Introduction
Airports in the US have stormwater management infrastructure ranging from open drainage ditches to more sophisticated methods of collection and processing stormwater. Large air carrier or air cargo airports often have extensive stormwater infrastructure systems, which are designed and constructed to manage the quantity and quality of stormwater runoff from their vast area of paved surfaces and buildings.
There have been several recent research studies on the environmental and toxicological impacts of airport stormwater, especially related to stormwater containing deicing contaminants. These published studies focus on analytical techniques, limitations, and interpretation of impacts of the 2012 US Environmental Protection Agency (EPA) Airport Deicing Point Source Category effluent limitation guideline (ELG); trends in the aviation industry (evolving fleets, reduced turn times, etc.) as they relate to deicer application, collection, and management practices; the applicable deicing planning and stormwater management strategies and tactics for airports of various size and category, climate, type of stormwater and receiving waters, regulatory requirements, storage and treatment capacity, site constraints, and budget constraints. Structured approaches, decision support tools or processes for the selection of deicer source reduction, containment/collection, conveyance/storage, or treatment/recycling solutions were studied as well.
According to many studies, stormwater runoff at airports is a significant and costly issue (ACRP 02-61, on-going project) [
1], especially for airports enduring winter weather. To better manage airport stormwater containing deicers, the solutions need to be implemented in both management and technology domains, considering the multiple dimensions of this issue and emerging solutions (as illustrated in Fig. 1).
In this context, this work provides an overview of the relevant issues by synthesizing the published literature, including: regulatory challenges in airport stormwater management, challenges in understanding the transport and environmental risks of airport deicing stormwater, opportunities in minimizing airport deicing stormwater footprint. The opportunities discussed include: deicer application minimization methods, deicer collection methods, alternative anti-icing and deicing agents, deicer treatment methods, and green infrastructure at airports. Furthermore, a guidebook and a decision tool are proposed to help airport practitioners evaluate sustainable and effective practices related to deicer application and the collection and treatment of stormwater containing deicers. The work presented in this paper aims to facilitate the update of
Airport Cooperative Research Program (ACRP) Report 14 (Deicing Planning Guidelines and Practices for Stormwater Management Systems) [
2].
Regulatory challenges in airport stormwater management
Many airports in the US are required to comply with the requirements of the 1977 Clean Water Act and its subsequent amendments. This includes the requirement to obtain a National Pollution Discharge Elimination System (NPDES) permit for point source discharges. Under the requirements, airports are usually permitted under Industrial Permitting and must prepare a Stormwater Pollution Prevention Plan (SWPPP) that requires regular monitoring, site inspections and reporting. Additionally, airport drainage design as directed through the US Federal Aviation Administration (FAA) Advisory Circular AC 150/5320-5D aims to safely and efficiently remove water from airport premises, to aid in safe travel on runways and other surfaces, and to discourage waterfowl and other wildlife. The quick and efficient removal of potentially polluted stormwater from airport facilities conflicts with the EPA’s intent to eliminate pollutants from waterways, unless stormwater treatment and management practices are utilized.
The original 2009 Request for Proposals (RFP) for what resulted in the publication of ACRP Report 14 (Deicing Planning Guidelines and Practices for Stormwater Management Systems) identified how US airports faced increasing public and regulatory attention and technical challenges in managing runoff from aircraft and airfield deicing and anti-icing operations. It was noted that while a large community of airports are affected by the effluent guidelines of the US EPA, a particular concern was how requirements affect smaller airports, which have more limited resources than large hub airports. ACRP Report 14 does a very good job of presenting planning guidelines and best management practices (BMPs) for deicing-runoff management systems that assist airports of all sizes and operational levels in designing (or re-designing) site-specific solutions. However, in presenting its very valuable information, the report required an airport manager to wade through all of the material to find what applications and recommendations may be best suited to their airport. This is because, as rightly stated in the report: “Each airport presents a unique combination of physical, climatological, operational, funding, environmental, and regulatory characteristics that must be evaluated as a whole when an effective deicing runoff management program is being developed (pg.2).” Except for the larger airports, an airport manager or staff may not have the expertise to make determinations about which deicing stormwater management practice may be applicable to them.
