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Heathrow reed bed retrofit sees dramatic lift in performance

Heathrow needed to consider new ways to store and treat its stormwater. ARM Group came up with the innovative and natural Forced Bed Aeration. After successful trials, the full-scale impact is now being seen with a dramatic lift in its treatment capacity.

Eighty thousand reeds were planted in the retrofit of Heathrow’s Mayfield Treatment PlantEighty thousand reeds were planted in the retrofit of Heathrow’s Mayfield Treatment Plant

Avoiding airport closures and travel delays has fuelled a surge in the use of de-icing fluids, partly due to an increase in severe winter conditions and also due to the increased concentration of the de-icer used on aircraft. European and American airport operators have to review their winter operations plans to support flight safety and schedule continuity.

One of the busiest airports in the world, Heathrow remains operational in harsh weather because of de-icers. But de-icers contaminating wastewater − coupled with London’s unpredictable weather pattern − make it difficult to manage biomass that is the critical success factor in natural processes involved in storing and treating it to meet discharge consents.

In 2001, the Mayfield treatment plant was built to store and treat the run-off from the southern catchment of the airport. A constructed wetland, it comprised 36,000m3 of storage: a primary reservoir, a floating reed bed section, a balancing lagoon and 12 constructed wetlands, each 0.5m deep.

Mayfield was struggling to cope with the challenges of harsher winters while maintaining optimum output and discharge compliance. The solution called for a step change in treatment capacity.

“Heathrow wanted to optimise its existing treatment system which was designed to store and treat much lower contaminant loads based on data from the 1990s,” says Tori Sellers, of ARM Group, who upgraded the new system. “They needed a natural treatment system to attenuate and treat the flow of stormwater before discharging to the local watercourse.

“The original treatment wetlands were designed to remove de-icers containing glycols using anaerobic processes with a design loading rate of 590kg BOD/d,” says Sellers.

Trials in the winter of 2009-10 revealed the wetland system was dealing with an actual load of 1,900kg BOD/d – almost three times the original design load.

Sellers explains: “The system was the right size physically for the volume of water but increased use of de-icer over the past few years had overloaded it. Oxygen and nutrient availability were the limiting factors. This, coupled with the fact that the run-off is nutrient-poor, resulted in a low microbial count in the system which meant it was struggling to maintain treatment performance. That led to effluent having to be tankered off-site for treatment.

“Glycols and acetates degrade more quickly in oxygen-rich conditions and we knew we were able to optimise the treatment capabilities by retrofitting the existing infrastructure at a fraction of the cost of replacing the entire system.”

Heathrow saw how it could upgrade its existing wetland infrastructure, which its in-house team would maintain with support from ARM.

The answer, as ARM recommended, lay in Forced Bed Aeration (FBA), which would optimise the performance of the site, reduce OPEX and lifecycle costs, increase biodiversity and improve monitoring to ensure the system’s effectiveness.

The Heathrow team had seen the results from New York’s Buffalo Airport, a US$9M (£5.6M), full-scale, airport wastewater treatment system that showed impressive removal rates.

Its aerated wetland system was right by the runway but it had no risk of bird strike as it had been planted with grass and has no areas of open water.

Mayfield treatment plant’s proposed re-design would use all the existing infrastructure of the constructed wetland with a retrofit of an aeration system. The team’s target: to remove a minimum of 3,500kg of BOD (Biochemical Oxygen Demand) per day.

Sellers says: “Significant benefits were shown with the FBA facility installed over traditional passive wetland treatment systems regarding removal of BOD, which was over ten times more effective than the airport’s passive systems. The critical success factor is ensuring the proposed aerated systems are correctly tuned to optimise both energy consumption and oxygen transfer rates.

“Harsh winters also need to be catered for as they generate both higher volumes and concentrations of contaminated wastewater.”

BAA commissioned ARM to optimise the Mayfield Farm Treatment Facility in October 2010. Works involved all 12 beds at Mayfield Farm being retrofitted with FBA’s networks.

No new assets were built as the existing infrastructure of lagoons and constructed wetlands were re-used yielding a much higher return on investment and increase in environmental performance.
Heathrow has improved its removal rates by a factor of 14 since using FBA even against a backdrop of harsh winters.

“Any natural wastewater treatment system requires management of nutrient and bacteria levels to ensure there is enough biomass to treat the contaminants. At Heathrow’s Mayfield Farm Treatment Site, we’ve installed new online instrumentation to control both storage and the various treatment processes so that they work together as an integrated whole.”

Minor adjustments, with minimal time demanded from airfield operations staff, help cope with swinging fluctuations in climate and treatment demands.

By upgrading its existing constructed wetland to the FBA standard, Heathrow cut both its construction costs and is set to save on running costs in the long-term. The SCADA system makes it easy for Heathrow’s in-house team to remotely monitor and control the system’s performance. By planting 80,000 reeds, Heathrow is meeting its biodiversity action plan by maintaining habitats for birds and bats.

“By staying within the footprint of the original facility, the dramatic results we saw in the trials have come true in the full-scale project so we’ve created headroom to cater for future climate fluctuations,” says Sellers.

Heathrow’s environmental operations manager, Russell Knight, says: “This facility allows us to continue fulfilling our commitments to running Heathrow responsibly and efficiently. We’re delighted this upgrade has improved treatment performance by a factor of which is a huge step forward in getting the most out of this innovative solution.” 

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