In order to comply with the conditions of its renewed National Pollution Discharge Elimination System (NPDES) permit, the Port of Seattle recently devised and implemented renovation plans at its Seattle-Tacoma (Sea-Tac) International Airport.
Numeric effluent limits for TSS, copper, lead and zinc within each of Sea-Tac’s regulated storm water outfalls became the main concerns in this undertaking. The Port and its consultant, R.W. Beck, studied the airport’s individual sub-basins and developed customized approaches for meeting new NPDES requirements.
Sea-Tac’s SDE4 sub-basin proved especially challenging because of its highly developed surroundings—parking lots, roadway entrances and exits, freeways, the main terminal and most the airport’s non-airfield operations.
The Port deemed a combination of source control and end-of-pipe treatment necessary in meeting effluent limits for this sub-basin. And with real estate at a premium, Seattle officials needed to find a way to consolidate the SDE4 storm water collection system.
The airport’s combined runoff needed to travel more than 2,000 ft to where the new end-of-pipe treatment system was being constructed. Construction crews accomplished this task using closed-face micro-tunneling technology. By installing a 54-in. pipe about 60 ft deep in the ground, they allowed for runoff to flow by gravity to the storm water treatment system. The project ultimately cost $10 million.
“The new system consists of an 11 acre/ft lined and netted detention and equalization pond with a forebay to collect heavy materials,” said Bob York, senior infrastructure engineer at the Port of Seattle. “The pond is designed with outlet orifices that regulate the discharge of the storm water from the pond, and, in turn, the water level in the pond rises and falls to equalize the discharge to the downstream treatment system.”
Hydrologic modeling demonstrated that at 11 cu ft/sec, at least 91 percent of Sea-Tac’s annual runoff volume would receive treatment. “All in all, the approach is an elegant solution to a difficult problem,” York said.
Solution Selection and Installation
Port officials and R.W. Beck consultants chose the storm water management StormFilter from CONTECH Stormwater Solutions to provide runoff treatment. The Port had previously conducted tests using different StormFilter media and recorded removal efficiency data for each of the parameters of concern.
The StormFilter installed at Sea-Tac measures 40 ft x 77 ft. Each 17.5-ft wide cartridge bay holds up to 160 cartridges and can be isolated for maintenance. Presently the filter contains 450 media-filled cartridges, and 150 more are to be installed at a later date. CSF and MetraIRX media are in place, being used along with two different cartridge flow rates—7.5 and 15 gal per minute.
The system went into service in May 2007, and ongoing monitoring will determine the combination of media type and flow rate for best life expectancy and pollutant removal efficiencies.