When Bigger is Better

A large-scale rental car facility at BWI airport uses an innovative treatment system to surpass stormwater regulations

• By Rex Hansen, PE
• Oct 01, 2004

Daily, an average of 52,089 people travel on flights in and out of Maryland's Baltimore/Washington International (BWI) Airport. To support this amount of passenger traffic, the 3,596-acre airport site must provide parking for departing passengers and access to an ample supply of rental cars for arriving passengers.

In December 2003, BWI Airport opened a new 50,000-square foot, two-story, 6,000-car rental facility. The facility applies a mall-like, one-stop-shop approach to car rentals, moving all car rental functions out of the airport and consolidating them in a single nearby area. To create this experience, the engineers at Michael Baker Corp., Alexandria, Va., faced a number of challenges, including the need to install a treatment system for stormwater runoff from the building's rooftop, the top floor of its 6,000-car parking garage, and from facility roadways and auxiliary parking areas. Through the selection of a passive, underground filtration system, the engineers were able to complete the rental car facility, meet Maryland's stringent water-quality requirements, and protect the local environment from pollutants such as sediments, nutrients, heavy metals, oil, and grease.

The Challenges
In order for this new one-stop-shop approach to be effective, the facility had to be located within close proximity to the main terminal. Previously undeveloped, the selected site was only a 10-minute shuttle ride from the main terminal, but was located at the end of a runway. Building code height restrictions stated that the facility would only be allowed to rise a certain distance above grade due to its proximity to the runway and concern about potential interference with the flight path. To work around this height requirement, the facility was designed with a larger footprint, supporting 3,000 cars on each of the two floors of the structure.

Stormwater Runoff
Before the facility was built, any stormwater runoff that reached the undeveloped site would naturally soak into the ground. However, once the land was developed and impervious surfaces created, any water that was discharged onto the site would run off the pavement rather than soaking in. When runoff is generated from impervious surfaces, such as parking lots and streets, it collects pollutants such as particulates (dirt, organic matter, and dust from atmospheric deposition), heavy metals (from car and truck exhaust, tires, brake linings, particles of weathered paint, and rust), oils and grease, and petroleum hydrocarbons. This particular site would be especially susceptible to these pollutants in runoff due to the high volume of daily rental car, bus, and employee vehicle traffic at the site, as well as from increased atmospheric deposition from jet exhaust.

U.S. Environmental Protection Agency (EPA) requirements set forth by the National Pollutant Discharge Elimination System (NPDES) permit program and administered in Maryland by the Maryland Department of the Environment, required that the facility have a form of stormwater treatment to address these runoff pollutants.

"This was a very unique project, and the regulatory agencies were very stringent about how the stormwater system was to be designed," said Joseph Bellini, PE, civil engineering tech manager at Michael Baker Corp.

The water-quality regulations in the State of Maryland require that any treatment system used must remove 80 percent of total suspended solids (TSS) and 40 percent of total phosphorus from stormwater runoff. In Maryland, phosphorus is a keystone pollutant of concern for the Chesapeake Bay because sediment, as well as nutrients such as phosphorus and nitrogen, promote algal blooms and result in a degradation of water quality and the health of the bay.

The first approach the engineers considered for management of stormwater was underground infiltration. Runoff from the facility's second-story parking lot, mall rooftop, and surrounding driveways and parking areas would be collected through a series of inlets or catch basins and conveyed by pipes to an underground drainage field where it would drain through a layer of rock and into the groundwater. A benefit of this approach is that rather than sending the runoff into an existing storm system off-site, the runoff (after infiltration) is used to recharge the groundwater. However, due to the volume of traffic at the facility, a concern was that the heavy pollutant load would contaminate the subsoil and groundwater and eventually clog the underground infiltration field. If this were to happen, it would be extremely difficult and cost-prohibitive to remedy, as access to the infiltration field would require excavation of the parking lot and disrupt activities at this high-traffic site.

However, if an additional system were added to remove solids, oils, greases, and nutrients prior to discharge to the underground infiltration field, concern over both polluting the soil and the groundwater system would be alleviated, and the service life of the total system would be lengthened.

One option for treatment before infiltration would be to install large, open sedimentation ponds. This option had several downsides. First, to be effective for this installation, the ponds would have to be quite large in order to treat the volume of runoff from the facility's impervious surfaces. Secondly, there was a concern, as there is at any airport, about installing a feature that would attract birds. Since water would be held in the open sedimentation ponds for extended periods of time, there was a great risk that the ponds would be a bird attractant and create a liability for planes using the nearby runway. Lastly, in some situations, reflection of the sun's glare off the exposed pond surface could potentially interfere with a pilot's vision, creating an additional safety hazard.

