Learn about biosolids

Biosolids are a nutrient-rich product of Washington’s wastewater treatment systems. They're a soil amendment farmers and land managers can use in place of commercial fertilizers. They're also used in the manufacturing of some fertilizers and as a component of compost, topsoil, and similar products.

We're responsible for regulating biosolids in Washington and maximizing their beneficial use. There are three choices for the management of biosolids in Washington: incineration, landfill disposal, and “beneficial use.” But only beneficial use is considered sustainable.

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What are biosolids?

Wastewater from homes and businesses flows to wastewater treatment plants where it undergoes a complex treatment process. The solid particles that settle out in the first treatment process continue along for more treatment. Part of the treatment process grows microorganisms that help clean the water. The added organisms themselves become more solid particles after their lifecycles end. The resulting treated solids become “biosolids,” after meeting federal and state standards.

Biosolids contain nutrients essential for plant growth. They can be treated enough to be suitable to sell or give away to the public. That level of treatment is more complex and costly, so most biosolids in Washington (and nationwide) are applied to agricultural and forested lands, where they improve soil and feed crops. 

This chart shows how biosolids are managed in dry tons. Beneficial use includes Class B biosolids land applied (60,782), EQ biosolids sold or given away (18,169), and Septage land applied (2,867). Disposal of biosolids includes incinerated (15,418) and landfilled (1,772). — Check out a spreadsheet version of this chart here.

By "beneficial use" we mean applying biosolids to the land to improve soil health and enhance vegetation growth in a manner consistent with protecting human health and the environment. Beneficial use preserves all of the nutrients and organic material in biosolids, returning them to land depleted of nutrients by practices such as farming and forestry.

The primary means for managing biosolids in Washington and elsewhere in the United States is through beneficial use. Beneficial use programs in the U.S. have excellent track records in human and environmental health.

Pathogens and pollutants are certainly things to be concerned about, but context is critical to understanding how they are managed in Washington’s biosolids system.

Pathogens

Pathogens are bacteria, viruses, or other organisms that can cause illness. They are everywhere people live, work and play. Because it’s not possible to escape the microbiology of the world, people have developed many natural defenses, such as skin and mucus that traps inhaled pathogens. People also have natural and acquired immunities to help prevent illness. Natural defenses and acquired immunities are both enhanced by proper hygiene like hand washing.

The simple presence of a pathogen does not lead to illness. There has to be a route of exposure. Then there has to be an infective amount of the pathogen, and a person susceptible to that pathogen. Washington's biosolids treatment and management practices are designed to break the chain of events that might lead to illness.

The wastewater environment is a jungle of organisms competing with each other for nutrients. They even eat each other. Many pathogens don’t do well in the treatment environment because that is not where they live long and prosper.

Biosolids are not feces that have been screened out of the wastewater. In fact, the microorganisms that treat the wastewater make up a lot of the biosolids that treatment plants produce.

Pathogen reduction is one measure of biosolids quality

Class B biosolids

Class B biosolids can be tested directly for pathogens or indicator organisms, or they can undergo treatment processes to significantly reduce pathogens, defined as a 99% reduction of pathogens.

Treatment examples include digestion—or liquid composting—and solar drying. Even with a 99% reduction of pathogens, Class B biosolids have restrictions on where they can be used, when crops can be harvested, and restrictions on access to the site.

Class A biosolids

Class A biosolids go through a more rigorous treatment called a Process to Further Reduce Pathogens (PFRP). A PFRP reduces pathogens to below detectable limits. Heat drying and composting are examples of PFRPs. Additionally, operators test all Class A products for pathogens or indicator organisms.

Biosolids are also treated to reduce attraction to pathogen-transmitting organisms like flies. Sometimes this is accomplished by simply mixing the biosolids into the soil. Or biosolids can go through stabilization processes that reduce their attraction before they are applied to the land.

Biosolids are not the same as the sewage that entered the wastewater treatment process.

Pollutants

A pollutant is something that can negatively affect people, plants, or animals when it is released into the environment. Pollutants in biosolids in Washington are generally far below limits established in federal regulations.

We regulate nine primary pollutants of concern in biosolids, consistent with federal requirements.

Arsenic
Cadmium
Copper
Lead
Mercury
Molybdenum
Nickel
Selenium
Zinc

PFAS and biosolids

Per- and polyfluoroalkyl substances (PFAS) are a group of thousands of manufactured chemicals that have been used in industry and consumer products since the 1940s. PFAS are resistant to breaking down over time, so they build up and are now found almost everywhere in the environment. PFAS are additives in a wide variety of everyday products like nonstick cookware, waterproof clothing, takeout food containers, carpets, and textiles.

