Chemosphere 43 (2001) 1051±1062
Best management practices for airport deicing stormwater Michael S. Switzenbaum a
, Shawn Veltman b, Dean Mericas c, Bryan Wagoner c, Theodore Schoenberg a
Department of Civil and Environmental Engineering, 224 Marston Hall, University of Massachusetts, P.O. Box 35205, Amherst, MA 01003, USA b Olver Inc., 116 S. Main St, Blacksburg, VA 24060, USA c Limno-Tech Inc., 501 Avis Drive, Ann Arbor, MI 48108, USA Received 13 April 2000; accepted 1 June 2000
Abstract With the advent of new regulations concerning aircraft deicing and management of spent aircraft deicing ¯uids (ADFs), many airports now face the dual challenges of simultaneously maintaining public safety and protecting the environment. This paper provides a theoretical assessment of the potential environmental impact of stormwater runo and oers detailed current information on alternative deicing ¯uid application methods and materials, collection and treatment practices. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Aircraft deicing ¯uid; Airport stormwater; Non-point source pollution; Ethylene glycol; Propylene glycol
1. Introduction To ensure wintertime ¯ight safety, large quantities of propylene glycol (PG) and ethylene glycol (EG)-based products are used to deice and prevent ice formation on aircraft. On an average, it takes 2±4 m3 of aircraft deicing ¯uid (ADF) to deice a large commercial aircraft (USEPA, 1995). According to Betts (1999), a mediumsized airport may use over 1000 m3 of ¯uid over the entire winter season. In addition, urea and a variety of acetate and formate-based products are used to deice and anti-ice runways and taxiways. The majority of these compounds, however, exert signi®cant oxygen demands when introduced into natural waterways. For example, Veltman et al. (1998a) measured BOD5 as high
* Corresponding author. Tel.: +1-413-545-5393; fax: +1-413545-2202. E-mail address: [email protected]
as 245,000 mg/l on airport runo. There are toxicity concerns associated with certain glycols and urea and with deicing and anti-icing ¯uid additives as well. As a result, the control of deicing runo is now being mandated by both federal and state regulatory agencies (i.e., stormwater permitting) to protect both human health and the environment. With the advent of these regulations concerning aircraft deicing and management of spent ADFs, many airports now face the dual challenges of simultaneously maintaining public safety and protecting the environment. Recently, a workshop was held at the University of Massachusetts/Amherst on best management practices for airport deicing stormwater (Switzenbaum et al., 1999). This paper (1) provides a summary of the information presented at the workshop, (2) provides an assessment of the potential environmental impact of deicing stormwater, and (3) oers detailed current information on alternative deicing ¯uid application methods and material, collection and treatment practices.
0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 0 ) 0 0 1 9 9 - 5
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2. Current practices 2.1. Aircraft deicing The purpose of aircraft deicing is to remove ice and snow from control surfaces (wings, rudders and fuselages). Airplanes are designed based on the predictable eects of air¯ow over clean wings. The accumulation of ice, snow or frost on the wings disturbs this air¯ow and results in increased drag, loss of lift, increased stall speed, and may cause an abnormal pitch characteristic. Ice, snow or frost only as thick and rough as medium sandpaper can signi®cantly reduce aerodynamic performance (Valarezo et al., 1993). Ice and snow are typically removed from the aircraft using a heated mixture of Type I ADF and hot water applied under pressure. Undiluted Type I ¯uids must contain a minimum of 80% EG or PG by weight, with the balance composed of water, buers, wetting agents and corrosion inhibitors. Deicing chemicals used in aircraft deicing must also be non-corrosive to prevent damage to aircraft and could interfere with sensitive electronic systems. Type I ADF is applied at gate areas or at areas speci®cally designed to collect ADF runo (centralized deicing facilities). During intense snow and/ or freezing rain events at airports where at-gate deicing is practiced, aircraft may be deiced again at the end of a runway immediately prior to departure (secondary deicing). The pilot, to determine if deicing is required again, then uses guidelines based on weather, type of ¯uid and holdover time (the amount of time between application and take-o). 2.2. Aircraft anti-icing Aircraft anti-icing may follow deicing as a means to prevent the further accumulation of snow or ice on the deiced surfaces either while aircrafts are waiting for take-o during especially severe weather or during overnight parking. Anti-icing is accomplished by applying Type IV anti-icing ¯uid (AAF) to clean (i.e., icefree) aircraft surfaces. Type IV AAFs are also composed of EG or PG, along with thickeners that allow the ¯uid to cling to the aircraft and provide prolonged protection and longer holdover times. Anti-icing is conducted at the same locations as deicing, with the exception of secondary deicing locations where anti-icing is not typically conducted. 2.3. Pavement deicing Pavement deicing is generally the responsibility of the airport operator. The purpose of pavement deicing/antiicing is to break the bond holding ice and snow to the surfaces of runways and taxiways, facilitating mechanical ice and snow removal to maintain adequate friction
