Recycling: DEN's Sustainable Approach

Large deicing pads were planned and built at Denver International Airport (DEN) for quick and efficient aircraft deicing near the departure runway thresholds. The pads are the cornerstone of a cold-weather management system that minimizes aircraft deicing at the gates, allowing gates to be more quickly available for inbound aircraft and minimizing interference with other gate operations, such as the loading of bags and food.

By concentrating the area in which full aircraft deicing is performed, the pads capture a more concentrated mixture of the spent glycol-based deicing fluid, allowing the captured fluid to gravity-flow to an onsite recycling plant, and minimize the amount of gate-applied deicing fluid that cannot be recycled economically because the fluid has mixed with precipitation from a much larger collection area, thereby diluting the glycol concentration. Diluted fluid (less than 1 percent glycol) must be sent to an offsite wastewater treatment facility.

In 2009, the recycling operation generated a cost savings of $1.4 million to DEN and resultant airline fees over the fees for disposal at the municipal sewage treatment plant, the Metro Wastewater Reclamation District (Metro). The combination of facility planning and operational improvements has steadily increased the amount of glycol captured and treated.

During the 2009-2010 deicing season, 71 percent of aircraft deicing fluid sprayed was collected, and 72 percent of the collected aircraft deicing fluid was recycled — 11 percent higher than the Environmental Protection Agency’s (EPA) proposed new rules, issued in August 2009, for Effluent Limitation Guidelines and New Source Performance Standards for the Airport Deicing Category.

Current FAA regulations in Parts 121, 125, and 135 prohibit a takeoff when frost, ice, or snow is adhering to the wings, control surfaces, or propellers of an airplane. DEN has strict and comprehensive Part 190 Aircraft Deicing Regulations, which provide a description of the aircraft deicing system and regulations, procedures, etc. The regulations are coordinated with the airport’s Stormwater Management Plan and ISO 14001-certified Environmental Management System.

Recycling — A sustainable strategy

DEN has substantial infrastructure in place to protect stormwater quality and maintain the separation of clean stormwater runoff from deicing waste (DIW) stormwater runoff. The major structural stormwater pollution control measures at the airport include the DIW stormwater collection/retention system, the West Airfield Diversion System (WADS), dedicated aircraft de-icing pads and an aircraft deicing fluid (ADF) recycling plant, secondary containment structures, oil/water separators within the clean stormwater system, and contained material storage and handling areas.

The airport operates four primary de-icing pads west of concourses A, B, C, and J, and a fifth pad on the west, known as WA, all strategically located enroute to the departure thresholds. Five to six aircraft can be deiced concurrently at each pad, for a total of 28 positions at one time.

Inland Technologies International Limited of Nova Scotia, Canada, began providing turnkey glycol services at DEN in 2004 with a full-time, 12-person team. Inland specializes in the design, development, and operation of petroleum waste treatment and airport glycol recycling services.


Type I ADF (89 percent propylene glycol) and Type IV ADF (50 percent propylene glycol) are used for aircraft deicing. Type I ADF requires additives that improve the spray’s performance but can harm the environment. Glycol is biodegradable, but will generate high biochemical oxygen demand (BOD) during the decomposition process, which can severely deplete oxygen levels in waterways, choking off aquatic plants and animals.

Virgin propylene glycol and Type I ADF additive pack are delivered by truck and stored prior to blending. A 153,000-gallon tank is used for storage of virgin propylene glycol, and a 20,000-gallon tank is used for storage of the additive pack. Type I ADF is blended onsite using a 23,000-gallon tank at the glycol recycling facility. Once blended and certified to meet the manufacturer’s specifications, the concentrated Type I ADF is stored in eight 20,000-gallon supply tanks. The blended, undiluted Type I ADF is delivered unheated to pads A, J, and WA via an underground 8-inch supply line. The underground supply line feeds four Type I load stations on pad WA, six Type I load stations on pad J, and a 20,000-gallon tank located in a deicing operations building (the icehouse) on pad A. The heated Type I ADF can be pumped to fixed booms on pad A, or loaded into trucks at four load stations at the icehouse.

Type IV is delivered to the airport by the fluid supplier ready to use; it does not require blending and is not mixed with water.

Collection, recovery

Spent ADF mixes with stormwater runoff on the deicing pads, and together they flow into the pads’ perimeter trench drain collection systems, which then gravity-feed via underground piping to the recycling plant. Five holding tanks with a capacity of 420,000 gallons each and an 800,000-gallon storage tank dedicated to pad WA stores the collected ADF until a minimum amount has been collected to begin operations at the plant.

A two-stage reclamation process includes sending the collected fluids through Glycol Concentrators and a Distillation System. The Glycol concentrators, which are proprietary equipment owned by Inland, were added to the airport’s existing distillation system, allowing for the minimum glycol concentration acceptable for recycling to be reduced from 8 to 10 percent down to 1 percent. Fluid with less than 1 percent glycol is sent to Metro.

In the first stage of reclamation, the concentrators separate excess water from the collected glycol/stormwater mixture, resulting in a concentrated feed stock of up to 50 percent glycol for the distillation stage, where it is converted into greater than or equal to 99 percent glycol. The distilled effluent is then run through a demineralization/deionization unit, or polisher, to remove trace airfield contaminants, and the resulting product is sold for industrial applications, such as reuse in glycol deicing fluid, heat transfer fluid, and automotive coolant.

In an effort to maximize the collection of spent ADF, aircraft are required to be positioned on the deicing pads so that all ADF runoff will fall within the waste collection trench drains. As required by DEN’s Part 190 Aircraft Deicing Regulations, notice and authorization requirements prior to conducting any de-icing operation are critical to ensuring that the system is capturing spent ADF that can be recycled, which protects the surrounding environment.

The goal is to capture at least 69 percent of ADF applied, a goal that has been achieved for the last seven deicing seasons. In October 2010, FAA awarded DEN four grants, one for $1.4 million to increase the operational efficiency and flexibility of the deicing facility.

Public/private partnerships

Recycling waste products is a common business opportunity suitable for a public-private partnership. Two key contractual variables make a glycol facility a viable enterprise: the quality of the untreated glycol mixture before it’s recycled and the quantity guaranteed to be delivered to the recycling plant.

Quality means the amount of glycol in the waste stream compared to other substances. Quantity is equally important because the recycler must have enough raw material to make enough saleable glycol product. Accordingly, the public partner (in this case the airport or a credit-worthy consortium of airlines) agrees to provide influent of a specific, minimum quality and quantity, and if it doesn’t, it pays the recycler a scheduled amount of money.

If the airport supplies a lot more than the minimum amount of raw product, the airport pays nothing to the recycler. Likewise, if the recycler is able to make a lot more saleable product than is necessary to break even, the airport often gets a share of the profits. Moreover, the contracts often include a sliding payment scale, whereby the airport pays the recycler less as the influent volume (and quality) increases above the minimum and gets a larger share of the profits (or a fixed payment per gallon) as the recycler sells more product.

Certifications; cost analysis

Another item that comes into play is the ability to get certification to reuse the recycled product for deicing purposes.

Airports should include in their analysis the following costs and considerations: the cost to capture and reclaim the glycol the cost to get initial certification; the cost to recertify the fluid every two years; the reliability and accessibility of having the same recycled product that received certification; the quantity of recycled product available; finding customers willing to use recycled de-icing fluid; and finding customers willing to contract for a volume of fluid to make it all worthwhile.

It’s also important to evaluate how much the recycled product is worth in other applications.