EPA Deicing Rule Targets Airport Stormwater Runoff
Managing stormwater from airports is critical to their operation. Robust drainage systems lie beneath the airfield and are designed to minimize standing water.
By Mark O. Liner, P.E.
Managing stormwater from airports is critical to their operation. Robust drainage systems lie beneath the airfield and are designed to minimize standing water. Up to now, the focus has been on flood prevention. This is likely to change for some airports when the Environmental Protection Agency (EPA) proposes a rule next year that regulates deicing activities. The rule is designed to limit the discharge of glycol and other pollutants from the airport. Airport managers are waiting to see how the regulations will affect their operations, but are aware that changes will be needed. For some airports, like Buffalo Niagara International Airport in New York, the changes are already underway. Design work is currently in process on a subsurface wetland system engineered to treat spent glycol. The system is in the design phase and is scheduled for construction in 2008.
Airports are evaluating how existing stormwater systems will be modified to meet EPA’s regulation.
How each airport takes on stormwater management will vary from site to site; the peculiarities of each airfield will drive the process. How much deicing activity is there? What are the options for disposal? What is the existing infrastructure at the airport? Each airport will need to answer these questions, which will then channel the solution in a different direction. There are many possible solutions for airports to consider, so it is imperative that airport managers and their engineers look at all the factors affecting their deicing operation and stormwater systems before breaking ground.
The impact of deicing fluids on the environment is most notably related to the high oxygen demand they exert when released to rivers and streams. A large slug of glycol can quickly deplete the dissolved oxygen in receiving waters, which can kill fish and other organisms that need aerobic environments. Removal of these “loads” prior to discharge is the primary basis for the use of sewage plants throughout the world. As a reference, typical household wastewater has a Biological Oxygen Demand (BOD) of around 200 mg/L, while some discharges from airports have measured over 100 times stronger than that at 22,000 mg/L (from EPA’s 2000 Preliminary Data Summary on Airport Deicing Operations). In most cases, surface water permits limit discharge of BOD to below 30 mg/L.
The depletion of oxygen in water bodies is only part of the story. Recent work by the United States Geological Survey (USGS) has examined the toxicity of airport deicing fluids. The research, presented by Steve Corsi of the USGS at the American Association of Airport Executives’ 15th Annual Deicing and Stormwater Management Conference, examined the toxic effects associated with the melt of glycol-laden snow banks. Further, the presentation probed the toxicity associated with the additives used in deicing liquids. It concluded that toxicity can be related to the additives used in deicing and anti-icing formulations. Although treatment for glycol may eliminate oxygen depletion in streams, it is the additives that may be of more concern in terms of toxicity to the environment.
Under the Clean Water Act (CWA), the EPA establishes technology-based national regulations, termed “Effluent Guidelines,” to reduce pollutant discharges from categories of industrial facilities that discharge to waters of the United States. The guidelines are designed to provide uniform guidance for NPDES permit writers across the United States. The new deicing fluid guidelines will establish a baseline with which all U.S. airports must comply. Currently the EPA is working with airports to collect survey data and conduct detailed sampling programs. This work will be used to identify the best available technology (BAT) that is economically achievable for treatment of deicing and anti-icing fluids. Typically, the BAT is the basis on which numerical discharge limitations are developed. The EPA plans to publish the proposed rule in June of 2008 and take final action two years after.
Airports are turning to engineered wetlands for the treatment of deicing fluid and stormwater runoff.
To complicate matters, a separate wing of the EPA is charged with regulating stormwater under the National Pollutant Discharge Elimination System (NPDES). The NPDES program has a well established permitting system for end of pipe discharges, typically associated with sewage plants and other pipe outfalls. This area of stormwater permitting is still evolving and undergoing implementation of a second phase. Currently, most airports have a NPDES permit that regulates the discharge of stormwater collected in and around the airfield. For these discharges, permit writers have little uniform guidance on how to establish limits for pollution from deicing. Effluent guidelines should fill the gap; however, such guidelines are rarely used to support stormwater permits. Permit writers will need to balance local stormwater conventions with national guidance.
