CONSTRUCTED WETLAND CONTROL OF BOD LEVELS IN AIRPORT RUNOFF

A surface water treatment system consisting of an aeration reservoir and pond (holding capacities 45,000 and 19,000 m³) and a network of 12 horizontal subsurface flow gravel-filled constructed wetland cells of different sizes (total surface area 2.08 ha) and planted with Phragmites australis, was commissioned at Heathrow Airport, London, United Kingdom, in the winter of 2002. Ongoing monitoring of the treatment system has shown significant reductions in the biochemical oxygen demand (BOD₅) throughout the system with levels decreasing by up to 76.7% across the constructed wetland cells following high anti- and de-icing fluid applications. However, continued exposure to BOD₅ concentrations exceeding the design target has resulted in anaerobic conditions in the wetland. The addition of nutrients to the treatment system has resulted in improved removal efficiency for elevated BOD₅ loadings in the aerated reservoir from 25.5% to 47.5%, The addition of different nutrient dosing regimes to complementary pilot-scale planted and unplanted vertical flow columns showed average but statistically insignificant BOD₅ removal percentage increases from 61.9 ± 21.1% to 70.8 ± 26.5%, respectively, in planted columns over a 7-day period. There is an overall improvement in the performance of the system, but operational reviews are continuing.