Mayfield Farm Treatment Works, commissioned in 2001 was designed to treat run off from Heathrow Airport’s Southern Catchment. The run off is contaminated with glycols resulting from the airports de-icing operations. Treating glycol contaminated run off is a challenge at every airport faced with varying cold-climate winter conditions. The run off can contain over 20,000 mg COD/L at 1ºC. The original system consisted of balancing ponds, aeration lagoons, rafted reed beds and 12 horizontal subsurface flow reed beds covering 2.08 hectares. Designed to treat influent COD at 170 mg/l with a design flow of 40l/s, the wetland cells consisted of open water zones and gravel zones and were planted with Phragmites australis. The original system operated passively with oxygen transfer rate of 2.4 – 7.7 g/m3 /d, removing 4 – 13 g/m3 /d of COD. This system was unable to treat current volumes of effluent and was failing to achieve compliance