Assessing Trash BMPs in North Carolina
The stormwater manager for the city of Fayetteville, NC, reported a recurring problem with trash and sediment entering a community lake and responded to an offer from Trash Guard. This simple device reduces debris, organic matter, sediments, regulated metals, and nutrients in stormwater systems, and the city was interested in assessing its performance in a sensitive lake area. Stormwater collection and treatment systems are a major element in water-quality improvement efforts, and these mandates are particularly sensitive where lakefront residents see impacts from stormwater systems.
As stormwater flows over terrestrial surfaces, pollutants are mobilized; these pollutants enter surface waters, and where residents see the impacts from stormwater they mobilize to facilitate necessary improvement. Although these systems are regulated as discharges to surface water and today most stormwater systems have allowable levels of pollution permitted for discharge, residents wanted more. Many systems are available to treat stormwater, and studies from the Water Environment Research Federation (WERF) and many universities have documented the effectiveness of these treatment systems. State agencies responsible for stormwater management and local units of government responsible for program management and implementation require reliable information upon which to base critical infrastructure decisions. Just as decentralization and distributed systems offer tremendous advantage to the wastewater industry, so too does decentralization and distributed system management offer advantage to the stormwater sector. Trash Guard installations are incorporated into catch basins throughout a community, trapping or retaining pollutants close to the source and preventing their migration into surface waters.
The simple device recently installed in target communities in North Carolina, Virginia, California, and elsewhere has been demonstrated effective in retaining pollutants in the small watersheds in which the stormwater originated. The Trash Guard was installed in a setting common in communities: a residential area with significant vegetative cover, small lots, a significant impermeable area, and a lake downgradient that was experiencing water-quality issues. The device has been demonstrated as effective in retaining stormwater solids in the source areas, and when solids are trapped, pollutants contained in these solids are removed from direct discharge to surface waters. A critical management component in the stormwater effort becomes removal of these solids from basins. To test the effectiveness of the Trash Guard in this watershed, a target basin was identified, a device installed, and a water testing/solids testing program implemented.
Over time the device traps solids and prevents discharge of pollutants to surface water. The solids retained at this site have been collected and analyzed, and a water-sampling device was installed to collect water entering the basin and exiting the basin following the device. Tables 1 and 2 show a summary of those results.
The watershed area for this site is approximately 8 acres and is approximately 40% impermeable. The neighborhood is residential, and the drainage from the area enters a community lake. A Trash Guard device was installed in late October 2009 and solids removed for a three-month test period from November 2009 to January 2010. Solids were removed and weighed during the three-month test period and samples analyzed to determine the amount of nutrients, regulated metals, and total organic carbons contained in the solids removed.
In addition to solids testing, a water-quality testing effort was initiated at this site for the three-month test period to document the pollutant removal. The site includes a residential area and associated roads. A summary of those water-quality results is presented in Table 2.
The goal of a stormwater system is pollutant removal. The retention of solids in the catch basin is a direct demonstration of pollutant removal. The water-quality testing of influent to the basin and effluent from the basin demonstrates water-quality benefits associated with a simple Trash Guard installation. The testing also affirms the need for efforts to manage and maintain stormwater systems as an effective element in water infrastructure.
Solids do accumulate in Trash Guard-equipped basins. To be effective, the solids levels must be managed. That requires assessment of solids levels and solids removal when the stormwater solids begin to accumulate. Preliminary assessments suggest that solids removal is necessary as the level of accumulation approaches 66% to 75% of the device height.
Solids accumulation has been significant. The test site was revisited in May 2011; solids were removed and are being analyzed. We do not have a record of amounts removed by the city since the three-month test period, but 824 pounds were removed during the May visit. The photos show the level of solids accumulation in the test site basin in May 2011 and the site once solids were removed.
Pollutant removal has increased as the level of solids accumulated in the Trash Guard-equipped basin increases. The initial activity of the Trash Guard is simply a screen, trapping solids in the basin. As solids accumulate, the basin serves as a filter rather than a simple screen. As the solids levels accumulate, pollutant removal improves, but as solids accumulate removal is necessary. Once solids are removed, the Trash Guard reverts to a screen until solids again begin to accumulate.
Pollutant removal has been demonstrated as effective in the installations monitored. Targeted use of Trash Guard units in communities can result in significant pollutant removal from stormwater systems. Communities should identify segments in the stormwater system where pollutant inputs are significant. If these pollutants include sediments, leaf and vegetative waste, or trash, a Trash Guard installation can result in significant removals of pollutant loads to receiving streams. Where lakefront owners see the impacts of sediment accumulation and nutrient enrichment, a Trash Guard may be a good proactive practice.