equivalent levels of public-health protection from pathogen exposure, EPA applied different use restrictions to each biosolids category.
A combination of treatment and site restrictions for Class B biosolids are intended to result in a reduction of pathogenic and indicator microorganisms (certain species of organisms believed to indicate the presence of a larger set of pathogens) to undetectable concentrations prior to public contact (Southworth 2001). Bulk biosolids applied to land must meet both treatment and use requirements (40 CFR 503.15[a]). EPA (1993) recognizes that those requirements do not necessarily consider risks to workers applying the biosolids at a site.
Class B biosolids must be treated to meet one of three criteria: a fecal coliform count of less than 2×106/gram (g) of dry solids at the time of disposal, treatment by a process to significantly reduce pathogens (PSRP), or treatment by a process that is equivalent to a PSRP. In the 1993 regulations, five processes were listed as PSRPs (and thus sufficient to meet the Class B treatment requirements):
Aerobic digestion at defined time and temperature combinations.
Air drying for 3 months, with at least 2 months at average ambient daily temperatures above freezing.
Anaerobic digestion under defined time and temperature conditions.
Composting under defined time and temperature conditions.
Lime stabilization so that the pH is greater than 12 after 2 h of contact.
These PSRPs were selected because they result in fecal-coliform concentrations of less than 2×106/g of dry solids, and they reduce Salmonella and enteric virus concentrations by a factor of 10 (EPA 1999).
The third treatment criterion requires that the permit authority approve the processes being used as equivalent to a PSRP. In practice, permit authorities have relied on the recommendations of the EPA Pathogen Equivalency Committee (PEC) (Cook and Hanlon 1993) when determining whether a particular treatment system should be designated PSRP. As of October 1999, PEC had recommended that two additional processes be designated PSRPs.