Performance, cost, and maintenance requirements of Austin sand filters

Five Austin-style sand filters were constructed by the California Department of Transportation (CALTRANS) as retrofit projects for maintenance yards and park-and-ride facilities in the Los Angeles and San Diego metropolitan areas. Each of these filter systems included storm-water monitoring equipment for collection of flow weighted composite samples. In addition, detailed records were compiled of the design elements, construction costs, and type and amount of maintenance required at each of the sites. The construction costs were relatively high because of the retrofit nature of the project and the integration of the CALTRANS storm-drain system with those of the adjoining metropolitan areas, which eliminated any opportunities for economies of scale. An analysis of performance using linear-regression techniques indicated that for sediment and almost all particle associated constituents, effluent concentration was independent of influent concentration. For instance, the average suspended solids concentration in treated runoff was 7.8 +/- 1.2 mg/L (at the 90% confidence level) regardless of observed influent concentration. The constant effluent quality produced for the particulate constituents indicates that the calculation of a percent reduction is more indicative of the influent concentration rather than the performance of the filter itself. Rejuvenation of the filter bed was required at three sites after 3 years of operation when the solids loading to the system was between 5 and 7.5 kg/m(2) of filter area. However, the clogging may have been accelerated by problems with the pumps that resulted in standing water on the filter for extended periods. Other maintenance activities consisted mainly of inspections, pump repair, and activities to reduce mosquito breeding. The main impediment to widespread implementation is the initial construction cost; however, modifications of the filter configuration and media may reduce these costs and increase effectiveness, thereby making the technology more attractive.