Biosolids are the treated solid materials that are left over from wastewater treatment plants (sewage plants.) Biosolids recycling presents low risk. Questions have been raised about the safety of biosolids that are recycled in King County. Public Health — Seattle & King County is providing this fact sheet to clarify this issue and describe our role in regulating the beneficial reuse of biosolids to protect public health.

Reusing and recycling biosolids

The recycling of biosolids has been extensively studied at universities throughout the U.S. for many years. The results of this extensive research show that biosolids can be reused in various land applications, with no or extremely low risk of environmental or public health problems. The U.S. Environmental Protection Agency issued new federal regulations in 1993 to guide the reuse of biosolids to avoid risks to plants, animals, and humans. King County fully complies with these regulations.

Bacteria or other pathogens

Biosolids may contain bacteria or other microorganisms, depending on how the biosolids are processed.

When biosolids are recycled and spread over land, any disease-causing microorganisms are exposed to a hostile environment and die. These organisms will die from factors including: exposure to ultraviolet light, exposure to heat/weather, acidity in the soil, moisture levels in the soil, or digestion from other organisms in the soil.

In King County, there has never been any reported illness associated with the recycling of biosolids.

Presence of E. coli in biosolids

Recycled biosolids may contain E. coli bacteria, but most strains of these bacteria do not cause disease. Recent news about E. coli O157:H7 outbreaks has resulted in confusion over the use of the general term E. coli and the specific disease-causing variety O157:H7.

E. coli is the name of a bacterium that normally exists in the intestinal tracts of warm blooded animals. There are many types of E. coli, and most do NOT cause disease. One strain of E. coli that can cause disease is called O157:H7; this is the relatively rare strain that is well known for being able to be passed though undercooked hamburgers and unpasteurized apple juice.

Since the O157:H7 strain of E. coli is rare, only very tiny amounts of this strain would ever make it to sewage treatment plants. Normal sewage treatment procedures will reduce the small level of most disease-causing bacteria in biosolids even further—by over 99%. This level will be virtually undetectable. Any biosolids containing these bacteria at this concentration would not be sufficient to transmit infection before they die during recycling application or further treatment.

When biosolids are recycled and commercially applied to land, Public Health — Seattle & King County requires that public access be restricted for a period of time.

Any biosolids that are sold or given directly to the public undergoes higher levels of treatment to make the product essentially free of any organisms that could cause disease. Composted biosolids have been safely used in backyards throughout King County for over 15 years.

Agency regulation

The Washington State Department of Ecology operates a regulatory program to ensure that local sewage treatment plants use biosolids in a manner that ensures the protection of public health. Our regulatory program includes the requirement for permits, biosolids monitoring, environmental monitoring and inspections of areas receiving biosolids.