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Equivalency Criteria

The Pathogen Equivalency Committee uses the following criteria to determine whether or not to recommend a process for equivalency. The foundation of every equivalency application must consist of these four criteria.

  1. Identification of Critical Process Parameters. The applicant must provide a well-defined and detailed process description, including diagrams, and discuss the disinfection stressors (physical, chemical, and/or biological) that are employed (For example: time, temperature, pH, solids concentration, ammonia, etc.).
  2. Verification of Pathogen Reduction. The applicant must demonstrate the minimum mandatory requirements listed in the table below with a high degree of certainty. Both process efficiency and process compliance parameters must be demonstrated. Applicants are highly encouraged to perform this testing in the laboratory or with a small-scale pilot unit. This suggestion is made because frequently pathogenic organisms must be added into the untreated sludge for testing to be possible. Practical aspects for demonstrating PFRP and analytical methods are discussed.

Criteria for Demonstrating Pathogen Reduction

Mandatory Minimum Requirements
  PSRP Equivalency PFRP Equivalency
Process Efficiency Parameters: ≥ 2 log reduction of fecal coliform bacteria
  1. ≥ 3 log reduction of total enteric viruses,and
  2. ≥ 2 log reduction of viable helminth (Ascaris) ova, and
  3. ≥ 3 log reduction of fecal coliform bacteria
Process Compliance Parameters
(The 40CFR503 Requirements):
< 2,000,000 MPN or CFU/g total solids (TS) of fecal coliform in the treated sludge Organism densities in the treated sludge of:
  1. < 1 pfu/4 g TS of total enteric viruses, and
  2. < 1 viable helminth (Ascaris) ova/4 g TS, and
  3. < 1,000 MPN fecal coliform / g TS or
  4. < 3 MPN Salmonella spp./4 g TS (applicant's choice)

Several optional organisms are listed in a separate table. EPA is collecting information on the fate of these microorganisms by different modes of treatment with the intent of gathering enough data to support their usefulness as surrogates in full-scale testing. Consider including as many of the optional analyses as feasible. The measurement of all or some of optional organisms will enhance your demonstration by increasing the level of confidence associated with the process under investigation and will be helpful later when the process is tested on a larger scale.

Optional Organisms for Process Efficiency¹

  PSRP Equivalency PFRP Equivalency
Any of the organisms listed can be quantified instead of or in addition to only the fecal coliform parameter listed in the mandatory table above ≥ 2 log reduction of E. coli bacteria
≥ 2 log reduction of Enterococcus spp. bacteria
≥ 3 log reduction of E. coli bacteria
≥ 3 log reduction of Enterococcus spp. bacteria
≥ 3 log reduction of Salmonella spp. bacteria
¹Note: Should any of the optional organisms be used in place of fecal coliform, fecal coliform (or Salmonella spp. in the case of PFRP) analysis on the treated sludge will still be required to meet the mandatory process compliance parameters.
  1. Demonstration of Successful Scale-Up. Equivalency recommendations are scale specific. The equivalency application requirements detailed above can be performed at any scale (laboratory-scale, pilot-scale, or full-scale). Applicants are encouraged to work at laboratory-scale up to this point due to ease of and increased control over the experiments. The Pathogen Equivalency Committee recommends performing the verification of pathogen reduction studies on pilot or full-scale only if an adequate number of helminthes and enteric viruses are naturally present. However, if all experimentation is performed at the lab or pilot-scale, only a lab or pilot-scale equivalency will be recommended. The applicant must show that the process is capable of performing reliably at the full-scale in order to receive a full-scale equivalency. This not only includes the microbial parameters above, but also the process and physiochemical parameters (stressors) identified as the critical control points in the process description. Experimentation at the full scale is where analysis of additional organisms may be very beneficial especially for PFRP equivalency recommendations.

As part pf a successful demonstration of scale-up, the applicant should put some effort into final development of the process. Details such as what process parameters will be measured, where, and how, and other operating parameters and additional conditions will need to be documented in an operation and maintenance manual should equivalency be recommended. In fact, an equivalency recommendation will not be finalized until such a document is received.

4. Appropriate Documentation of Field and Laboratory Procedures. Documentation that all data collection efforts submitted for equivalency determinations were conducted in accordance with EPA approved quality assurance/quality control requirements and all assays were conducted using EPA or EPA approved methods of analysis. This criteria is best satisfied by the preparation of a quality assurance project plan (QAPP) prior to the collection of data.

