There are places in the arid West where the use of greywater is a force multiplier – taking the water you already used to wash your clothes and piping it into the garden rather than “wasting” it by dumping it down the sewer. But it does not always make as much sense as our intuition might suggest.
I’m reminded of the issue by a nice piece by Ben Preston in Miller-McCune about Art Ludwig, guru of California’s greywater movement:
Although nothing new, diverting greywater — water from washing machines, showers and sinks containing far less bacteria than the funky brew toilets and kitchen sinks emit — for irrigation has become one of the primary tools in a growing arsenal of conservation methods being examined.
Greywater is great in places where the effluent is “wasted”, such as coastal cities where outflow from sewage treatment plants is discharged into the ocean. But in a place like Albuquerque, the saving is not as great as it appears. Here, the effluent from my washing machine goes through a sewage treatment plant, then into the Rio Grande, where it is available to the ecosystem and downstream water users. (Albuquerque’s sewage treatment plant is the largest “tributary” on the Middle Rio Grande.) Every gallon I might divert from the washing machine to my garden results in one less gallon in the river.
It’s another example of the fact that there is very little free water left in the system.
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Right. But every grey gallon means one less groundwater gallon for a tree or garden that will be watered either way. Qualitative issues must matter.
It definitely matters where that gallon of water came from in the first place. If it’s just returning to the place of diversion (with a small deduction for consumptive use) it probably makes sense to flush it. Reusing the water just increases your consumptive use, but it should reduce the initial diversion. You can get into all kinds of arguments over whether that water would have been diverted anyway.
I continue to believe that when supplies are coming from groundwater (especially deep groundwater) the losses from pumping the water in the first place greatly exceed the benefit of returning the water to the environment – if the effluent is recharged rather than put into ecological restoration projects, again assuming foregone pumping by adopting reuse.
But the most basic argument against encouraging graywater here in Tucson – reduced flows in the sewers can result in flow problems where the gradient in the sewer does not allow for flushing of solids with the reduced liquid volume. so the county ends up using potable water to flush out the sewer lines. Sometimes ya just can’t win.