Generally, the first flush involves the first couple of inches of rainfall and carries off 90 percent of the pollution load and will have the highest concentrations of contaminants. Additionally, the low-flow-rate runoff in stormwater collection systems which flows between storm events can sometimes be more polluted than the more dilute streams flowing later during and after storms. Therefore, to achieve optimum contaminant removal from airport runoff, a stormwater treatment system must be able to deal year-round with these two flows (low rate flows and first flushes) as well as with the cold weather contamination by surface and aircraft deicing/anti-icing chemicals.
Sub-surface flow wetlands are typically used where the wastewater being treated is noxious or odorous; where a higher degree of freeze protection is desired; where the attraction of wildlife (especially waterfowl) may be undesirable (e.g., at airports); and/or where ample, economic supplies of suitable substrate material are readily available.
These wetlands consist of submerged gravel beds constructed below ground level. To the untrained eye, they are difficult to discern from open fields. Technically, they are wetlands because wetland plants can grow in them, though there is no open water. Their water surfaces are typically 12 inches below their mulch and unsaturated gravel surfaces.
They can be operated either with the wastewater flowing horizontally through the bed or with the water percolating down vertically through the gravel. Bacteria attached to the gravel are responsible for pollutant removal. For high-strength deicing liquids, aeration of the bed is required to assist the bacteria in metabolizing the glycol.
Although there have been several smaller, pilot-scale wetland systems, there are currently only three large, existing SSF constructed wetlands now operating that treat glycol-contaminated stormwater at airports: at Edmonton International Airport in Alberta; at Heathrow International Airport in London; and, at Air Express Airport in Wilmington, OH. The first two are horizontal flow SSF wetlands, while the third is a reciprocating (tidal) flow, vertical SSF wetland. All three are associated with surge ponds in front of their multiple wetland basins (“cells”).
- The wetland at Edmonton treats stormwater contaminated with ethylene glycol; at Heathrow, a variety of glycol types; and, at Wilmington, propylene glycol.
- The Edmonton wetland operates only part of the year, being frozen in the coldest weather. The Heathrow wetland can operate year-round. The Wilmington wetland attempts to operate most of the year, but tends to impound water in the very coldest periods.
- The Edmonton wetland is vegetated with transplanted cattails; the Heathrow wetland is planted with reeds; and the Air Express wetland is not vegetated.
- At Edmonton and Wilmington, influent contaminated runoff flow rate is keyed to water temperatures with lower throughputs occurring when the water is colder. None of the wetlands is insulated.
- All three wetlands use gravel as their substrates: it is 2.3 feet thick at Edmonton, 2.1 feet thick at Heathrow; and, seven feet thick at Wilmington.
The horizontal sub-surface flow wetland at Edmonton, which commenced operation in 2001, consists of 12 square, gravel-filled cells with sides measuring 156 feet each, arranged in six trains of two cells each. Wetland gravel surface area is seven acres and its “footprint” is eleven acres. Design conditions for the wetland were for the treatment of stormwater runoff contaminated with up to 1,350 milligrams (mg) of ethylene glycol per litre at flows of up to 330,000 gallons per day (gpd). The wetland system there includes the wetland cells; a lift station; associated ponds and ditching; piping between existing and new ponds and the wetland; an outlet weir system; mandated continuous sampling facilities; and, fencing and diversion facilities that allow less contaminated water to bypass the wetland into a normally dry, very large stormwater detention pond.
Operating and maintenance costs for the wetland system are quite low, and it’s designed for unattended operation, except for periodic monitoring. The sub-surface flow wetland at Edmonton International Airport continues to operate successfully with minimal operator attention, meeting effluent targets.
A Solution: Engineered Wetlands
Nevertheless, there still are problems with the use of these ordinary constructed wetlands for treating glycol-contaminated stormwater runoff at airports. They all tend to be relatively large.