Established on September 9, 1958, the Municipality of Metropolitan Seattle (METRO), a regional, intergovernmental agency, was created to address water quality and wastewater issues in the Seattle area. What it is most remembered for is the clean-up of Lake Washington.
When METRO began recycling sludge to use as fertilizer in 1987, there were concerns over the perceived notion that this fertilizer was toxic because sludge is the byproduct of treating sewage. However, studies indicated that sludge fertilizer was not toxic because it had been treated first. It showed great promise as a plant fertilizer and a soil rehabilitator. Even 50 years later, some people and researchers express concerns about the possible toxicity of sludge fertilizer and the dangers of using it.
Lake Washington’s sewage problem
Before METRO began managing wastewater treatment, untreated sewage was being dumped into Puget Sound, Elliot Bay, and the Duwamish River. This polluted the water and made swimming and fishing unsafe. In certain weather conditions, sewage would wash back onto the shore, leading to the closure of beaches due to the risk of bacterial infection. Additionally, rain would lead to sewer overflows that contaminated Lake Union, Lake Washington, and Green Lake.[1]
A relatively small amount of sewage was treated at this time, but even treated sewage posed environmental problems. In the early 1950s, treatment plants around Lake Washington dumped their treated sewage into the lake, increasing the growth of algae, creating a foul odor, and reducing water visibility.[2] Not only was the water pollution highly unsanitary, leading to various health concerns, but it negatively impacted the natural ecosystems in the Seattle area, including fish and plant life.
When METRO was formed, its primary goals were to treat a higher percentage of sewage and to clean up Lake Washington. To accomplish these goals, METRO organized volunteer programs for ecosystem restoration and built wastewater treatment plants at West Point, Richmond Beach, Renton, and Carkeek Park.[3] However, a by-product of the sewage treatment process was sludge
What’s the deal with sludge?
Sludge is made up of semi-solids that have been separated from the sewage. Multiple substances comprise sludge, including organic matter, microorganisms, and inorganic solids. While the sludge was no longer being dumped into Lake Washington, it still needed to be stored or used.
Starting in the 1970s, METRO began recycling sludge instead of dumping or burning it. Organic sewage sludge could be treated and turned into a biosolid fertilizer. Multiple companies, including GroCo and SilviGrow, purchased sludge from METRO to make fertilizer, which was used in forests, gardens, and farms. Some reports indicated that fertilizers made from sludge assert that it is more environmentally friendly, than traditional fertilizer, since it’s made from a renewable resource.
More fun with sludge
In addition to selling sludge,[4] METRO began a research project in collaboration with the University of Washington (UW) to learn how sludge fertilizer affected tree growth. They conducted their research at the UW site, Pack Forest, and the results were positive for soil rehabilitation and energy recovery.[5] However, there was some public concern over whether the leakage of nitrate and nitrogen from the biosolids could potentially infect the groundwater. Researchers at Pack Forest concluded that, even in the worst-case scenario where 100 percent of the nitrogen in the fertilizer leaked into the groundwater, there were no substantial negative effects.[6]
UW and METRO also researched the possible agricultural applications for sludge. Research conducted at Pack Forest showed positive results when composting sludge with sawdust. After being disinfected, the sludge-sawdust fertilizer could be safely handled and easily applied. METRO even used this fertilizer at the UW Arboretum.[7]
Research at Pack Forest yielded promising results for sludge use to rehabilitate soil. In areas where the soil was so unusable that trees and plants previously had a near 100 percent mortality rate, the application of sludge resulted in a complete turnaround, even increasing the growth rates of trees by 180 percent.[8] This use of sludge was incredibly successful and opened the door for further use in different environments.
Sludge and beyond…
Between 1992 and 1996, METRO merged with King County, which assumed its responsibilities. However, METRO’s impact is still visible through a swimmable Lake Washington and several transit and environmental programs, and one of METRO’s lesser known but still important projects was sludge. Today, King County turns sludge into a biosolids product known as Loop, a nutrient-rich fertilizer that’s used in forests and on farms throughout Washington state to support plant growth.[9] Although METRO no longer exists, its impact on water quality and sludge recycling was vital for the overall improvement of King County’s environment. As of 2026, its efforts have been continued by the King County’s Wastewater Treatment Division, which is part of the Department of Natural Resources and Parks.
Bibliography
“History of Our Mission.” King County, Washington, n.d. Accessed January 13, 2026. https://kingcounty.gov/en/dept/dnrp/waste-services/wastewater-treatment/about/history.