between aircraft tires and the runway. Residual deicing materials on the pavement provide anti-icing protection. There is a variety of Federal Aviation Administration (FAA)-approved pavement deicing materials (PDMs) available, in both liquid and solid forms. Liquid PDMs are primarily applied in anticipation of major deicing events while solid PDMs are primarily applied to existing ice and snow. Solid PDMs include sodium acetate, sodium formate and urea; liquid PDMs include potassium acetate (KAc) and potassium formate (KFor). 3. Opportunities for source reduction 3.1. Optimized ¯uid mixtures The FAA mixture requirements for ``clean aircraft'' are based on the dierence in temperature between the outside air temperature and the freeze point temperature of the deicing mixture. This is known as the ``buer''. For example, a typical 50/50 mixture of a standard Type I deicing ¯uid and water has a freeze point of )28°C and, therefore, can be used when the outside air temperature is as low as )18°C, allowing for the 10°C buer. Airlines typically use a 50/50 mixture of Type I ADF/ water for deicing purposes. However, the blend of undiluted ADF and water required to achieve the necessary buer is dependent on ambient temperature, with the ratio ranging between 60%ADF/40% water for temperatures below )18°C and 20%ADF/80% water for temperatures above approximately )4°C. Hot water alone can be eective at temperatures above )2°C. This fact allows signi®cantly more dilute ADF solutions to remain eective when used at airports located in regions where the temperature rarely falls below )7°C. At these airports, regular monitoring of air temperature allows the use of deicing mixtures with less than the typical 50% concentration of glycol; thereby reducing the overall amount of ADF applied. This technique provides for a direct reduction in the total amount of ADF used with minimal impact on airlines operations. In general mixtures are made and stored at predetermined ratios rather than being made as needed. 3.2. Hybrid-deicing systems Allied Signal has developed an innovative aircraft deicing system utilizing forced hot air in combination with a low-¯ow deicing ¯uid nozzle. The system is con®gured similar to a conventional deicing truck with an operator bucket mounted on a boom and dual tanks for Type I and Type II/IV deicing and anti-icing ¯uids. In addition, the unit has a turbine compressor that provides a high velocity air stream. The heart of the system is an applicator turret mounted on the bucket.
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The applicator head has a ring-shaped nozzle for the air stream and dual (0.57 and 1 l/s (9 and 16 gal per min)) nozzles in the center of the ring for deicing ¯uid (conventional deicing nozzles ¯ow at 2.8 l/s). Separate controls are provided for each of the nozzles. The applicator head has an operating range of about 3 m at the 0.57 l/s ¯ow and 4.5±6 m at 1 l/s. The system can be operated in either deicing or anti-icing modes (Allied Signal Corporation, Morris Town, NJ). Delta Air Lines has used the hybrid unit exclusively during deicing operations at General Mitchell International Airport (Milwaukee, WI) during the 1998±99 deicing season. Overall, the hybrid-deicing unit appears to have resulted in an approximate 80% reduction in deicing ¯uid usage. Reductions in biochemical oxygen demand (BOD) loads from glycol were even greater than this because variable ¯uid mixtures were used to suit ambient temperatures, rather than using a single mixture (e.g., 50:50 mix of glycol and water). 3.3. Other aircraft deicing methods There are several other methods being developed for aircraft deicing including infra-red heaters, resistive heating, computer gantry, deice boots and hot air blast deicing (USEPA, 1998, 1999).
4. Environmental consequences of deicing Approximately 75±80% of Type I ADF applied to an aircraft is deposited on deicing area pavement, either through overspray or drippage. In the absence of collection, the majority of this material ¯ows to apron stormwater systems. The remaining 15±20% of Type I ADF applied to an aircraft is sloughed during taxiing and take-o (Mericas and Wagoner, 1994). This material is dispersed and deposited on the air®eld, with some portion eventually reaching the air®eld stormwater system. The majority of Type IV ADF remains on the surface of the aircraft until rotation and take-o, at which point sloughing similar to that experienced by Type I ADF occurs. Consequently, a much larger fraction of Type IV ADF is dispersed on the air®eld than is the case with Type I ADF. Pavement deicing chemicals are applied to large impervious areas, and as a result, tend to be entrained in large quantities of stormwater and are present in lower concentrations than ADF. Due to environmental concerns, federal and state regulators are increasingly mandating the collection and treatment of deicing runo to ensure that the environmental impacts are controlled to the extent necessary to protect water quality.
