National Academy of Sciences. Biosolids Applied to Land:
Advancing Standards and Practices

Chapter 6. Evaluation of EPA's Approach to Setting Pathogen Standards

Treatment of domestic sewage sludge is required to minimize the risk of adverse health effects from pathogens in biosolids applied to land. In 1993, EPA published regulations establishing the processes and conditions it deemed necessary to minimize these risks. Unlike the chemical standards, the pathogen regulations are not risk-based standards but are operational standards intended to reduce the presence of pathogens to concentrations that are not expected to cause adverse health effects. The standards include treatment requirements, site restrictions, and monitoring requirements.

This chapter reviews the pathogen standards for land-applied biosolids in light of current knowledge of the potential pathogens in biosolids, how humans might be exposed to those pathogens, and factors that affect exposure (environmental fate, regional variations, and host factors). It also reviews approaches for conducting microbial risk assessments and discusses how those approaches might be used to improve EPA's pathogens standards for biosolids. This chapter does not review health effects studies (see Chapter 3).

PATHOGEN STANDARDS

EPA established two categories of biosolids: Class A biosolids, which have no detectable concentrations of pathogens, and Class B biosolids, which have detectable concentrations of pathogens. With the goal of providing equivalent levels of public-health protection from pathogen exposure, EPA applied different use restrictions to each biosolids category.

Class B Requirements

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.

Treatment Requirements
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):

1. Aerobic digestion at defined time and temperature combinations.
2. Air drying for 3 months, with at least 2 months at average ambient daily temperatures above freezing.
3. Anaerobic digestion under defined time and temperature conditions.
4. Composting under defined time and temperature conditions.
5. 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.

Site Restrictions

The site restrictions for Class B biosolids (listed in Box 6–1) were developed on the basis of the time attenuation required to reduce the levels of pathogens (bacteria, viruses, and helminths) to below detectable concentrations at the time of public exposure (equivalent to those achieved by Class A biosolids) (Southworth 2001). The use restrictions correspond to important exposure pathways (Table 6–1).

Several potential exposure routes do not appear to have been considered when those use restrictions were developed. For example, inhalation of dust was presumed to occur only on-site, and controlling access to the site was intended to prevent such inhalation. The potential for off-site exposure to wind-blown dust and aerosols does not appear to have been considered. Nor was the potential transport of pathogens in runoff from the site to neighboring properties considered.

In addition, regulations require that public access to the site be restricted for either 30 days or 1 year, depending on the probability of public exposure. This restriction is vague, however, and has been interpreted by some state agencies as a requirement for posting warnings but not necessarily providing access barriers. In other contexts, such as municipal solid-waste landfills, EPA has been more specific about access controls, "Owners or operators [of landfills] must control public access…by using artificial barriers, natural barriers or both, as appropriate to protect human health and the environment" (40 CFR 258.25). Furthermore, there is no requirement that on-site measurements be taken to confirm that the treatment and site restrictions for Class B biosolids result in pathogens concentrations below detection.

Class A Requirements

For biosolids to be categorized as Class A with respect to pathogens, they must meet one of six criteria:

1. Time and temperature requirements based on percentage of solids in the material.
2. pH adjustment accompanied by high temperature and solids drying.
3. Monitoring of enteric viruses and helminths after a treatment process to ensure below-detection concentrations.
4. Monitoring of enteric viruses and helminths in the biosolids at the time they are distributed or applied to land.
5. Treatment by a process for the further reduction of pathogens (PFRP).
6. Treatment in a process deemed equivalent to a PFRP. There are seven processes that are designated PFRPs for Class A biosolids: (a) composting with minimum time and temperature conditions, (b) heat drying with specified temperature and moisture conditions, (c) high-temperature heat treatment (no moisture content condition), (d) thermophilic aerobic digestion at specified time and temperature, (e) beta irradiation at specified dosage, (f) gamma irradiation at specified dosage, and (g) pasteurization. As with Class B biosolids, PEC has the authority to recommend to permit authorities that additional processes be designated PFRP. As of October 1999, nine additional processes were granted PFRP status by PEC (EPA 1999).
   

The goal of the treatment processes to achieve Class A biosolids is to reduce pathogen densities to below the following detection limits for these organisms: less than 3 most probable number (MPN) per 4 g of total solids for Salmonella sp.; less than 1 plaque-forming unit (PFU) per 4 g of total solids for enteric viruses; and less than 1 viable ova per 4 g of total solids for helminths. When the Part 503 regulations were developed, Class A certification was generally based on the presence of either Salmonella or fecal coliforms (indicator bacteria) (Southworth 2001), because only a few laboratories were capable of conducting virus and helminth analyses and more time was required for these analyses (2–4 weeks). Since then, the number of laboratories capable of such analyses has increased dramatically, and analysis time has decreased.

Class A pathogens requirements must be met before or at the same time that vector-attraction reduction requirements are met. For any criteria, the microbial agents are measured when the biosolids are used, disposed of, or prepared for distribution. At that time, Class A biosolids must meet one of two requirements: either the density of fecal coliforms is less than 1,000 MPN per gram of total solids or the density of Salmonella sp. is less than 3 MPN per 4 g of total solids.

NOTE:  THE BULK OF CHAPTER 6's MATERIAL IS NOT INCLUDED, JUST THE FOLLOWING FINDINGS AND RECOMMENDATIONS

FINDINGS AND RECOMMENDATIONS

Finding: The pathogen standards of the Part 503 rule are technologically based requirements intended to reduce the presence of pathogens. The standards consist of treatment, use, and monitoring requirements. Classification of Class A and Class B biosolids are based largely on fecal coliforms as indicator organisms. Class A biosolids do not have detectable concentrations of pathogens (determined by indicator organisms) and, therefore, risks from them are expected to be lower than those from Class B. Pathogens are normally present in Class B biosolids, but the risk they pose is unknown, because no risk assessment has been performed.

