New research confirms graywater safe for irrigation use

A graywater system in California filters water through sand before using it for outdoor irrigation. Credit: Water Environment Research Foundation.

As you’ve likely noticed, we’re big fans of graywater reuse. But we confess the same question has crossed our minds as has likely occurred to many of you: Wait…reusing water from showers or washing machines to irrigate plants?  Isn’t the soap and detergent in graywater bad for plants? Could there be a health risk to using this lightly used water?

As common-sense as the idea of graywater recycling is – using the appropriate water source and quality for a given purpose, rather than using water treated to drinking standards for all of them – we can understand how some people would have reservations about adopting graywater technologies in their home.

Indeed, those concerns about public health have slowed the widespread adoption of graywater recycling in Illinois, despite separated piping and filtration systems that eliminate any risk of cross contamination with drinking water. Fortunately, 2013 looks to be the year when the Ill. Dept. of Public Health will update the state plumbing code to allow graywater reuse, both for flushing toilets and for outdoor irrigation.

A recent study by researchers at Colorado State University with funding from the EPA and the American Cleaning Institute confirms what many early adopters of graywater for irrigation already know: the plants do just fine, thank you.

The controlled study of households in California, Colorado, Arizona and Texas indicates that, in general, plants and soils watered with graywater do not suffer from higher concentrations of harmful salts, minerals, or bacteria. In fact, overall, plants grew better – measured based on their total biomass – with graywater, probably because of higher nitrogren input, which acts as a fertilizer.

Of five key chemical metrics, only two were significantly different for graywater-irrigated plants, as compared to the control group watered with tap water. Those two were nitrates (NO3) and antimicrobials, the latter coming from the many antibacterial personal care products now on the market.  In theory, a build-up of antimicrobial chemicals in the soil could kill off beneficial microbes.

Overall, the effects of graywater on plants were highly species-specific. A few types of trees and shrubs grew poorly, while others flourished from the additional nitrogen. Gardeners planning to begin using graywater for irrigation should avoid avocado, lemon, and Scotch pine, which are highly sensitive to changes in water and soil chemistry.

In some cases, abnormally high levels of surfactants – the chemicals that cause soaps and cleaners to foam in water – were detected in the soil where graywater had been applied. However, surfactant concentrations typically plateaued after the first year and did not pose a threat to plant health. Homeowners can easily counteract this problem by using low-foam or biodegradable products.

The study also helps assuage another common concern about graywater: the possible presence of harmful bacteria. Across all the households tested, the presence of indicator bacteria, like e. coli, was extremely rare and not statistically significant.

As the study reminds us, for properties with outdoor landscaping, especially in regions with hot summers, graywater reuse for irrigation has the potential to cut total water bills by 30 to 50 percent.  While we hope to see more studies on this topic in the future, the research from Colorado State is useful knowledge for many water-conscious homeowners, as well as larger municipal and corporate water users looking for ways to cut back on potable water consumption. With an overall haul of the Illinois state plumbing code on tap for 2013, this study is more evidence that should help water consumers feel confident about the safety and health aspects of reusing graywater for irrigation.  

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