Since the publication of
ACRP Report 14 in 2009, significant progress has been made in areas of policy and rulemaking. Airport deicing ELG, addressed by the EPA in 2012, requires NPDES permits to address issues associated with deicer-laden wastewater from airports and established new standards for wastewater discharged from airports that is associated with aircraft and pavement deicing [
3]. The new rule requires the proper collection, treatment, and disposal of deicer laden wastewater by airports to be addressed in the NPDES permit on a case-by-case basis. For instance, for airports with 1,000 or more annual jet departures, the guidance is to use non-urea based deicers or to meet a set numeric effluent limit for ammonia (daily maximum of 14.7 mg/L for Nitrogen). For larger airports with 10,000 annual departures located in areas where airport pavement and airplanes are commonly deiced, the rules require collection of 60% of aircraft deicing fluid following deicing (or the use of deicing pads). Additionally, if these airports discharge the collected deicer-laden wastewater into “waters of the U.S.,” the wastewater must meet chemical oxygen demand (COD) standards (daily maximum of 271 mg/L, weekly average of 154 mg/L; using sampling protocol for soluble COD) [
4].
The goal stated by EPA in justification of the new rule is the reduction in pollutant discharge by 16 million pounds per year, at a minimum, with a cost of about $3.5 million annually [
5]. EPA also estimated that this guideline will apply to 198 existing airports national-wide [
6]. Comments received by the EPA have led to ongoing communication between stakeholders and the FAA following the comment period, and acknowledgment of concerns expressed by industry regarding “feasibility, safety, operations, delays, and financial impacts of collection and treatment” of deicer-laden wastewater [
4]. After data were submitted by airports and other stakeholders to the FAA on the proposed rule, the final ELG was revised and is actually less stringent than what was proposed initially, which required a reduction of 44.6 million pounds of contaminants at a cost of $91.3 million annually [
6]. Canada and European countries also have environmental regulations regarding airport deicing runoff to nearby soils and water bodies. Canada Environmental Protection Act requires an upper limit of 100 mg/L of glycol discharge from aircraft deicing activities at airports [
7]. The European Aviation Safety Agency [
8] published a report which developed 26 recommendations that would result in a beneficial reduction in the risks associated with deicing / anti-icing activities in airports.
It has been anticipated that “new regulatory developments may have their greatest effect on midsize and smaller airports, where deicing operations and runoff may have been previously considered to be too small by environmental regulators to be of significant concern” (ACRP Report 14 [
2]). A flowchart (Fig. 2) illustrates the temporal evolution of de-icing contaminated stormwater regulatory management.
Challenges: transport and environmental risks of airport deicing stormwater
Airport stormwater may not only induce increased runoff volumes and changes to peak flow rates and timing, posing a risk for downstream receiving waters (e.g., severe erosion and degradation of river and stream channel banks and riparian areas), but also introduce the pollutants into receiving water systems. Typical pollutants may include anti-icing and deicing chemicals (a.k.a., deicers) used on
airfield pavement, such as potassium acetate, potassium formate, sodium formate, sodium acetate, corrosion inhibitors and other additives and deicers on
aircraft, typically propylene glycol (PG) and ethylene glycol (EG) based products with corrosion inhibitors, thickeners, etc [
7,
9–
12], along with other elements from unpainted metal surfaces such as zinc and copper. They may also include petroleum-related compounds, sediment (indicated by total suspended solids), metals (Cu, Zn, etc.), and nutrients (indicated by biochemical oxygen demand, total P, total N, etc.) [
13], as a result of aircraft and other vehicle refueling, vehicle maintenance, material storage and stockpiles, and jet fuel exhaust deposits. The primary pollutants of concern identified by the EPA include ammonia, which is associated with the use of urea, and elevated COD in wastewater being discharged [
4].
The contaminant sources from aircraft deicing and anti-icing practices are summarized in Fig. 3. Generally, the snow/ice control operations at different airports may differ as a function of the weather conditions, operation procedures and airport policies [
14]. The quantity of deicing spray ranges from 40l/plane to 15000/plane depending on operational and climatic factors [
15]. Note that pavement deicing (or anti-icing) products (PDPs) are typically applied over large areas, and thus more difficult to collect than aircraft deicing fluids (ADFs) and aircraft anti-icing fluids (AAFs). As such, they will eventually be washed away by stormwater.
According to
ACRP Report 14, efforts can be made to reduce the stormwater discharges of contaminants by reducing deicer usage, increasing the amount of deicer collected (either recycled or sent to treatment), and reducing fugitive losses. An understanding of application, dispersion and transport of deicers across the airport landscape is very important to evaluate their environmental implications [
16]. Deicers can be transported by storm and melt water runoff and discharged into nearby water system. The environmental impacts are mainly from their adverse effects on aquatic life, which include low dissolved oxygen, aquatic toxicity, bacterial growth, and odors in receiving water systems [
16–
20].
