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. We have three basic choices for the management of biosolids: incineration, landfill disposal, and “beneficial use.” But only beneficial use is considered sustainable.

What are biosolids?

When you think about a wastewater treatment plant, you probably think about the effluent — the cleaned water returned to rivers, bays, and lands. But a great deal of a treatment plant’s operation is devoted to producing and treating the solids from that wastewater.

Treatment plants remove solids early in the process, but they also grow microorganisms that help treat the wastewater. The organisms themselves become more solids. All of those solids are eventually collected, treated, and tested.

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 soils and nurture crops.

“Beneficial use” of biosolids is a sustainable solution

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.

Alternative version

Biosolids and health

Treatment plant operators follow complex rules and requirements to produce clean water and biosolids that are safe to use. We are taught from an early age that anything we flush is dirty, and that’s usually a safe bet. But biosolids produced from wastewater treatment are not the same as sewage. They are a treated and tested product. 

Why is there opposition to biosolids?

Anything that goes into a sewage system has potential to end up in the biosolids. Some pollutants do end up in biosolids, but in extremely small amounts. Treatment plants maintain cultures of helpful bacteria and microorganisms. Those microorganisms help treat the wastewater, but there are pathogenic organisms (things that could make you ill) in the system, too.

Pathogens and pollutants

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 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.


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 

Industrial wastewater

Some large manufacturers operate their own wastewater treatment plants and do not discharge to public sewers. The residual solids they produce from the treatment of industrial wastewater is called sludge, which is not biosolids.

In Washington, significant industrial dischargers are required to meet pollutant limits before they discharge to the sewer. This is called pretreatment, and it is an important part of water quality permitting. We also authorize some communities to implement their own pretreatment programs. They can work directly with businesses to reduce the amounts of harmful pollutants that might be released. In cases where this authorization doesn't exist, we do this directly.