Treatment of urban stormwater runoff by sedimentation

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Virginia Polytechnic Institute and State University


Laboratory-scale settling units were used to determine the degree of treatment that could be achieved by sedimentation of stormwater runoff. Seven runoff samples were collected from shopping centers, which were selected because of their large impermeable surfaces resulting in high pollutant concentrations. The sampling sites were also representative of locations where detention basins would be constructed to control runoff flows and/or sediment loads. Approximately twenty liters of stormwater runoff were placed in each of four Plexiglas columns, and samples were withdrawn from column sampling ports immediately following sample addition, and after two, six, twelve, twenty-four, and forty-eight hours. The settling of the first runoff sample collected was terminated after only twenty-four hours. Sampling depths along the column, were either at one, two, and three feet, or at one, two, and four feet. Each sample was analyzed for total and volatile suspended solids, total and soluble Kjeldahl nitrogen, total and soluble phosphorus, orthophosphate, ammonia, oxidized nitrogen fonns (nitrites and nitrates), the particle-size distribution, and six heavy metals. Organic matter and total and fecal coliform bacteria were also measured but with less frequency. Dissolved oxygen measurements were made during settling of two of the seven experiments.

Sedimentation reduced the concentration of most pollutants significantly, although pollutant concentrations composed mainly of soluble forms were not readily removed. Also examined was the use of settling data for determining particle removals in basin design criteria by the relationship between the reduction of particle surface area and various pollutants. The greatest majority of surface area in the runoff samples was associated with particles that were between 15 to 35 microns in diameter.