Similar to many other areas around the nation, metropolitan Nashville-Davidson County, TN, has struggled to improve its stormwater quality since the late 1970s. The Metropolitan Department of Public Works (MDPW) issued its first Stormwater Management Manual in 1979 and has revised it four times since then. As you might expect, each new addition has tightened the noose on pollution and polluters. Along the way, MDPW has learned a lot that might help other areas covered by either Phase I or Phase II National Pollutant Discharge Elimination System regulations.
The latest edition of MDPW’s stormwater manual includes 120 pages of detailed information, all of it supporting the basic requirement that postdevelopment stormwater runoff must not differ significantly from that of predevelopment conditions. The details make it clear that “not differ” refers to quantity and flow rate as well as the pollution content. Postdevelopment rainfall has to run off a site in the same amount, at the same velocity, and just as clean as it did before the land was developed.
In practice, this simple-sounding requirement is difficult to achieve, especially where real estate is expensive. Detention ponds are certainly acceptable to MDPW, but they require a lot of land, usually prohibiting that alternative for small, freestanding businesses such as fast-food outlets, drug and grocery stores, and even midrise office buildings and shopping centers. To deal with these issues, many developers are being forced underground–literally.
Looking for BMPs
Design engineers are constantly looking for best management practices to help them meet environmental regulations. Underground detention/retention systems are becoming more common for controlling runoff quantities, but the increased size necessary to control the quality of runoff can make them less feasible. In some cases, it is more cost-effective to follow underground detention with a stormwater-quality treatment unit to remove the floatables and suspended solids before the runoff is released from the system.
Nancy Lee, P.E., project manager with Barge Cauthen & Associates Inc. in Nashville, explains that although six or seven water-quality treatment methods are approved by MDPW, most are not feasible for her clients. “We don’t have room for detention/retention ponds, grass swales, sand filters, or other large treatment facilities on small sites, but underground mechanical treatment units have worked well for us.
“CDS Technologies manufactures a low-maintenance, one-manhole treatment unit that uses physics to separate pollutants from stormwater runoff,” Lee continues. “The technology seems to be a very effective treatment practice, which is something municipalities like too.”
CDS (continuous deflective separation) units are cylindrical underground structures that can be designed to treat flows from 1 to 300 cubic foot per second (cfs) and higher. Flows greater than the design capacity of the unit pass over a weir and bypass the separation chamber. All other flow enters the unit’s separation chamber and begins a circular motion that is hydraulically designed to allow the flow to pass through a cylindrical stainless steel screen while the screen traps and retains pollutants. The circular flow is balanced so that the tangential flow around the interior of the screen keeps the screen clean. Floatable and neutrally buoyant debris, sediment, and other pollutants collect in the center of the chamber. Heavier pollutants settle into a central sump where they can be removed by vacuum, as needed (one to four times each year). Floatables are retained in the separation chamber above the submerged screen. After the water passes through the screen, it moves under an oil-skimmer baffle and continues downstream in the stormwater drain. The baffle prevents oil and grease from escaping the CDS unit.
CDS units permanently remove virtually 100% of floatables from stormwater flows up to their treatment capacity. Once trapped in a CDS unit, debris cannot be flushed back into the downstream system. The units also remove 100% of all particles greater than one-half the size of the screen opening. Studies have also shown that, with the addition of sorbents, the capture efficiency of free oil and grease is approximately 80-90%. “We’ve used sorbents in designs that involve sinkholes,” Lee relates. “In Tennessee, if the stormwater discharge flows into a sinkhole, oil concentrations can’t be greater than 2 parts per million.”
Precast and Cast-in-Place Units
Sherman-Dixie Concrete Industries Inc. manufactures the precast structures that contain the CDS components. In addition to being a manufacturer, Sherman-Dixie aids engineers during site-specific design and assists contractors in the field to ensure an accurate installation. “After lengthy study of every stormwater-quality technology on the market, we chose CDS for several reasons,” explains Mike Kusch, director of technical marketing. “It has a small footprint, and it’s easy to install and maintain. CDS is the only process that uses a screening mechanism, which we consider an effective method for pollution removal. We offer 16 different-size precast units. Our largest units can handle as much as 300 cfs.”
“To tell you the truth, I was scared when we set our first CDS unit,” remarks Brett Wesnofske, a corporate officer with Sherman-Dixie’s client Sunrise Contracting Inc. in LeVergne, TN. “They started showing up on plans a couple of years ago, so we checked into it to find out what they were and how much they cost. Since we’d never installed one, we figured we’d better allow a full day for the job, so that’s what we put in our bid. As it turned out, it didn’t take more than an hour and a half. There was really nothing more to it than setting a standard precast manhole, then bolting the screen inside.
“There are several other ways to deal with stormwater, and we’ve had experience with most of them,” Wesnofske continues. “In my mind, CDS is the best system out there. Just one unit can handle an incredible amount of runoff, and it has a big sump, so it doesn’t have to be cleaned out but once or twice a year. Of course, one of my favorite things is the fact that I have never had a callback on a CDS unit.”
Some Phase II Cities Start Early
Robert Haley, assistant manager of the Permit Section of Tennessee’s Department of Environment and Conservation, points out that the screws are tightening on stormwater pollution and that time is running out for Phase II cities to submit permit applications. “In fact, time runs out at midnight on March 10, 2003,” he states. “By that date, Phase II localities should be in the process of putting new stormwater ordinances in place, or at least have a timetable for enacting those ordinances. We will expect more immediate ordinances in areas of high growth.”
Jeff Hooper, P.E., a project manager with Barge Cauthen & Associates, notes that some Phase II cities where he does work already comply with the new regulations. “Some cities are more advanced than others in considering these new requirements. As a result, those cities have already implemented regulations that bring them into compliance.“Clients sometimes wonder why they have to meet these standards now, when it wasn’t required last year. But we have to start somewhere. If we continue to develop, to put down more asphalt, we’ll continue to pollute the environment unless we install technologies like CDS to clean the runoff before we release it. This is a reasonable step we can take, which will pay significant environmental dividends to all of us.”