Nearly 53,000 people fly in and out of Maryland’s Baltimore/Washington International Airport (BWI) daily. The
3,596-acre airport provides both parking for departing passengers and access to thousands of rental cars for
arriving passengers. Recently, BWI opened a new 50,000-square-foot, two-story, 6,000 car parking facility and
one-stop car-rental mall.
Engineers at Michael Baker Corporation, Alexandria, Va., installed a treatment system for stormwater running
off the building’s rooftop, the top floor of its 6,000-car parking garage, facility roadways and parking areas.
Using a passive, underground filtration system, the engineers met Maryland’s stringent water quality
requirements while protecting the local environment from pollutants, including sediments, nutrients, heavy
metals, and oil and grease.
Located at the end of a runway, the new parking lot and rental car structure could not interfere with the flight
path. To meet this restriction, engineers designed the structure with a footprint holding 3,000 cars on each of
Before building the facility, any stormwater runoff reaching the undeveloped site soaked naturally into the
ground. Once the land was developed, discharged water ran off the impervious pavement surfaces. This made
the site susceptible to runoff pollutants from the high volume of cars, buses and the atmospheric deposition
from jet exhaust.
EPA requirements established by the National Pollution Discharge Elimination System and administered by the
Maryland Department of the Environment required stormwater treatment addressing these runoff pollutants.
Maryland water quality regulations require treatment systems to remove 80 percent of total suspended
solids and 40 percent of total phosphorus from stormwater runoff. In Maryland, phosphorus and nitrogen are
keystone pollutants for the Chesapeake Bay because they promote algae blooms that degrade the health of
At first, engineers considered underground infiltration for stormwater management. Rather than sending
the runoff into an existing storm system off-site, recharging the runoff into the groundwater was a benefit of this approach. But, with the facility’s high traffic volume, a heavy pollutant load could potentially contaminate
the subsoil and groundwater and eventually clog an underground infiltration field. Remedying that would be
difficult and costly, because access to the infiltration field would require excavation of the parking lot and a
disruption of activities.
Adding an additional system to remove pollutants prior to discharge into the underground infiltration field
would alleviate both polluting the soil and over-straining the groundwater system, thereby extending the life of
One option for pre-infiltration treatment involved installing large, sedimentation ponds. Because open
sedimentation ponds hold water for extended periods of time, the ponds would attract birds, an aircraft liability.
Also, the sun’s glare off the pond might interfere with pilot’s vision. A sand filter that would strain runoff before
it entered the underground infiltration field was another option. To treat the sizeable volume of runoff, the sand
filter would require a large footprint and frequent, costly and difficult maintenance. Both the sand filter and the
infiltration field would eventually become filled with pollutants and oils.
The Maryland Department of the Environment had approved a passive filtration system from Contech
Stormwater Solutions that removes pollutants from stormwater runoff using media-filled, surface-cleaning
The Stormwater Management StormFilter® is an underground structure that houses rechargeable, media-filled,
siphonic filter cartridges. Polluted water passes through the cartridges, which trap particulates and adsorb
To address the complex drainage system for the large facility, Contech. engineers recommended an innovative
box-culvert system design using five StormGate™ high-flow bypasses. These provided treatment for higher peak
water quality flow rates while offering a less costly alternative to a large cast-in-place unit. Rather than directing
all the runoff to one point for treatment, engineers used a series of StormFilter systems to treat it at multiple
All of the 439 filter cartridges used CSF® leaf media because of its excellent ability to remove a wide variety of
contaminants from the site. Its granular nature allows for physical filtration of sediments and total nutrients,
and its chemistry allows it to remove complex metals and the anthropogenic organic contaminants through
adsorption. It also removes soluble metals by cation exchange
The StormGate directs low flows to the treatment system and bypasses any flows exceeding the design water
quality flow rate for the system to prevent the potential re-suspension of collected sediments and oil and grease.
Each box culvert at the car rental facility was installed in one day.
Underground installation with a ground level, traffic-bearing lid makes the system accessible. System
maintenance is done annually to maintain optimum efficiency. However, site conditions, pollutant loading and
rainfall patterns ultimately define the maintenance frequency.
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.