U.S. Water News Online
YUMA, Ariz. -- The water that just washed your clothes may
end up being used to keep your grass green in the near future.
It all depends on testing that a University of Arizona researcher
in Yuma is doing to see if "gray water" from laundry can be recycled
as irrigation water.
The objective was to determine whether children playing on grass
wet from watering with gray water would be at risk of picking up
pathogens that could make them sick, said Jorge Fonseca, vegetable
specialist at the UA Yuma Valley Experiment Farm.
Another objective was to determine the impact of gray water on
soil, he said.
What he learned is that the research "is worth pursuing," he said.
"Our project so far has shown us that gray water from laundry does
not increase the amount of aerobic bacteria and coliform bacteria in
grass or vegetables," he said.
In some cases the laundry gray water from volunteer households has
fewer microbes and coliform than water from the irrigation canals, he
said. In fact, he added, in some cases, the gray water has fewer
pathogens than the canal water. It may even be as safe as tap water.
And the water savings would be substantial, he noted.
"There's a potential to save 45 gallons of water a day per person
using all sources of gray water," he said. "There are estimates that
60 to 65 percent of average household water use is gray water."
That includes not only laundry water but also wastewater from such
sources as showers and kitchen sinks.
However, in the study, only laundry water was included because of
concerns the other sources might have more pathogens, Fonseca said.
For example, kitchen water might include juice from raw meat that
could be a health hazard.
"I think gray water exclusively from laundry machines is a pretty
good way to save thousands of gallons of water a year," he said.
Currently, some Tucson households and 3,000 families in Maricopa
County are using gray water to irrigate their lawns, he said.
State and federal regulations recommend that edible plants not be
irrigated with gray water. But the description of these regulations
vary from state to state and many are vague, "so we still wanted to
look at how much risk there is," Fonseca said.
One concern, he said, is that if someone were sick, those
pathogens could contaminate the gray water. A collaborative study
from Florida found that bacteria in the gray water form a biofilm --
like what people might see lining their kitchen sink -- that is hard
"So the implication is that we need to be careful if someone is
sick," he said.
As for the soil, one concern is the sodium in the detergent, he
said. "Excessive accumulation of sodium in soil results in compacted
soil that makes it very hard for roots to grow."
Another issue is that stored gray water can develop a sulfur-type
odor. Ironically, when one starts smelling this is when the microbial
population, including potential pathogens, diminishes. The odor is
caused by the anaerobic bacteria converting sulfites from the
detergents into hydrogen sulfide, he said.
These preliminary results are positive, but Fonseca and other UA
researchers are wary about getting too excited.
"We can't be as confident with our results as they pertain to the
soil. Some scientists have proposed that the soil will deteriorate in
nutritional make-up after watering with gray water. We have not yet
observed the results from our last evaluation, but we believe that
even if there is some potential deterioration of soil properties, the
amounts of gray water someone may use on their plants will not be
enough to erode the soil significantly," Fonseca explained.
He feels that in a typical situation, gray water would provide no
more than 10 percent of all the water used to irrigate a lawn or
garden. Additional watering with tap water could wash the excess
salts deposited from the gray water, he said.
Also, applying gypsum will help prevent most soil deterioration,
The potassium in some detergents actually is beneficial to plants,
Future UA research will include more examination of the effect of
gray water on different types of soil and soil exposed to different
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