4.1. Dissolved oxygen concerns Miller (1979) summarized the literature concerning the fate and persistence of glycols in the environment. According to Miller (1979), glycols are capable of being degraded by a variety of acclimated and unacclimated soil, water and sewage organisms. Complete degradation, depending on testing conditions, occurred within 3±20 days. Therefore glycols should neither persist in the environment nor bioaccumulate. The principal concern regarding the environmental impacts of deicing activities relates to oxygen consumed during the decomposition of deicing materials, principally glycol, urea, acetate and formate contained in runo. In some situations, the impact of nitrogen (due to the use of urea as a pavement deicer) may be signi®cant. The BOD5 concentrations associated with EG and PGs are both relatively high. Table 1 (data assembled from the ®les of Limno-Tech, 1999) displays a summary of available literature data concerning the biodegradability of undiluted glycols and glycol-based ADFs. As a point of comparison, 1 l of pure PG has the equivalent chemical oxygen demand (COD) to approximately 6000 l of domestic wastewater. PDMs also exert an oxygen demand. Table 1 presents the BOD5 strength of the commonly used pavement deicing agents. While urea has been the primary pavement deicing agent, other less polluting chemicals such as KAc and sodium formate are replacing it. In addition to oxygen demand, the release of urea (which is subsequently converted into ammonia) into the environment may result in toxicity to ®sh and stimulation of aquatic productivity. 4.2. Toxicity concerns There are toxicity concerns associated with the use of EG and with some of the additives in ADFs. A petition Table 1 Summary of reported biodegradability of various aircraft and PDMsa
PG (pure) EG (pure) Type I ADF ± PG based Type I ADF ± EG based Type II ADF ± EG based Type II ADF ± PG based Sodium acetate Sodium formate KAc KFor Urea
1,000,000 mg/l 400,000±800,000 mg/l 810,000 mg/l 870,000 mg/l 370,000±463,000 mg/l 335,000±380,000 mg/l 410 mg/g 230 mg/g 180,000 mg/l 40,000 mg/l 2100 mg/g
Source: Limno-Tech ®les.
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has been submitted to the Environmental Protection Agency (EPA) by the Natural Resources Defense Council, Defenders of Wildlife, National Audubon Society and the Humane Society of the US. The petition states that each year the 17 busiest airports in the US release a total of 26 million kg of EG in deicing operations (Federal Register, 1998). The petition requests that airports should be required to report their yearly chemical releases to the Toxic Release Inventory since EG creates ``signi®cant'' risks to human health and the environment. In addition, Ren et al. (1996) found that EG is estrogenic in rainbow trout. From an environmental perspective, the primary additives of concern are triazoles, a group of heterocylic compounds. Triazoles are widely used as yellow metal corrosion inhibitors (Rao et al., 1997). In terms of ADFs, benzotriazoles and tolytriazoles are commonly added as corrosion inhibitors. Cancilla et al. (1997) isolated these components from ADF and found that they had signi®cant toxicity when tested using the Microtoxâ toxicity test. Analysis of benzotriazole and mono-and dimethyl-substituted benzotriazoles showed increasing toxicity with increasing methylation. Cancilla et al. (1998) reported ®nding tolytrizaoles at environmentally signi®cant levels in the groundwater beneath a major North American airport. Tolytriazoles were found at concentrations approximately 25 times higher than the reported EC50 levels in the Microtoxâ assays. Pillard (1995) also evaluated the toxicity of ADF and found that the ADF formulations were signi®cantly more toxic to the water ¯ea, Ceriodaphnia dubia, and the fat head minnow, Pimephales promelas, than pure EG and PG. He suggested that the ADF additives were the cause for the increased toxicity. Similar results were reported by Fisher et al. (1995) following a study of acute toxicity in deicing runo from the Baltimore-Washington International Airport. There are very little data available on the degradability of aromatic heterocyclic compounds such as the triazole compounds (Pitter and Chudoba, 1990). Biodegradability will depend on the number of heteroatoms in the molecule and the type of substitutions, and therefore compounds with three nitrogen atoms in the molecule such as triazoles are the most dicult to degrade (Robinson and Callely, 1986; Pitter and Chudoba, 1990). 5. Collection and conveyance options With increasing regulatory emphasis on the control of runo from airport operations, there has been a signi®cant increase in the number of airports that have taken measures to capture spent deicing ¯uids. Two general deicing waste collection methods have been developed: (1) at-gate deicing, and (2) the central pad
concept. The use of at-gate vs. a central pad is not an ``either/or'' scenario. Many airports have moved towards systems that include elements of both. In addition, the use of glycol recovery vehicles (GRVs) may be used in conjunction with these two types of collection. 