Hey lazyweb – anybody know if someone’s looked rigorously at the question of greywater use in the context of a Jevons-like paradox?
Putting together some notes for a talk this weekend to the Xeriscape Garden Club of Albuquerque (Sat. 10 a.m. at the Garden Center if you’re in town), I’ve been thinking anew about the question of greywater use – running a second plumbing system from the sinks and other non-poopy waste producers in your house out to the garden.
It seems obvious that this is a winner, because you’re making use of the wastewater rather than just throwing it away. Except that in many settings, it’s not thrown away, but rather returned via the sewage treatment plant for use by others. Here’s the new National Research Council report on reuse:
[E]xtensive reuse has the potential to affect the water supply of downstream users and ecosystems in water-limited settings.
The NRC is talking about larger-scale reuse, but the principle is the same. If you’re on the coast, and your sewage outfall goes directly into the ocean, greywater reuse seems like a no-brainer. But in desert communities like Albuquerque, it’s a more complicated question. My sewage goes back into the Rio Grande, where it is used by others. A gallon I divert from my wastewater stream to water my garden is one less gallon we remove from the Rio Grande at the north end of town, and one less gallon we return at the south end of town. There may still be good reasons for doing this – energy, system losses, treatment costs, etc. But “saving” that gallon of water is not one of them.
So here’s the Jevons question. For those not familiar, in brief, the Jevons paradox is the notion that, as energy technology gets more efficient, rather than simply dropping our energy consumption, we at least in part do more of whatever it was we were doing with the energy. So as lighting got more efficient and therefore less expensive, one of the things we did was light up a lot more of our spaces.
What if, by using greywater, I figure that since I’m using water that’s kinda “free” in the context of both money and supply, I figure I can use more in the garden?
If I NOW use 100 units of water straight into the sewer, will I use more if I recycle it in the house? Not likely. If I use 50 units outside and 50 inside and THEN put in greywater, will my sewer discharge fall; almost certainly. What’s left in the system will be “denser” in pollutants. Then you need to know if it costs more to treat 2 units @ 20% load or 1 unit @ 40% load. Don’t know…
Great question. My experience has been (I own a rainwater catchment business in Texas) that when I install a greywater system, all that reclaimed water is offsetting use of treated water in the garden. This water then soaks back into the ground and can either recharge an aquifer or provide water for deep rooted plants and trees.
It’s also been my experience that when people install a rainwater catchment system or a greywater system, they are more informed about the need for water conservation and will therefore use less water overall, in and out of the home, than the average American.
Greywater offsets treated water use for outdoor irrigation.
Imagine if everyone did this. There would be less water taken out of the river, more of it and of better quality, for the folks down river, and less expense in treating sewer that would be pumped back into the river.
As for the Javons Paradox, water is very close to free already. It’s conveniently piped to our homes and we don’t bother to monitor our usage during the month, we just blindly pay our water bill without much thought. If our needs for water are being met, why use more of it?