Parking lots, patios and other paved areas could one day collect rainwater, purify it then channel it to underground tanks for reuse, say researchers. Professor Simon Beecham, a civil engineer from the University of South Australia, says special porous pavers made of concrete containing specific additives would purify the polluted run-off. He says the water could then be captured in large underground tanks and be used for irrigation, cleaning and flushing the toilet."We're trying to harvest a resource that we've not been able to tap into before," Professor Beecham said. Roads, driveways, pathways and the like make up 60 per cent of impervious urban surfaces. The run-off from them causes flooding and pollutes waterways. Professor Beecham says until now, harvesting rainwater from them has proved more difficult than from roofs. His team is developing a system in which porous concrete pavers allows run-off to seep into underground tanks made of galvanised 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.
Professor Beecham says a layer of granulated activated carbon traps dissolved organic matter from leaf litter that is responsible for algal blooms in rivers. He says the pollutants can accumulate in the pavers over 25 to 30 years, allowing usable water to be caught and pumped above ground for reuse.
He says 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.
The pavers could be seeded with low maintenance native vegetation including sedges. Professor 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 per cent more than conventional paving.