Study examines the benefits of using rainwater to flush toilets

Stormwater runoff problems could be reduced by collection of roof run-off to flush toilets
CC BY 2.0 eutrophication&hypoxia

Rainwater collection - such a simple proposition, it would seem. Free water from the sky. Reduced demand for water treated to the quality of drinking water for purposes like flushing a toilet, or even gardening. Should that be the end of the discussion?

So why is rainwater collection so controversial, even illegal in some cases? There is a lot more to the proposition than first meets the eye.

Laws designed to protect people by ensuring clean water supplies may need to be reversed to enable people to collect and use rainwater, especially for domestic purposes like toilet flushing. And even if many homeowners set up a rainwater collection system for the occasional bit of gardening, only the extremist is willing to haul buckets of water in from the garden for use in their bathroom?

Getting the topic into the sphere of public discussion could be the first step to conceiving a sustainable infrastructure for the constructive use of rainwater. One topic in the conversation will have to be: is it even worth doing? Especially in cities, where population density means less roof space per toilet flush.

A recent study led by Dr. Franco Montalto, of Drexel’s College of Engineering, looks at this question. The team used a mathematical model, inputting climatic records and water mass balance calculations for four major cities (New York City, Philadelphia, Chicago, and Seattle), to answer: Yes, a properly sized rainwater collection system could reduce potable water demand by 65%.

The study points out an even lesser known benefit. Before our buildings, roofs, and roads covered the earth, rainwater would infiltrate into the ground. Now it rushes off our impermeable surfaces, massing into quantities that must be managed to prevent erosion and flooding, and which pick up all the pollutants we leave -- tire wear residues, fuel combustion particles, pesticides, fertilizers, and more -- carrying this contamination to our surface waters.

The Drexel study, published in Resources, Conservation and Recycling, estimates that run-off could be reduced by 75%, offering an even bigger benefit than just the reduction in potable water demand.

A lot more will be required from urban water managers to create a vision of a future in which rainwater joins with our modern plumbing to provide a convenient flush in a more ecologically beneficial manner. The concept needs to be part of sustainable city planning.

Study examines the benefits of using rainwater to flush toilets
New York City, Philadelphia, Chicago, and Seattle would all see dramatic reductions in potable water use by rainwater collection, in spite of the population density. There are other benefits too.

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