Protection of the natural environment is at the heart of the Heathrow constructed wetlands project.From design to operation, the reed bed filtration scheme has been geared to minimise environmental impact.
Local flora and fauna were threatened by the presence of glycol, which is used to deice the wings of planes, in surface water run-off.
Glycol is more inert than, for instance, salt, and is relatively harmless but it is slow to biodegrade. 'Glycol takes several weeks to break down naturally, ' explains Gerry Prickett, an engineer with BAA framework consultant TPS, 'and in the process it uses a lot of oxygen.' While it does not kill fish, it can damage smaller life forms and causes foaming.
Via two storm drain systems at Heathrow, glycol was reaching the River Crane and Princess Lake, both in an urban conservation area. Unchecked, this could have led to penalties from the Environment Agency.
In 1994 BAA began looking at options for containing the contaminated run-off.
Ecological consultant Penny Anderson suggested that reed bed treatment could be a costeffective solution. Reed beds had been used successfully to treat sewage. TPS confirmed construction and operation of a biological filtration system would offer long-term cost savings.
One treatment plant processes run-off from two airport catchments. It is located 3km south of the airport on land owned by BAA at Mayfield Farm.
Both run into existing holding lagoons which are fitted with biological oxygen demand detectors linked to valves controlling flow. When water oxygen levels are low, indicating high levels of glycol, flows are diverted to holding reservoirs.
Run-off from the southern catchment is conveyed via a relatively short new pipeline from its lagoon to a new 45,000m 3reservoir at the reed bed treatment area. But storing and transferring water from the lagoon serving the larger eastern catchment proved more challenging. It is located in an ecologically sensitive area, and there was not enough space to create a dirty water storage reservoir.
There was, however, a 100,000m 3capacity reservoir close to the collection lagoon.
TPS devised an innovative system for holding up to 43,000m 3ofglycol-laden water in the reservoir without contaminating the clean water alongside.
A sheet pile wall was installed with a 300m gap at mid-point, sealed by a curtain formed from a specially imported hydrocarbon-resistant butyl material. It is anchored at the base by timber piles and is held above surface level by floats.
'The curtain is folded back on itself, allowing it to 'balloon' outwards or inwards by some 60m, depending on changes in the clean and dirty water volumes, ' Prickett explains. A 20mm differential between the clean water and dirty water head is enough to push the curtain floats across the surface.
To transfer the dirty water to the Mayfield Farm plant the design team shares the large diameter ring main used by Heathrow's fire-fighters. Water can be injected into the ring and, using additional pumps, moved to a new section of pipe connected to the reed bed treatment site. Computer simulations linked with computerised controls on pump times monitor the location of the glycol-contaminated water in the ring.
Some glycol inevitably is dispersed around the system. Airport fire teams demanded research to demonstrate that glycol would not compromise fire-fighting capability.
At the discharge point, the dirty water is oxygenated by aerators in a holding pond before flowing into the reed beds. Biological filtration is first carried out as water passes through 'floating' beds - concrete channels containing reeds planted into special rafts of 150mm drainage pipe. Water passes directly over the roots, allowing bacteria in the nodules to break down the glycol.
Flows then pass to secondary gravel reed beds for 'polishing' before the water goes into the River Crane.
Sustainable construction has been a feature of the project. BAA framework contractor Laing used recycled gravel from one of the Eton school boating lakes and old brick material to fill gabions.
Environmental innovation has been complemented by landscaping and tree planting.