Hope, Jane. “Sewage Sludge Disposal and Utilization Study – Full Report.” Washington State Institute for Public Policy, January 1986. https://www.wsipp.wa.gov/ReportFile/1127/Wsipp_Sewage-Sludge-Disposal-and-Utilization-Study_Full-Report.pdf.
“Loop Biosolids.” King County, Washington. Accessed January 13, 2026. https://kingcounty.gov/en/dept/dnrp/waste-services/wastewater-treatment/resource-recovery/biosolids/loop-biosolids.
“Loop Compost Is on the Horizon.” Loop Biosolids, August 15, 2022. https://www.loopforyoursoil.com/gardens-landscapes/.
Oldham, Kit. “Metro: Municipality of Metropolitan Seattle.” Metro: Municipality of Metropolitan Seattle, June 18, 2006. https://www.historylink.org/file/7813.
“Pack Forest History.” Pack Forest, University of Washington College of the Environment, Accessed January 13, 2026. https://www.packforest.org/history.html.
“Pack Forest.” University of Washington State Relations. Accessed January 13, 2026. https://www.washington.edu/staterelations/web/pack-forest/.
Peterson, Neil. “Metro Tests Land Disposal of Sludge.” International Right of Way Association, May 1980. https://eweb.irwaonline.org/eweb/upload/web_0580_Metro_Tests.pdf.
“Silvigrow Project.” King County, Washington, March 1989. https://your.kingcounty.gov/dnrp/library/1989/mr6973.pdf.
Clean Up Water Poster. Office of Information Resource Management: Service Development / Printing and Graphic Arts, Series 1147, Container 6, Folder 32. King County Archives, Washington.
Pollution Sign, 1987. Office of Information Resource Management: Service Development / Printing and Graphic Arts, Series 1147, Container 5, Folder 6. King County Archives, Washington.
Cigarettes in Water, 1986. Office of Information Resource Management: Service Development / Printing and Graphic Arts, Series 1147, Container 5, Folder 6. King County Archives, Washington.
Sludge Recycling. Office of Information Resource Management: Service Development / Printing and Graphic Arts, Series 1147, Container 4, Folder 15. King County Archives, Washington.
Research Sign. Office of Information Resource Management: Service Development / Printing and Graphic Arts, Series 1147, Container 4, Folder 15. King County Archives, Washington.
Sludge Wheat. Office of Information Resource Management: Service Development / Printing and Graphic Arts, Series 1147, Container 4, Folder 15. King County Archives, Washington.
Sludge Spraying. Office of Information Resource Management: Service Development / Printing and Graphic Arts, Series 1147, Container 4, Folder 15. King County Archives, Washington.
[1] Kit Oldham, “Metro: Municipality of Metropolitan Seattle,” Metro: Municipality of Metropolitan Seattle, June 18, 2006, https://www.historylink.org/file/7813.
[2] “History of Our Mission,” King County, Washington, n.d., https://kingcounty.gov/en/dept/dnrp/waste-services/wastewater-treatment/about/history.
[3] Kit Oldham, “Metro: Municipality of Metropolitan Seattle,” Metro: Municipality of Metropolitan Seattle, June 18, 2006, https://www.historylink.org/file/7813.
[4] “History of Our Mission,” King County, Washington, n.d., https://kingcounty.gov/en/dept/dnrp/waste-services/wastewater-treatment/about/history.
[5] Peterson, Neil. “Metro Tests Land Disposal of Sludge.” International Right of Way Association, May 1980. https:eweb.irwaonline.org/eweb/upload/web_0580_Metro_Tests.pdf.
[6] Peterson, Neil. “Metro Tests Land Disposal of Sludge.” International Right of Way Association, May 1980. https:eweb.irwaonline.org/eweb/upload/web_0580_Metro_Tests.pdf.
[7] Neil Peterson, “Metro Tests Land Disposal of Sludge,” International Right of Way Association, May 1980, https://eweb.irwaonline.org/eweb/upload/web_0580_Metro_Tests.pdf.
[8] Neil Peterson, “Metro Tests Land Disposal of Sludge,” International Right of Way Association, May 1980, https://eweb.irwaonline.org/eweb/upload/web_0580_Metro_Tests.pdf.
[9] “Loop Biosolids,” King County, Washington, n.d., https://kingcounty.gov/en/dept/dnrp/waste-services/wastewater-treatment/resource-recovery/biosolids/loop-biosolids.
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