The primary environmental concerns with several deicers in products manufactured and used historically were high organic content, resulting in high Biochemical Oxygen Demand (BOD), and aquatic toxicity. There is a wide range of chemicals potentially used in deicers, including freezing point depressants (FPDs), surfactants, corrosion inhibitors, thickening agents, defoamers, pH modifiers, dyes, oils, and antioxidants and antimicrobial agents (ACRP Web-Only Document 3, 2008 [
21]). Nonetheless, not all of these component categories are present in all deicing and anti-icing products, and aquatic toxicity data are likely unavailable for a large portion of these chemicals (e.g., proprietary additives). Some of the fate and transport characteristics of primary constituents of concern in deicers are now better understood, but many likely have not been studied as extensively or in the correct medium and environmental setting (e.g., airport stormwater runoff versus treated municipal effluent). Also, most research on the fate and transport of deicers historically focused on FPDs and, to a lesser extent, on benzotriazole-derived corrosion inhibitors and alkyphenol ethoxylate (APE) surfactants. The components for which environmental characteristics were not well understood included dyes, thickeners, pH modifiers, defoamers, other corrosion inhibitors and surfactants, and even the FPDs used in certain airport-deicer related products (e.g., pavement deicer).
Since the mid-2000s, new research at the time designed to identify or confirm constituents of concern in aircraft deicers in earlier formulations presented the compelling case that toxicity from Type IV aircraft anti-icing formulations is of most concern, and that the majority of acute and chronic toxicity to fish and aquatic invertebrates is likely from APE and alcohol ethoxylate surfactants, with secondary contributions from benzotriazole-derived corrosion inhibitors (ACRP Web-Only Document 3, 2008 [
21]). That research also confirmed previous assertions that toxicity in pavement deicers appears to be mostly associated with the FPDs in the product formulations in general, and glycol-based FPDs in particular. Propylene glycol and ethylene glycol are a primary source of high BOD in waterways/conveyances receiving runoff from airports. Sulej et al. [
15] summarized that runoff waters generated by airport operations are hazardous to the environment because they contribute to the contamination of the water and soil. The quantity of runoff waters has increased greatly due to worldwide increase in air traffic. Such runoff waters require special management procedures such as advanced deicer collection methods, alternative AAF/ADF agents and new recycling methods.
Of particular relevance to the evaluation and consideration of environmental and toxicological characteristics and potential impacts of aircraft and airfield deicers is a presumption that North American deicer manufacturers have eliminated, or made great strides toward removal of APE and triazoles from deicer formulations.
Opportunities in minimizing airport deicing stormwater footprint
As mentioned above, the airport deicing management faces many challenges in the regulation development, contaminant sources control, transport of deicing stormwater, environmental risks and etc. Airports urgently need resilient and affordable solutions to address the deicing stormwater issues. These issues essentially pose great opportunities for the academia and industry to improve the current application, collection, and treatment techniques and technologies of airport deicing stormwater management.
Deicer application minimization methods
Since the publication of
ACRP Report 14 in 2009, significant progress has been made in technologies, practices and products that allow for reduced application of aircraft and airfield deicers. By conventional practice, over 1,000 m
3 of ADFs may be used at a medium-size airport over a typical winter season [
22]. The US EPA estimated that 21 million gallons of ADFs/AAFs are discharged to surface waters per year in the United States [
10]. Vasilyeva [
23] identified the following deicing treatment options as current practices that may reduce the amount of ADFs needed:
• Hot water aircraft deicing – the use of 140°F water rinse of the aircraft, followed by the application of anti-icing fluid. Works best in mild winter conditions, and at temperatures above 27°F.
• Forced air aircraft deicing – uses a high-pressure air blast and can effectively remove dry, powdery snow. ADF can be added to the air stream as well, for removal of ice and wet snow. This method has been shown to aid in reducing aircraft cleaning time and reduce ADF usage. At this time these systems do not appear to be cost-effective in areas with icy conditions and wet-heavy snow.
• Hangar storage – hangar storage of aircraft when not in use prevents the accumulation of ice and snow on aircraft surfaces, precluding the need for deicing but may still require anti-icing of the aircraft surface.
Vasilyeva [
23] identified the following technologies which may be available in the future to aid in reducing the amount of deicers needed:
• Infrared (IR) deicing systems – uses natural gas or propane systems to create IR energy to melt frost, ice, and snow from the aircraft surface. This technology is currently in limited use.
