An elutriation apparatus has been proposed for determining the settling velocity distributions of suspended solids in combined sewer overflows (CSOs). The apparatus consisted of a series of interconnected cylindrical settling chambers, through which the sample was drawn using a peristaltic pump attached to the outlet of the last settling chamber. Since the diameters of the cylinders were progressively increasing, sediment with different settling velocities settled in different cylinders. By measuring the amount of sediment in each cylinder, the settling velocity distribution was deduced. The apparatus was tested using samples from a CSO outfall in Hamilton, Ontario, Canada. The test results showed that the apparatus performed well for determining settling characteristics of suspended solids and chemical distribution in different settling fractions. The effectiveness of polymer addition to enhance flocculation and settling of CSOs was also tested in this apparatus. The elutriation apparatus proposed here offers several advantages over the traditional methods (i.e., settling columns) of measurements of settling velocity distributions. The main advantage is its ability to measure the distributions under dynamic conditions, i.e., in the presence of flow. Consequently, the elutriation apparatus reproduces flow conditions in actual settling tanks better than conventional settling columns.

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