Radial-Flow Filtration Dramatically Reduces Stormwater Zinc From Airport Building

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By Calvin Noling, P.E.
Director of Industrial & Remediation Business, Stormwater Management, Inc.

Management for Seattle-Tacoma (Sea-Tac) International Airport reports favorable reduction of stormwater zinc in an on-going program at a cargo building through deployment of a radial-flow filtration system at a zinc-aluminum alloy coated roof-runoff source. The program was undertaken in an effort to help establish more effective alternatives for mitigating zinc in stormwater runoff.

Filtration units, a filter media option, and technical support are being provided by Stormwater Management Inc. (SMI) of Portland, Ore. Initial results show zinc reduction of 60 percent to 90 percent at initial concentrations ranging from 0.4 mg/L up to 15 mg/L (i.e., 12 mg/L for dissolved Zn, 15 mg/L for total Zn).

"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'," said York. "We want to get at the problem in its most concentrated form, and obtain the highest removal efficiency."

In the late 1990's, the Puget Sound Regional Council concluded its only option for expanding commercial aviation capacity was at Sea-Tac Airport. Then the Port 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 showed aquatic toxicity.

Despite the fact that this runoff was found to be non-toxic at its eventual discharge point to Miller Creek, the Port elected to investigate the source of toxicity in the upgradient runoff.

Subsequent source testing and forensic investigation revealed toxicity attribution to zinc leaching into runoff from steel roofing coated with a zinc-aluminum alloy. The zinc concentration in the roof runoff, at 1-10 mg/L, was one to two orders of magnitude higher than zinc in typical urban runoff at 0.1-0.5 mg/L.

Stormwater runoff is generally more toxic to aquatic organisms than natural waters due to rainfall's inherently low pH and hardness. Heavy metals like zinc in stormwater from roof runoff are generally more bioavailable and toxic to aquatic organisms when the pH and hardness are low. Organic substances that would be present in a natural environment tend to buffer (neutralize) the pH and contribute hardness to the water. These natural processes are absent in urban settings covered by impervious surfaces such as building roofing.

Measurements were taken up-gradient from the discharge point, before the concentration would be greatly reduced by the Port's stormwater detention facility. Alternatives considered to mitigate the zinc and toxicity were treatment by filtration with an organic media, treatment with chemical addition, or by source removal such as 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. "We later updated the painting cost to $0.75 per square foot, but have not determined its useful life, and we were also concerned because the zinc-aluminum alloy coating was warranted to perform only in an unpainted state."

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

"Within the filtration option," Tobiason continued, "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."

The Port knew of Stormwater Management 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, and offered a reasonable fixed treatment rate, and a built-in bypass feature for high flows. "We opted to deploy it beginning last spring," said Tobiason. "We liked the idea that these units were an adaptation of a stormwater treatment system already proven nationwide and recognized by many regulatory agencies."

Initial screening of alternative filtration media options had begun 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 (including experimental methodology) 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.

Full-scale testing of the filtration delivery system, as well as continued testing of alternative media, began late in 2002. It was set up at a cargo building with a one-acre steel, zinc-aluminum coated roof, at a limited-space site where cargo trucks were continuously arriving and departing at loading docks on a paved surface.

The particular building was chosen from among a group of similar zinc-aluminum coated options because it was the most recently constructed, and sampling had shown a consequent higher rate of zinc leaching into runoff. 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 EPA standard stormwater benchmark limit for zinc that is applicable to many other permitted facilities nationally.

"We wanted to pick the worst case scenario," noted Tobiason.

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 Port 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 short flow paths on the roof (about 100 feet).

Installation of the four filtration units on the north side of the building, all containing the leaf compost medium, called for fitting them into five-foot spaces between active truck loading bays. In an effort led by Albert Shen, project manager for the Port, this was accomplished by placement on concrete mounting pads, with each pad featuring two protective bollards (posts).

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.

"We started testing last August, and will continue for about a year, so we can determine media longevity," said Tobiason. "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."

"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 whole new ‘best management practice' alternative for dissolved metals, and for roof runoff in general."

Stormwater Management Inc. says its radial-flow, siphonic DownSpout StormFilter is customarily designed with two cartridges, allowing it to treat runoff from rooftops up to 15,000 sq. ft. The company adds that the number of filter units, as well as the cartridge configurations, are tailored to individual site requirements, and that for applications requiring a higher degree of treatment, a two-stage unit is typically used.

SMI reports typical metals reductions achieved with its single-stage units are zinc, 50 percent to 85 percent; copper, 60 percent to 90 percent; and lead, 65 percent to 90 percent on a mass basis. Both particulate and dissolved zinc and other heavy metals are removed. The StormFilter is described as broadly effective for removal of suspended solids, metals, oil and grease, organics, and other pollutants.

SMI says other configurations of the StormFilter are available, and are used in the industrial, commercial, residential, and transportation sectors. The company also provides catch basin inserts, screening, and oil/water/grit separator systems to treat pollutant loading at a particular site. An electrocoagulation system is available for washwater treatment and heavier loading applications. A suite of accessories is offered to complement the company's core products.

Source: Stormwater360

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