• Tempered Steam Technology (TST) – uses air and “water vapor steam-infused air to melt” frost, snow, and ice on aircraft surface.
• Improved weather forecasts– improved weather forecasts, and timely and accurate updates can be used to limit the application of ADF to only when necessary.
• Technology under development – using warm fuel in the wing fuel tanks, pulse electro-thermal deicing (small pulses of electricity to break up the ice), and surface coatings that provide anti-icing protection for the aircraft surface (and thus eliminate the need for ADF applications).
D'Avirro and Chaput [
24] also identified in their
ACRP Report 45 (Optimizing the Use of Aircraft Deicing and Anti-icing Fluids) 18 promising aircraft deicing and anti-icing source reduction strategies or technologies. Then, they focused on three of the promising ones: Holdover time variance across an airfield; Increased use of spot deicing for aircraft frost removal; and increased use of aircraft deicing/anti-icing fluid dilutions.
Deicer collection methods
Recent years have also seen the identification of effective practices and technologies to collect deicing stormwater from airports and thus reduce the need for reactive treatments. The Transportation Association of Canada (TAC [
25]) requires Airport Operators and Service providers to prepare a
Glycol Management Plan (GMP), similar to a deicing management plan that is part of a Snow and Ice Control Plan in the US GMPs detail deicing operations and methods used to prevent environmental damage from deicing operations. Input into the GMP and its final approval are required by all parties including “airport operators, deicing service providers, air carriers using the airport, companies or individuals responsible for disposal of used deicing fluid.”
With the 2012 rulemaking by the EPA on ELG, commonly used technologies to collect sprayed ADF and their effectiveness were identified, including-
glycol collection vehicles which can remove 20% of the remaining ADF,
“plug and pump” technology that when combined with glycol collection vehicles can remove about 40% of the remaining ADF, and the use of
constructed deicing pads which allow for collection of about 60% of the remaining ADF [
4].
Alternative ADF/AAF and PDP agents
Alternative AAFs with reduced aquatic toxicity and BOD have also been developed in recent years (ACRP Web-Only Document 3, 2008 [
21]; ACRP Web-Only Document 8, 2010 [
26]) to replace conventional ADF/AAF products conforming to the aerospace material specifications 1424 and 1428E [
16]. Potassium acetate, sodium acetate, sodium formate, potassium formate or calcium magnesium acetate can be used as alternatives, which are biodegradable, have lower BOD and toxicity characteristics than glycol based solutions. Also the formates have been gradually replacing the acetates as they have proven to be more effective and environmentally friendly.
Deicer treatment methods
Developing on-site treatment facilities or sending to off-site treatment facilities becomes one of the feasible options for the airport as well [
7], since most publicly owned treatment works (POTWs) may not be able to handle the volumes of deicer stormwater. Some capable POTWs still may not be willing to treat such stormwater considering the high BOD. The treatment charges are also very high.
As a part of
ACRP Report 99, Guidance for Treatment of Airport Stormwater Containing Deicers, the following technologies were identified for treatment of airport stormwater contaminated with deicers [
27]:
On-Site Biologic Treatments
• In aerobic conditions- Activated Sludge, Aerated Gravel Beds, and Aerated Lagoons,
• In
anaerobic conditions- Anaerobic Fluidized Bed Reactors, which has been identified by the EPA [
4] as the technology with the greatest pollutant reduction prior to discharge.
• Aerated – Moving Bed Biofilm Reactors
• Aerobic – Passive Facultative Treatment Systems
On-Site Physical Treatments
• Evaporation – Distillation, Mechanical Vapor Recompression
• Membrane Filtration – Reverse Osmosis
Off-Site Biologic Treatment
• Public Wastewater Treatment Facilities (POTW)
Off-Site Physical Treatment
• Private Recycling Facilities
Some natural treatment systems, sustainable low-tech and low-cost ecotechnologies have also emerged as ideal tools to manage and treat storm runoff from airports. These systems are particularly suited to treat aircraft deicing/anti-icing fluids with high COD/BOD5 (between 1.5 to 3), such as vegetated wetlands and rock biofilters to remove nutrients and pathogens from stormwater. Airports face a number of unique challenges related to the application of natural treatment systems to treat deicer-laden stormwater. One issue is cold weather, which tends to depress biologic process and associated pollutant removal in natural treatment systems. Another issue is minimizing open water associated with treatment wetlands to reduce the threat of bird-aircraft strikes. Over the past 15 years, a number of natural treatment systems have been tested or installed at airports. Examples include a vertical subsurface flow treatment wetland installed at Toronto’s Pearson International Airport, a horizontal subsurface flow treatment wetland installed at Edmonton International Airport [
28], a horizontal subsurface flow treatment wetland installed at Westover Air Reserve Base in Massachusetts, and a hybrid surface-subsurface treatment wetland at London’s Heathrow Airport [
29]. To overcome limited removal kinetics in cold weather and high oxygen demand in stormwater with deicers, some researchers have proposed the use aerated wetlands, which were recently tested at Buffalo-Niagara International Airport [
30].
