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Technical Papers
The RCFS was designed as a captureand-treat system. The storage component of the system (tank) is a 750mm diameter corrugated metal pipe (CMP) network designed to capture 75% of the calculated water quality volume (i.e. the runoff associated with a 25mm event). The treatment component (StormFilter) was designed on a mass-loading basis required to meet the annual pollutant loading requirements of the site with a minimum estimated interval between maintenance of one year. The estimation of the yearly maintenance was based
The RCFS system contained a total of eight, 460mm high, media-filled filter cartridges operating at a flow rate of 0.5L/s per cartridge. Each of the filter cartridges was filled with an activated alumina media. The media used for this study was a granular perlite coated with activated alumina; this was done to aid in the attenuation and/ or capture of nutrient pollutants by cation exchange and adsorption. With the exception of the surface coating, coated and uncoated perlite media were
determined to be identical with respect to physical characteristics and therefore the media should be considered equivalent with respect to expected solids removal performance.
METHOD The Mitchell Community College RCFS installation was evaluated over a 20-month period following system maintenance in November 2010. Independent oversight of all aspects of the project was provided by Ryan Winston, MS, Extension Associate Engineer in the Department of Biological and Agricultural Engineering at North Carolina State University. Sample handling services (sample retrieval, system reset and sample submittal) were provided by Pace Analytical Services (Pace) and laboratory work was conducted by Pace and Test America. Precipitation was measured with a tipping bucket-type rain gauge. Influent and effluent water quality samples were collected by portable automated samplers simultaneously collecting flow, precipitation and water quality samples. Each automatic sampler was connected to the cellular network for remote communication and data access. The influent sampler was equipped with an area velocity flow module with low profile sensor for flow analysis and influent sample pacing. The effluent sampler was equipped with a bubbler flow module used in conjunction with a weir for flow analysis and effluent sample pacing. Sample strainers and flow
SEPTEMBER 2014 WATER
STORMWATER TREATMENT
Figure 2. Aerial view of the testing site.
on a predictive probabilistic assessment of the sediment load entrained within the stormwater runoff from the site. Although the cartridges were maintained on a yearly basis, a qualitative assessment undertaken at the time of the cartridge exchange did not indicate that any bypassing of treatment occurred due to filter occlusion. The yearly maintenance frequency is unremarkable, given the detention tank upstream and the RCFS’s passive surface cleaning mechanism, which activates at least once during every runoff event and deposits the waste on the floor of the cartridge bay.