New study examines potential for recycled water to address irrigation needs

WSU Researcher Doug Collins holding kale that was grown with  
river water (left) and recycled water (right)  

Fresh water availability is a growing concern as snowpack storage and rainfall decrease during our dry summer months. Access to a stable water source is important for farmers who rely on river water during the summer months to irrigate their crops. Yet this use, impacts the habitat of the aquatic life that rely on the river streamflow to survive and thrive.  

A possible solution is to use recycled water, also known as reclaimed water, for crop irrigation instead of river water. This alternative water source has been used by other states for years to solve their growing water needs. Using recycled water offers an opportunity to provide farmers with a sustainable and reliable water source, while keeping water in the river for salmon.  

Recycled water has been produced for more than 20 years in Washington state, including in King County, where the Brightwater Treatment Plant produces class A recycled water – the highest class of recycled water. This water is distributed via the recycled water pipeline that runs along the Sammamish River and is currently used to irrigate golf courses, parks and sport fields as well as industrial uses like pipe flushing and motor cooling.   

Recycled water is an important water supply for agriculture in many parts of the United States and is used irrigate food and non-food crops. A significant portion of the produce in supermarkets has been grown with recycled water. However, locally, recycled water is a relatively new water source, which farmers and consumers are less familiar. When farmers around our region began asking more questions about recycled water and the safety of using it on food crops, the Washington Water Trust (WWT), a nonprofit leader in river and streamflow restoration, developed a research project to examine safety concerns about using recycled water for food crop irrigation. Washington Water Trust joined with King County to partner with researchers at the University of Washington and Washington State University to better understand the impact of using recycled water on food crops as well as the perceptions of recycled water among farmers and consumers  

The group focused on Sammamish Valley agriculture and recycled water use, because of the proximity to Brightwater and the Sammamish River.  

The Sammamish River provides critical habitat for salmon and other aquatic species; however, warm water temperatures, low dissolved oxygen, and habitat degradation are having negative impacts on fish populations, according to Nicole Gutierrez, Project Manager at Washington Water Trust. 

The Sammamish Valley is also home to hundreds of acres of farmland that require irrigation – particularly during the late summer when rainfall and soil moisture decrease. During these warmer months is when more water is pulled from the river to irrigate those lands and produce crops for our community.  

“During the late summer months, that’s when fish need the [instream flow] the most so there’s some conflicting priorities,” Gutierrez said. “We need to keep land in production and have enough water in stream to support this really significant and important aquatic life, while also meeting the demands of our community members.” 

Although river water irrigation can help with overcoming a farmer’s challenges with drought, it’s not a sustainable option, especially in terms of supporting endangered fish populations in the Sammamish River.  

One way to improve conditions for fish and wildlife and provide a climate resilient water source for our food producers is to use recycled water for irrigating crops. Using recycled water in place of river water would improve conditions for salmon by leaving more water instream, particularly in the late summer, when flows are at their lowest. If irrigators who currently divert water from the Sammamish River switched to recycled water, the resulting increase in instream flows could aid in lowering the water temperatures, increasing dissolved oxygen levels, and result in more usable habitat for fish.   

A demonstration garden using recycled water was created to study this approach, including potential impacts and community perceptions.  

The demonstration garden consisted of carrots and kale to help researchers understand the different ways that potential containments of emerging concern (CECs) can move into crops. Researchers tested recycled water and river water for containments every step of the process.   

Edward Kolodziej an associate professor at the University of Washington, who is an expert in tracing chemicals around the environment, was one of the researchers who examined the potential contaminants of emerging concerns and measured their levels in the water, the soil and in the crops themselves. He tested the samples against 206 CECs, including pharmaceuticals, antibiotics, personal care products, herbicides and industrial chemicals. 
According to Kolodziej, less than 10 percent of CECs were found in the river water and recycled water crops. In a presentation given to farmers in the Sammamish Valley, he concluded that although CECs, specifically pharmaceuticals, were found in the crops, you would need to eat an extensive amount of the crop irrigated with recycled water to get a single dose of any particular pharmaceutical.  

Another important finding was the difference in crop yield. According to Doug Collins, an extension specialist at Washington State University, the recycled water irrigation soil had greater concentrations of phosphorous, sulfur, potassium and other important nutrients that crops need to grow and flourish than river water. The recycled water crop yield for kale was 147% more than the river water crop yield, while the recycled water crop yield for carrots was 25% greater than the river water crop yield.  

A side-by-side comparison of kale grown with river water on the left and Kale grow with recycled water on the right. 

Improving the understanding of producer and consumer perceptions of recycled water was an additional component of the research and offered an opportunity to educate the public about the study and address any misconceptions about using recycled water to irrigate crops.  

Nicole Gutierrez of Washington Water Trust, tabling at a local farmers market.  

The researchers engaged 500 community members and created a survey to better understand consumer sentiments, and about 86% of those surveyed said they would eat crops grown with recycled water. Survey participants expressed a greater willingness to eat crops grown with recycled water when asked if they would do so knowing using recycled water to irrigate crops would help protect endangered salmon.  

Researchers created focus groups to learn more about consumers’ concerns regarding recycled water, and to identify public education areas. This feedback has helped guide year two of the study.  

What’s next? 

One of the long-term goals of the study is creating a roadmap for better understanding the benefits and challenges to using recycled water for farmers and improving instream habitat. Gutierrez said data from the second year of the study is still being evaluated and will be used to validate the findings from the first year of the study. This summer, the project will also be conducting a second year of outreach to consumers to better understand and respond to community members’ concerns.  

For more information about the project, visit  

This project is funded by the King County Wastewater Treatment Division, King Conservation District, and the Bullitt Foundation.

A group of researchers and local producers around the demonstration garden. 

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