Substitution of environmentally preferred propylene-glycol-based deicing solutions for ethylene-glycol-based solutions may help mitigate environmental problems. Ethylene glycol, long the standard component of antifreeze and deicing products, is a highly soluble chemical that greatly increases the BOD of receiving waters, and it is also quite toxic to mammals at relatively low concentrations. Propylene glycol actually exerts a higher BOD, but it is non-toxic; in fact it is a common additive to cosmetics, medical products, and pet foods. Use of propylene glycol-based products represents the current best choice for aircraft deicing unless, of course, the mission can be delayed during inclement weather or heating systems (for example, infrared heaters) can be carefully deployed. The military specification covering aircraft deicing fluids is MIL-A-8243, which specifies two products: military type I deicers that are propylene glycol-based, and military type II deicers that are ethylene glycol-based (three parts ethylene glycol and one part propylene glycol). The military type I fluid has been the deicer of choice, but according to the San Antonio Air Logistics Center, the US Air Force is currently converting to the use of AMS fluids for aircraft anti-icing/deicing products, therefore, the Military Type I fluid is no longer the deicer of choice.

According to Air Force T.O. 42C-1-2, MIL-A-8243 Type I fluid is the only approved MIL-A-8243 fluid for new procurements to installations which have yet to convert to AMS fluids. As of 11/99, AMS fluids are still awaiting aircraft specific technical order authorization. However, MIL-A-8243 Type I should not continue to be purchased once the aircraft specific Technical Orders have included authorization and application procedures for the AMS 1424 Type I fluid. At that time, new procurements for a deicer fluid shall be for AMS 1424 Type I. Also, under current Air Force policy, only propylene glycol based-fluids shall be procured due to environmental concerns.

Other options are available for replacing ethylene glycol (and propylene glycol) in certain applications. Deicing products made from potassium, magnesium, sodium, or calcium magnesium acetates and urea pellets are better suited for airfield pavement application than the glycols, both of which are being phased out for direct pavement deicing. Isopropyl alcohol is also approved for airfield deicing, but is operationally limited, due to its high volatility that results in fumes that can be carried inside aircraft. The preferred replacement products for airfield surfaces are the acetates because they are non-toxic and almost entirely biodegradable. Potassium acetate is formulated in a 50% solution and applied as a liquid. Sodium acetate is a granulated product that is applied to pavement in the same fashion as urea pellets. Urea products are still used, but they are not the products of choice because of the ammonia nitrogen’s toxicity to aquatic life. To eliminate the toxicity, somewhat complicated nitrification/ denitrification treatment is required for wastewater containing ammonia-nitrogen-rich compounds like urea. In addition, urea products are not as versatile as acetate products, especially for low temperature situations, due to the relatively high temperature to which urea depresses the melting point of ice, at best about 15°F. Functionally, urea pellets are usually not applied when temperatures are below 25°F. Urea pellets are a USAF approved airfield deicer, but their use is discouraged.

The U.S. Air Force (USAF) has identified certain deicers that are approved for specific applications. USAF-approved aircraft deicers include propylene glycol-based fluid (type I) and ethylene glycol-based fluid (type II). However, according to the Headquarters Air Force Reserve Command, ethylene glycol is prohibited for new purchase and the only ethylene glycol that can be used are existing stores, which are diminishing quickly. Propylene glycol based SAE fluid (non-shear sensitive) can also be purchased, although the shear sensitive is still in the process of being approved. USAF-approved airfield deicers include isopropyl alcohol, potassium acetate, sodium acetate, and sodium formate. According to the San Antonio Air Logistics Center, USAF installations are required to provide snow and ice control per AFI 1045, May 98. For roadway pavements away from surfaces used by aircraft, sodium chloride and calcium chloride may be used.

Methods of controlling and/or capturing used deicers may include:

1. Blocking or closing storm drain sewers during dry weather;
2. Conducting deicing operations in areas where fluids can easily be retained;
3. Installing lined detention basins or underground storage tanks;
4. Using mechanical vacuum sweepers or similar devices to capture runoff;
5. Installing aircraft wash racks.

