In the late 1990s, the Puget Sound Regional Council concluded its sole option for expanding commercial aviation capacity was at Seattle-Tacoma International Airport (Sea-Tac). At the time, the Port of Seattle determined it could not establish a third runway without wetland encroachment and needed permits requiring control of both quantity and quality of stormwater discharge.
In 1999, while conducting Whole Effluent Toxicity (WET) 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 was toxic to aquatic organisms. The port elected to investigate the source of toxicity in the runoff.
Using forensic investigation and source testing, Seattle-Tacoma International Airport investigators discovered zinc leaching into runoff from a cargo building roof coated with zinc-aluminum alloy. At 1 – 10 mg/L, the zinc concentration in the roof runoff was one to two orders of magnitude higher than zinc in typical urban runoff at 0.1 – 0.5 mg/L. The port considered several alternatives to mitigate the zinc toxicity. They included re-roofing, applying an inert paint coating, treatment with chemical additives and filtration treatment using an organic media.
According to Scott A. Tobiason, surface water manager in the Port Aviation Division’s Environmental Programs Group, painting costs were approximately $1.00 per square foot cost for an inert paint coating. But the Port was concerned because painting the zinc aluminum alloy coating of the roof invalidated its performance warranty.
The Port has tested several painted surfaces and found that zinc is still present. Said Tobiason, “Painting is not perfect. Painting has a percent effectiveness too.”
Later the port determined the filtration cost to be $0.50 per square foot, allowing for startup expense, and was able to determine filtration media with a life between one to two years and nominal replacement charges. This option was determined to cost less than painting the roof. Meanwhile, chemical treatment to induce precipitation, coagulation or flocculation was regarded as a generally less proven and more costly technology — and with uncertain regulatory acceptance compared to filtration as a recognized solution for stormwater.
“Within the filtration option we knew of the choice between upflow and downflow schemes for running the stormwater through filtration media,” Tobiason said. “But we didn’t have the space available in the testing area for the construction of an underground vault.”
The port learned about a radial flow delivery system that could be installed above ground from Contech Engineered Solutions. The Port decided to deploy the solution with a compact cartridge filter, reasonable fixed treatment rate and a built-in bypass feature for high flows.
Collecting samples from a recently constructed zinc-aluminum coated building, the port sampling showed a high rate of zinc leaching into runoff. According to the Port’s latest NPDES permit, the building and other airport areas drain to a discharge area subject to a zinc benchmark limit of 0.117 mg/L by 2007 — the limit corresponding to the Environmental Protection Agency standard stormwater benchmark for zinc applicable to other permitted facilities.
Eight DownSpout StormFilter units were specified, each holding two filtration cartridges. Conventional stormwater design guidance suggested fewer cartridges, but the port opted for a design to treat a higher total annual runoff percentage.
“It is a proven treatment technology that can be cost effectively deployed for a rooftop treatment system,” Tobiason said. “It’s something that doesn’t require excavation - you can do it above grade - and you can install them inexpensively by just intercepting the down spouts.”
Four filtration units containing CSF® leaf compost medium were installed on the north side of the building in fivefoot spaces between truck loading bays. Each was placed on concrete mounting pads.
On the building’s south side there are sidewalks and employee parking. The Port placed one unit with the leaf media directly on the sidewalk and the remaining three on a custom steel structure that supports the units for comparative testing of filtration media.
Stated Tobiason, “In our case we used it to treat zinc from a metal rooftop, but it could be extended to treat any other type of constituents depending on the type of media.”
Testing results showed a zinc reduction between 60 and 90 percent with initial concentrations ranging from 0.4 mg/L up to 15 mg/L (that is, 12 mg/L for dissolved zinc, 15 mg/L for total zinc).
“The results to date are promising, and have led us to consider using the technology at other potential sources at the airport, while we continue the program at the cargo building,” 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, so we are implementing best management practices. We started testing last August and will continue for about a year, so we can determine media longevity,” Tobiason explained. “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 mg/L. We’re encouraged, because we are seeing good removal toward our goal of 80 percent. We’re typically seeing that level or better.”
Testing was completed for about a year to allow the Port to determine media longevity. Dissolved zinc reduction was found to average between 81 and 76%. The Port was also able to confirm that the effective life of the filter media is between 12 and 18 months.