Green infrastructure at airports
Federal and state regulatory agencies are also promoting green infrastructure strategies as a promising approach to complying with their water regulations and requirements. Airports are increasingly exploring the use of green infrastructure strategies and tactics for addressing their stormwater challenges, as the appropriate integration of green infrastructure into new construction or planned improvement projects can result in significant cost savings and environmental benefits [
31]. Green infrastructure solutions (e.g., bioretention systems, rain gardens, vegetated filter strips, permeable asphalt or concrete pavement, drainage wells, and amended topsoil) aim to supplement or replace conventional gray infrastructure (e.g., impermeable pavements and curbs, inlets and pipes that inhibit water filtration or infiltration and quickly move the stormwater offsite). Local municipalities have increasingly implemented these innovative solutions and demonstrated their potential at greatly reducing the water contamination and quality impacts of stormwater [
32]. Green infrastructure techniques, technologies, and elements typically follow the principles of low impact development (LID) in design and project development [
33–
35], and they work by weaving the natural (e.g., bioinfiltration) or simulated natural processes (e.g., pervious concrete) into the built environment.
While green infrastructure presents great opportunities for airports in their stormwater management efforts, the implementation of green infrastructure must consider constraints related to safety and operations (e.g., standing water or risk of wildlife attraction, accessibility issues, and restrictions on some facilities in specified runway zones). To minimize the attraction of wildlife, stormwater facilities at airports are commonly designed to limit open water. For example, subsurface wetlands are favored over less complex surface-flow wetlands and infiltration systems are designed to minimize the duration of ponding.
As mentioned above, many new developments of regulatory drivers, practices, technologies, and products related to the management of deicer stormwater from airports, along with a growing body of airport user experience have been made in last decade. Nonetheless, the related information has not been synthesized into a national guidance document or distilled into a decision tool to facilitate the adoption of cost-effective strategies into airport deicing planning and stormwater management. Furthermore, experience and knowledge regarding the cost-effectiveness and benefits of various best practices may vary from airport to airport, often related to: the airport size and category (level of aircraft activity), local climate (e.g., precipitation intensity and frequency, deicing/anti-icing frequency and duration), characteristics of stormwater and receiving waters, local or regional regulatory requirements on stormwater discharge, storage and treatment capacity, site constraints, budget constraints, etc.
Development of guidebook and decision tool
To address the challenges mentioned herein, there is an urgent need to develop a concise and practical guidebook and a decision tool for airports across the nation to adopt specific practical stormwater management strategies while balancing their priorities in environmental, economic, and social values and operational constraints. The guidebook and decision tool would reflect “a growing understanding of the environmental impacts associated with deicing activities and the effectiveness of existing stormwater management practices.” They will also provide the latest information that airport practitioners need to better evaluate, select and implement the strategies and tactics for managing deicer stormwater.
The guidebook should include topics related to the context of the deicer stormwater issue (its regulatory drivers, environmental concerns, recent developments, and other dimensions), guidelines for developing integrated deicing-runoff management systems, guidelines for selecting individual practices (non-structural or structural BMPs), deicing factsheets, and appendices (including the decision tool). The Guidebook should also incorporate/address comments discussed and received during the agency/industry interviews and illustrate the selection, application, and evaluation of various cost-effective and sustainable deicer-stormwater management best practices.