5.1. At-gate collection At-gate deicing is the preferred method of deicing for most airlines conducting non-hub operations, primarily because it allows the same personnel that load baggage to also deice aircraft. However, the collection of deicing waste from airports that practice at-gate deicing can be complicated and expensive. At-gate deicing leads to deicing activity over relatively large areas, resulting in collection of signi®cant quantities of precipitation with the deicing ¯uids. A variety of techniques are applied in controlling deicing runo associated with at-gate deicing. These generally involve hydraulic isolation of the deicing area and collection and storage of the deicing runo. Isolation may be accomplished by several means. Options include diversion of existing storm sewers during deicing periods, installation of new storm sewers, and inlets speci®cally designed for deicing runo; or installation of temporary blockages that trap the runo either on the surface of the ramp or in the storm sewer pipes for subsequent removal and disposal. Relatively simple pumps may be used to collect runo trapped in sumps or blocked pipes. Trench drains are often used as well. 5.2. Central deicing pads The central pad concept for deicing waste management is based on the concept of minimizing the volume of deicing waste by restricting deicing to very small areas. Pads are typically located near the gate areas or at the head of the runways to allow deicing to be completed just prior to take-o. While the central pad concept is preferable to the at-gate concept because it minimizes the size of the collection area and the incident rainfall that must be collected, it has not been universally accepted by the airlines because it interferes with scheduling. In addition, although the name implies a small collection area, central pads designed to accommodate more than one commercial aircraft generally encompass several acres. Central deicing facilities (CDFs) can provide a closed circuit where all the ADF is recovered. A deicing pad is specially graded and it captures and routes highly contaminated runo to storage ponds or tanks. Hancock International Airport in Syracuse has used an automated valve linked to a total organic carbon (TOC) analyzer to accomplish this separation. The pad should
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be located near the ends of departure runways to improve the safety of the deicing process. The collected contaminated runo by this process may be suitable for recycling. On-site recovery is discussed in more detail later in this manuscript. 5.3. Vacuum sweeper trucks Vacuum sweeper trucks may be used for removing ADF from the pavement of either gate areas or a central deicing pad. A limitation of vacuum sweeper trucks is that they add to the number of service vehicles in the gate areas where trac ¯ow is already congested. The cost of these trucks is from $200,000 to $400,000 each. Dierent models of vacuum sweeper trucks exist. The VQuip Sweeper picks up the majority of the glycol but leaves a residue on the pavement. The 1550 SRS scrubbers are another kind of vacuum trucks made by Tenant of Minneapolis, Minnesota. The cost of a sweeper and associated equipment is $400,000 (Cook, 1992). 5.4. Conveyance Existing stormwater systems can often be altered to convey or store deicing wastes. However, caution is advised when using existing stormwater systems because storm sewers are not generally constructed using watertight joints and glycol ex®ltration of groundwater may occur. One use of existing stormwater systems has been to block the pipes leaving a deicing application area, allowing deicing wastes to be stored in the existing piping system. After the deicing event has been completed, the waste can then be pumped out of the storm sewer for treatment or disposal. The system works for small collection areas, but pavement ¯ooding may occur during heavy precipitation in larger systems. Existing stormwater systems can also be used to convey deicing wastes directly to a sanitary sewer connection. At Toronto's Lester B. Pearson International Airport, manhole inserts are used to prohibit the contaminated runo from entering storm sewers. The inserts are manufactured in standard valve diameters of 6, 8 and 10 in. (15.24, 20.3 and 25.4 cm) and cost between $1200 and $1800 (USEPA, 1999). Inserts are pumped out periodically during deicing activities. This type of system relies upon pavement storage and it may not be practical during heavy precipitation due to the safety concerns associated with pavement ¯ooding. In addition, the use of such a system requires that the pavement joints be sealed to prevent in®ltration under the increased head. Maintenance is intensive requiring constant attention during deicing events.
5.5. Storage Storage is a required component of most deicing waste collection systems. Storage is used to hold concentrated deicing wastes destined for recycling and to hold diluted runo for treatment. Lined lagoons, glass fused to steel storage tanks and concrete tanks have all been used. In small applications, underground storage in large diameter pipes or buried steel tanks has also proven eective. Sizing storage systems for deicing waste management requires information on the area of collection, storm intensity and duration, and rate of treatment or discharge. Since storage may be used to equalize both ¯ow and mass loading, each factor should be considered in system design. 6. Treatment alternatives There are several alternatives for the treatment and disposal of deicing runo. These alternatives are divided into three primary categories: o-site, on-site and recovery options. The o-site category consists of alternatives where the airports discharge stormwater into a collection system or pay for trucking to a treatment/ disposal facility. The on-site category consists of aerobic and/or anaerobic treatment facilities that can be constructed at the airport for the pretreatment or treatment and disposal of the deicing runo. The recovery category utilizes ®ltration, reverse osmosis, and distillation to recover glycol from runo. 6.1. Aerobic processes Aerobic systems are widespread, typi®ed by most municipal and many industrial wastewater treatment plants. Aerobic systems are well known to environmental engineers and have been proven to be eective and robust treatment systems for numerous water pollution control eorts. Although quite eective in converting wastes to harmless end products, these aerobic systems in general (depending on the mode of operation) consume large amounts of energy (pumping of air or oxygen) and/or produce large volumes of residual biomass (sludge) that must be further treated before disposal. Since EG and PG are widely used for various purposes, the aerobic degradation metabolism of both is well understood (Pasternak, 1993). It is known that PG is metabolized in the liver by alcohol dehydrogenase to lactic acid, and then to pyruvic acid. Both of these metabolites are normal constituents of the citric acid cycle and are further metabolized to carbondioxide and water. Numerous studies have shown that both EG and PG are biodegradable under aerobic conditions (Fincher
