The Mayfield Farm Treatment Works, established in 2001, was designed to manage water runoff from Heathrow Airport. During colder weather conditions, Heathrow needs extensive de-icing to ensure that planes can take off and land safely; the runoff from this process contains large amounts of contaminants from the de-icer, which needs treatment before discharge. The challenge for airports is that runoff needs to be continually treated to reduce contaminant levels, and it needs to account for the colder weather that creates these conditions – sometimes as low as 1°C.

The original treatment system included balancing ponds, aeration lagoons, rafted reed beds, and 12 horizontal subsurface flow reed beds over 2.08 hectares. It was designed to treat inflows with 170 mg/l of COD at a flow rate of 40 l/s; the passive system originally built, however, was unable to treat at this rate (it could only remove 13g/m3/day of COD at its peak) and it was struggling to meet compliance as a result.

ARM compared the existing reedbed design with a modified version and a Forced Bed Aeration™ (FBA™) system – this was done to ensure an appropriately designed system, free of unnecessary modifications, and therefore expenses. The trial revealed that the size of the system was appropriate, but the limited oxygen transfer rate was why it wasn’t treating the runoff to standard.

ARM upgraded the distribution and collection system and retrofitted FBA™ into the existing reed beds. The open water zones were filled with gravel, and the gravel depth was increased by 500mm.

The upgrade at Mayfield Farm also included the following enhancements:

  • Primary reservoir aeration and nutrient dosing

  • Fully aerated mixing zones with nutrient dosing

  • Partially aerated mixing zones

  • Aeration and nutrient dosing in the balancing pond

  • Forced Bed Aeration™ enhanced horizontal subsurface flow reed beds

  • Variable speed transfer systems

  • A primary treatment bypass facility with a motorised shut-off valve

Thanks to these modifications, the system allows the reed beds to transfer up to 165 g/m3/day of oxygen and treat 3,500 kg of BOD per day at an average flow of 40 l/s.