In determining the pathogen standards for biosolids, EPA considered a variety of potential bacteria, viruses, protozoa, and helminths that might be present in biosolids, their fate and transport in the environment, and the potential for human contact. The committee found that EPA considered an appropriate spectrum of pathogens and indicator organisms in setting its standards, but new information on those and other pathogens not considered is now available for conducting a national sewage sludge survey of pathogens and updating hazard identification. Because of the variety of pathogens that have the potential to be in biosolids, the committee supports EPA's use of pathogen-reduction requirements, use restrictions, and monitoring of indicator organisms, rather than pathogen-specific concentration limits, in its regulations.

Recommendations: EPA should conduct a national survey of pathogen occurrence in raw and treated sewage sludges. Important elements in conducting the survey include use of consistent sampling methods, analysis of a broad spectrum of pathogens that could be in sewage sludge, and use of the best available (preferably validated) pathogen measurement techniques.

Additional indicator organisms, such as Clostridium perfringens, should be considered for potential use in regulation of land-applied biosolids. Such indicators and other operational parameters (e.g., time, temperature, pH, and chemical dose) may be suitable for assessing day-to-day compliance with the regulations.

Finding: As with the chemical standards, EPA based its pathogen standards on selected pathogens and exposure conditions that were expected to be representative and conservative enough to be applicable to all areas of the United States and for all types of land applications. However, pathogen survival in soils may range from hours to years, depending on the specific pathogens, biosolids-application methods and rates, initial pathogen concentrations, soil composition, and meteorological and geological conditions. In addition, very few data are available to estimate the occurrence, transport, and decay rates of pathogens and endotoxins in bioaerosols.

Recommendation: Site restrictions, buffer zones, and holding periods for land-applied Class B biosolids should consider geographic and site-specific conditions that affect pathogen fate and transport.

Finding: Regulations for Class B biosolids include use restrictions. These restrictions are intended to limit animal and human contact with land-applied biosolids until environmental factors reduce pathogens to concentrations that are not expected to cause adverse effects. Because there are no requirements for on-site monitoring of pathogens, there is little information available to evaluate the reliability of use restrictions in achieving their intended minimum exposure levels or to verify that those desired levels are maintained over an extended time.

In addition, the committee found that some potential exposure pathways were not sufficiently considered when the use restrictions were developed. For example, potential off-site inhalation of dust and aerosols does not appear to have been considered. The potential for groundwater contamination by pathogens was not sufficiently addressed. This is a concern in geologically sensitive areas, where there is the potential for leachate from application sites to contaminate subsurface-water resources. In addition, the potential for runoff to contaminate surface waters was not adequately addressed.

Recommendations: Studies should be conducted to determine whether the site restrictions specified for Class B biosolids in the Part 503 rule actually achieve their intended effect with regard to pathogen levels.

As recommended in Chapter 5 for chemicals, EPA should develop a conceptual site model to identify the major and minor exposure pathways (including secondary transmission) by which humans might come into contact with pathogens in biosolids.

Finding: Substantial advances in detection and quantification of pathogens in the environment have been made since the promulgation of the Part 503 rule. For example, new molecular techniques for detecting pathogens, such as PCR, are now available. In addition, new approaches to environmental sample collection and processing are available. However, no consensus standards have been developed for pathogen measurements in biosolids and bioaerosols.

Recommendation: EPA should foster development of standardized methods for measurement of pathogens in biosolids and bioaerosols. EPA should include round-robin laboratory testing to establish method accuracies and precisions at the various pathogen concentrations expected in raw sewage sludge and partially and fully treated biosolids. These new detection methods should be used to verify that EPA's prescribed pathogen reduction techniques are reliable in achieving their intended goals. Mechanisms should be developed for incorporating new methodologies into the verification process as they become available.

Finding: Microbial risk-assessment methods similar to those used in chemical risk assessments have been developed for pathogens in drinking water and food. These methods are not as well-established as those for chemicals, and there are important differences between the two. For example, a microbial risk assessment must include the possibility of secondary infections, either through person-to-person contact or from transmission of the pathogen to others through air, food, or water. The importance of secondary transmission depends in part on the level of acquired immunity to the pathogen in the community, a phenomenon that has no analog in chemical risk assessment.

The committee believes quantitative microbial risk assessment (QMRA) is a feasible approach to setting standards for pathogens in biosolids. The committee does not recommend that QMRA be used to establish pathogen-specific regulatory concentration limits but recommends that it be used as a tool for developing treatment, use, and monitoring requirements (or for validating current requirements) to meet acceptable risk levels. However, there are still substantial data gaps, such as characterization of dose-response relationships and transport and fate of pathogens and endotoxins in biosolids and bioaerosols. Monitoring of compliance with the regulations should continue to be conducted using indicator organisms and operational parameters and practices (e.g., temperature, buffer zones, and pH) to ensure that tolerable risk levels are not exceeded.

Recommendation: QMRAs should be developed and used to establish (or validate) regulatory criteria (treatment processes, use restrictions, and monitoring) for pathogens in biosolids. They can also be used for sensitivity analyses and identifying critical information that is needed to reduce uncertainty about the risks from pathogens in biosolids. To conduct these risk assessments, consideration must be given to assessing risks from all potential routes of exposure (e.g., bioaerosols, groundwater), dose-response relationships, pathogen survival, and secondary transmission of disease. In some cases, research will be needed to fill gaps in knowledge of those inputs. As additional information is gathered on exposure, dose-response relationship, and pathogen survival, the risk assessments should be reviewed and updated as necessary.