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and Payne, 1962; Kaplan et al., 1982; McGahey and Bouwer,1992; Staples, 1996; Verschueren, 1996). In addition, Rusten et al. (1999) reported that monopropylene glycol runo from the Oslo airport was a feasible external carbon source for denitri®cation at a new wastewater treatment plant. Williams (1998) summarized the results of various standard tests and industrial experience concerning the biodegradability of EG and concluded that it can be eectively treated in biological wastewater treatment plants (e.g., activated sludge plants) as long as the receiving plant has the capacity to handle any increased organic loading. Other investigators have reported on the successful use of aerobic processes for both EG and PG (Nitschke et al., 1996; Gallagher, 1998; Saerman et al., 1998). However, several investigators have reported operational problems at publicly owned treatment works (POTWs) attributed to ADF-laden waste streams (Jank et al., 1974; Krumsick et al., 1994). Nitschke et al. (1996) found that wastewater from aircraft deicing containing diethylene glycol (DEG) can cause severe disturbances in sewage treatment plants, especially in cases of shock loadings and at low wastewater temperatures. It was stated that adaptation to DEG is needed along with the avoidance of shock loadings to avoid these disturbances. Based on BOD5 tests, it was found that PG is more degradable than DEG. Gallagher (1998) reported on lab and ®eld demonstrations of an aerobic pilot scale on-site treatment system. Using appropriate mixes of inocula and necessary nutrients, BOD concentrations were reduced to acceptable discharge levels in less than three days, even under winter conditions. The ®eld demonstration was conducted at the Dane County Regional Airport in Wisconsin and consisted of a preheater tank followed by a sequencing batch reactor (SBR). Saerman et al. (1998) evaluated the use of a batchloaded aerobic ¯uidized bed reactor for stormwater contaminated with EG. Good removals were observed for lab scale testing of batch ¯uidized bed reactors, but the ultimate initial BOD levels of the stormwater were rather dilute (at 240 mg/l). Aerobic pretreatment may also be carried out in storage lagoons by the addition of aeration equipment and provisions for nutrient augmentation. Examples include the Greater Rockford Airport (Rockford, IL) and the Duluth International Airport (Duluth, MN) (USEPA, 1999). However, the presence of large bodies of water at airports raises concerns in that such bodies attract wildlife that can interfere with ¯ight operations. 6.2. Anaerobic processes Anaerobic systems oer several advantages over aerobic systems that may be signi®cant for the proper
management of aircraft deicing wastewater at certain locations. In anaerobic treatment systems, microorganisms ferment the wastes to methane and carbon dioxide in the absence of oxygen. In general, less sludge is produced compared to aerobic systems. Anaerobic processes have a long history of use in wastewater engineering. The most common and earliest uses of anaerobic processes have been for on-site individual treatment systems (septic tanks) and for managing sludge at municipal treatment plants (sludge digestion). The development of high rate processes such as the anaerobic ®lter, the anaerobic expanded or anaerobic ¯uidized bed reactor (AFBR) and the up¯ow anaerobic sludge blanket (UASB) enabled greater opportunities for using anaerobic processes (Switzenbaum, 1995). With the development of high-rate treatment systems, anaerobic processes are being used for industrial wastewater treatment and, in some cases, municipal wastewater treatment (Speece, 1996). The number of full-scale anaerobic systems has greatly increased over the past 25 years. Totzke (1998) has estimated that over 1800 non-lagoon installation exist worldwide. In addition, the variety of wastewaters found to be amenable to anaerobic treatment has greatly increased. The advantages of anaerobic treatment over aerobic treatment have long been known (McCarty, 1964). These include lower sludge production, no need for oxygen, lower nutrient requirements, and production of methane gas as a potentially valuable byproduct. These advantages are particularly relevant for deicing wastewater treatment in that such wastewaters have a large oxygen demand and are usually nutrient-de®cient. Jewell (1987) noted that anaerobic biomass can remain dormant for long periods of time without any signi®cant loss in activity. This is a positive consideration for any seasonally generated waste streams, such as deicing wastewater. Disadvantages associated with anaerobic treatment processes include the slower growth rate of the methanogenic organisms, the perception that the systems are more prone to upset and dicult to operate, the production of hydrogen sul®de, and the management of methane if it cannot be economically used. Perhaps the most signi®cant obstacle is lack of experience with the process (Switzenbaum, 1995). While aerobic biodegradation of glycols has been demonstrated in numerous studies, the anaerobic biodegradation of glycols has not been widely studied. A limited number of earlier studies demonstrated that glycols are biodegradable under anaerobic conditions (Cox, 1978; Kaplan et al., 1982; Dwyer and Tiedje, 1983). Veltman et al. (1998b) conducted anaerobic biodegradability experiments on both EG and PG-based ADFs and demonstrated that both are completely biodegraded under anaerobic methanogenic conditions.
