During winter months at Department of Defense (DoD) air bases, large amounts of aircraft deicing fluids (ADF) that contain primarily propylene glycol, ethylene glycol, and additives are used to ensure flight safety throughout certain adverse weather conditions. Due to its high biological oxygen demand (BOD), the runoff from these events must be directed either to stabilization ponds, the sanitary sewer, or an on-site treatment system, which may result in high treatment costs or possible overload conditions. This project validated that constructed wetlands are an efficient, cost-effective technology for the treatment of ADF runoff by installing and monitoring a 0.6-acre, pilot, subsurface-flow treatment wetland.
The subsurface-flow treatment wetland was constructed at Westover Air Reserve Base (ARB), Massachusetts, by excavation, installation of a liner, and provision of a porous medium, through which the wastewater flows, either vertically or horizontally. The upper layer is a finer medium, into which fast-growing, opportunistic species (e.g., Phragmites species) are planted to develop dense root systems. These root systems provide a complex microenvironment for a multitude of naturally occurring physical, chemical, and biological removal mechanisms to occur (i.e., sorption, retention and microbial degradation), attenuating the ADF contaminants. A subsurface flow design was selected because it provided insulation from cold temperatures, greater media area for microbial attachment, and no susceptibility to bird air-strike hazards.
Heavy snows during 2002-2003 at Westover ARB provided the opportunity to assess the treatment wetlands. A BOD removal rate (i.e., loading rate) of 220 kilograms per hectare per day was achieved during the first season of operation. Peak BOD input concentrations between 1,000 and 15,000 milligrams per liter (mg/L) were reduced by more than 50 percent to as low as 133 mg/L during 5 of the 10 deicing events. However, the pilot-constructed wetland was too small for the higher than anticipated frequency and severity of deicing events encountered during this first season. Improved performance is anticipated during the 2003-2004 deicing season, when the plant life will be more mature and acclimated to the incoming ADF runoff.
Constructed wetlands are a low-cost alternative for the sustainable treatment of ADF runoff. The nature of the technology is passive and requires very little operation and management effort, especially when compared to more active and resource-intensive alternative treatment technologies currently available off-the-shelf. The cost of a full-scale installation at Westover ARB was estimated at $800,000, with an operating cost of $4,000 per year. On an annualized basis, this would represent a one-third cost savings over alternative treatment options. (Project Completed - 2004)