by Alex Garland

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In eastern King County, a mountainside forest holds an unlikely approach to sustainability and ecological restoration. There, a decades-long project converts poop into profit, yielding bigger trees and potentially lowering taxes.

For over 40 years, the King County Loop Biosolids Program has transformed our sewage and wastewater into Loop®, a nutrient-rich organic fertilizer that covers the floor of Marckworth Forest and nurtures farmland in Eastern Washington.

At its core, the King County Department of Natural Resources and Parks is leading a significant environmental effort. Although every state has a biosolids program somewhere within its borders, King County’s Loop® is one of the oldest. It repurposes wastewater treatment byproducts to enhance landscapes and address pressing environmental challenges, with the biosolids product playing a pivotal role in reevaluating approaches to forest management, agriculture, and environmental conservation.

Loop’s journey — from its origin in us, to our wastewater treatment facilities, to its impact on Marckworth Forest — is long, but uncomplicated.

From Sewage to Soil

The odyssey begins when we flush the toilet. Wastewater flows to treatment plants, where gravity efficiently separates liquids from solids. Some of the liquids are treated and repurposed for uses like irrigation, while the solids, including organic waste, undergo a transformative process in digester tanks.

Biologically speaking, these tanks function the same way that our stomachs do, using heat (98 degrees Fahrenheit) and naturally occurring microorganisms that break down waste. The generation of biogas, which is captured and utilized for fuel and electricity, is an added benefit.

This closed-loop approach mirrors nature’s own nutrient cycle, allowing the nutrients we extract from food to return to the land. The result is improved crop yields, reduced fertilizer expenses, erosion control, carbon storage, and soil restoration for farmers and foresters.

Dr. Sally L. Brown, a research associate professor at the University of Washington who specializes in, among other things, soil amendments and carbon sequestration, heralds Loop. She says it serves as a comprehensive plant food, containing the “essential 16 elements for plant life.”

A Sustainable Mission

Recognizing the value of biosolids, the Washington State Department of Ecology mandates their diversion from landfills and King County maintains its dedication to the full utilization of Loop.

The program, guided by scientific research, has demonstrated its positive environmental impact by storing carbon and reducing greenhouse gas emissions. These benefits outweigh emissions related to transportation and contribute to a more sustainable wastewater treatment process.

The goal of the program is to make the most of biosolids, the byproduct of wastewater treatment, according to Benjamin Axt, biosolids’ project manager for Resource Recovery at King County Wastewater Treatment. These biosolids, he said, are “recovered from the wastewater stream and recognized as a beneficial product” for forest growth, productive forests, and agriculture.

Brown commends Loop’s sustainability: “Loop is a well-balanced fertilizer, albeit low on potassium, that effectively recycles nutrients already in circulation.”

This recycling of essential nutrients is more environmentally friendly than conventional fertilizers, which often require energy-intensive industrial processes, leading to greenhouse gas emissions and depletion of finite resources. Loop’s environmentally friendly quality and sustainability are rooted in its ability to recycle nutrients and mirror natural nutrient cycles.

Benefits for the Forest

Research on biosolids and their effects on tree growth began in 1973, in Marckworth Forest, and were more widely used on commercial forest lands in the late 1980s, Axt said.

The forest, which is County trust land, is managed by the Department of Natural Resources (DNR) but yields timber that provides financial returns for county residents. “We’re growing trees, and then we’re reaping the rewards,” Axt said. Loop has emerged as a key component in this equation, facilitating the growth of trees that contribute to the county’s economic prosperity.

The available timber in Marckworth Forest, covering approximately 103 acres, is currently available for a total minimum bid of $1.886 million, which could make a positive impact on county coffers. The collaborative land management by DNR and King County has roots dating back to 1995, marking an essential contribution to carbon storage and lower tax rates.

Brown said using Loop in agriculture and forestry leads to multifaceted benefits: “bigger trees, higher yields, soil-carbon sequestration, improved water infiltration, reduced bulk density [and] improved water-holding capacity.”

These outcomes represent a significant enhancement of both environmental sustainability and productivity, with larger trees leading to higher yields, while carbon sequestration plays a vital role in mitigating greenhouse gas emissions.

Carbon sequestration is the process of capturing and storing carbon dioxide to prevent it from warming the atmosphere while reducing our carbon footprint. Approximately 25% of global carbon emissions are absorbed by plant-rich areas like forests and grasslands. However, this carbon can be re-released into the atmosphere or stored in the soil when plants decompose. Wildfires and deforestation can reduce forests’ capacity to store carbon.

