Filtering Stormwater in Bethesda to Protect the Chesapeake Bay
Tuesday, October 31, 2000
Stormwater runoff misdirected, permits pulled, delays potentially costing a company hundreds of thousands of dollars - and behind it all, one of the country’s most rigorously protected bodies of water. These elements came together in April 1999 during the construction of Bethesda Place II, a 12-story, 300,000-ft.2 Maryland office building.
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| The concrete treatment vault was cast in place before construction of the building could continue. |
The building’s design originally called for the stormwater runoff from the underground parking garage to flow into the sanitary sewer system, as is the practice in the District of Columbia. About half the district is served by a combined sewer system that accommodates both sanitary flow and stormwater runoff. Just a few miles away in the city of Bethesda, however, stormwater runoff must be directed into a separate stormwater system. By the time the problem came to light, construction of the garage was well under way.
The project fell under the authority of theWashington Suburban Sanitary Commission (WSSC), which has stringent stormwater requirements, and Maryland’s Montgomery County Department of Permitting, which halted the project when the drainage problem was discovered. Before the general contractor for the garage portion of the project could divert the parking garage runoff to the stormwater system, it had to demonstrate to both authorities that the stormwater would be adequately filtered.
Joe Morrison, project manager for contractor John J. Kirlin Inc. of Rockville, MD, estimates that delays would have cost the company $150,000 per day. "It was ugly. We were stopped dead if we didn’t give them an approved method of controlling or purifying the stormwater."
Bethesda lies within the Chesapeake Bay watershed. "We’re really sensitive here because of the bay," Morrison acknowledges. The WSSC exercises stringent control over its portion of the watershed, reflecting efforts in recent years to clean up the bay. Chesapeake Bay is North America’s largest estuary and the most biologically diverse, with more than 3,600 species of plants, fish, and animals. Pollution threatened many of these species during the 1970s, and in the 1980s the bay became the first estuary in the United States to receive intensive government-sponsored restoration. Besides parts of Maryland, the 64,000-mi.2 watershed that drains into the bay encompasses parts of Delaware, the District of Columbia, New York, Pennsylvania, Virginia, and West Virginia.
The Department of Permitting offered a suggestion for John J. Kirlin’s problem, however: a device called the StormFilter for removing stormwater pollutants. Manufactured by Stormwater Management of Portland, OR, the StormFilter is a passive treatment system that uses chemical—not mechanical—processes to reduce pollutants. Precast treatment vaults of different sizes and configurations are available, but Stormwater Management and John J. Kirlin chose a 6- x 12-ft. cast-in-place unit for Bethesda Place II. Shipping a precast concrete vault from the West Coast didn’t make much sense, Morrison points out, so Stormwater Management provided build-in-place engineering drawings and communicated the plan to WSSC. "We had the permit in three days," says Morrison. "The whole turnkey installation was probably a seven-day form-out, pour-out process."
The vault contains cylindrical filter cartridges—in this case, four - each capable treating peak flows of 15 gal./min. Once the StormFilter is operating and stormwater flows into the vault, a float-actuated priming system prevents it from flowing out again until the water level rises high enough to purge air from the filter cartridges and create a siphon in the cartridges’ central drainage tubes. As the filters start to become plugged with debris, the siphon effect increases, helping to maintain a high flow rate. Hoods prevent air from entering the cartridges and breaking the siphon effect until the water level has dropped.
Morrison points out that another potential delay, if the drawings had not been immediately available, was the placing of the underground piping for the water exiting the vault. "We had to know where—what elevation. You can’t put piping in unless you know where the end is going to be. Once we got the drawings, we knew what elevation we needed to run to."
The rechargeable cartridges accept different filtering media, depending on the pollutants expected at a particular site. Filter media, which can be used in combination for maximum effect to target a variety of pollutants, include a pleated fabric insert, perlite, zeolite, and a patented leaf media made from processed deciduous leaves. At Bethesda Place II, where oil, grease, and radiator coolants from the parking garage were the main concern, perlite filters were used.
"We used to just put concrete interceptors in with baffles," Morrison points out. "This is a little bit more sophisticated."
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