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Rainwater harvesting

Sustainable construction awards Commended

An unpromising environmental scheme was turned around by innovation

A massive 70% use of nonpotable water, replacing highly processed drinking water, has been a key element for BAA's new Terminal 5 at Heathrow.

The company has been determined to integrate sustainability into the project from design stage onwards, in water consumption, power use, local transport and architecture.

But a detailed look at the overall 'sustainable water strategy' in 1999 showed that initial cost and engineering assessments for one element, rainwater collection or 'harvesting', just did not pan out, says the water engineering team leader Ed McCann, principal engineer with consultant Binnie, Black & Veatch.

Just how his team turned this negative starting point into a positive outcome, by innovation and lateral thinking, was the content of his entry.

Terminal 5 is a big project and the water system alone will involve some £10M of engineering work as it is built over the next six years.

To reduce environmental impact and help control costs, BAA wanted the terminal and associated facilities to use as little drinking quality water as possible, delivering instead less processed water via a separate pipe network.

Drinking water has environmental, social and economic costs for the whole country because it demands space, energy and facilities for reservoirs, treatment works and distribution, as well as the daily financial and environmental costs of processing it. But for use in toilets, cleaning, and for plant irrigation, there is no reason to process water too much.

Most of the non-potable supply will come from boreholes in nearby chalk aquifers. Early designs for the system also involved gathering 36,000m 3of rainwater from the 6ha roof of the central terminal building. However piping, and an underground storage tank put capital outlay at £20.80/m 3/year on a so-called 'normalised' cost basis - simply not sustainable, 'especially as this kind of engineering expenditure reflects environmental cost as well', says McCann.

Borehole water has a capital expenditure of £1.40/m 3/year.

Limited processing and pumping costs, such as basic chlorination, are about the same for each supply, around £0.05p/m 3.But BAA was prepared to invest time and resources to explore this issue further, he says 'so we looked at going smaller'. Roof collection area was reduced 'and we had the idea of eliminating the concrete storage tank'. Heathrow T5 will have a lot of contained gravel underground and this can serve as a storage area, charged and discharged by pipe. But even this, collecting just 8,000m 3ofusable rain each year, would cost £18.8/m 3/year.

'You need higher input relative to the infrastructure, ' says McCann.

'Where could we get a lot of water?'

The answer seemed to be the airport itself. All the runways and flat areas have to drain off and the nearby north side looked ideal. It drains into a holding pond.

Why not use that? Quantities were large, several hundred thousand cubic metres annually, and storage already existed. Once the now unnecessary gravel storage was removed, it costed out well at just £2.30/m 3/year.

But the holding pond drains ultimately into a nearby catchment, the Wraysbury River. Taking water out could be seen to be depriving it of flow, at least in dry months, 'though we would not extract at those moments', points out McCann. Even though the scheme did not have any detrimental impact on the river system, this option was rejected, to mitigate any unforeseen social or environmental impact.

Attention turned to the southern airport catchment which is even larger, draining to a 39ha holding pond. Water running there will also be supplemented by the general drainage from the whole T5 project.

'Currently that either evaporates or drains out via the Perry Oaks sewage treatment system; after T5 is built it will be 'extra' run-off that no-one really wants.'

The bad news is that this holding area is some 4km from the airport;

building a pipeline and pumping it back would cost more.

Except that a pipeline already exists. 'Because of possible contamination from aircraft deicing, there is a special diversion pipeline running back through the main drainage tunnel, ' explains McCann. ' When glycol levels are too high a valve is tripped and outlet water is diverted through this back to a processing facility. Due to the minimal potential of glycol contamination, the diversion pipeline is actually usable for 98% of the year.

By reworking the valve system - 'a relatively cheap, energy and resource efficient process' says McCann - it can be pressed into service to bring back 'any amount' of clean water, he says, 'though in the event we estimate some 300,000m 3/year will be used.'

Bingo. Lots of usable water with almost no infrastructure outlay. 'Now it costs out at £0.50p/m 3/year for capital expenditure, less than borehole water.'

He says the team's willingness to look at the problem from all directions has produced the 'biggest rainwater harvest scheme in the UK', successfully helping to 'match the quality of supply with need' as BAA requested.

Project team BAABinnie Black & Veatch WSP Environmental

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