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Resource Recovery

Creating resources from wastewater

For more information about Resource Recovery, please send us an email message or contact us at:

King County Wastewater Treatment Division
Resource Recovery
201 S. Jackson Street
Mail Stop: KSC-NR-0512
Seattle, WA 98104
Phone: 206-477-5557
Fax: 206-684-2057

Water droplets on plant shoots.

Reclaimed Water Research
Local water + Local studies = Local results!

Although reclaimed water has been researched and used in other parts of the world for decades, the King County Wastewater Treatment Division wants to be sure we fully understand the quality, potential uses and safety of our own locally generated reclaimed water in our local environment.

With science rapidly changing, King County keeps current on national research and news just in case something relevant or interesting crops up, but for the most part it’s the local results that matter the most to our customers.

Because we play on local soccer fields and golf courses and because we eat produce and buy flowers at our farmers markets, King County contracted with researchers at the University of Washington since 2007 to perform local and independent research to determine the safest and best uses for our reclaimed water. Where else to obtain the best-available science about public health and environmental impacts about our own backyards than right here in our own backyard?

The results have been reassuring! Studies from the University of Washington have confirmed our Class A reclaimed water uses are safe for people and the environment.

lettuce head

Safety of Reclaimed Water for Edible Food Crops: Pathogens and Metals / Greenhouse and Field

University of Washington researchers conducted a two-year study to determine the food safety and nutrient value of food crops irrigated with reclaimed water. In 2008, strawberries, lettuce and carrots grown in a greenhouse were irrigated with tap water, reclaimed water or commercial fertilizer diluted with tap water. At the end of the growing season washed and unwashed samples were tested. A year later, the study was replicated with strawberries, lettuce and potatoes grown in outdoor raised beds. Researchers monitored each plant for food safety parameters, including total coliform, fecal coliform, Escherichia coli (E. coli), arsenic, cadmium, lead and nickel. Soils were also sampled for carbon, nitrogen, phosphorus, salinity and pH.

The study concluded that food crops irrigated with reclaimed water are safe for human consumption based on federal standards. It was concluded there is no significant difference between watering choices in terms of the presence of microbial pathogens or metals in the plant tissues or water leached through the soils. Total coliform, which should not to be confused with E. coli or fecal coliform, was detected during the field study only in the soil of the outdoor raised beds toward the end of the growing season. The source of the coliforms was determined to be a family of rabbits nesting nearby, not the reclaimed water, thus reinforcing the common practice of always washing produce prior to consumption. Soil tilth was also preserved in the samples receiving reclaimed water – pH, salinity and nutrient levels were within the range deemed suitable for successful gardening. For the full report, click here.


Growth and Physiology of Horticulture Crops Irrigated with Municipal Reclaimed Water in King County, Washington

Irrigating with reclaimed water is a common practice in many parts of the world. However, since each region’s geology and climate are unique, it is important to address local horticulture issues such as how to integrate the use of reclaimed water with existing fertilizer practices and soil conditions. It is also important to determine the potential impacts of reclaimed water on plant growth and soil properties. In 2008, scientists from the University of Washington began a two-part study to determine the safety of vegetables irrigated with reclaimed water. This study also examined the growth, physiology and aesthetics of ornamental and vegetable plants irrigated with reclaimed water.

The study began in a greenhouse, where amaranth, sunflower, delphinium, lettuce, strawberry and carrot were monitored for biomass, height, leaf area, photosynthesis and chlorophyll content. Soil pH and salinity were also measured at the end of the greenhouse study. For each plant species, there were 15 pots. Five of each received either tap water, reclaimed water or fertilizer diluted with tap water. The second phase of the study occurred in an outdoor garden in 2009 with the same plant species tested for the same attributes.

In the end, it was determined that plants irrigated with reclaimed water performed fairly well and all growth and physiological measures showed equal or better results with reclaimed water when compared to tap water. This finding indicates there is some nutritive benefit to reclaimed water, though where growth was not substantial compared to tap water plants additional fertilizer may be desired. This study also determined that salinity in the soils, a concern for many growers, does not appear to be a problem with reclaimed water. For the full report, click here.

Turf grass

Fate of Personal Care Products and Pharmaceuticals and Growth Response for Reclaimed Water Irrigated Turf Grass

Turf grass irrigation has the largest potential use for Class A reclaimed water in King County. In July 2008, researchers at the University of Washington conducted a six-month greenhouse comparison study to address concerns from potential customers regarding turf growth response, salinity, and the fate and transport of personal care products and pharmaceuticals. This study chose to focus on the fate and transport specifically of ibuprofen (a commonly used pain killer), triclosan (a widely used antibacterial chemical), and three types of estrogen (hormones) in turf grass and soil samples collected from a publicly-owned golf course in Tukwila. The Class A reclaimed water used in this study was produced by a sand-filter process from the South Treatment Plant in Renton.

The results of the study indicate that the estrogens, triclosan and ibuprofen did not persist in the soils and were not detected in the plant tissue or the water that drained through the soil. These findings indicate that applying reclaimed water to turf and soil systems does not result in any accumulation of the hormones and personal care product chemicals studied. These findings agree with the national research monitored by King County. Also, unlike other areas of the country irrigating with reclaimed water, there were no changes observed to the salinity of the soils due to the low salinity of our reclaimed water.

Lastly, this study demonstrated that reclaimed water does not negatively affect the growth of turf grass or the nutrient quality of the soil. Rather, since sand-filtered Class A reclaimed water retains nutrients essential for plant growth, this study suggests that turf growers may consider decreased fertilizer use as an additional benefit with reclaimed water irrigation. For the full report, click here.

Soil sample

Ecological Use of Reclaimed Water: Potential Benefits and Impacts to Groundwater and Soil

Local scientists are now exploring the feasibility of using reclaimed water to replenish flows in compromised watersheds thus reducing the County’s reliance on marine discharges. Since Washington State has strict regulations regarding the purity of water used to recharge groundwater sources, this study will test the proposition that soil essentially acts like a filter that influences the fate and transport of chemicals introduced by infiltrated water. University of Washington researchers are engaging in a two-year study to test the effects of the two types of locally produced reclaimed water on two of the soil types found in King County.

The first year of the study will examine the effects of reclaimed water on core samples of soil from the Alderwood Series – a glacially modified till common to King County foothills, woodlands, orchards and field crops. The second year of the study will focus on the soil type found in the Sammamish Valley which drains slower and is higher in organic material than the Alderwood Series.

Over each year, soil core samples will receive a designated type and amount of either sand-filtered or membrane bioreactor water based on an agronomic rate of 2.5, 5 or 10 centimeters per week. Water will be collected from each core sample and analyzed for salinity, total nitrate, ammonia, total phosphorus and metals (silver, arsenic, cadmium, copper, lead and zinc). Scientists will also be testing for endocrine disrupting compounds. Results from the first year of the study will be available in summer 2011. These results will be used by King County to develop design protocols regarding the use of reclaimed water for environmental enhancement projects near groundwater sources. This information will be useful to reclaimed water utilities designing projects and to regulators in assessing the environmental risk of projects.