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A Coventry University professor has designed an environmentally friendly system for tapping rainwater and recycling domestic water supplies which could cut average water bills by a third.

Chris Pratt, dean of the university's school of the built environment, has produced a storage container for the typical driveway by enveloping the hardcore beneath the special porous surfacing with an absorbant membrane. The collected water - totalling 2,000 litres, enough for two weeks - is then pumped through filters and ultra-violet sterilisers.

Professor Pratt is experimenting with microbes as another way of remediating the oil-polluted water which seeps through the surface of the driveway. He has calculated that a household of four people uses 560 litres of water per day. Using stored rainwater from the roof and driveway, and reusing washing machine water for use in toilet cisterns, a saving of 201 litres per day is possible. The system would not allow the reuse of water for drinking or washing, but would prove a boon for car washing, toilet flushers and for gardeners, who find that regular tap water often contains too much residual chlorine.

But the main beneficiary would be the environment. "It would mean that surface water would not be going into the rivers carrying pollutants which cause erosion and which downgrade the general environment," he said.

Installation costs around Pounds 1,000 but Professor Pratt says this would be offset quickly.