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problems associated with the separation of the different fluids and from fuels. Reducing the volume of pollutants which require treatment is far preferable and a less costly option. In some cases, airports were able to recover 80% of deicing fluid but even that process leaves a residue which then needs to be treated.

WATER TREATMENT STRATEGIES Many airports store the effluent and send it for treatment at wastewater treatment works (WwTWs) or use soft areas as a soakaway. WwTWs have loading restrictions which means that during peak events, airports need to store the effluent before bleeding it back into the WwTW during low flows.

ADDRESSING THE ISSUE Heathrow Airport constructed its own WwTW at Mayfield Farm in 2000/2001 to treat and store surface water run-off from its southern catchment. It was designed based on 1990s’ data and comprised 36,000m3 of storage: a primary reservoir, a floating reed bed section, a balancing lagoon and 2.08 Ha of constructed wetlands, 0.5m deep. These passive

horizontal, sub-surface flow wetlands had a design loading of 590 Kg COD/day. The constructed wetland was no longer achieving optimal performance due to excessive treatment demands on the system caused by changes in deicer concentrations coupled with a marked increase in the use of deicer as a result of more severe winters. Since the primary chemicals used for deicing at Heathrow (propylene glycol and ethylene glycol) do not contain any nitrogen, phosphorus or potassium, the runoff is inherently weak in nutrients. Nutrients are required to grow the bacteria required for biological treatment of the deicing runoff. Without nutrients, treatment capacity is significantly limited. Having evaluated the efficiency of the treatment plant, Heathrow decided a step change was needed to dramatically enlarge Mayfield’s treatment capacity whilst using the existing infrastructure.

EVALUATING ITS OPTIONS The airport wished to optimise its existing treatment system so that it was able to treat heavier contaminant loads. Among others, they consulted ARM Group Ltd (ARM) –

specialists in natural wastewater treatment. ARM director Tori Sellers said: “Heathrow needed a system which would attenuate and treat the flow of storm water before discharging to the local watercourse. Physically, it was the right size for the volume of water but it was overloaded with deicer therefore low nutrient levels and microbial counts were compromising its treatment capacity so effluent was tankered off site. “We know that glycols degrade more quickly in aerobic conditions so we needed to get more oxygen in to the system. We also knew that we could retro-fit the existing infrastructure at a fraction of the cost of replacing the entire system and one that Heathrow would find easy to maintain themselves only needing us for support,” says Tori.

HIGHER TREATMENT CAPABILITIES IN A SMALLER FOOTPRINT ARM recommended installing artificial aeration into the wetlands using Forced Bed Aeration™ (FBA™) which increases the oxygen availability within the wetland. Heathrow’s system was configured in

The team behind Heathrow’s innovative Mayfield Treatment Works with FBA™ (Tori Sellers from ARM Group and Russell Knight Heathrow’s environmental operations manager in the centre).

12 / AF / January 2014

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