503 Regulations of 40 CFR was promulgated as part of the Clean Water Act to provide general requirements, pollutant limits, management practices and operational standards, for the final use or disposal of sewage sludge generated during the treatment of domestic sewage in a treatment works.

Standards are included in this part for sewage sludge applied to the land, placed on a surface disposal site, or fired in a sewage sludge incinerator. Also included in this part are pathogens and alternative vector attraction reduction requirements for sewage sludge applied to the land or placed on a surface disposal site. Presented in 503 Regulations are reporting requirements for Class I sludge management facilities publicly owned treatment works (POTWs) with a design flow rate equal to or greater than one million gallons per day, and POTWs that serve 10,000 people or more.

The Part 503 rules requires monitoring of sewage sludge that is land applied for metal concentrations, pathogen densities, and vector attraction reduction. In addition, soil testing for nutrients (Nitrogen, Potassium, and Phosphorus) may be useful at land application sites to help determine plant nutrient needs. Additional monitoring (water quality and vegetation) is not required by Part 503 for land application sites because the rule protects these resources through pollutant limits, management practices, pathogen reduction requirements, etc.

Determining the suitability of sewage sludge for land application by characteristic properties is often the first necessary step in planning and designing a land application system. Important properties of sewage sludge that need to be characterize include:

Quantity
Total solids content
Volatile solids content
pH
Organic matter
Pathogens
Nutrients (Nitrogen, Phosphorus, Potassium)
Metals (10 Metals)
Organic chemicals
Hazardous pollutants (Toxicity Characteristic Leaching Procedure, TCLP), if any.

Subpart B of the 503 rule prohibits the land application of sewage sludge that exceeds pollutant limits termed ceiling concentrations for 10 metals, and places restrictions on the land application of sewage sludge that exceeds additional pollutant limits specified in the rule (pollutant concentrations, cumulative pollutant loading rates (CPLRs), or annual pollutant loading rates (APLRs).

Arsenic, Cadmium, Chromium, Copper, Lead, Mercury, Molybdenum, Nickel, Selenium, and Zinc

Subpart D of the 503 rules describes requirements for land application of sewage sludge (and domestic septage) that reduce the potential for the spread of disease, thus protecting public health and the environment. Subpart D requirements cover two characteristics of sewage sludge - Pathogens and Vector Attractions.