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Several anaerobic treatment lab and pilot scale studies have been conducted (Cummings, 1995; Mulligan et al. 1997; Darlington and Kennedy 1998). More recently, a pilot scale AFBR was successfully tested at the Albany International Airport with actual ADF-laden airport stormwater (Komisar et al. 1998; Veltman et al. 1998a). With a projected increase in the volume of glycol-contaminated stormwater as a result of airport expansion, the Albany International Airport decided to test anaerobic on-site pretreatment of collected stormwater as a means of lowering stormwater management costs. The project included design, construction, and operation of a 700 l AFBR at the Albany International Airport. The study was conducted over a 12 month period. Based on the results of this pilot study, a full-scale anaerobic ¯uidized bed system has been installed at the Albany International Airport. Another recently explored possibility is direct anaerobic discharge. Direct discharge is an o-site alternative to traditional discharge at the front end of a wastewater treatment facility. This concept has been tested at the Milwaukee airport. The idea is to truck concentrated deicing runo to plants for discharge to anaerobic digesters. This method of treatment oers several bene®ts including: · increased production of methane gas, which is used by POTWs as an energy source; · reduced sanitary treatment charges for the airport; · minimal risk of POTW upset. 6.3. O-site wastewater treatment plant O-site treatment of airport deicing stormwater is usually accomplished by discharging deicing runo through the sanitary sewer system to a wastewater treatment plant for treatment and disposal. Many airports currently discharge or are considering discharging spent deicing ¯uid to the local wastewater treatment plants through existing sewers for aerobic treatment and disposal including Chicago (IL) O'Hare International Airport, the Kansas City (MO) International Airport, the Des Moines (IA) International Airport and the Baltimore (MD) Washington International Airport (USEPA, 1999). In order to use o-site treatment of a local wastewater treatment plant, the plant must be able to handle extra hydraulic and organic loading resulting from the airport stormwater discharge. In addition, the plant must be able to process the extra sludge resulting from the treatment of the stormwater. As noted previously, deicing wastewater can have a very high organic content. If the wastewater treatment plant is not able to handle the increase in hydraulic and BOD loading, then on-site storage facilities such as lagoons or basins may be needed for ¯ow and mass equalization. On-site pre-
treatment may also be needed to lower the BOD mass loading. The local wastewater treatment plants may charge the airport or the airlines on a ¯ow volume or BOD mass basis. Aerobic treatment charges typically range between $0.55 and $0.77 per kg of BOD above a speci®ed threshold (275±350 mg/l) plus a base¯ow charge, approximately $0.264 per m3 . Other treatment plants charge a ¯at rate, regardless of concentration ranging from $16 to $238 per m3 . The costs to truck the runo for disposal range from $198 to $528 per m3 (Evans, 1995). A large airport can use from 662 to 5677 m3 of glycol per season (Mericas and Wagoner, 1994). The cost for this option can vary from $131,000 to $3 million per winter season. Therefore the feasibility of the o-site treatment alternative depends on the plant location, capacity and charges for treating high BOD waste. For airports using, collecting and discharging large quantities of ADF to a POTW, treatment costs can be signi®cant (USEPA, 1999). However, centralized treatment (where possible) will in general be less costly than most on-site methods. At present, on-site treatment costs are scarce and are therefore not presented for purposes of comparison. 6.4. On-site soil treatment system Land application of wastewater is a proven technology and has been used for numerous types of industrial wastewaters (Loehr et al., 1979). Land application of ADF stormwater may be a good alternative for numerous airports since airports often have signi®cant amounts of available land. Several studies have shown that glycols are readily degradable in soil (Haines and Alexander, 1975; McGahey and Bouwer, 1992; Klecka et al., 1993). Bausmith and Neufeld (1996) conducted soil pan studies to examine land treatment as a viable approach for managing ADF waste on-site. PG followed ®rst-order removal kinetics, with degradation achieved to practical quanti®cation levels (e.g., 10±17 mg PG/Kg soil (dry weight)). Biodegradation of PG was severely inhibited when 40% by weight solutions were applied to the soil, but good removals were observed in soils receiving ADF solutions of 5%, 10% and 20% (initial PG concentrations of 7016, 14032 and 28064 mg/kg). 6.5. On-site recovery A typical recovery process to reclaim the glycol in runo from aircraft deicing operations includes pretreatment to remove dirt and debris, nando®ltration to remove the high molecular weight additives, and distillation to increase the concentration of glycol in solution (Vanderlinden, 1998). Reverse osmosis may also be used to concentrate dilute streams for subsequent recovery
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and granular activated carbon is used for decolorization and trace compound removal. Typically, waste solutions must be at least 10% glycol to make recovery feasible (USEPA, 1995). Recovery of glycols is costly and may be applicable only at the largest airports (USEPA, 1994). EG and PG are prohibitively expensive to separate by distillation because of the closeness of their boiling points (USEPA, 1995). Since there is little market for a mixed glycol product, glycol recovery systems are employed at airports where a single glycol is used. Recovered glycols are reformulated and sold for use as drilling ¯uids, as automobile anti-freeze, as industrial or commercial coolants or as windshield washer solutions. The Society of Automotive Engineers (SAE) performance-based standards for ADF indirectly limit the on-site reuse of glycols since all recovered product must be returned to the glycol suppliers for performancebased recerti®cation (USEPA 1995). In general, the reuse of deicer ¯uid on aircraft is problematic due to quality control and the cost issues associated with storage and treatment (USEPA, 1998). Several companies are currently collecting and processing ADF runo to recover glycol. These companies include: the Environmental Quality Company (EQ), Inland Technologies, and A-R Plus. Generally, these companies prefer to process runo streams containing only PG-based ADF due to the higher resale value of reclaimed PG vs. EG. Collection and recycling of ADF runo oers several bene®ts including: · a potential revenue stream for the airport; · reduced loadings and costs associated with sanitary treatment. Reclamation of glycol from ADF runo is currently being successfully conducted at Denver International, Detroit Metropolitan, Minneapolis/St. Paul, Pittsburgh International, and General Mitchell International (Milwaukee, WI) airports, among others. 