Biosolids Recycling Basics

What Are Biosolids?

Biosolids are the nutrient-rich organic product of the wastewater treatment process which can be recycled as a soil amendment. At the treatment plant, solids are removed from wastewater and treated in large digesters where the organic solids are stabilized, reducing the volume by about half. After digestion, a portion of the water is removed, leaving a semi-solid material ready for recycling.

King County's Role in Biosolids Recycling

King County recycles biosolids in forestry, agriculture and soil improvement projects; a portion is composted with sawdust for use in landscaping and gardening. 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 (external link) which provides collaborative research, technical assistance and public information for biosolids managers in the region.

Why Recycle Biosolids?

Biosolids are an excellent source of essential plant nutrients and organic matter. The addition of organic matter can reduce erosion by improving soil texture and structure and it also increases the soil's ability to hold moisture. By recycling biosolids, nutrients are returned to the soil where they can enhance plant growth.

Is Biosolids Recycling Safe?

Long-term scientific studies have repeatedly demonstrated that biosolids recycling is safe. Monitoring of biosolids, soils, water resources and plants continue to show benefits from recycling. These studies formed the basis for federal and local biosolids regulations.

The U.S. Environmental Protection Agency (EPA) issued its biosolids rule in 1993; it is the most comprehensive set of regulations ever. EPA and a panel of scientists with biosolids expertise examined all aspects of biosolids in the environment, including potential effects on ground water, air and soil quality, surface runoff and food crops.

The biosolids rule set quality limits for trace metals and requires pathogen and odor reduction. 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.

What's the difference between Class A and Class B biosolids?

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, 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.