Toilet flushing accounts for one-third of all potable water use in households across the U.S. and the U.K, making it the biggest use, or waste, of water. 

However, environmental researchers from Drexel University found that precipitation levels are high enough in major cities like Philadelphia, New York, Seattle and Chicago that rainwater recycling could be used to flush toilets instead. This could save both natural resources and money, as it would reduce the demands for water treatment plants

"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," lead researcher Franco Montalto said. 

Data from the U.S. Environmental Protection Agency (EPA) shows older toilets use as much as 3.5 to seven gallons of water per flush, and even the latest standard water-conserving models use 1.6 gallons.

Montalto is an associate professor in Drexel's College of Engineering and director of the university's Sustainable Water Resource Engineering Lab.

"The study looks at four of the largest metropolitan areas in the country to see if it rains enough to make implementation feasible and, if everyone did it, what effect it would have on domestic water demand and storm water runoff generation in those cities," he explained

Processes of collecting and using roof runoff, otherwise known as rainwater harvesting, have been implemented across California in the wake of the state's water crisis. However, most are small-scale water recycling systems, so researchers were interested in the feasibility of larger, more comprehensive systems. 

Researchers created a model that allowed them to easily analyze annual rainfall patterns, residential population and roof square footage in major cities. Their simulations revealed that households in Philadelphia, New York, Seattle and Chicago would be able to offset 80 percent of toilet flushes with a 1,000-gallon home rain harvesting storage tank, and, as a result, reduce their water bill by as much as 25 percent. 

Researchers also note that adopting this technology would lessen the amount of rainwater that runs off and causes downstream flooding and pollutes local waterways.

"Think of it this way. Before the building was on the site, the rain was intercepted by vegetation canopies, and/or infiltrated into natural soils. Either way, the rain ended up replenishing soil moisture, allowing the plants to grow, and recharging the local groundwater aquifer," Montalto said. "The more buildings that go up, the more we need to consider how to manage the water that would have landed in the drainage area they're displacing."

Their findings were recently published in the journal Resources, Conservation and Recycling.