Treatment performance of an aerated constructed wetland treating glycol from de-icing operations at a UK airport

• Review of literature regarding using wetlands to treat glycol based de-icers from airports.
• Description of the wetland treatment system at Heathrow Airport which was converted from primarily anaerobic treatment to aerated treatment.
• Increasing the BOD treatment capacity of the HSSF wetlands without increasing the wetland area: the benefits of aerated wetlands and associated performance results presented.
• Results for the first 2 years of ‘aerated’ operation presented and inherent issues related with event-driven treatment systems and nutrient/biomass management discussed.

Mayfield Farm Treatment System receives effluent collected from the Southern Catchment of Heathrow Airport, a total of 290 ha. During winter operations, weather-related de-icing activity results in large shifts in the flow and concentrations of airport run-off. The biological treatment of spent de-icing fluids is challenging because it involves the treatment of a cold and nutrient-deficient wastewater of variable flow and strength. After a successful full scale trial to determine the efficacy of aerated wetlands, the entire treatment system was upgraded and optimized in 2010. It consisted of a primary reservoir, partial and complete mix zones, a balancing lagoon, and 12 aerated horizontal sub-surface flow (HSSF) treatment wetlands operating in parallel, with a total wetland area of 2.08 ha. The original non-aerated HSSF wetland system, commissioned in 2001 had a design cBOD5 loading rate of 374 kg/day at a flow rate of 40 l/s (Revitt et al., 2001 and Richter et al., 2003). The aerated HSSF wetlands now have a design cBOD5 loading rate of 2073 kg/day at a maximum flow rate of 80 l/s. Due to the weather conditions of the UK, the system is event-driven with highly variable flows and loads, with peak cBOD5 loadings of up to 12,188 kg/day.The entire treatment system is now designed to remove 3500 kg/day cBOD5 at a design flow of 40 l/s. It has a combined capacity of >44,000 m3 of wastewater with a retention time of approximately 21 days, depending on the pump flow rate which varies between 40 and 80 l/s. Nutrient dosing points have been positioned at strategic points throughout the whole system to prevent nutrient deficiency becoming a limiting factor in the development of a stable and viable mass of bacteria necessary to achieve the design criteria.