Should We Use Rainwater to Flush Toilets?

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Here in Ireland, we get rained on a lot. But so do several cities in America. Philadelphia, New York, Seattle and Chicago all have a lot of rain. And this has led to a lightbulb moment for researchers at Drexel University. They think that citizens of these rainy urban environments are letting the benefits of a plentiful natural resource go right down the drain. And now Irish people are being charged for  water, it might be a good idea to look at rainwater in a new light here in Ireland.

Environmental engineers say that if homeowners had a way to collect and store even just the rain falling on their roofs, they could flush their toilets often without having to use a drop of municipal water. Toilet flushing is the biggest use of water in households in the first world, accounting for nearly one-third of potable water use. But there is no reason that clean, treated, municipal water needs to be used to flush a toilet – rainwater could do the job just as well.

“People have been catching and using rain water for ages, but it’s only been in the last 20-30 years that we have realised 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,” said Franco Montalto, director of the Sustainable Water Resource Engineering Lab at Drexel. “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 stormwater runoff generation in those cities.”

The process of collecting and using roof runoff is called rainwater harvesting. It is now in vogue among urban planners and water managers in the wake of the California water crisis and in other places like Melbourne, Australia, where dry weather has made water worth its weight in gold.

Drexel’s study was the first to crunch the numbers and sort out just how feasible it would be to lessen the use of potable water for non-potable purposes. And, it would reduce undesirable urban stormwater runoff. This is important for Ireland in light of recent flooding. You know the type: it floods Cork every year and has started to flood lower-lying areas of Dublin as increasingly larger stretches of paved ground mean that the water can’t sink into the soil.

“When the natural landscape is replaced by a building, rain can no longer infiltrate into the ground,” Montalto said. “It runs off, is captured in drains, where it can cause downstream flooding, carry pollutants that settle out of the air into local water bodies or it can cause the sewer to overflow. This 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 water boffins studied annual rainfall patterns, residential population and roof areas. They then gave each three-person family an imaginary 4,000-litre home storage tank. They discovered that a family with an average roof size would have enough water to cover most flushes throughout the year simply by diverting their downspouts to collect storm water.

This would reduce overall household potable water demand by approximately 25 percent, which could mean slashing the municipal water bill for an average-sized home by as much as one quarter.

Says Montalto, “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. 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.”

There are plenty of companies in Ireland that offer equipment to harvest rainwater… go on, have a google. And soon, you might just be using the April showers to flush away your water bills!

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