The regulatory environment that applies to stormwater infrastructure projects and potentially affect their costs, benefits, and risks should be considered in Guidebook. Stormwater regulation has evolved over the past 40 years since passage of the Clean Water Act in 1972. Today, there are three main categories of regulated stormwater discharges: municipal, industrial, and construction. Many of the day-to-day operations of airports fall under the purview of several NPDES permits. For example, transportation facilities, identified under the Standard Industrial Classification (SIC) code 45, often conduct vehicle maintenance, equipment cleaning, or aircraft deicing operations. These activities meet requirements for one or more categories defined under industrial discharges. Areas of airports that might not be subject to the industrial program may be regulated by the municipal program such as parking lots, access roads, and commercial operations. Other activities might require permits addressing construction discharge. Navigation of the rules and requirements required by airports when addressing stormwater discharge can be difficult. Over the past few decades, stormwater regulation has shifted from volume control to prevent flood inundation in urban areas to a more focused collection of rules and policies that address the more diffuse nature of nonpoint runoff and associated pollutants that become entrained from source to sink.
Some emerging BMPs featuring the in situ treatment of stormwater at airport facilities should be incorporated in the Guidebook as well. For instance, LID, or what is often referred to as green stormwater infrastructure (GSI), attempts to maintain or replicate the predevelopment hydrologic regime through use of design techniques that create a functionally equivalent hydrologic landscape. Some LID practices include, but are not limited to, decreasing impervious surfaces through permeable pavements, creating micro-scale stormwater retention and detention areas such as rain gardens or bioretention cells, increasing flow paths, preserving highly permeable soils, incorporating vegetated swales and permeable paving into the building plan, and preventing soil compaction by discouraging the use of heavy equipment [
36,
37]. Such BMPs may or may not be suitable for the treatment of stormwater containing significant amounts of airport deicers.
A decision tool should also be developed to assist managers in their determinations, which will help them evaluate and select from among options and promote the use of sustainable, efficient management systems that reduces the potential environmental risks of deicer-laden stormwater from aircraft or airfield pavement snow/ice control operations. A number of these same solutions may be feasible for smaller airports, which face unique problems of their own.
As envisioned by authors, the tool would be using common off-the-shelf (COTS) software and allow an airport manager to select from a dropdown menu or data entry point the following:
1) their current level of annual or winter operational aircraft activity;
2) their historical or anticipated use of deicers;
3) typical aircraft size or ground area affected;
4) the historical or anticipated frequency and duration of winter storm and deicing events;
5) the characteristics and classification of receiving waters;
6) the stormwater characteristics for their airport (flow, frequency, volume, pollutant);
7) the type of NPDES permit type and discharge requirements;
8) budget parameters assigned to deicing operations;
9) available storage and treatment capacity;
10) description of the local climate conditions during winter; and
11) regulatory considerations.
The tool will increase functionality and use of all the material available in the guidebook by allowing airport managers to quickly find what applications and recommendations may be best suited to their airport –from small general aviation (GA) to larger air carrier airports. Upon selecting or entering the various inputs, the decision-tool would generate a list of practices and/or factsheets contained within the report that would be appropriate for consideration by the airport manager. The only factor not anticipated in the tool would be site constraints and considerations, as those would be difficult to quantify in the decision tool matrix considered. However, an airport manager could easily evaluate the recommended tool output options for whether they would be appropriate or not to their individual site particulars.
In recognition that deicing operations and their resultant stormwater runoff activity could involve different solutions, as applied to the location of activity, size and amount of runoff, and type of deicers used, separate worksheets within the tool could be developed for the different area(s) of the airport affected. For instance, separate worksheets would allow for addressing the deicing storage and delivery area, another for the terminal or cargo ramp area, another for the runway and taxiways, and still another for the terminal roadways, for instance. The tool could also make reference to applicable federal regulations, or allow for entry of state or local requirements.
The decision tool will help users select practices of deicer stormwater management based on their unique conditions and constraints. Some of the key considerations are discussed as follows.
1) Level of aircraft activity
The use of deicers depends on the aircraft operational levels, aircraft size, and types of deicing fluids being used. Aircraft wanting to depart during inclement weather would want to be deiced to remove frost, snow or ice build-up from the aircraft. Airlines operate schedules needed to retain their connections, therefore deicing aircraft quickly would enable aircraft to stay on time and on schedule. Business aircraft rely heavily on General Aviation airports to stay open to meet their business schedules.
The amount of deicers used on runways would increase or decrease depending on the weather conditions and amount of aircraft operations. As aircraft activity increases and weather conditions change, airports tend to use more deicers. Increasing the amount of deicer usage increases the opportunity for deicers to leach into the ground off the pavement edge. Runways have edge drains to prevent the water table from eroding the runway subsurface. Deicing fluids can seep into the ground and into the edge drains then out to the stormwater system. High ground water tables can be contaminated depending on the hydrological characteristics of some airport soils. The ongoing new industry technologies and practices with deicers and stormwater infrastructure that minimizes or removes the risk to the environment and to remain in compliance with federal and state laws should be evaluated.
2) Frequency and duration of deicing events
Not all airports in the snow-belt regions receive significant amounts of snowfall or icing conditions to warrant the use of deicers on the airport surfaces. The frequency and duration of applying these deicers depend on the number of operations, weather conditions, and chemicals being used.
3) NPDES permit type and discharge requirements
Standard Industrial Classification (SIC) code (Air Transportation; SIC Code 4581) is used to identify the regulated facility. Stormwater or Stormwater Discharge General Permits provide authorization to discharge under the National Pollutant Discharge Elimination System (NPDES) which may be issued from EPA or its designated agency. The stormwater sampling may occur monthly or quarterly dependent on the language in the permit and or state requirements. Since state discharge requirements may vary, this research would include an updateable database of each state. Local governments may develop and enforce ordinances that are more stringent than the federal and state laws. The project will provide a glossary of terms including individual industrial stormwater and general industrial stormwater.
4) Site constraints and considerations
The location of an airport operations area (AOA) and any deicing facilities in relationship to open ditches, inland and coastal areas, near lakes, streams, tributaries, and wetlands would dictate where and the amount of product that can be applied. Referral to the SWPPP would display the stormwater basin areas and direction of water flow to creeks, streams, and rivers or the ocean.
5) Budget constraints
Unpredictable snow and icing events are a big strain on all airports, particularly the medium size general aviation airports. When a snow or icing event occurs for the smaller general aviation airports, most of these airports close until the weather conditions would warrant the airport to reopen. Lack of funding, the number of snow events they receive over a year would stifle any chemical purchases and equipment to apply these chemicals. For an economically friendly solution, some of the airports use sand on the ice to provide better traction for aircraft, although airlines with aircraft engines low to the ground prefer sand not be used as it can be ingested by the engines and pit the engines blades.
The smaller the airport, the more constrained the budget. One of the methods a smaller airport could use instead of aircraft deicing products are hangaring aircraft for the night. Some of the smaller airports that use PG or EG-based deicing fluids can contact their municipal sanitary sewage facility for quantities of deicers to be deposited into the sanitary sewer for treatment at the sanitary facility. These facilities need to know when the fluids are going to be released and the amounts being released. There are limits to the amounts that can be released for treatment in any given day.
Budget constraints may not allow for the acquiring of Glycol recovery vehicles for the collection of aircraft deicing fluids remaining on the airport surface. Collecting the overspray of aircraft deicing fluids can be rather challenging for these airports. For airfield snow/ice control, Potassium Acetate is more expensive for the smaller airports on a tight budget. One large icing event for a small airport could use up the entire annual budget. Although not as environmentally friendly as Potassium Acetate, those airports would be more inclined to use a less expensive alternative such as glycol or urea. Smaller general aviation airports may not deice the AOA areas at all, in fact, the only deicing that may occur would be the Fixed Base Operators (FBO) deicing aircraft.
6) Storage and treatment capacity
All chemical storage should follow manufacturer’s storage and handling requirements as indicated on the manufactures material safety and data sheets (MSDS). Most solid deicers need to be stored under dry conditions, protected from precipitation and elevated from the floor on pallets for example. When aircraft deicers are used and mixed with other fluids such as rain, and melting snow and ice, the mixture would need to be treated and disposed of before releasing into the storm sewers. Glycol recovery vehicles or deicing pads can collect the majority of the fluids. Constructed wetlands can collect and treat glycol deicing fluids, deicing pads can collect and store deicing fluids and green glycol removal processing plants can treat the chemicals before releasing them into the stormwater systems.
7) Local climate
There are times when airports not located in the Snowbelt are inundated with snow fall and ice storms. In some geographical areas, the weather can change drastically. Some airports in the southern regions would receive enough snow/ice conditions that an airport snow and ice control plan would be beneficial.
A graphic mockup of the framework that drives current decision-making processes related to the development and implementation of a deicing runoff management strategy is outlined in ACRP Report 14 and replicated below (Fig. 4).
A decision tool proposed should have a similar framework but will embody updated and state-specific regulations, new and available technologies, new products, and new sustainable LID treatments. Information to update the framework will be based on published literature and interviews of airport managers that use the above framework. A revised graphic mockup of the decision-making process will be an essential aspect of developing the decision tool.
