Design of a Constructed Wetland for the Treatment of Glycol Contaminated Stormwater
The applicability of treatment wetlands for reducing the concentration of ethylene glycol is being considered at several airports, Including Cincinnati and Chicago in the United States, and is being pilot-tested at the Airborne Express airport in Wilmington, Ohio and Heathrow Airport in London, England. The types of treatment wetlands being considered or implemented at these sites are surface flow and subsurface flow systems.
The Greater Toronto Airports Authority (GTAA) is constructing the first full-scale vertical-flow treatment wetland in Canada for reduction of ethylene glycol in stormwater. A two-cell treatment wetland preceded by a sedimentation forebay is under construction. The wetland facility collects water dis-charged from terminal aprons, taxiways, and runways where aircraft receive de-icing compounds particularly glycol, The facility provides 24 to 48 h of retention to attenuate storm flows and provide removal of suspended sediment. Microbial metabolism of glycol within the wetland system is expected to reduce the BOD of discharged water. Cell No. 1 of the wetland is a subsurface vertical -flow system and Cell No.2 is a surface horizontal-flow system. Both treatment cells will be planted with Phragmites australis reeds. This emergent hydrophyte is able to withstand the periodic flooding conditions characteristic of a stormwater management system. In addition, the reeds provide oxygen transfer to the subsurface microbe population and maintain hydraulic conduc-tivity in the vertical-flow system. The reeds provide very little habitat benefit for gulls and waterfowl and will therefore not attract birds that may cause hazards for aircraft.
The level of treatment in the wetland system will be variable because of the nature of event flows. The water quality of effluent discharged from the facility is monitored automatically using an online TOC (total organic carbon) analyzer. The GTAA uses a previously established relationship between TOC and BOD (biological oxygen demand) to determine if the effluent quality is acceptable for discharge to the adjacent watercourse. If the TOC:BOD concentrations exceed the discharge criteria for the creek, effluent is diverted to a sanitary sewer. The mass glycol loading reduction using a vertical-flow treatment wetland, especially during low-flow and high concentration events, is expected to be significant and will contribute to the improvement in the water quality of Etobicoke Creek and provide a cost savings to the GTAA with respect to sewer use charges.
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