Reviewed by Zhiqiang Hu
Professor of Civil and Environmental Engineering

Biosolids is domestic wastewater sludge that meets standards for use as a fertilizer or soil conditioner. Biosolids standards include limitations for metal and other compounds, pathogen reduction, vector requirements and best management practices.

EPA regulations, under Title 40 Code of Federal Regulations Part 503 (40 CFR 503), establish the minimum national standards for the use and disposal of domestic sludge. These standards include limitations for the land application of biosolids. This publication outlines biosolids standards for pathogens and vectors.

40 CFR Part 503 divides biosolids into “Class A” and “Class B” biosolids in terms of pathogen reduction requirements. However, the terms are more commonly used to describe the suite of requirements and treatment methods for pollutants, pathogens, and vector attraction reduction.

Please refer to the United States Environmental Protection Agency for more on: 

Pathogen limitations for Class A biosolids

Class A biosolids are required for application onto public-use sites and certain food crops. This includes residential areas, road banks, parks, golf courses, schools and other similar areas.

Class A is also required for applying biosolids to turf, vegetable crops, root crops or home gardens.

Alternatives for Class A pathogen treatment

Class A biosolids must meet one of the following bacteria limitations and one of the process treatment alternatives:

Bacterial limitations

Biosolids must comply with one of the following:

  • Fecal coliform is less than 1,000 Most Probable Number (MPN) per gram of total solids (dry weight) (MPN/g-TS).
  • Salmonella sp. bacteria density is less than 3 MPN per 4 grams total solids (dry weight).

Class A Process treatment alternatives

Biosolids must also meet one of the following:

  • Maintain the sludge at the time, temperature and percent solids determined by using the formula in [(EPA Class A alternative 1, per 503.32(a)(3)].
  • Maintain the temperature of the sludge above 52 degrees Celsius for 72 hours. The sludge must be above pH 12. Air dry the sludge to 50 percent solids or higher, (EPA Class A alternative 2, per 503.32(a)(4).
  • Use other treatment process to achieve the following:
    • Enteric virus density must be less than 1 Plaque Forming Unit (PFU) per 4 grams of total dry weight solids.
    • Viable helminth ova density must be less than 1 per 4 grams of total dry weight solids, [EPA Class A alternatives 3 or 4, per 503.32(a)(5) and (6)].
  • Use a Process to Further Reduce Pathogens (PFRP) or equivalent treatment process approved by the permitting authority, [EPA Class A Alternative 5 or 6, per 503.32(a)(7) and (8)] (Table 1).

Table 1
Processes to Further Reduce Pathogens (PFRP)


Pathogen limitations for Class B biosolids

Apply Class B biosolids on grain and forage crops, pastures, grassland, fallowland and timberland.

The alternatives for Class B biosolids consist of either a treatment process, such as a Process to Significantly Reduce Pathogens (PSRP) or a bacteria limitation, based on fecal coliform.

Fecal coliform testing is recommended for all treatment processes for at least two years of operation in order to verify PSRP performance. Conduct tests during seasons of the year when biosolids will be applied. Equivalent PSRP processes must be approved by the permitting authority.

Alternatives for Class B pathogen treatment

  • Fecal coliform test the biosolids. The geometric mean of the density of fecal coliform must be less than 2,000,000 Colony Forming Units (CFU) or 2,000,000 MPN, per gram of total dry weight solids. Collect seven grab samples of sludge within one day.
  • Use PSRP or an equivalent treatment method approved by the permitting authority (Table 2).

Table 2
Processes to Significantly Reduce Pathogens (PSRP)


Table 3
Processes approved by EPA as equivalent to PSRP. (Control of Pathogens and Vector Attraction in Sewage Sludge, EPA/625/R-92/013, Dec. 1992.)


Alternatives for vector attraction treatment

All biosolids (Class A and Class B) must meet one of the following alternatives for vector attraction reduction:

  • Reduce volatile sludge solids to 38 percent.
  • Alternate sludge testing for volatile solids:
    • Digest sludge samples in laboratory (30 days for aerobic sludge and 40 days for anaerobic sludge). The resulting volatile solids reduction during the testing must be less than 15 percent for aerobic sludge and less than 17 percent for anaerobic sludge.
  • SOUR (Specific Oxygen Uptake Rate) is less than 1.5 milligrams oxygen per hour per gram of total dry weight solids at 20 degrees Celsius. For anaerobic sludge, the sample must be aerated in the lab until dissolved oxygen saturation is reached before testing.
  • Aerobic sludge digestor at an average temperature of greater than 45 degrees Celsius for more than 14 days and at least 40 degrees Celsius.
  • The pH must be greater than 12 for two hours and greater than 11.5 for at least 22 hours.
  • Dry sludge to less than 25 percent moisture for stabilized sludge or less than 10 percent for primary sludge.
  • Subsurface inject the sludge.
  • Incorporate the sludge into the soil within six hours after surface spreading.
  • If the sludge is domestic septage only, the pH must be greater than 12 for 30 minutes.
  • Use an equivalent method approved by the permitting authority.


Original authors: Ken Arnold, Chief of Land Application, Missouri Department of Natural Resources; John H. Dunn, Environmental Engineer, Environmental Protection Agency Region VII; Dennis Sievers, Department of Agricultural Engineering