The tool should also be both in a graphical decision tree format and as a computer-based decision tree on CD-ROM or as an application. Operators or airports from general aviation to larger air carrier or air cargo can review or enter data about their airport size; their type and volume of current or anticipated deicing runoff generation; stormwater capacity; and other known factors that will be determined during the tool development. Following the decision tree or internal logic of the application program, an airport manager or environmental compliance officer will be able to ascertain applicable regulations (to include the opportunity to add state or local regulations or criteria); intervals for inspection; testing criteria and record-keeping requirements; any actions steps to be taken if gaps in compliance are determined, and other outcomes as deemed necessary.
To test the decision tool, a methodology, which is commonly used for shared decision making in the medical world allowing medical patients to make informed decisions based on the best available science, their values, and prognosis for recovery, should be adopted. Per this protocol [
38], a four-step protocol for decision tool development and testing is outlined. A decision tools are developed iteratively, and then it is critical that the information is easy to understand, unbiased, and fits current needs. A summary of the four steps is outline as follows:
Step 1: Understanding the decision – What does the data show in terms of risks and benefits associated with decision options. What is the context for tool development?
Step 2: Developing a first draft of the decision tool – Based on user inputs from the previous step develop a draft graphical mockup of the decision tool, and build a prototype of the spreadsheet based decision tool.
Step 3: Iteratively modify the tools based on end user input while ensuring that the tool is accurate, understandable, and balanced.
Step 4: Testing the tool in a real world setting – With this final step, the tool will be tested on a limited basis based on real world conditions. Results from this testing will be shared with end users to ensure that the tool conforms to end user needs.
The decision tool should also be tested by the stakeholders from the original focus groups, case examples, and supporting airport organizations (Such as AAAE committee airports). Individuals will be asked to test the tool given their particular airport. Comparisons will be made to their existing circumstances for congruence or betterment. A questionnaire about the decision tool will help to determine whether it provides the appropriate information, is functional, and is useful.
Concluding remarks
Airport stormwater may induce increased runoff volumes and changes to peak flow rates and timing, posing a risk for downstream receiving waters. Typical pollutants in airport stormwater include anti-icing and deicing chemicals and other additives along with other elements from unpainted metal surfaces. They may also include petroleum-related compounds, sediment, metals, and nutrients. The environmental impacts of these pollutants are mainly from their adverse effects on aquatic life, which include low dissolved oxygen, aquatic toxicity, bacterial growth, and odors in receiving water systems.
To manage the airport stormwater, airports are usually permitted under Industrial Permitting and must prepare a SWPPP that requires regular monitoring, site inspections and reporting. Additionally, airport drainage design as directed through the FAA Advisory Circular AC 150/5320-5D aims to safely and efficiently remove water from airport premises. Since the publication of
ACRP Report 14 in 2009, significant progress has been made in areas of policy and rulemaking. Airport deicing ELG, addressed by the EPA in 2012, requires NPDES permits to address issues associated with deicer-laden wastewater from airports and established new standards for wastewater discharged from airports that is associated with aircraft and pavement deicing [
3].
Since the publication of ACRP Report 14 in 2009, significant progress has been made in technologies, practices and products that allow for reduced application of aircraft and airfield deicers. Many new developments of regulatory drivers, practices, technologies, and products related to the management of deicer stormwater from airports, along with a growing body of airport user experience have been made.
ACRP Report 14, including the factsheets should be updated. Since its publication in 2009, a number of additional ACRP research reports and syntheses have been performed on a number of different aspects of airport and aircraft deicing management, stormwater management, sustainability, and environmental management systems, which could be considered in the update of ACRP Report 14. A number of other research studies or airport documents have been published as well, to include airports that have developed sustainability or environmental management system plans.
A concise and practical guidebook and a decision tool for airports across the nation should be developed to adopt specific practical stormwater management strategies while balancing their priorities in environmental, economic, and social values and operational constraints. The guidebook and decision tool will reflect “a growing understanding of the environmental impacts associated with deicing activities and the effectiveness of existing stormwater management practices.” They will also provide the latest information that airport practitioners need to better evaluate, select and implement the strategies and tactics for managing deicer stormwater. This is part of the scope by the ACRP 02-71 project which will “update ACRP Report 14 and develop a decision tool to help airport practitioners evaluate sustainable and effective practices related to deicer application and the collection and treatment of stormwater containing deicers.”
Higher Education Press and Springer-Verlag Berlin Heidelberg
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