Project Case Study: Treating Roof Runoff at the Source

June 2004 » Feature Articles
In the late 1990s,the Puget Sound Regional Council concluded that its only option for expanding commercial aviation capacity was at Seattle-Tacoma (Sea-Tac) International Airport. But the Port of Seattle's Aviation Division determined that construction of a third runway would encroach on wetlands, requiring control of both the quantity and quality of stormwater discharge.
Calvin Noling, P.E.

In the late 1990s,the Puget Sound Regional Council concluded that its only option for expanding commercial aviation capacity was at Seattle-Tacoma (Sea-Tac) International Airport. But the Port of Seattle's Aviation Division determined that construction of a third runway would encroach on wetlands, requiring control of both the quantity and quality of stormwater discharge.

In 1999, while conducting whole effluent toxicity testing as part of implementing requirements for its National Pollutant Discharge Elimination System (NPDES) permit, the Port's surface water management team discovered that stormwater at a particular location in the drainage system showed aquatic toxicity. This runoff was non-toxic at its eventual discharge point to an adjacent creek, but the Port elected to take measurements up-gradient from the discharge point, before the concentration was greatly reduced by the Port's stormwater detention facility.

Subsequent testing and forensic investigation revealed that zinc was leaching into runoff from steel roofing that was coated with a zinc-aluminum alloy. The zinc concentration in the runoff, at 1.0 mg/l to 10.0 mg/l, was one to two orders of magnitude higher than the 0.1 mg/l to 0.5 mg/l zinc in typical urban runoff.

The Port considered three alternatives to mitigate the zinc and toxicity-treatment by filtration with an organic media, treatment with chemical addition, and source removal by re-roofing or applying an inert paint coating.

“Our initial estimates revealed a cost of $2 to $4 per square foot for an inert paint coating, compared to about $1 per square foot for treatment,” said Scott A. Tobiason, surface water manager in the Port Aviation Division's Environmental Programs Group (EPG). “We later updated the painting cost to $0.75 per square foot, but have not determined its useful life. We also were concerned because the zinc-aluminum alloy coating was warranted to perform only in an unpainted state.”

The EPG then updated filtration costs to $0.50 per square foot, allowing for startup expense. They determined filtration media life at one to two years, with nominal replacement charges. Compared to filtration's status as a recognized option for stormwater, chemical treatment to induce precipitation, coagulation, or flocculation was regarded as a less proven and more costly technology, with consequent uncertain regulatory acceptance.

“Within the filtration option,” Tobiason said, “we knew of the choice between upflow and downflow schemes for running the stormwater through filtration media. But we didn't have the space available in the testing area for the construction of an underground vault required for either, and preferred not to suffer the cost escalation in other potential treatment areas where there might be more room.”

Port officials knew of Portland, Ore.-based Stormwater Management, Inc. (SMI) from preliminary work on alternative media, and learned the company had a proven radial flow delivery system that was amenable to above-grade installation, with a compact cartridge filter. The system offers a reasonable fixed treatment rate and a built-in bypass feature for high flows. Radial-flow filtration system minimizes heavy metal pollution

Filtration media options
Initial screening of alternative filtration media options began in 2000, comparing commercially available options of a patented leaf compost from SMI, a perlite/zeolite mixture, and a polyamine sponge with a soybean hull medium that was available in experimental quantities. Results, detailed in a paper that was presented at the Watershed 2002 Conference in Ft. Lauderdale, Fla., prompted continued interest only in the soybean and leaf options.

The EPG began full-scale testing of the filtration delivery system and continued testing alternative media late in 2002. The tests were set up at a cargo building with a one-acre steel roof (zinc-aluminum coated), at a limited-space site where cargo trucks arrived and departed continuously at loading docks on a paved surface.

Filtration units on the north side of the building fit into 5-foot-wide spaces between loading bays.

The building, along with other airport areas, drains to a discharge location subject to a zinc effluent limit of 0.117 mg/l in 2007, according to the Port's latest NPDES permit. This limit corresponds to the U.S. Environmental Protection Agency standard stormwater benchmark limit for zinc that is applicable to many other permitted facilities nationally.

Eight DownSpout StormFilter™ units from SMI were specified, each holding two filtration cartridges. Conventional stormwater design guidance would have resulted in fewer cartridges, but the EPG opted for a design that would treat a higher percentage of total annual runoff. Instead of using the local design guideline that has a one-hour time step for modeling rainfall runoff, a 15-minute time step was used in deference to the relatively small drainage area and the short flow paths on the roof (about 100 feet).

SMI's radial-flow, siphonic DownSpout StormFilter usually is designed with two cartridges, allowing it to treat runoff from rooftops up to 15,000 square feet. The number of filter units, as well as the cartridge configurations, are tailored to site requirements. Typically, a two-stage unit is used for applications requiring a higher degree of treatment.

Installation of the four filtration units on the north side of the building, all containing the leaf compost medium, called for fitting them into 5-foot-wide spaces between active truck-loading bays. The units were placed on concrete mounting pads, with each pad featuring two protective bollards.

On the south side, where there were only sidewalks and employee parking, one unit with the leaf medium was placed directly on the sidewalk, while three others were placed on a custom steel structure that supports the units for comparative testing of filtration media.

“Results to date are promising, and have led us to consider using the technology at other potential sources,” said Robert York, senior project manager in the Stormwater Program for the Port of Seattle's Aviation Division. “While the zinc concentration in untreated stormwater is greatly reduced by the time it gets to the point of compliance, we still want to take care of it at the source. We want to get at the problem in its most concentrated form, and obtain the highest removal efficiency.”

Last August, Port officials started testing and will continue for about a year in order to determine media longevity. “So far, we've seen that the leaf option and a peat type not previously tested have comparable dissolved zinc removal, at 60 percent to 90 percent in the input range of 0.4 mg/l to 12.0 mg/l,” said Tobiason. “We're encouraged because we are seeing good removal toward our goal of 80 percent. We're typically seeing that level or better.

“Now we're hoping to also see consistent effluent quality near, or ideally under, 0.117 mg/l, which is a secondary goal. We are pleased to be a part of pushing the envelope toward a new best management practice alternative.”

Calvin Noling, P.E., is the director of industrial and remediation business at Stormwater Management, Inc., Portland, Ore. He holds a master's degree in environmental engineering, has broad industrial stormwater and wastewater treatment experience, and is a co-author of “Industrial Water Reuse and Management,” published by the American Institute of ChemicalEngineers. Noling can be contacted at 800-548-4667; e-mail: calvinn@stormwaterinc.com.


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