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General Questions

Biosolids are the nutrient-rich organic product of treating wastewater from homes, businesses and industries. The solid, organic portion of wastewater is separated and sent to digesters at the treatment plant, where beneficial bacteria consume the raw material. When this biological conversion process is complete, excess water is removed, leaving a soil-like product full of essential nutrients and organic matter.
Wastewater flows to treatment plants from homes, businesses, and industries. In King County 95% of the wastewater is from homes and businesses, with only 5% from industries. Wastewater pumped from septic systems is also delivered to the treatment plant. Refer to the system to view the extent of the sewered system in our region.
No - not any more than bread dough is the "same" as bread. Biosolids are what you get after you subject the solids to additional treatment processes, including anaerobic digestion at warm temperatures. The process of digestion converts a large portion of the organic solids into gases, such as methane, which is captured at our wastewater treatment plants and used as a source of energy for the operation of the facilities. The solids only becomes "biosolids" after digestion. Digestion also kills nearly all of the pathogenic organisms commonly found in wastewater, and helps speed the degradation of contaminants sometimes present in wastewater. After digestion, biosolids meet Washington Department of Ecology requirements for beneficial uses, such as habitat restoration, forest soil amendment, and crop fertilization. The final step in treating biosolids involves removing excess water, in a process known as dewatering.

King County does not apply any "raw sewage" or "sewage sludge" as part of any of its biosolids management program. 
Dewatered cake -- the kind of biosolids product King County produces -- is 21 to 25% solid matter. It looks like grey-black damp mud, somewhat crumbly but also packable and sometimes "shiny" in places (due to struvite, a mineral). When dried, biosolids look like fine-textured dark soil.
Fresh, damp biosolids can have an odor, usually described as "earthy" or "organic," similar in some respects to bone meal or manure; people often describe a hint of "ammonia" to the odor as well. After being worked into the soil the odor typically dissipates within a short time.

Composted soil amendments containing biosolids, like GroCo, have been further composted with sawdust, so have much less odor -- usually quite similar to potting soil or any other organic mulch.

GroCo made with Loop

GroCo is a biosolids compost sold for public use in the Seattle area. It is similar in appearance, texture, and odor to other popular organic soil amendments, and can be used in a similar manner. Composting allows GroCo to meet regulatory requirements to be classified as "Class A" (no remaining pathogens) that can be used safely in landscapes and home gardens.
GroCo is produced by mixing 3 parts sawdust with 1 part biosolids. At the composting facility, this sawdust-biosolids mixture is piled into large mounds. The mounds are turned regularly to keep them aerated and the temperature is monitored carefully to meet pathogen reduction requirements. The mounds are then allowed to mature for about a year, much like a backyard compost pile, but on a larger scale. Once the GroCo compost is ready, it's bagged for individual sale, or sold in bulk at the facility.
GroCo is made from sawdust and biosolids. Other common composts are made from yard trimmings, food scraps, or manure. Recently, an independent lab tested GroCo compost, a compost made from sawdust and steer manure, and a compost made from yard trimmings and food scraps. Each compost was tested for 173 compounds, including metals and organic compounds that EPA considers to be priority pollutants. Very few compounds were found in GroCo.
All of the nutrients found in biosolids are also found in GroCo compost, although at smaller concentrations because of the blending with sawdust. It may be necessary to add fertilizer, especially after the first growing season. GroCo compost provides organic material, or carbon, to soils and improves the physical properties of sandy or clayey soils, just like any other organic compost amendment.
Call GroCo, Inc. at 206-622-5141 for information on purchasing this popular soil amendment. Options include delivery within the greater Seattle area.
GroCo compost made with Loop is an effective soil amendment for lawns, gardens, and landscaping. GroCo provides nutrients for plants and builds soils with organic matter, helping retain soil moisture.

Biosolids recycling

Using biosolids in farms, forests, and gardens recycles the nutrients and carbon, providing real benefits to crops and soils. Because the safety of this practice has been demonstrated for decades, Washington State Dept. of Ecology encourages recycling of biosolids, conserving landfill space for waste materials.
King County's biosolids management program recycles biosolids as a source of nutrients and organic matter in forestry, agriculture and other soil improvement projects; a portion of our biosolids is composted with sawdust for use in landscaping and gardening. King County is committed to recycling 100% of the biosolids we produce -- about 116,000 tons annually.

The biosolids program emphasizes beneficial use of this resource and pursues environmental stewardship through diverse public-private partnerships. King County is a member of the Northwest Biosolids Management Association which provides collaborative research, technical assistance and public information for biosolids managers in the region.

In practical terms, the difference between Class A and B is in where and when treatment to reduce pathogens occurs. King County biosolids are anaerobically digested at the treatment plant to meet Class B pathogen reduction. To meet Class A standards and the exceptional quality designation, biosolids must be further processed (usually by some form of heating or composting) to virtually eliminate remaining pathogens. With Class B biosolids, we rely on the natural environment to provide this further treatment. Factors such as heat, wind and soil microbes create harsh conditions for pathogens; field monitoring has shown a rapid die-off of pathogens when biosolids is applied to forest soils or stored at agriculture sites. Class A and B designation also determines how biosolids can be used. Class A is required for biosolids that are sold or given away in a bag or container, or applied to home lawns and gardens. Additional permitting and site management are required for Class B biosolids.
In the greater Seattle area, biosolids are composted with sawdust and sold under the name, GroCo. Call GroCo, Inc. at 206-622-5141 for information on purchasing this popular soil amendment. Other cities may also have biosolids products available for garden use, such as a soil mix or compost.
  • Tacoma makes several products under their trademark name, TAGRO. Info at: 
  • Pierce County dries and pelletizes their biosolids, marketing it as SoundGRO. Info at:
Biosolids are an excellent source of essential plant nutrients and organic matter. The addition of organic matter can reduce erosion by improving soil structure and increasing the soil's ability to hold moisture. By recycling biosolids, nutrients are returned to the soil where they can enhance plant growth.
In farm applications, the biosolids will normally be plowed into the top six inches of soil, leaving little visible trace on the surface. In forest applications, the biosolids are normally left on the surface to break up and blend naturally into the soils over time. In all cases, the biosolids release plant nutrients slowly, following the normal process of decay. Their exposure to air and sunlight destroys any remaining pathogens in the biosolids within a short time. As the biosolids break down they release their nutrients into the soil, where plants can absorb them through their root systems. Biosolids are applied at "agronomic" rates (carefully controlled to match plant needs) slowly providing the nutrients to the growing plants, and ensuring that surface runoff and deeper groundwater are unaffected by the application.
Although federal and state rules encourage recycling, cities and towns make their own decisions about how to manage biosolids. Options include application to agricultural or forest lands, composting, land reclamation, incineration, or landfilling. All 50 states practice some land application. Nationwide, more than half of all biosolids are applied to agricultural crops; however, this includes less than 1% of the nation's agricultural lands. In Washington and Oregon, a very large proportion of biosolids is land applied or directly marketed to the public.

Quality and Safety

The best way to protect biosolids quality is to prevent putting certain wastes down the drain. The following items should go in the trash, not in the toilet or the sink:
  • Stickers from fruits or vegetables
  • Plastic or other non-organic materials like condoms or tampon applicators
  • Expired/unwanted prescription or over-the-counter drugs - ideally, these should be returned to "drug take-back" sites, if available.

Also avoid dumping these down the drain:

  • Grease and other fats from the kitchen
  • Products labeled "danger", "hazardous" or "toxic"

Try to use fewer cleaning products and chemicals at home, and switch to "biodegradable" or more natural alternatives.

Remember: only toilet paper and human waste should get flushed!

Yes.  Long-term scientific studies have repeatedly demonstrated that land application of biosolids is safe. These studies helped form the basis for federal and local biosolids regulations and best management practices. The U.S. Environmental Protection Agency (EPA), with the help of scientists from universities and the U.S. Dept. of Agriculture, studied biosolids for more than 10 years. These experts examined many aspects of biosolids in the environment, including potential effects on ground water, air and soil quality, surface runoff and food crops, and then developed regulations that would protect human health and the environment. The EPA issued its biosolids rule in 1993; it is the most comprehensive set of regulations ever.
The biosolids rules set quality limits for trace metals, requires pathogen and odor reduction. The standards include site management requirements such as buffers from streams and other water resources. Biosolids are carefully applied in amounts that provide only the amount of nitrogen needed by crops. These practices provide for safe and effective recycling of biosolids, while being protective of environmental quality and human health. In an independent study, the National Academy of Sciences reviewed current regulations, management practices, and public health concerns and concluded that "the use of these materials in the production of crops for human consumption when practiced in accordance with existing federal guidelines and regulations present negligible risk to the consumer, to crop production and to the environment."

Biosolids have been recycled safely by King County for more than 30 years. Monitoring of biosolids, soils, water resources, and plants continue to show benefits to plants and soils with no effect on surface water or groundwater.

