An elutriation device to measure particle settling velocity in urban runoff

Urban runoff is primarily treated by settling particles. One important parameter in the design of these settling basins is particle settling velocity. Yet, this parameter is rarely measured. A modified elutriation device is developed to measure particle settling velocity distribution for use in stormwater runoff treatment design and performance evaluation. The elutriation device has distinct advantages over settling column measurement, including (1) less time requirement to make measurements, and (2) flexibility to operate at various flow rates to cover wide ranges of particle settling velocity. Major modifications of the existing elutriation devices include using a variable speed pump, changing the glass column to plastic, and adding screens at the flow inlet for more uniform velocity distribution while making the column shorter. The results of the experiments showed that the particles retained in each column of the modified elutriation device could be predicted by assuming a fully-developed, laminar velocity profile across the cross-section of each column. Operation of the device under two flow rates and multiple columns increased the range of settling velocities measured. The information presented in this paper may be used to develop standard protocols for the evaluation of particle settling velocity in stormwater.