Melt water from airport snow piles may require treatment under the new EPA regulation.
Along with current stormwater permitting and the new Airport Deicing Effluent Guidelines, airports will also be subject to constraints related to water-quality-based discharge limits and total maximum daily loads (TMDLs). These constraints vary from state to state and place to place and will add complexity to the permitting process.
Due to their footprint and paved surface area, airports are already in the business of stormwater management. Every airport has a strategy and an infrastructure dedicated to ushering rainfall off the airfield. After installation, most airport stormwater collection pipes and tanks have led a passive life with minimal attention. For some airports, reengineering these passive systems to meet the new EPA guidelines will be a serious and formidable task.
Flood prevention requires storage and most airports and their engineers have figured out how to capture, store and channel flow so as to minimize standing water. Big pipes and big tanks make it happen. The challenge is to rework these big, established systems to aid in the collection and treatment of contaminated stormwater. In some instances, large plugs or valves isolate deicing areas and provide effective collection. In other instances new, parallel collection systems are installed and dedicated to spent deicing fluid. While plugs and valves may seem simple, the cost of constructing a parallel collection system can be daunting for an airport.
While flood prevention involves managing volume, water quality protection aims to capture and control the first flush of airfield runoff generated in a storm. It is a different storage volume for a different purpose. And, yes, in an ideal world that would require two tanks: one for flood prevention and one for collecting the first flush.
For airports, this first flush volume can also be considered equalization for the treatment system, which will result in a smaller, less costly system. Storms can come and go with the first flush held and paced into the treatment system afterwards. The benefits of equalization cannot be understated; it smoothes out the peaks, normalizes the flow, and provides flexibility to airport operations. Most importantly, it will reduce the cost of the treatment system. Engineers know that a gallon of storage is cheaper than a gallon of treatment, particularly if the wastewater has a high concentration.
The Safety Factor
There is little question that safety is a critical issue for airports. During severe weather, no limits are placed on keeping airplanes safe to fly. This is of course as it should be. But there is a cost associated with safety, which is partly borne by the airports in cleaning up after deicing. When it comes to the environment, do we need to pose the same question? Is a permit violation acceptable during a 20-year storm? A 10-year storm? Or never?
Before starting work, a wastewater engineer must establish a basis of design that includes the flow, the concentration, and limit to be met. The mindset is that each day is the same and that flows and loads are well behaved. The stormwater engineer, though, knows that airport flows and concentrations are highly erratic and best understood via hydrographs and statistics. In the end, both will concede that there is a point of diminishing returns for the system, a point beyond which additional infrastructure will yield less and less benefit. So, as part of the basis of design, both engineers must agree on an acceptable confidence level for the design.
Construction of engineered wetlands for an airport wastewater management system.
Historical flow and concentration data can be analyzed to tease out 80%, 90%, and 100% confidence levels and, with some effort, an engineer can cost out the burden of building a stormwater system for each case. Such a statistical exercise is interesting, but may result in more questions than answers. A deterministic approach, in which a design storm event is selected, may provide a shorter, more reasonable route to a basis for design. In this case, historical flows and concentrations are reviewed and a “bad” deicing event picked. Flows and concentrations are modeled for the event and a system is optimized for the given conditions of the scenario. The confidence level is backed out in this approach, but it is easier to explain the project costs to airport executives by referencing a particular storm, rather than citing an 85% confidence level.
EPA’s deicing regulation is uncharted territory. It will require airport managers to balance the realities of stormwater management with limitations on stormwater quality. Taken separately, the tasks are routine. Integrating both, however, will require patience and a sizable pocket book. When the rule is proposed next summer, the burden of compliance will become partly clear. The costs associated with modifying airport infrastructure to reach compliance will be less so. Some airports are ahead of the game and will provide valuable guidance on managing both stormwater quality and quantity.
About the Author:
Mark Liner, P.E., is a senior engineer at Jacques Whitford NAWE, LLC, where he specializes in the design and retrofit of wastewater treatment for industrial facilities, particularly in conjunction with natural systems. Liner received his masters in engineering from Clemson University. He can be reached at firstname.lastname@example.org.