Airport run-off treatment, Mayfield Farm Treatment Works

Challenge

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 airport's 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 20,000mg COD/L at 1°C.

The original treatment 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 170mg/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 2.4–7.7g/m3/d, removing 4–13g/m3/d of COD. This system was unable to treat current volumes of effluent and was failing to achieve compliance.

ARM conducted a full-scale trial comparing the existing reed bed design with a re-engineered reed bed and one fitted with Forced Bed Aeration™ (FBA™). The results indicated that there is enough wetland volume at Heathrow, but the limited oxygen transfer rate is limiting treatment performance.

Solution

ARM re-engineered the distribution and collection system and retrofitted FBA™ into the existing reedbeds. The open water zones were filled with gravel and the gravel depth increased by 500mm. The upgrade works at the Mayfield Farm included the addition / modification of:

• Primary Reservoir Aeration + nutrient dosing point
• Complete aerated mix zone + nutrient dosing
• Partial aerated mix zone
• Balancing pond aeration and nutrient dosing points
• Horizontal subsurface flow reedbeds with FBA™
• Variable speed transfers
• Primary treatment by pass facility (motorised shut-off valve)

The upgraded reedbeds now transfer up to 165g/m3/d of oxygen and are capable of treating 3500kg/BOD/day at 40l/s average flow.