Organic residuals, especially biosolids, may contain pathogens - microorganisms that can cause disease.  By regulation, biosolids applied to soils must be treated to reduce or eliminate pathogens.  The U. S. EPA Part 503 regulations define two classes of biosolids: Class A and Class B:

  • Class A biosolids undergo a "Process to Further Reduce Pathogens (PFRP)." Pathogens are reduced to a level similar to the native soil and environment. Class A biosolids products can be used on home lawns and gardens, parks and golf courses, and other places where public contact is likely.
  • Class B biosolids undergo a "Process to Significantly Reduce Pathogens (PSRP)." This means that, while pathogens are significantly reduced to levels that are often below those found in animal manures, additional best management practices ("BMPs") and restrictions on public access are required at the site where they are used.



Prions are twisted proteins associated with "mad cow disease" (BSE) and other neurodegenerative diseases in mammals.  Some claims on the internet suggest biosolids transmit prions.  There is no evidence of this.  Read more...

Additional resources on pathogens in biosolids:

Contemporary Perspectives on Infectious Disease Agents in Sewage Sludge and Manure (James E. Smith, Patricia D. Millner, Walter Jakubowski, Nora Goldstein, & Robert Rynk, editors, 2005). An excellent compilation of findings of a 2001 seminar of experts on biosolids and manure pathogen risks. Contact BioCycle or NEBRA for details.

Problem Formulation for Human Health Risk Assessments of Pathogens in Land-Applied Biosolids; includes an excellent literature review.

The NEBRA office has additional information, references, and referrals.