Between 50 and 80 percent of global household water goes down the drain with plenty of useful life left.
So-called "graywater" that drains away after clothes-washing, bathing or showering could be reused to flush toilets or to water outdoor landscaping, putting an increasingly precious resource to second use, say experts.
"It's a waste to use high quality potable water for things like flushing our toilets or watering our flowers," said Meena Palaniappan of the Pacific Institute in Oakland California, who authored a recent report on graywater reuse.
"We're in a world that is facing more constraints on water availability, with climate change impacts on precipitation," she added. "We need to look at all options available to us to deal with these real limits."
The most basic graywater reuse does not store the water, using it immediately for flushing or irrigation. A common setup pipes water directly from a washing machine to an outdoor system to water fruit trees or non-edible landscaping, Palaniappan said.
More complicated systems store and treat water with filters, sedimentation tanks or other equipment. Water treatment is necessary for systems that store water because even the small amounts of soap, dirt or other organic materials can cause the water to smell as microorganisms begin consume the organic matter.
"The simple systems work best," said Laura Allen of Greywater Action, an educational group in Oakland, Calif.
As graywater use expands, more companies may install and support reuse systems, making it easier for consumers to operate more complex systems, Planiappan said.
One concern is whether it is safe to put water that could contain traces of human waste or other contaminants into the soil, although Palaniappan says there have been no documented public health issues.
It helps that the water is applied directly on the lawns of its makers. "The graywater you use on your property is the graywater you produce in your house and you know the quality of it," Palaniappan said.
"The risks should be minimal if the graywater is used at the household level, and the graywater is prevented from running off the property into surface water, or contaminating groundwater," agreed Kara Nelson of the University of California at Berkeley.
Another concern is that soil can suffer if inappropriate soaps and detergents are used. Soaps containing salt or boron and bleach can damage soils, so graywater reusers need to avoid such products.
When compatible soaps are used, they can actually fertilize plants. "The healthy topsoil treats water better than a sewage treatment plant ever could," Allen said. "It relies on natural microorganisms. As long as you're using healthy products, the soil will remove all those nutrients and turn it into plant food."
Just as rainwater harvesting has become more common in recent years, especially in water-stressed areas such as parts of India, graywater is likely to see wider use in the near future, Palaniappan said.
In Australia, for instance, people can receive rebates for installing graywater reuse systems, and several groups in the Middle East are interested in expanding its use, she said. In the United States, Arizona, Texas and New Mexico have favorable laws for graywater reuse, Allen said.
However, many states have such strict regulations on graywater reuse that they effectively prohibit it for irrigation. Better incentives and more realistic pricing of water are needed to expand graywater reuse, Palaniappan said.
The Pacific Institute estimates that graywater reuse in California alone could save enough water to supply 6.7 million Californians, all the grain grown in California each year, or 18 large water desalination plants.
Graywater reuse doesn't just save water. It also saves energy. "Some of the largest uses of energy are for the transport of water," she said.