Another option considered was use of a sand filter. The runoff would filter gravitationally through the sand and then to the underground infiltration field. Again, to treat the volume of runoff, the sand filter would require a large footprint and maintenance would be frequent, costly, and difficult. Similar to the maintenance issues facing the infiltration field, the large bed of sand would eventually become entrained with pollutants and oils.

The Solution
A final option considered was a passive filtration system, which is approved by the Maryland Department of the Environment, that uses media-filled filter cartridges with an integrated surface-cleaning function to remove pollutants from site stormwater runoff.

The product, the Stormwater Management StormFilter®, offered by Stormwater Management, Inc., Portland, Ore., is an underground structure that houses rechargeable, media-filled, siphonic filter cartridges. Polluted water is passed through the cartridges, which trap particulates and adsorb pollutants such as dissolved metals, nutrients, and hydrocarbons. The cartridge media is customized for each project to target site-specific pollutants.

The StormFilter is installed underground, but unlike the other options considered, the system is easily accessible using a traffic-bearing lid installed at ground level. Maintenance of the system is straightforward and typically required on an annual basis to maintain optimum system efficiency, although site conditions, pollutant loading, and rainfall patterns ultimately determine the maintenance frequency. Also, an integrated surface-cleaning mechanism on each cartridge hood maintains the permeability of the filter surface and enhances the overall performance and longevity of the system.

The StormFilter's approval in Maryland, combined with the system's ability to be installed underground, easily maintained, and customized to target a wide variety of pollutants made it the ideal solution to the challenges facing the engineers.

"In order to get the project approved, we selected the Stormwater Management Inc. filters," said Bellini. "This we felt went the extra step to meet or exceed the criteria of the regulatory agencies. The systems we used went beyond minimum regulatory requirements, but they were appropriate for the project."

Installation
Once the StormFilter had been chosen for the site, the next step was to integrate it into the site plans. Because of the facility's large size, the drainage system was fairly complex. Rather than directing all of the runoff to one point for treatment, the engineers from Michael Baker Corp. decided to use a series of StormFilter systems to treat runoff at multiple locations on the site.

To address the complex drainage system, the engineers recommended an innovative box culvert design, which provides treatment for higher peak water-quality flow rates while offering a less costly alternative to a large cast-in-place unit or to multiple precast units.

Five individual box culvert systems were installed along with five StormGate™ high-flow bypasses. This Stormwater Management system is used to direct low flows to the treatment system and bypass any flows that exceed the design water-quality flow rate in order to prevent the potential re-suspension of collected sediments, oil, and grease.

Each box culvert structure is 10 feet wide by 50 feet long. Unlike a typical installation of a cast-in-place structure, which requires time for excavation and concrete work (building and taking down forms, pouring, and curing), the installation of a box culvert structure involves preparing the excavation and lowering a series of precast concrete box structures into the ground. Once in place, the boxes are connected together and the area around the total structure is backfilled. Because of this reduction in installation time, each structure at the car rental facility was installed in one day, a surprisingly fast installation time for a structure of this size.

To address the pollutants of concern coming from the site, CSF® leaf media was selected for use in all of the 439 filter cartridges because of its ability to remove a wide variety of contaminants. While its granular nature allows for physical filtration of sediments and total nutrients, CSF's chemistry allows it to remove complexed metals and anthropogenic organic contaminants through adsorption, and remove soluble metals by cation exchange and chelation.

Expectations
While the filtration system units were installed in various locations across the site over a period of months in 2002, overall site construction and stabilization was completed in late 2003. In November 2003, after construction was completed, the filter cartridges were installed in the units, effectively making them ready to address stormwater runoff and ensuring that the systems would perform at optimum system efficiency.

From a cost, space, and accessibility standpoint, the StormFilter has so far proven to be the right solution for the BWI car rental mall. From a performance standpoint, it has proven in similar installations that it is able to meet and surpass the water-quality requirements for the site, and also is expected to address the pollutant concerns at BWI.

"It met the needs of the project and exceeded the requirements the state had as far as how it was applied," comments Bellini on the StormFilter; and the result is a big building that makes big steps towards protecting the environment.

This article originally appeared in the 10/01/2004 issue of Environmental Protection.