Axt said that thanks to a 2021 study led by Brown, it’s become clear that soil quality plays a huge role in how much carbon it can store. According to Axt, applying biosolids to good soil yields the equivalent of 1:1. Put in simpler terms: Good soil can store as much carbon as you put in it.

But here’s the surprising part: “If you’re on a poor [soil] site, you put down a ton of Loop, and you get five tons of carbon sequestered.” That’s like giving a five-fold boost to poor soil’s carbon-storing abilities.

But it’s about more than carbon storage: It’s also about making soil healthier, assisting farmers, and reducing the amount of carbon dioxide in the air. Axt points out that this carbon storage doesn’t only happen near the surface. It also extends deep into the ground, promising even more carbon-capture potential.

Loop’s sustainability, said Axt, stems from its unique production process. “We have a continual supply coming to us. So that supply is as sustainable as our food sources,” he said. “It’s sustainable as long as we have people flushing their toilets. We recover all the stuff that plants like out of it and then bring it out to the forests. It’s a pretty simple system when you really go far back from it. When I talk to friends, I tell them that I’m working on King County’s composting toilet, and it’s just bigger than you think it is.”

Unlike traditional fertilizers, Loop is a soil amendment that utilizes the continuous supply of nutrients from wastewater, effectively recycling resources and reducing environmental impact. “I should say we’re not technically a fertilizer under state law, because that comes with its own set of regulations,” Axt explained. “We’re a soil amendment. Most people in their gardens would say it does fertilization, but I’m not allowed to say that.”

Protecting Rivers and Streams — and People

The safety and efficacy of Loop are of paramount importance to the program, Axt said, highlighting the rigorous monitoring and testing processes. “We monitor the influent before it reaches our treatment plants and throughout the treatment process,” Axt said.

The complex, strictly regulated treatment process is complemented by routine testing, both by UW’s environmental lab and the County’s internal labs. Brown further emphasized comprehensive testing, including for nutrients, metals, and pathogens, in compliance with EPA regulations. Scientific studies and university research support the product’s safety and benefits.

Proactively addressing concerns and misconceptions about Loop and the program is a key effort. Axt said the program maintains strong education initiatives and conducts outreach to concerned citizens. When concerns arise, the program provides information and data, including detailed reports. The program takes a proactive stance in preventing the introduction of toxic chemicals into the wastewater system; it also works to keep incorrect information out of people’s thoughts. Axt said educating the public is often a more cost-effective solution than removing specific substances from wastewater.

Responsible management of applying Loop significantly reduces harmful runoff into waterways. Buffering standards and practices are closely followed, even in forest environments with smaller buffers that exceed required standards. These well-vegetated forest buffers effectively prevent runoff, with ongoing monitoring of nitrogen and nitrates as proxies for various contaminants.

The presence of per- and polyfluoroalkyl substances (PFAS) — a category of chemicals employed in the fabrication of fluoropolymer coatings and items designed to repel heat, oil, stains, grease, and water — in biosolids is a growing concern. “The ongoing PFAS sampling project is in progress, and we are currently waiting for results,” Axt said. “We expect that it’s going to come back at levels that aren’t of extreme concern.”

Brown agreed, highlighting the thorough testing Loop undergoes, for nutrients as well as metals. “Pathogens are also tested for, as required by law. The program regularly participates in scientific studies to assure the safety of the product. King County also supports research at a number of universities, including UW and WSU, to assure the safety and benefits of the practice.”

Wastewater for the World

Looking ahead, the program envisions a future of continued innovation and sustainability. Axt said their long-term goals include maximizing the beneficial reuse of biosolids and diversifying their applications. “Our vision for the biosolids program is to continue doing what we’re doing and putting biosolids to 100% reuse,” Axt said.

A compost project is currently in the works, eventually making compost from biosolids available to the public. As the Biosolids Program expands its applications, the County’s commitment to sustainability and environmental well-being serves as a potential model for communities globally. Brown believes Loop is an invaluable resource: “The biosolids are some good sh*t!”

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📸 Featured Image: Exploring the sustainable frontiers of waste conversion, Marckworth State Forest stands as a testament to King County’s innovative Loop Biosolids Program, transforming sewage into a thriving environmental and agricultural boon. (Photo: Alex Garland)