|Storm Water Treatment at Critical Areas: The Multi-Chambered Treatment Train (MCTT) (EPA/600/R-99/017) March 1999
This is the first volume in this report series. It describes the work conducted during the early years of this project through recent full-scale tests. Other volumes in this report series describe the results of field investigations of storm drain inlet devices and the use of filter media for storm water treatment.
The first project phase investigated typical toxicant concentrations in storm water, the origins of these toxicants, and storm and land-use factors that influenced these toxicant concentrations. Nine percent of the 87 storm water source area samples analyzed (using the Microtox toxicity screening procedure) were considered extremely toxic. Thirty-two percent of the samples exhibited moderate toxicity, while 59 percent of the samples had no evidence of toxicity. Only a small fraction of the organic toxicants analyzed were frequently detected, with 1,3-dichlorobenzene and fluoranthene the most commonly detected organics investigated (present in 23 percent of the samples). Vehicle service and parking area runoff samples had many of the highest observed concentrations of organic toxicants. All metallic toxicants analyzed were commonly found in all samples analyzed.
The second project phase investigated the control of storm water toxicants using a variety of conventional bench-scale treatment processes. Toxicity changes were monitored using the Azur Environmental Microtox bioassay screening test. The most beneficial treatment tests included settling for at least 24 hours (up to 90 percent reductions), screening and filtering through at least 40-micrometer screens (up to 70 percent reductions), and aeration or photo-degradation for at least 24 hours (up to 80 percent reductions). Because many samples exhibited uneven toxicity reductions for the different treatment tests, a treatment train approach was selected for testing during the third project phase.
The third project phase included testing of a prototype treatment device (the multi-chambered treatment train, or MCTT). However, the information provided in this report can also be used to develop other storm water treatment devices. Through pilot and initial full-scale testing, this device has been shown to remove more than 90 percent of many of the storm water toxicants, in both particulate and filtered forms. The MCTT is most suitable for use at relatively small and isolated paved, critical source areas about 0.1 to 1 hectares (0.25 to 2.5 acres) in area. These areas would include vehicle service facilities (gas stations, car washes, oil change stores, etc.), convenience store parking areas and areas used for equipment storage, along with salvage yards.
The MCTT is an underground device that has three main chambers:
A typical MCTT requires between 0.5 and 1.5 percent of the paved drainage area, which is about a third of the area required for a well-designed wet detention pond.
A pilot-scale MCTT was constructed in Birmingham, Alabama, and tested over a six-month monitoring period. Two additional full-scale MCTT units have been recently constructed and are currently being monitored as part of Wisconsin's 319 grant from EPA. During monitoring of 13 storms at a parking facility, the pilot-scale MCTT was found to have these overall median reduction rates:
The color was increased by about 50 percent due to staining from the peat. The pH decreased by about a half pH unit, also from the peat media. Ammonia nitrogen was increased by several times, and nitrate nitrogen had low reductions (about 14 percent). The MCTT, therefore, operated as intended: it had very effective reduction rates for both filtered and particulate storm water toxicants and suspended solids. Increased filterable toxicant reductions were obtained in the peathand mixed media sorption-ion exchange chamber, at the expense of increased color, lowered pH, and depressed COD and nitrate reduction rates. The preliminary full-scale test results substantiate the excellent reductions found during the pilot-scale tests, while showing better control of COD, filterable heavy metals, and nutrients, and less detrimental effects on pH and color.
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