This graphic shows how PFAS travels from household and business sinks, washing machines, toilets, and drains to wastewater treatment plants.

Municipal wastewater treatment plants (WWTPs) do not produce PFAS. They receive them from upstream sources like our homes and businesses, and are charged with cleaning them up. The wastewater that leaves our homes and industry carries PFAS from products we contact and use.

Unlike many other contaminants, PFAS do not break down into simpler forms during the wastewater treatment process. In certain instances, these contaminants can transform into different types of PFAS during treatment. This complicates identifying their presence and results in some PFAS moving through the treatment process and into treated wastewater, or effluent, and biosolids.

The most efficient and effective way to prevent contamination from many toxic chemicals–in water and soil –is to not generate the waste in the first place. Learn more about PFAS and biosolids. Ecology’s efforts to restrict the use of PFAS in products and find safer alternatives will prevent the many opportunities for exposure to this toxic chemical, a prospect that is especially important given the challenges in destroying PFAS. Learn more about Ecology’s PFAS strategy.

Source reduction efforts will lead to less PFAS in the waste stream – this means cleaner water and less contamination in biosolids.

Ecology's biosolids PFAS sampling efforts

In 2024, we partnered with 44 wastewater treatment plants across Washington to run voluntary testing for PFAS in biosolids. The study was a one-time sampling event that included wastewater treatment plants of differing sizes, with different upstream dischargers, from each region in Washington. Ecology staff collected samples in coordination with facility staff and had them analyzed as outlined in the project’s Quality Assurance Project Plan. Ecology’s focus in this report is on perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) as these are the PFAS chemicals that EPA and other state agencies are currently focusing on in biosolids.

What we found

PFAS are widespread. Almost every sample from 44 treatment plants contained PFAS, regardless of the facility’s size or location.
Method limitations. We discovered a limitation when using EPA method 1633 to measure PFAS in liquid biosolids. Other states have reported this same limitation.
Dewatered biosolids were more consistent. Results showed low levels of PFAS, similar to findings in other states without PFAS manufacturing.
Multiple sources. PFAS were found in both large and small facilities, showing that household wastewater could contribute as much as industrial discharges.
Future sampling. All facility types – not just major urban or industrial plants – should be included for further analysis.

Read the full report, Assessment of PFAS levels in Washington state biosolids, or you can read the summary, Biosolids, Focus on Assessment of PFAS levels in Washington state biosolids.

Next steps

The passage of Senate Bill 5033 “Concerning sampling or testing of biosolids for PFAS chemicals” during the 2025 legislative session will help Ecology gather additional data. The law requires temporary PFAS sampling for all biosolids facilities. We are working to address the limitations identified with using EPA Method 1633 on liquid biosolids. Facilities must provide their sampling results to Ecology for analysis. We will then draft a report to the legislature including recommendations for next steps. These expanded testing efforts and data will inform the path forward for Washington’s biosolids program.

Learn more about Senate Bill 5033 implementation.

Source reduction efforts

Wastewater treatment facilities provide critical services to their communities. The wastewater from homes and businesses flow to these facilities that use a number of treatment processes before discharging treated wastewater, called effluent, to the water or ground. These facilities receive all types of contamination, from pharmaceuticals and bacteria to PFAS and PCBs. Each facility also has permits from Ecology that regulate their discharges. Learn more about contaminants of emerging concern and how we're managing them.

In other states, source investigation has proven successful in greatly reducing the amount of PFAS coming from industrial sources and into wastewater treatment facilities. Ecology requires wastewater treatment plants to identify and control industries discharging wastewater to them. As Ecology issues and reissues permits to wastewater treatment plants, we are adding requirements to identify industrial sources of PFAS, where applicable. We are also including PFAS monitoring requirements as needed for wastewater treatment facilities. Learn more about Ecology’s work to address PFAS in wastewater.

While Ecology regulates wastewater treatment plants, the water quality standards used to regulate PFAS in wastewater must be approved by the Environmental Protection Agency (EPA) before Ecology can put related pollution limits into water quality permits. There are currently no federal water quality standards for PFAS. Ecology established some standards for PFAS in freshwater in 2024, which is currently awaiting EPA approval.

At the same time, the options for treating or removing PFAS in wastewater are evolving. Right now, the technologies are prohibitively expensive for most communities who pay for wastewater treatment facilities and any necessary technology investments. Preventing PFAS from entering wastewater and going to WWTPs in the first place can reduce the need to build costly new treatment systems and avoid placing the burden for treatment on rate payers.

Contact information

Biosolids program contacts