Because of these hydraulic problems, it has not been possible to accurately determine the pollutant removal effectiveness of these systems. The use of sand and geotextile fabrics in the vertical sand filters makes it difficult to predict the drainage rate of these runoff control systems. Drainage of the contents of the runoff control system through the vertical filters is not controlled solely by the sand, but also is affected by the geotextile fabric that is used to support the sand between the rock gabions. Therefore, control systems designed based only on the hydraulic behavior of the sand may not drain in 24 hours as called for in the design. Therefore, the HMT cannot function as a hazardous materials collection basin during runoff events when the roads are wet and the chance for an accident is higher.
Sedimentation is the most important pollutant removal mechanism for the runoff control systems.
Modifications of runoff control systems which focus on extending the detention time of the basins may be more effective in controlling suspended solids in runoff than enhancing the filter performance. Scour and resuspension of sediments was observed in the detention basins. This phenomenon causes increased suspended solids loadings on the filters resulting in discharge of higher concentrations of suspended solids in the filter effluent and in clogging of the sand filter. Sediment and suspended solids removal efficiencies can be increased and maintenance requirements reduced by the installation of rock gabions, baffles or another device which reduces resuspension of solids. Laboratory Filtration Experiments Laboratory, bench-scale filtration columns using various media were investigated at the Center for Research in Water Resources.