal efficiency can be increased by 10 to 15%. A dimensionless scaling parameter was derived for the performance of standard sumps. The scaling parameter incorporates the most influential independent variables. A sediment removal efficiency function and a washout function were developed from the experimental data using the scaling parameter. The functions can serve as a tool for the selection of sumps and the evaluation of other settling devices. The uncertainty in the model predictions has been evaluated by utilizing the bootstrap method. The data collected prove that with a proper maintenance schedule, and/or by the addition of the SAFL Baffle, standard sumps can be successfully used as a pre-treatment device for stormwater.