7. System evaluation criteria This section describes the primary issues that are currently driving decisions regarding the types of deicing runo collection and treatment systems being constructed in North America. 7.1. Central pad vs. at-gate The bene®ts and drawbacks of centralized deicing pads are principle issues that have been the focus of deicing ¯uid collection system design decisions at many airports. The primary bene®t associated with the use of CDFs is that a majority of intense aircraft deicing is contained in relatively small area. This allows much more concentrated runo to be collected than is typi-
cally possible using at-gate collection systems which, in turn, allows recycling to be a more viable option. In addition, other treatment methods such as o-site transport to WWTPs are more economical at higher concentrations. An additional bene®t aorded by CDFs is the ability to utilize gates at a higher frequency than is possible using at-gate deicing. This is particularly bene®cial at hub airports where the predominant airline typically operates on a very tight schedule that requires gates to be open when banks of incoming aircraft arrive. Centralized deicing allows aircraft to be moved from gate areas, freeing the gate for arriving aircraft. At-gate aircraft deicing usually delays departures and can result in systematic slowdowns. Slowdowns may also result in multiple applications of deicing ¯uids to meet holdover time requirements. Finally, CDFs facilitate the use of Type I ADF and reduce the use of Type IV ¯uid due to the fact that CDFs are usually located directly adjacent to departure ends of runways and aircraft can easily make holdover time limits. Because a large portion of Type IV ¯uid is deposited along runway areas at take-o, stormwater generated in these areas can be adversely impacted by this sloughed ¯uid. If runo generated at a CDF can be eectively removed from the airport, the reduction in Type IV use can improve the overall quality of runo leaving an airport. There are several drawbacks to using CDFs. During inclement weather, undersized CDFs can become congested with aircraft waiting to be deiced. This in turn can cause congestion at gates and on taxiways. This is a particularly problematic concern for cargo carriers such as FedEx that must refund delivery charges for cargo not delivered on time. Smaller carriers have also expressed concerns that they will not receive equal priority during snowstorms and will experience greater delays than larger carriers. A related concern regarding CDF use involves who applies ADF at the pad. Typically, airlines prefer to use their own equipment and employees to deice their aircraft and often have their own operation procedures regarding deicing operations. If several airlines use one pad, this would require that each airline has trucks, operators and ADF storage positioned at the pad, which could cause congestion. The principal bene®t of at-gate deicing is that airline employees who conduct deicing can conduct other tasks such as baggage handling and aircraft departure duties. This is particularly important for airlines whose operations are limited at an airport. A secondary bene®t is that at-gate deicing frequently allows components of existing collection system infrastructure to be incorporated into a new collection system. This can reduce the costs associated with designing and constructing the system.
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Finally, at-gate deicing allows for morning defrosting and cold soak deicing to be conducted at the gate, reducing departure delays. The primary drawback to at-gate deicing is the fact that in general much more dilute runo is collected than is the case with CDF deicing. This reduces the feasibility of ADF recycling. If recycling is necessary to reduce loads to treatment plants, a GRV may be necessary as well. 7.2. Regulatory issues The EPA is currently conducting a study to determine if the regulation of deicing ¯uid runo is warranted on a nationwide basis. This study is being conducted as a result of a lawsuit ®led against EPA by the National Resources Defense Council (NRDC) (Federal Register, 1998). A draft version of this study was published on 5 October 1999 (USEPA, 1999). The ®nal version has not yet been published. EPA sta have been forthright in stating that conducting the study is not a ``guarantee'' that limits will be developed. Potential components of nationwide regulations include: · minimum collection and treatment standards that all airports must achieve; · recommended collection and treatment alternatives; · thresholds for imposition of discharge limits; · toxicity limitations. Due to the possibility of nationwide standards, many airport managers are currently reluctant to commit to capital-intensive deicing runo collection and/or treatment systems. 7.3. Toxicity Historically, the principal concern associated with deicing runo has been dissolved oxygen impacts due to elevated BOD concentrations. However, during the last several years, acute and chronic toxicity impacts have also been documented at several large airports. It is believed that materials added to PG or EG to prevent corrosion, improve ¯uid viscosity, and promote shear from aircraft during take-o are signi®cantly more toxic than either glycol. The possibility of toxicity limitations is an additional concern for airports facing the issuance of a national pollutant discharge elimination system (NPDES) permit or renewal.
