Removal of Pollutants by a CDS Unit at a Major Storm Outfall in Florida

A major storm drain pipe was retrofitted with a Continuous Deflective Separation (CDS) unit and a linear pond to help treat stormwater discharged from an urban drainage basin in Tampa, Florida. The CDS technology is designed to remove large sized particles such as litter, leaves, twigs, sand and paving residue from storm runoff. Results of this research suggest that it removes these gross solids very well, but it does not remove the dissolved and suspended particles present in the water column. The CDS unit did remove levels of polycyclic aromatic hydrocarbons (PAHs) at concentrations many times higher than levels considered toxic to benthic organisms. Since PAHs do not easily dissolve in water, they are rarely measured in water quality studies, but are considered a serious problem in sediments in portions of Tampa Bay. The data did not support the idea that the leaves collected by the CDS unit leached nutrients and increased their concentrations in the water that left the CDS unit, but this result may be influenced because leaching had already occurred while the leaves and discharge water traveled through the storm drain together. If litter and large sized particles are the pollutants of concern in a drainage basin, a CDS unit is a good solution, but if dissolved or suspended particles, especially nutrients, are a problem, a CDS unit will not reduce those pollutants. A CDS unit is probably best suited as the first element in a series of stormwater treatment methods.