Summary

The King County Wastewater Treatment Division is building on its longstanding Loop biosolids program to produce high-quality compost that is suitable for use in residential gardens. Through a new pilot program, the county is distributing Loop Compost to community organizations across the Central Puget Sound region on a limited basis while exploring potential retail opportunities.

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The King County Wastewater Treatment Division is introducing Loop Compost, a new locally produced compost made from materials recovered during the wastewater treatment process. Currently produced through a pilot-scale operation, Loop Compost builds on the success of the state’s longest running biosolids program, which King County has operated for nearly 50 years. 

Loop Compost is a nutrient-rich, odor-free soil amendment created by blending one part Loop biosolids with three parts clean woody material. It can be widely used in established vegetable gardens, flower beds, lawns, shrubs, trees, and even indoor plants. 

The new compost offers many of the same benefits as Loop biosolids, which have transformed soil conditions on farmland and forestland for decades. Loop Compost helps improve soil health, supports plant growth, lowers greenhouse gas emissions, and reduces stormwater runoff by allowing soils to absorb rainwater more effectively. It also contains essential nutrients, including nitrogen and phosphorus, though at lower concentrations because it is blended with woody materials.

“Loop Compost is the latest example of our Wastewater Treatment Division converting waste into opportunity, this time supporting community gardens that provide nutritious, culturally relevant produce to families throughout our region,” said Mo McBroom, Interim Director of the King County Department of Natural Resources and Parks. “I’m grateful for the ingenuity of our employees who are contributing to a resilient local food economy, building on decades of success we’ve had helping farmers.” 

“Loop Compost is the next step in our long history of promoting a circular economy, while protecting ratepayer dollars,” said Kamuron Gurol, Director of the King County Wastewater Treatment Division. “Producing a high-quality product diversifies our ability to meet the Washington State Department of Ecology’s 100 percent beneficial use requirement for biosolids and reduces the environmental and financial costs of sending them to landfills.” 

Conveyer belt located at a treatment plant
The Wastewater Treatment Division’s Loop composting facility is located at its South Treatment Plant in Renton.

The launch of Loop Compost revives a legacy familiar to many local gardeners. From 1976 to 2020, GroCo compost — produced by a private composter using Loop biosolids — enhanced farms and landscapes throughout the region. After the partnership ended following the closure of GroCo, the Wastewater Treatment Division launched a pilot program to once again produce compost using the same core ingredients and a process that returns valuable resources to the communities that contribute to the wastewater system every day.

King County’s pilot composting facility at South Treatment Plant in Renton started making high-quality compost in 2025 and currently produces about 35 cubic yards each week, or roughly 1,500 cubic yards annually. The compost, along with recycled water produced on-site, is already being used to grow fruits and vegetables at CitySoil Farm, located at the treatment plant.  

Starting with nonprofits and community gardens while exploring retail opportunities  

This spring, the county began distributing more than 215 cubic yards of Loop Compost to more than two dozen nonprofit organizations and community gardens, while also exploring potential retail opportunities.  

Through the King Conservation District, World Relief Western Washington was among the first public recipients to use Loop Compost at its Paradise Parking Plots Community Garden in Kent. The garden plays a vital role for immigrant and refugee families adjusting to life in their new community. Each year, about 40 households grow crops, such as African nightshade, maize, beans, and various greens — food connecting them to their home countries and traditions while growing community in the Puget Sound region. 

A man uses a rake to manage a compost pile
The staff at South Treatment Plant in Renton has launched the Loop Compost pilot project, offering biosolids to community organizations.

“We are excited and grateful to partner with King County in this mutually beneficial initiative that helps reduce waste, gives 45 refugee and immigrant families the ability to grow even more food, and to do even more to mitigate flooding for the local community,” said Medard Ngueita, Executive Director of World Relief Western Washington. “Loop biosolids was one of the main ingredients in the compost we used to establish our community garden beds in 2017 and has been integral to insuring healthy soil and food for our refugee and immigrant community members. I am excited that this Community Garden will continue to thrive and grow food for many families in our community through the use of Loop Compost.” 

Through a Partners in Water Program grant, the Wastewater Treatment Division is collaborating with nonprofit organizations to develop an equitable distribution plan for Loop Compost, and create and test inclusive outreach, education, and engagement activities to build awareness across King County. 

As access to Loop Compost expands, the product continues to undergo rigorous testing and quality controls to ensure it meets strict state and federal standards. King County’s Loop products are among the most highly regulated and tested soil amendments available today. In addition to meeting the Washington State Department of Ecology’s Class A standards, they are free of pathogens and regularly tested for heavy metals and trace chemicals.  

Compost rolls off a conveyer belt
Loop Compost is a nutrient-rich, odor-free soil amendment created by blending one part Loop biosolids with three parts clean woody material.

In April 2025, the Wastewater Treatment Division published results from a year-long study that found PFAS levels in wastewater and biosolids are low and have similar levels to most municipal waste systems across the country that serve residential and commercial users. Because Loop products are generated by cleaning, recycling, and transforming materials that go down toilets and drains, keeping harmful chemicals and contaminants out of the wastewater system is critical to maintaining product quality. 

Loop Compost and Loop biosolids are part of the Wastewater Treatment Division’s Resource Recovery, a program that transforms wastewater byproducts into renewable resources that include recycled water, sewer heat recovery, and renewable natural gas. 

The Wastewater Treatment Division is one of four divisions at the King County Department of Natural Resources and Parks. 

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Contact

Akiko Oda, Wastewater Treatment Division, 206-263-4190