Engineers research feasibility of flushing toilets with rainwater

It’s one-part comedy and two-parts tragedy. While California homeowners have had to cut their water use by 25 percent, nearly 100,000 people in Flint, Michigan are afraid of drinking their tap water for fear of lead poisoning. 

Meanwhile, we all use treated water every time we flush our toilets. Flushing is the largest single use of household water consumption in the U.S., so all that purified water is wasted in toilet bowls. For an alternative, researchers recently looked to the heavens above.

A team of Drexel University environmental engineers conducted the research. Their research indicated that it rains enough in Philadelphia, New York, Seattle and Chicago for homeowners to collect and store the rain to use to flush their toilets and not use a drop of municipal water.

Franco Montalto, PhD, associate professor at Drexel’s College of Engineering and director of the university’s Sustainable Water Resource Engineering Laboratory, used a mathematical model. The model analyzed annual rainfall patterns, residential density, and roof square footage for the four cities. He found that a properly sized rainwater collection system could not only handle the flushes, but reduce potable water use by 65 percent.

Rainwater harvesting

“People have been catching and using rain water for ages, but it’s only been in the last 20 to 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,” Montalto explained in a press release.

The process of collecting and using roof runoff, which researchers call rainwater harvesting, has been working its way into vogue among urban planners and water managers over the last couple decades. It has been implemented widely in California in the wake of its water crisis.

The research team calculated that with a 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 more 80 percent of its flushes throughout the year simply by diverting their downspouts to collect rain. With even bigger storage tanks, people might be able to virtually eliminate using potable water for flushing. And the good news for residents is not only does the switch have positive environmental impacts, it could cut the water bill for an average-sized house by as much as 25 percent. But even without installing a storage tank capable of holding a year’s worth of flushing water, a scaled-back version would still help chip away at the water bill.

“In general, greater potable water savings are estimated in cities with either larger roof areas or lower population density,” the study showed. “However, such savings would be accompanied by smaller reductions in runoff. Philadelphia and Seattle are the two cities where percent water savings would be greatest if residential neighborhoods were all equipped with rainwater harvesting systems.”

Upstream and downstream benefits

This “upstream” benefit could conserve drinkable water and reduce the strain on city watersheds. However, the research also suggests that there’s a “downstream” benefit, too, since it would lower the amount of unwanted storm water runoff. Before we paved paradise and put up a parking lot, all that rainwater would seep into the ground. Before you knew it, rainwater would become groundwater, one of the biggest sources of fresh water on the planet.
“When the natural landscape is replaced by a building, rain can no longer infiltrate into the ground,” Montalto said. “It runs off and is captured in drains, where it can cause downstream flooding, carry pollutants that settle out of the air into local water bodies, or in the case of a city like Philadelphia or New York, cause the sewer to overflow, which leads to a discharge of untreated wastewater into local streams and rivers. So capturing rainwater can help to reduce the demands on the water treatment system and ensure that it will still function properly even during heavy rainfall events.”

The study estimates that an average residence with a 1,000-gallon rainwater harvesting system could reduce runoff by over 40 percent. Among the cities studied, Philadelphia would see the largest percentage of runoff reduction if rainwater-harvesting systems were installed in residences citywide. This is because the average roof size in Philadelphia is the smallest of the cities surveyed, so there is less runoff to manage from a single roof. The researchers found that larger percentages in runoff reduction from a rainwater harvesting system can be the result of either small roof sizes or high population densities. But managing storm water is a concern for all urban areas.

“Think of it this way,” Montalto said. “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. 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.”

Content Type: