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What are biosolids?

Biosolids are the nutrient-rich product of the wastewater treatment process. Biosolids improve soil fertility and enhance plant growth and crop yield.  Loop® is the brand name for biosolids produced by King County. Loop is used as fertilizer and soil amendment for commercial forestry and agriculture, and as an ingredient in compost for landscaping and home gardening. 

Making Loop

When wastewater goes down drains in the greater Seattle area, it flows to one of our treatment plants. These facilities treat incoming wastewater to remove solids and ensure that the resulting water is clean enough for discharge to Puget Sound or use as recycled water. The solids are further treated to make biosolids.

digesterDigester tank 

The biosolids production process begins when the solids are pumped into large heated tanks, known as digesters, where they become food for beneficial microbes (bacteria and other microscopic organisms). Over the course of several weeks, the digestion process kills off 95-99% of the disease-causing organisms that may be found in raw solids and reduces the mass of the solids by about half.

Microbial activity in the digesters also produces methane gas, which is recovered and used as a source of energy for the treatment plants. When digestion is complete the solids are centrifuged to remove water, producing a charcoal-colored soil-like material – Loop biosolids.

Using Loop

King County produces about 120,000 tons of Loop each year, the majority of which is used as fertilizer and soil amendment for agricultural crops in eastern Washington or commercial forests in east King County. Loop is also used as an ingredient in making GroCo compost, a high quality compost for gardens and landscapes.

Applying Loop to wheat fields before
By amending soils with Loop, we turn what might otherwise have been waste into a valuable resource. Loop contains large concentrations of carbon (organic matter) and nutrients that are needed for plant growth, such as nitrogen, phosphorus, potassium, sulfur, and zinc. Using biosolids increases organic matter in the soil, which improves soil structure and soil water-holding capacity.

If biosolids were disposed in a landfill or incinerated, none of these benefits would be realized. Using Loop as a soil amendment is also the most cost-effective means of managing our biosolids and has effectively reduced our carbon footprint.

Managing Loop

King County’s biosolids program is responsible for managing Loop recycling, including transportation and delivery, permitting and managing Loop applications, research and monitoring, and public outreach. Since 1973, we have worked with local organizations, farm groups, and university scientists to develop an award-winning program that serves as a model for safe, sustainable biosolids recycling.

Biosolids Regulations

In the early 1980s, the US Environmental Protection Agency (EPA) began an effort to determine regulatory standards for biosolids programs nationwide. This process involved compiling years of field research and conducting detailed human health risk assessments, followed by careful review by a national panel of scientists with biosolids expertise.

EPA evaluated hundreds of pollutants during development of the national biosolids rule. The resulting standards (EPA 40 CFR 503, 1993) require treatment to reduce pathogens and set safe limits for nine elements (arsenic, cadmium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc) in biosolids. These pollutant limits are based on highly conservative risk assessment assumptions designed to protect the environment and human health. The biosolids rule also includes requirements for managing land application sites, including setbacks from water and using agronomic application rates that are based on the amount of nitrogen needed by crops and other vegetation.

Biosolids are designated either Class A or Class B, which refers to pathogen content. Loop is anaerobically digested, reducing pathogens by 95-99%, and meet Class B standards. To meet Class A standards, biosolids must be further treated (usually by additional heating or composting) to eliminate all pathogens. Class B biosolids such as Loop are subject to site management constraints, such as harvest or public access restrictions; this allows time for all remaining pathogens to die off naturally in the soil environment. Only biosolids products that meet Class A pathogen reduction requirements are available to the general public for use in landscaping or gardening. GroCo compost made with Loop meets Class A requirements.

Washington State Department of Ecology (Ecology) adopted these standards in 1998 (WAC 173-308) and is responsible for managing the state’s biosolids program. See Ecology's website for more information.

How Biosolids Are Made process diagram

For more information about Loop biosolids, please contact us at: