Treatment Of Deicing Fluids

EPA has heightened its focus on commercial airports — a primer and a look at CAST


After treatment in the CAST system, the effluent is separated into high purity water, a high-quality concentrated solution of the process chemical, which can be reused or resold, and a slurry or solid waste product from the bottom of the still. Pre-treatment components include conventional precipitation and flocculants to precipitate dissolved phosphates and nitrates, as well as granular-activated carbon to remove dissolved organic acids.

In a vacuum, the boiling point of liquids is lowered, so the water evaporates at a much lower temperature, typically 100-140°F, and the heat required to generate the vapor is approximately 1040 Btu/LB of vapor. These low temperatures allow CAST systems to be manufactured with engineering plastics and simple technologies, such as hot water heaters, cooling towers, and pumps.

In contrast, high temperature distillation requires specialty materials and complicated components, such as Freon-based refrigeration systems, or high pressure steam boilers. Additionally, CAST systems can operate on low temperature heat sources, such as hot water heaters, low pressure steam, or waste heat supplies. Operating costs are thus lowered through savings on fuel, and maintenance is reduced through the use of simpler technologies.

The CAST system for propylene glycol, a common ADF, recovers 95 percent of the propylene glycol, meeting ASTM standards at a purity of 99.9 percent. This material can be sold to recover some of the cost of the system. The waste product, which contains five to six percent propylene glycol and dissolved salts, can be disposed of as non-hazardous waste or incinerated.

CAST systems have distinct advantages over alternative options, which include other distillation methods, biological treatment, or reverse osmosis. The reverse osmosis systems are more expensive to install and operate, and the propylene glycol recovered in this manner does not meet ASTM standards, and does not have a value for resale.

In comparison with other distillation techniques, CAST systems have a built-in advantage since they operate at lower pressures. Thus, the system can operate at 125°F instead of at 212°F, as required for other distillation systems. As a result, CAST systems can be constructed from less expensive materials, and require less energy to operate. Since there are no internal parts other than the atomizer and some mist eliminators, scaling is kept to a minimum and maintenance is reduced. Finally, the processing time is better performance, with more gallons of effluent treated per minute.

The advantages of the flash distillation over biological treatment include greater reliability, less variability and sensitivity to operating conditions, and the ability to recover the deicing fluids for reuse or resale. The biological treatment destroys the propylene glycol or ethylene glycol by using it as a carbon source (food) for the biomass.

CAST recovers the ADF for reuse or recycle, with an economic value (currently about $3.50 a gallon). This revenue can reduce the cost of meeting the rule. CAST systems have a smaller footprint than biological systems, have lower capital and operating costs, and produce less greenhouse gases. The biological systems are sensitive to low temperatures and do not work well in the winter, and therefore incur higher storage costs so they can process the PG in the warmer temperatures. CAST systems can operate effectively and reliably under a wide range of conditions, and are appropriate for cold weather operations.

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