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Reap the harvest

Drainage - Effluent

It sounds like something from 'The Good Life', but harvesting rainwater can reduce flooding and save cash. Alan Sparks reports.

Over a third of domestic potable water is flushed down the toilet. Yet half of the UK's total water need falls as rainwater onto our roofs.

As the nation grapples simultaneously with growing water shortages and increased flood risk, this anomaly is starting to catch the government's attention.

Purified water is required for drinking, bathing, cooking and dishwashing. But rainwater harvesting can meet demands such as toilet flushing, clothes washing and watering the garden - free of charge.

Collecting rainwater is simple.

Traditional gutters and drains feed via a filtration system into a collection tank. Water can then either pumped to a header tank or directly to the point of use.

It is also low cost: The 'Eco-Vat' system produced by supplier Polypipe can typically be installed for around £10,000, and by reducing mains water consumption this initial outlay can be recouped through savings on metered water.

The greater the demand for nonpotable water, for example in buildings used for retail or commerce, the faster the return.

Payback can typically be achieved within five years.

Clients with large roof areas - for example on retail parks - are looking at the potential to speed return on outlay by selling excess runoff to smaller businesses nearby for non potable uses, says Polypipe Civils environmental product manager, Chris Brewster.

As a boost to the take-up of water harvesting, the government's recently launched planning policy guidance on development in flood plains, PPG25, recommends that sustainable drainage is used to reduce flood risk, and the newly drafted Part H of the building regulations demands that rainwater on all new developments be diverted away from main drains.

Given the legislative pressure and strong environmental case for sustainable drainage, and in view of water harvesting's simplicity and clear cost benefits, collection and recycling of rain water should have caught on. But it hasn't.

The reason lies in the hazard posed by the Legionnella bacteria which can develop in stagnant water, giving rise to the potentially fatal Legionnaires Disease.

Stagnant zones can form in the corners of rectangular tanks, and water harvesting schemes typically opt for circular tanks which minimise risk.

And to help promote rainwater harvesting systems, a British Board of Agrément certificate has been drawn up to assure clients that schemes are water tight, clean, and easy to install and maintain.

Polypipe Civils secured the first certificate and has since been joined by Klargester Environmental.

More are expected to follow.

INFOPLUS www. eco-vat.co.uk

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