Learn about biosolids

Many people have questions about how biosolids are produced, tested, and used. This information explains the technical, regulatory, and scientific work that goes into ensuring biosolids are safe for people and the environment.

What are biosolids?

Wastewater from homes and businesses typically goes to a wastewater treatment plant. At the plant, aquatic bacteria and gravity are used to remove solid particles from the water. These small particles contain both organic matter and mineral material such as silt and clay. Once separated from the wastewater, these particles are treated and then tested. If they meet certain standards, they are called "biosolids," meaning they can be recycled as a fertilizer and soil amendment.

How are biosolids used?

Farmers across Washington use biosolids to grow a variety of crops, including wheat, corn, grass hay, and hops. Biosolids can be applied as a liquid, dewatered so they resemble soil, or dried out completely.

Research and evaluation of biosolids

Biosolids have been studied extensively in hundreds of peer-reviewed scientific studies. Studies on trace elements and other compounds have looked at a variety of impacts to soil, plants, wildlife, and people. Investigations have evaluated toxicity of compounds, leaching, and contaminant transfer to plants and people. These studies have found biosolids to be safe when managed under current guidelines.

In Washington, scientists from the University of Washington, Washington State University, and other institutions have tested crops fertilized with biosolids. The U.S. Environmental Protection Agency and the National Academy of Sciences have also evaluated biosolids and found them to be safe when properly managed:

How wastewater treatment plants make biosolids

Typically, the water used for showers, flushing the toilet, laundry, and other household uses is piped to the local wastewater plant for treatment. Wastewater treatment plants use aquatic bacteria to consume or digest organic material in wastewater. The bacteria consume and digest the suspended and dissolved organic matter, along with many of the chemicals that get poured down the drain.

After secondary treatment, the bacteria die and settle out at the bottom of large tanks or basins. The digested organic material sinks to the bottom, leaving clear water on top. That water is disinfected and then returned to groundwater or a surface water, such as a local river or Puget Sound.

The layer of organic matter at the bottom is what becomes biosolids. The organics are dewatered, sampled, and analyzed. If the results meet the scientific standards set in the rule they are considered biosolids and approved to be applied to the land.

Are biosolids poop?

It's a funny question, but one many people ask: Are biosolids poop? No, they're not. During the wastewater treatment process, bacteria digest the organic material that we flush down the drain. Then the bacteria themselves die, becoming a stabilized material that can be a valuable source of fertilizer and help improve soil quality.

Pollutants and pathogens in biosolids


Biosolids contain trace elements that also occur naturally in soils and animal manure. Risk assessments based on university and EPA research identified nine trace metals as potential risks to human health and the environment. Biosolids containing these metals above EPA limits may not be applied to land.


Pathogens are disease-causing organisms such as bacteria, viruses, and fungi. Wastewater treatment plants destroy pathogens by chemical, physical, and biological processes:

  • Bacterial digestion
  • High temperatures
  • Chlorine
  • UV lights
  • Drying processes

Any remaining pathogens are destroyed by the sun's ultraviolet rays, by drying out, or by beneficial soil microbes after biosolids are applied to land.

Biosolids use in Washington

Land application

About 85 percent of the biosolids produced in Washington are used as fertilizer and a soil amendment. The remainder are incinerated or disposed of in landfills. Biosolids are only applied to land after a permitting process that includes guidelines for buffers, application rates, and site management.

Water quality

Research shows that land application of biosolids at agronomic rates (the rate at which plants can make use of the nutrients) poses little or no risk to groundwater. Biosolids contain nitrogen mostly in an organic form that is less likely to leach into groundwater or run-off into surface water. Guidelines control how, where, how much, and how often biosolids can be spread on any particular field. Soil microbes breakdown biosolids and slowly release nitrogen needed by growing plants. This gradual process helps prevent surface runoff and leaching into groundwater.

Soil quality

Biosolids contain important nutrients and organic matter that improve plant growth and soil fertility. They are an excellent source of nitrogen and phosphorous, as well as nutrients such as calcium, copper, iron, molybdenum, and zinc. Biosolids also add organic matter that improves soil's structure and its ability to hold water. By testing biosolids and applying them at agronomic rates, metals in soils will not accumulate at levels significantly higher than those found naturally.

Biosolids improve soil and crops in the following ways:

  • Improved plant growth and quality
  • Creating a better balance of water and air in the soil for plant growth
  • Improved ability of sandy soils to hold water
  • Improved ability of heavy clay soils to drain
  • Helping prevent wind and water erosion
  • Increased soil organic matter
  • Reduced need for chemical fertilizers
  • Protecting ground and surface waters