The use of biosolids has been one of the most extensively studied practices in the U.S. In some parts of the country, biosolids have been used for over 70 years. The Water Environment Federation has investigated several long-term application programs. They found that sites with cumulative biosolids loadings equivalent to more than 600 years of annual agronomic rates showed very little, if any, negative impact on the soil-plant system or on water resources. The University of Arizona Water and Environmental Technology Center has also confirmed the positive benefits of biosolids in a 20-year long research project.
Loop biosolids are always tested using the most current federally-approved laboratory methods. All testing is performed by the state-accredited King County Environmental Lab. Other labs and public entities routinely seek the services and expertise of this lab, which has been noted by Washington State Department of Ecology as a premier lab in the region. Biosolids data is available to the public here.
In the Seattle area, industries are required to remove toxic contaminants before the wastewater is discharged to sewers. (For more details about this program, see King County Industrial Waste Program)

The county's biosolids easily meet EPA's most stringent standards for safe use on land and crops. Many metals and organic chemicals that are unsafe in large amounts also occur naturally in the environment, and some, such as zinc and copper, are necessary for the health of plants and animals. King County's biosolids contain only very small amounts of metals and organic chemicals, thanks to the effective monitoring and pre-treatment requirements placed on industries. Less than 3% of the wastewater flowing to county treatment facilities are from industrial sources.

Countless consumer products contain substances that would be toxic at higher concentrations. It's the concentration of a substance that is important. Biosolids contain tiny amounts of organic chemicals from these products, but not enough to be harmful to humans or the environment. That is why the only substances regulated in biosolids are a few metals and nutrients.

Research to-date shows that most organic chemicals are degraded quickly, are not taken up by crops, do not move through soils, and pose much lower risk to humans than our everyday activities. For example, it would take more than 200 years of daily contact with biosolids for a farm worker to be exposed to the same amount of the anti-bacterial compound Triclosan as they would encounter by washing their hands just once with antibacterial soap. 

King County biosolids, Loop, contain very small amounts of metals, thanks to the county's Industrial Waste Program, which requires key industries to clean their wastewater in order to protect biosolids quality. Moreover, less than 3% of the flow into county treatment plants is from these industrial sources.

There are pathogens in raw wastewater that enters the treatment plant. During treatment, the wastewater solids go through a biological digestion process that kills 94-98% of the pathogens. Biosolids from King County contain no detectable parasites and viruses are only rarely detected. Any remaining pathogens die off quickly when exposed to conditions at field sites. Unless a person actually ingests fresh biosolids, the treated material poses little health risk to humans. Public health is protected by site management procedures such as wait times for harvesting crops and public access restrictions. (See what public health specialists have to say about the safety of biosolids: Public Health- Seattle & King County.
Biosolids enrich the soil with essential nutrients that are released gradually as plants need them. These nutrients include not only organic nitrogen, phosphorus, and potassium, but also minor elements such as calcium, copper, iron, magnesium, manganese, selenium, sulfur, zinc and others that are necessary for plant growth. Biosolids also add organic matter, which improves the structure of the soil and helps the soil absorb and retain moisture. Recycling biosolids on land replaces nutrients that are lost when crops are harvested and maintains the productivity of soil, one of our most vital resources. Biosolids are a valuable resource that can be used productively rather than treated as a waste to be dumped and buried at considerable cost.
Biosolids enhance soil fertility and growth of plants, including food crops. Safety of crops has been studied by agricultural scientists for decades and this topic was a major consideration in setting the standards for biosolids for agriculture use. Research and field experience confirm that crops and soils benefit when biosolids are used. King County applies biosolids to wheat, hops, and other crops in eastern Washington. Scientists from WSU have repeatedly tested grain crops fertilized with biosolids and found that content of trace metals were similar to those treated with other commercial fertilizers.
Biosolids do not affect surface or ground water quality near application sites. Application sites are monitored regularly for any sign that biosolids constituents have leached into groundwater or have been picked up by surface water. In addition to using buffers to separate application areas from water resources, biosolids are applied at low agronomic rates to match the nutrient needs of crops. These management practices protect both groundwater and surface waters.
Scientists at the University of Washington began studying the effects of biosolids on wildlife in the 1970s. They did not find any animals that are harmed by this practice. Because biosolids makes vegetation grow faster, animals from deer to mice benefit by having more nutritious food and better habitat. 

The National Marine Fisheries Service (NMFS) evaluated the county's biosolids program and determined that biosolids application posed no risk to chinook salmon and, in fact, provided an environmental benefit by enhancing forest growth.
King County has funded biosolids research for decades as a means to continually improve our biosolids field practices. We work exclusively with university scientists who have expertise in various fields associated with biosolids use—such as soil chemistry, soil ecology, forest ecology, and crop science. These individuals teach, design and conduct research, analyze data, publish papers, and collaborate with their peers worldwide. Their methods and results must pass the critical assessment of their peers and those on the review boards of technical journals.

Loop biosolids have been safely used for nearly 40 years as a soil amendment by King County's customers. The proof? Decades of monitoring data and studies by UW and WSU scientists, demonstrating:

  • Increased crop yields
  • More productive soil microbial communities
  • Increased organic carbon in soils
  • Water quality is protected