You’ll be shocked to hear this, perhaps, but the biggest single way that you use water in your household is to flush the toilet.

Older toilets use as much as 3.5 to 7 gallons per flush, and even the latest standard water-conserving models still use 1.6 gallons each time you do your business, according to the EPA. That accounts for about a quarter of your household use, on average.

There’s a big problem with that, because we still cling to the practice, which dates back to 19th century municipal systems, of having one water supply pumped into the house. That means that we’re flushing waste down the toilet with water that’s been treated at considerable expense to be drinkable. And in places where water is scarce, it’s an even bigger problem.

Future Cities Go Off the Water Grid

In drought-ravaged California, for example, the population has been flushing about 120 billion gallons per year. One water expert told the Los Angeles Times that high-efficiency toilets don’t save as much water as expected, because people tend to flush them multiple times to get the job done.

But Drexel University engineers have a solution — at least, for cities where there is ample precipitation. Instead of using potable water in toilets, why not utilize rainwater collected from rooftops?

In a study just published in the journal Resources, the researchers report that it rains enough in Philadelphia, New York, Seattle and Chicago that if homeowners simply had a way to collect the water falling on their roofs, they could flush their toilets often without having to use potable water from municipal systems.

Video: How Much Water Do We Really Need to Drink?

“People have been catching and using rain water for ages, but it’s only been in the last 20-30 years that we have realized that this is something that could be done systematically in certain urban areas to ease all different kinds of stresses on watersheds; potable water treatment and distribution systems; and urban drainage infrastructure,” Franco Montalto, an associate professor in Drexel’s College of Engineering, and director of its Sustainable Water Resource Engineering Lab,said in a press release.

The solution would also help eliminate another big problem — what to do with stormwater runoff in cities, which often picks up chemicals and refuse and contaminates the bodies of water into which the drains eventually empty.

What If California Runs Out of Water?

The team calculated that, with enough water storage capacity — a little more than a standard 1,000-gallon home storage tank — a three-person family in a home with the city’s average roof size would have enough water to cover over 80 percent of its flushes throughout the year simply by diverting their downspouts to collect stormwater. With even bigger storage tanks, people might be able to virtually eliminate using potable water for flushing.

If you like this idea, offers a guide to installing a rainwater-supplied toilet.