The water could then be captured in large underground tanks and be used for irrigation, cleaning and flushing the toilet, he says.

"We're trying to harvest a resource that we've not been able to tap into before," says Beecham.

Roads, driveways, pathways and the like make up 60 percent of impervious urban surfaces. And run-off from them causes flooding and pollutes our waterways.

?EUR??,,????'??Until now harvesting rainwater from them has proved more difficult than from roofs,?EUR??,,????'?? says Beecham.

His team is developing a system in which porous concrete pavers allows run-off to seep into underground tanks made of galvanized metal or a flexible plastic lining filled with gravel.

A special bonding material ensures the porous pavers are strong enough to withstand the heavy weight of cars and trucks. Additives mixed into the pavers, or into the sand and gravel bedding material beneath them, enables the system to trap pollutants.

A paver injected with ferrous hydroxide, for example, traps toxic and persistent heavy metals like lead, zinc and cadmium that come from sources such as car tyres, brake-linings and exhaust. A layer of microbes on fabric beneath the pavers can trap and degrade hydrocarbons such as oil. And a layer of granulated activated carbon traps dissolved organic matter from leaf litter that is responsible for algal blooms in rivers, says Beecham.

According to Beecham, the pollutants can accumulate in the pavers over 25 to 30 years, allowing usable water to be caught and pumped above ground for reuse. The pavers could also allow trees, which themselves soak up and recycle water, to grow more freely because their roots have access to more water and air.

Problem tree roots could be avoided by using a special concrete device that directs the roots away from the pavers, he says. The pavers could also be seeded with low maintenance native vegetation including sedges.

Beecham says one of his PhD students Baden Myers is about to construct a full-scale prototype of the complete water harvesting and reuse system, which he predicts will cost 10 to 30 percent more than conventional paving.