JFS Engineering
Land Development and Water Resources Engineering
Rain: A Renewable Resource

Here in New York City, the primary issue with stormwater runoff is the flow and volume of water generated, and our overloaded sewer systems are hardest hit during rain events. The natural way to correct this is to create more green space to soak up that water, or implement rainwater harvesting systems to act as a buffer. We’ve heard of floodplain restoration professionals and guerilla green roofers; these folks work in their own ways to solve this problem.

We are blessed with 44 inches of rainfall a year, spread out pretty evenly across the entire year, with about 90% of this runoff occurring in storms yielding 1.2 inches of rain or less. Let’s look at a hypothetical residence in Brooklyn. If we get an evenly distributed 44 inches of rain a year, let’s assume we get 3.67 inches of rain a month. If we set our sights on that 90% rain event on the 800 square foot roof, we can compute the average monthly rainfall capture as about 1650 gallons per month.

We can do quite a bit with that amount of water; a well-designed rainwater harvesting system could run a typical garden hose for 22 minutes a day. Well, we don’t all have to water our lawns or make ice sculptures to help save our fair city. We can use it to flush our toilets!

As the manufacturers of gray water systems will conveniently point out, toilet flush and bathroom sink water volumes are nearly identical, so we could use the bathroom sink water to flush the toilet. It’s a great idea, but in many jurisdictions like New York City, gray water is not the problem; it’s the runoff from large rain events.

When we look at the water use for a typical three bedroom residence, we typically assume two persons per bedroom and four toilet flushes per day. Using the now-standard 1.6 gallon per flush toilet, we can assume our house uses 38.4 gallons per day, or 1168 gallons per month just to flush away our bodily waste. Buried or ground-mount tanks in the 1200 to 1600 gallon size range are relatively inexpensive and readily available. With that, even in a severe drought or water service interruption we can still flush for a month if we start with a full tank.

The current regulatory environment is positive with regards to reducing the complexity of rainwater harvesting systems, so in the future these systems may be even easier to maintain and more popular, driving the future price down more. In cases of new construction, these systems can be integrated into the building and site design to potentially reduce development costs.

This is a re-post of an article originally featured in The Green City Challenge, the original article can be found here.

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