Some facilities contract with recyclers to recover and process glycol for reuse. Two recovery methods are commonly used. The diluted glycol fluids are either concentrated using evaporation or membrane separation to enable their reuse as antifreeze agents, or they are concentrated and then purified using fractional distillation. The concentrated glycol solutions can be used where quality considerations are not important; for example, to spray onto coal so that it will not freeze in a railcar when ready to unload. The purified glycol can be used for blending with virgin feedstock to make new antifreeze and other new glycol-based products. However, according to the Headquarters Air Force Reserve Command, where possible the AF tries to discharge the spent glycol to the wastewater treatment plant, as the least expensive alternative. Recycling is generally not feasible and portable to semi-portable small treatment units are not economical.

Because of the high BOD exerted by the glycols, they are not readily sewered either to surface water or to a wastewater treatment plant. Thus, if the spent deicing fluids can not be recycled easily or economically, they should at least be contained and collected to allow treatment and disposal. Some treatment options are available to handle spent deicing fluids; for example, biological degradation using a semi-continuous anaerobic process to convert glycols and low concentrations of any other organic contaminants, such as fuel, oil, or grease, into carbon dioxide and methane.

Users may also seek to minimize the use of deicing agents whenever possible. This may be accomplished by: (a) monitoring current and predicted weather conditions and adjusting flight schedules to avoid those times most prone to freezing conditions; (b) placing aircraft in hangars to avoid icing or positioning aircraft to take advantage of natural melting from sunlight or to minimize icing winds; or (c) using brooms, squeegees, ropes and forced air blowers to remove ice.

The compliance benefits listed here are only meant to be used as general guidelines and are not meant to be strictly interpreted. Actual compliance benefits will vary depending on the factors involved, e.g., the amount of workload involved.

Commercial S.A.E. type I (fluid with deicing attributes only, not shear sensitive) glycol deicing fluids are generally compatible with common materials of construction. Therefore, stainless steel, mild steel, lined steel, or reinforced plastics are acceptable. Piping and any ancillary equipment should also use only the same compatible materials. One incompatible material, galvanized steel, should never be used in any glycol service.

Commercial S.A.E. type II (shear sensitive fluid with anti-icing attributes) glycol fluids are also relatively compatible with common materials of construction, but have a slightly narrower range of material compatibility. Stainless steel or reinforced plastics are the preferred materials of construction for shipping or storage containers for commercial type II fluids by at least one vendor. However, mild steel can be used for commercial type II fluids, provided the container has a chemically resistant liner made from polyurethane or phenolic epoxy resins. Piping and any ancillary equipment should also use only compatible materials, such as stainless steel or plastic. S.A.E type II is currently being phased out by its replacement S.A.E Type IV.

Potassium acetate formulations should meet all the standards established under AMS 1432 for corrosion and materials compatibility. Its product formulation does contain some corrosion inhibitors which, when exposed to low alloy metal or steel surfaces, forms a barrier on the metal surface to prevent accelerated corrosion. To prevent depletion of the inhibitors, shipment and storage is recommended in plastic (polyethylene) or stainless steel containers. Shipment or storage in low alloy or steel containers can result in fluid degradation.

Granulated sodium acetate products should meet all the standards established under AMS 1431A. Granulated sodium acetate is typically supplied in polyethylene lined containers, but is also compatible with mild steel, as well as stainless steels, plastics, glass, and polyester-reinforced fiberglass.

When selecting an appropriate deicer, users should also consider the compatibility with the surface or substrate to which the deicer is to be applied. For example, sodium chloride and calcium chloride can damage aircraft and are also not approved for use on runways or taxiways.

• Reduces the amount of hazardous waste generated.
• Saves thousands of dollars in hazardous waste disposal costs annually.
• Eliminates hazardous material cleanup costs associated with spills, leaks or soil and groundwater contamination.

Note: Comparisons with and evaluations of anti-freeze recycling equipment for aircraft deicing fluid recycling should not be made. Deicing fluids are always more dilute than anti-freeze solutions and contain different types of contaminants. Deicing fluids come in contact with external surfaces of aircraft and are likely to contain various soils, but no glycol oxidation products. On the other hand, antifreeze can contain heavy metals, glycol oxidation products (primarily organic acids), and precipitated salts from prolonged use.

For the Air Force, USAF activities should refer to Technical Order (T.O.) 42C-1-2, Anti-icing, De-icing, and Defrosting of Parked Aircraft. In the case of potassium acetate, sodium acetate, or sodium formate, approval by the aircraft and subsystems single managers is required prior to use.