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Practical Aspects for PFRP Equivalencies

Over the last few decades the concentrations of naturally occurring enteric viruses and viable helminth ova routinely observed in raw sludge have dropped well below the levels necessary to demonstrate the required log reductions for the foundation of a PFRP equivalency as described in the table above. While this is good news from a public health standpoint, it has been problematic for entities applying for PFRP equivalency. Remember, PFRP equivalency must demonstrate removal efficiencies not just compliance with 40CFR503 limits. When the untreated sludges used in PFRP equivalency testing are below 1,000 pfu per gram total solids (dry weight) of enteric viruses and/or 100 viable helminth ova per gram total solids (dry weight) one of two options is available:

  1. Spiking. At the laboratory scale, spiking should not present much of a problem, however, at pilot or full-scale, appropriate spiking can be costly and difficult to carry out. In some cases, it may even be outright prohibited due to the potential health risks of an accidental release of pathogens. For these reasons, the Pathogen Equivalency Committee does not recommend use of this option for the purposes of demonstrating successful scale-up unless the spiking can be completed safely and effectively. If spiking is the preferred option, several issues must be addressed.
    • To determine the necessary number of organisms contained in the spike to ensure that the desired density of organisms in the untreated sludge is reached; it is recommended that a preliminary matrix spike be performed ion the sludge to be used. At a minimum, the spike density must be calculated using a conservative (i.e., highest reasonable) percent solids concentration expected and a generous safety factor to account for sludge toxicity and analytical method recovery. Determining the appropriate spike density and volume is critical to a successful outcome. If the number of organisms recovered from the untreated sludge after spiking is not adequate to demonstrate the required log reductions the effort and expense of the test will be wasted.
    • The applicant must document the source of the spiking organism, how it was collected and maintained, and how it was added to the untreated sludge.
    • The viability or titer of the spiking solution should be verified just prior to the actual spiking.
    • The spiked sludge must be assayed for the organisms of interest just prior to treatment. Estimates of the density of spiked organisms in the sludge based upon the number per unit volume of the organisms added to a given mass or volume of sludge is unacceptable.
    • Flow-through (typically continuously fed and completely mixed) treatment units present additional difficulty in ensuring that there is no short circuiting and that the spiked sludge is what is collected at the outfall. In these situations it is possible that any observed reduction in numbers can be attributed to, at least partially, dilution of the spiked sludges with unspiked sludges. Two suggested ways around this problem are adding a tracer to the spiked sludge and monitoring the tracer for dilution effects or extending the length of time over which the spike is added to greater than the calculated hydraulic residence time, then taking the treated samples following one full hydraulic residence time. Again, all conditions under which such experiments are performed must be specified and verified. It must be shown that measured log reductions are the result of treatment efficacy, not dilution.
  2. Surrogates. This option may require some spiking of pathogens, but only at the laboratory-scale. Appropriate use of surrogates is shown in the table below. During laboratory-scale testing one or more additional organisms native to the untreated sludge used would be enumerated alongside of the spiked enteric viruses and/or helminth ova. In scale-up, the reduction of the surrogate organisms can be used along with the established relationships to helminth ova and/or enteric viruses to support successful scale-up and equivalency. It is currently not possible to identify one single surrogate organism that will work in all cases as the levels of naturally-occurring organisms will differ from location to location and relationships between organisms will differ depending on treatment. However, some examples of possible surrogates for helminth ova include aerobic endospores and Clostridium perfringens, while some examples of possible surrogates for enteric viruses include reoviruses and somatic bacteriophages. An appropriate surrogate must be identified on a case-by-case basis. Applicants are welcome to suggest other organisms as surrogates as long as data supporting the choice of surrogate is obtained for the sludge and treatment process under evaluation.

Appropriate Use of Surrogate Organisms

  Enteric Virus Density in Feed Sludge Helminth Ova Density in Feed Sludge
  >1,000 pfu/4g TS ≤1,000 pfu/4g TS >100 ova/4g TS ≤100 ova/4g TS
Laboratory-Scale Analysis enteric viruses seeded enteric viruses
somatic bacteriophages
helminth ova seeded helminth ova
aerobic endospores
Clostridium perfringens
Full-Scale Analysis enteric viruses somatic bacteriophages
helminth ova aerobic endospores
Clostridium perfringens

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Conditions and Restrictions

The Pathogen Equivalency Committee may require additional terms, conditions, or restrictions to an equivalency recommendation when the mandatory supporting data in an equivalency application is substantial, but not sufficient to support a full equivalency or when uncertainty in the data is at an unacceptable level.

Additional terms and conditions are usually requirements for additional microbial monitoring at the full-scale above that which is required for routine reporting as required by 40CFR503. This additional monitoring is typically for a defined period of time and, following the collection of additional supporting data, the conditional term may be dropped from the equivalency.

A restriction on an equivalency recommendation would place limitations on access to and use of the land receiving the biosolids. These site restrictions would be similar to those associated with Class B sludge, such as crop harvesting, animal grazing, turf harvesting, and public access periods. You may read additional information regarding Class B site restrictions in Chapter 5, Section 5.5 (page 38), of EPA report Control of Pathogens and Vector Attraction in Sewage Sludge.

As mentioned above, measurement of some or all of the alternative organisms can be used to bolster the level of confidence associated with the process under investigation. This additional data could be used in these cases to make the determination of conditional terms and/or restrictions. Positive additional supporting data will lessen (or in some cases even eliminate the need for) terms, conditions, and/or restrictions when the evidence of the mandatory minimum disinfection verification requirements is weak.

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