An Evaluation of Storm Drainage Inlet Devices for Stormwater Quality Treatment

The activities summarized in this paper included the testing of three representative stormwater control devices that were located at storm drainage inlets. The two proprietary devices utilized screening and filtering (using filter fabric and a coarser mesh). A conventional catchbasin inlet, having a sump, was also tested for comparison. These devices were monitored in Stafford Township, NJ, to evaluate their removal effectiveness for stormwater pollutants. Twelve paired samples collected at each device represented composite inflow and outflow stormwater. The samples were split into filtered and unfiltered components for extensive analyses of conventional and toxic pollutants. The experimental design was capable of identifying significant pollutant removals of at least 15 to 50% at a 95% confidence level, depending on the pollutant. The only significant pollutant removals were found during tests of a conventional catchbasin having a suitable sump. The median removal rates were about 30% for suspended solids, about 40% for turbidity, about 15% for color, and about 20% for total solids. No other pollutants were found to be significantly reduced. However, the coarse screened inlet device was found to significantly reduce the discharges of trash and other large debris. Unfortunately, flows passing through trapped material caught on the screen had increased concentrations of suspended solids and volatile solids, probably due to washing of decomposing large organic material through the screen. The filter fabrics tested in the laboratory showed about 50% removals for suspended solids and COD, but they rapidly clogged, significantly shortening their run times and minimizing any benefit from their use. This research was conducted in partial fulfillment of cooperative agreement no. CR 819573 under the sponsorship of the U.S. Environmental Protection Agency. BACKGROUND Storm drainage system inlet structures can be separated into three general categories. The first category is a simple inlet that is comprised of a grating at the curb and a box, with the discharge located at the bottom of the box which connects directly to the main storm drainage or combined sewerage. This inlet simply directs the runoff to the drainage system and contains no attributes that would improve water quality. However, large debris (several cm in size) may accumulate (if present in the stormwater, which is unlikely). The second type of inlet is similar to the simple inlet, but it contains a sump that typically extends 0.5 to 1 m below the bottom of the outlet. This is termed a catchbasin in the U.S., or a gully pot in the U.K., and has been shown to trap appreciable portions of the course sediment. The third category is also similar to the simple inlet, but contains some type of screening to trap debris. These include small cast iron perforated buckets placed under the street grating, as used in Germany, large perforated and lipped stainless steel plates placed under the street grating, as used in Austin, Texas, and a number of proprietary devices incorporating filter fabric or other types of screening placed to intercept the stormwater flow. Over the past 85 years, there has been extensive use of catchbasins for coarse material removal from stormwater runoff (Lager, et al. 1977), mainly to reduce sedimentation problems in the storm drainage system. Catchbasins have also been utilized in Europe for over a century. The purpose of catchbasins historically has been to prevent the clogging of sewer lines with sediment and organic debris, and to prevent odors from escaping from the sewers by creating a water seal. Over the years, many different styles of catchbasins have been used, and many different enhancement devices have been added to increase their