7.4.1. Alternate deicing agents As noted previously, KFor is a liquid PDM that has recently been introduced in the US. This material exhibits approximately 25% of the BOD exhibited by KAc. At airports where signi®cant quantities of PDM are used, use of this material would translate directly to lower BOD concentrations in stormwater generated in runway and taxiway areas. While KFor is currently more expensive than KAc, the additional cost is negligible compared to savings due to lower BOD loadings. In addition to alternate PDMs, several researchers are currently working toward developing ADFs that either exhibit lower BODs or degrade at an accelerated rate. None of these alternate ADFs has yet been approved by the FAA, however, if approval is obtained, the potential impact that these ¯uids could have on the design of collection and treatment systems is enormous. 7.4.2. Alternate application systems As stated earlier, several companies are currently developing hybrid-deicing systems that entrain low ¯ow/ high pressure ADF in a stream of compressed air. These systems have been successfully tested and are expected to be integrated into the deicing programs of several airlines as early as the 1999/2000 deicing season. The ADF savings associated with the use of this type of equipment is very signi®cant. The airlines have a direct incentive to invest in such equipment because of the savings in ADF that result. 7.4.3. Upset provisions A component of cost reduction that is not always evaluated when considering deicing runo collection and treatment system design is the expected number of permit violations that will be incurred under various system designs. For instance, if an agreement can be reached with regulators whereby collection and treatment will only have to be sucient to handle BOD loads generated during a speci®ed design storm, (e.g., 15 cm of snow or 1.3 cm freezing rain) the size of the system will be greatly reduced compared to a system designed to handle the ``storm of the century''. Careful crafting of upset provisions that acknowledge the fact that deicing is a weather-based phenomenon can have a signi®cant impact on the types of collection and treatment systems designed and constructed at airports.
7.4. Alternate materials and application methods
8. Summary and conclusions
Innovative ADF application methods and alternate materials that exhibit reduced BOD concentrations relative to the materials they replace may have a profound impact on the type of collection system necessary to meet regulatory requirements. Several of these methods and materials are discussed in this section.
To cope with dual constraints of passenger safety and environmental protection, new strategies have been developed for management of airport stormwater. This paper has reviewed many of the signi®cant developments over the recent past associated with the management of airport stormwater.
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The following bullets highlight signi®cant issues associated with the current state of deicing system design in the US: · Central deicing pads provide better collection eciencies than most at-gate types of deicing runo collection and require minimal associated labor. However, airlines operating non-hub operations and cargo airlines are generally unwilling to support pad construction due to concerns over delays. · Central wastewater treatment plants are almost always the most economical method of treating deicing chemical runo, if sucient biological loading capacity is available. Direct anaerobic discharge or anaerobic on-site pretreatment may be economical in capacity-limited situations. · Hybrid-deicing equipment and alternate PDMs oer signi®cant near-term BOD reductions that will almost assuredly translate to lower management costs. · Careful crafting of permit upset provisions can significantly reduce the cost of management systems. · The EPA is currently conducting a study to determine if the regulation of deicing ¯uid runo is warranted on a nationwide basis. Preliminary results of this study have been published (USEPA, 1999) but the ®nal version has not been published at the time this manuscript is being prepared. For airports currently considering large capital investments in collection and treatment systems, it is probably prudent to ``wait and see'' what conclusions are reached in the ongoing EPA study.
Acknowledgements The work on which this paper is based was ®nanced in part by the US Department of the Interior, Geological Survey, through the Massachusetts Water Resources Research Center. Contents of the publication do not necessarily re¯ect the views of the US Department of the Interior, nor does mention of trade names or commercial products constitute their endorsement by the US Government.
Appendix A. List of acronyms AAF ADF AFBR BOD CDFs CDP COD
aircraft anti-icing ¯uid aircraft deicing ¯uid anaerobic ¯uidized bed reactor biochemical oxygen demand central deicing facilities central deicing pad chemical oxygen demand
DEG EG EPA FAA Gpm KAc KFor NPDES NRDC PG PDM POTWs SAE SBR TOC UASB
diethylene glycol ethylene glycol environmental protection agency federal aviation administration gal per min potassium acetate potassium formate national pollutant discharge elimination system national resources defense council propylene glycol pavement deicing materials publicly owned treatment works society of automotive engineers sequencing batch reactor total organic carbon up¯ow anaerobic sludge blanket
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