The Water Environment Federation (WEF) formally recognized the term biosolids in 1991 and it is in common use thoughout the world as of 2004. Biosolids refers to the soil-like residue of materials removed from sewage during the wastewater treatment process. During treatment, bacteria and other tiny organisms break sewage down into simpler and more stable forms of organic matter. The organic matter combined with bacterial cell masses, settles out to form biosolids. According to USEPA, biosolids that meet treatment and pollutant content criteria "can be safely recycled and applied as fertilizer to sustainably improve and maintain productive soils and stimulate plant growth."

Biosolids in their liquid from look like muddy water and contains 1-10% solids. Biosolids may be dewatered in a second step of the treatment process, which turns it into a "cake" with the texture of a wet sponge. In this stage it contains 11-40% solids.

In the United States as of 2002, about 60% of all biosolids is beneficially applied to land as fertilizer for growing crops. Biosolids that meet the Class B treatment and pollutant criteria, in accordance with USA "Standards for the use or disposal of sewage sludge," Title 40 Code of Federal Regulations Part 503 (40 CFR 503) can be land applied with formal site permit restrictions and strict record keeping. Biosolids with lower pollutant content have less restrictions. Biosolids that meet Class A pathogen reduction requirements or equivalent treatment by a Process to Further Reduce Pathogens (PFRP) have the least restrictions on use. PFRPs include composting, heat drying, heat treatment, thermophilic aerobic digestion, beta or gamma ray irradiation and pasteurization.

The National Research Council published "Biosolids Applied to Land: Advancing Standards and Practices" in July 2002. They concluded that there is no documented scientific evidence that biosolids regulations have failed to protect public health, but there is persistent uncertainty on possible adverse health effects. The NRC noted that further research is needed and made about 60 recommendations for addressing public health concerns, scientific uncertainties, and data gaps in the science underlying the sewage sludge standards. The USEPA has had insufficient funding to implement NRC recommendations as of 2004.

In the United States municipal wastewater treatment plants annually produced about 7.7 million dry tons of biosolids in 1997, about 6.8 million dry tons in 1998 according to sources relying on USEPA estimates. According to the NRC, about 5.6 million dry tons was the normal US annual biosolids production rate as of 2002.

External links

• BioCycle magazine, 2000 article
• National Biosolids Partnership page
• Disinfopedia article critical of biosolids
• Environmental Health Perspective, 1997 article
• National Research Council, July 2002, Biosolids Applied to Land: Advancing Standards and Practices
• USEPA biosolids page