Recent studies completed by Burns & McDonnell at a large hub airport produced a long-term spent aircraft deicing fluid and storm sewer system management plan that is consistent with the airport's master plan.
Most major airports must actively manage spent aircraft deicing fluid (SADF) and stormwater runoff. But the way they do it is unique to each facility. The recovery, collection, conveyance, storage and treatment of SADF and stormwater can be accomplished in several ways. National Pollutant Discharge Elimination System (NPDES) requirements for permitted stormwater outfalls limit the level of biological oxygen demand (BOD5) in discharges. Propylene glycol used in deicing fluid is a major source of BOD5 in stormwater and snow melt runoff at airports. Modeling the runoff and mass balance of BOD5 loading throughout the complex SADF and stormwater systems is difficult because of the variability in frequency and quantity of rainfall, snow melt and deicing fluid use. This presents challenges in developing appropriate management strategies to reduce the potential for non-compliance with permit requirements.
The recommendations and procedures outlined in Airport Cooperative Research Program (ACRP) Report 14, Deicing Planning Guidelines and Practices for Stormwater Management Systems, are a good starting point for SADF and stormwater management. The report outlines the standard industry approach for developing and maintaining a management plan to reduce pollutant discharges.
Once a management plan has been developed and implemented, the report recommends a trial-and-error approach to maintaining compliance. The performance of the plan should be evaluated at the end of every season and deficiencies addressed. If the system performs effectively, then the guidance suggests that no additional action is needed.
One key to a successful management plan is capturing real data from the operating system and combining it with data from modeling tools. That allows for more precise and effective system management.
Various methods are available to airports for stormwater and SADF management. These can include deice pads, catch basins, diversion valves, storage tanks, pump stations, detention/retention ponds and treatment/recycling facilities. The most appropriate method is selected based on permitting requirements, off-site treatment/disposal options available, and capital improvement and operational costs. Developing site-specific hydrologic and hydraulic models as well as mass balance and BOD5 loading models can be important and useful tools in selecting the appropriate stormwater and SADF management options.
Recent studies completed by Burns & McDonnell at a large hub airport produced a long-term SADF and storm sewer system management plan that is consistent with the airport's master plan. The study consisted of evaluating the airport's SADF recovery and treatment/recycling systems by a highly specialized staff of stormwater, wastewater, aviation and hydraulic engineers. The project engineers had to step out of the everyday mindset of a consultant. Each airport has a set of unique problems; therefore, it must develop unique solutions.
The SADF and storm sewer systems were complex and hydraulically connected, creating challenges from operational, maintenance and compliance perspectives. Adding to the complexity was the challenge of meeting the NPDES BOD5 discharge limits using a traditional stormwater collection and conveyance system not designed to achieve that level of performance.
It would not be economical to capture and treat all precipitation that hits a deice pad during deicing operations. Therefore, like any other collection system, level of service or return periods need to be used. Traditional rainfall return periods could not be used because deicing operations only occur during the winter. The design team developed a modified return period to only include deicing months, in addition to modifying the intensity of the storm to more closely match a typical winter storm distribution.
Beyond the Weather
Once the weather calculations were determined, the team needed to model the expected glycol use and determine how much of the SADF is captured in the glycol recovery system versus how much is lost to the environment. The expected glycol use was calculated using historical flight patterns and historical glycol use levels. A relationship was developed relating the quantity of glycol used per aircraft to rainfall depths. This allowed for the development of a water quality model to be used in conjunction with the hydraulic model.
One type of loss to the environment, known as a fugitive loss, occurs when fluid adheres to aircraft. It shears off during takeoff, drips, tracks on the wheels of ground support equipment, gets carried off as wind drift, or biodegrades in pavement surfaces and soils. Fugitive losses can constitute 20 percent to 60 percent of the total deicing fluid used. Because of this wide variation, strategic surface water sampling may be needed to capture data from smaller watershed sub-basins as part of the larger overall basins. Airport watersheds may comprise thousands of acres. Testing only at the NPDES discharge locations will not determine where fugitive losses occur. The sampling plan should establish locations throughout the airport for capturing all major sub-areas where fugitive losses could occur. This data can then help determine where fugitive loss hot spots are. Future infrastructure could then be constructed to capture and treat runoff from these locations.
Capturing as much data as possible and combining that data with the overall system hydraulic and mass balance loading (water quality) models can be an extremely useful tool for ongoing SADF and stormwater management.
Sidebar: Wetland Mitigation and Natural Treatment Systems
Jurisdictional waters and wetlands exist on most airport properties. Airport development not only increases the amount of stormwater and SADF runoff, it also potentially affects federally regulated wetlands and waters of the U.S. Compensatory mitigation driven by compliance with the Clean Water Act may be necessary based on results of environmental assessments.
Wetlands mitigation can be incorporated into natural treatment systems such as submerged bed wetlands, bioretention cells and wetland swales to achieve the necessary level and type of stormwater treatment. Alternatives should be evaluated based on effectiveness, cost, operation and maintenance, and wildlife impact, as well as their potential to establish mitigation banks for future development.