ENHANCING CONTAMINANT REMOVAL IN STORMWATER DETENTION BASINS BY COAGULATION

The Washington State Department of Transportation designs, operates, and maintains stormwater detention basins for flood control. Historically, detention basin design was based primarily on hydraulic characteristics such as storage capacity. Initiatives by the Washington State Department of Ecology have prioritized the consideration of stormwater quality as well as quantity for stormwater treatment. Consequently, current operation and future design should consider water quality in addition to flood control. A scale model of a typical basin was constructed to investigate contaminant removal capabilities of existing detention basins. Particular attention was given to the enhanced removal of smaller sediment size fractions and heavy metal contaminants adsorbed to those fractions. Using typical contaminants and concentrations, a simulated highway stormwater runoff was formulated and applied in scale model detention basin testing over a range of flow rates. Four coagulants were evaluated for their ability to enhance removal of sediment and metals. Each coagulant was initially evaluated in laboratory scale batch experiments with continued model tests on selected coagulants. In the model detention basin, coagulant addition resulted in significant increases in metal removal over the range of stormwater flow rates studied. The greatest improvement was observed at the higher flow rates. Enhanced metals removal resulted from increased removal of small particles and their adsorbed metals. Further improvement in contaminant removal was observed following the addition of an influent baffle. This baffle resulted in an increase in hydraulic detention time by reducing short circuiting with an associated improvement in contaminant removal.