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In dry or arid areas, from Israel to California, water use can be tightly regulated, especially for gardens. But there’s another source of water that’s going unused, and a new study suggests we should take a closer look at it.

Water conservation types like to talk about the three basic types of water for use in agriculture: blue, green, and grey. Blue water is clean, withdrawn from reservoirs, groundwater, and rivers. Green water is rain. And grey water is used, or dirty water, but not so used that it has no possible reuses. So that disqualifies toilet water, which has to be treated more intensely (sewage is known as “black water”), but the leftovers from your sink or bathtub has a lot of possibilities.

A new study from Ben-Gurion University of the Negev in southern Israel—a place where water concerns are paramount, to be sure—tested the health of people over a long period of time using treated grey water in irrigation. Grey water can be treated in many different ways; some methods are as small as a few-hundred-dollar filtration system you can buy at Home Depot.

This particular study had one group of home gardeners use treated grey water and another group of home gardeners use regular tap water in their respective gardens for a year, writing down their health symptoms (specifically those of gastroenteritis) each week. Each group ate food from their gardens as usual, and the researchers kept an eye out for any kind of contamination; any ill health from possibly polluted water. Seems like kind of a risky study but, you know, gotta do it!

Anyway, the researchers found absolutely no indication that the treated grey water caused any health problems whatsoever for the subjects; in fact, the group that used tap water actually had a higher rate of illnesses, but the researchers didn’t delve into why.

A second element of the study was an in-depth microbial analysis of the grey water, which found that the levels of microbes in the treated grey water was not any higher than in tap water. So it isn’t really any surprise that the grey water was no more harmful to the subjects than the tap water. Either way, the researchers say this could be an important baseline to encourage more studies for using treated grey water in agriculture.
  1. I might add that if anyone is concerned about greywater having a bad effect on plants or wood there is lower hanging fruit to be concerned with. I will spare everyone the list, but rest assured. A baseline of dangers burry anything Grey or Rain water has to offer, including potable tap water in your local famous city.

  2. Long-term exposure to inorganic arsenic, mainly through drinking of contaminated water, eating of food prepared with this water and eating food irrigated with arsenic-rich water, can lead to chronic arsenic poisoning. Skin lesions and skin cancer are the most characteristic effects.

    • I am curious whether you have data which indicates there is any arsenic in treated greywater. I am not aware of any data showing higher concentrations than in any other water source.

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