Although it continues to run well, the SSF wetland at Edmonton only operates outside of frozen conditions and must impound water/collect contaminated snow in winter. The horizontal SSF wetland at Heathrow has been experiencing plugging problems in the shallow gravel of its primary cells. During very cold weather, cold air drawn into the beds of the wetland cells at Wilmington freezes the bacteria used for contaminant removal and water must be impounded during these periods.
What was needed was a wetlands technology that could overcome such limitations. The answer lies with engineered wetlands.
Engineered wetlands are new types of semi-passive constructed wetlands designed so that operating and process conditions can be modified, manipulated, and/or controlled, in contrast to the more passive operation of ordinary constructed wetlands. With engineered wetlands, higher levels of contaminant removals are possible at higher throughputs and with much shorter residence times. Constructed wetland systems can be “engineered” in many ways to greatly improve performance.
An aerated vertical SSF engineered wetland is one kind in which air (supplied by blowers) is introduced under thicker gravel substrate (4–12 feet thick). Aeration air flows up through the gravel from a buried fine bubble diffusion system, countercurrent to downward percolating wastewater. The vegetated gravel surfaces of engineered wetlands are insulated with layers of mulch or compost to prevent freezing problems, and the systems are designed to operate throughout northern winters — whatever the ambient air temperatures.
The aerated sub-surface flow engineered wetlands technology is demonstrated and proven. Dozens of smaller wetlands of this sort are already in operation for sewage treatment in northern U.S. areas, and have operated successfully during summer and winter for many years. As well, several very large systems for flow rates of up to 4.5 mgd and contaminant concentrations of up to several thousand mg/L now have been designed and are under construction for treating a variety of wastewaters.
The Buffalo Example
A large new aerated vertical sub-surface flow engineered wetland system is now in the final stages of design at Buffalo Niagara International Airport. Previously, concentrate from around the deicing pads had been collected for disposal at a local wastewater treatment plant. This facility has given notice that it wishes the airport to find an alternative disposal option. Also, the large fraction of the glycol that still ends up in the airport’s stormwater sewers can exceed the airport’s NPDES permit, which mandates a 30 mg BOD5/L limit.
The airport’s owner, the Niagara Frontier Transportation Authority, decided to proceed with the design and engineering of a sub-surface flow engineered wetland to treat both the concentrate (the spent deicing fluids from the deicing pads) and stormwater together prior to discharge. The objective of the project is to ensure that no water leaving the airport exceeds regulatory requirements.
The project already has involved the successful off-site treatability testing of propylene glycol-spiked stormwater from the airport at high (70ºF) and low (40ºF) design basis temperatures at off-site pilot-scale wetland test facilities to determine kinetics and other scale-up parameters for subsequent full-scale facilities. The results of the pilot-scale testing determined that a 12-acre aerated vertical SSF engineered wetland would do the job. It demonstrated very good treatment (96-97 percent removal of target pollutants) at both the high and low design basis temperatures.
The new engineered wetland for BNIA will treat glycol-contaminated stormwater runoff and other wastewaters during the deicing season, and stormwater sewers’ base flow and rainfall event first flush runoff year round. The wetland will consist of ten earthern-bermed, rectilinear wetland cells excavated from an existing open area near the airport’s main runway. At ground level, only a field of wetland grasses will be visible, growing from a “dry” mulch surface.
Design is based on treating up to 0.2 mgd of concentrate from the deicing pads generated by up to 300,000 gallons of pure propylene glycol use annually during the average 190-day deicing season, and an average stormwater flow rate of 1 mgd year-round. The design allows for the treatment of a mix containing up to 2,150 mg BOD/L during the deicing season and uses existing underground glycol and stormwater tanks for flow equalization.
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