Biosolids Standards for Metals and Other Trace Substances

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 trace substances, pathogen reduction, vector requirements and best management practices. This publication outlines requirements for metals and other trace substances.

Applying biosolids to land uses the available nitrogen, phosphorus and potash as fertilizer for growing crops. It is an environmentally sound practice sanctioned by the U.S. Environmental Protection Agency (EPA) and the Missouri Department of Natural Resources (DNR).


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. The standards include limitations for the land application of biosolids.

The DNR incorporated the EPA standards into the state requirements under the Missouri Clean Water Law and regulations. The state rules also include additional requirements that are not covered in the EPA standards. Complying with state regulations automatically meets the EPA standards.

Standards for biosolids land application

The amount of metals and other pollutants in the biosolids determines the acceptability for land application and the appropriate loading rates to protect crops, soils and the environment.

Limitations consist of concentration-based limits and pounds per acre limits. State guidelines for trace compounds also apply when an EPA standard has not been published for a specific pollutant.

Ceiling concentration

The ceiling concentration is also referred to as ceiling concentration limits (CCL). This is the maximum concentration of each pollutant allowed in biosolids for land application. According to Part 503, biosolids containing any metal that exceeds the CCLs cannot be land applied. You may apply biosolids on land if the ceiling concentrations are not exceeded (Table 1). When necessary, it is permissible to mix the material with lower concentration biosolids or other suitable materials, such as sawdust, to meet the concentration limits.

Table 1
Concentration-based limits (40 CFR 503 and Missouri Department of Natural Resources, Permit Standard Conditions Part IV, June 1993.)

Biosolids ceiling concentration1
PollutantMilligrams per kilogram dry weight
1Land application is not allowed if the sludge concentration exceeds the maximum limits for any of these pollutants.

Low metals concentration

The low metals concentration is also referred to as pollutant concentration limits (PCL). The low metal concentration biosolids has reduced requirements because of its higher quality (Table 2). Biosolids with metal concentrations below PCL can be land applied without obtaining a permit. You may safely apply these biosolids for 100 years or longer at typical agronomic loading rates. Records are required for each application site. Concentration limits for unrestricted use (non-tracking of sites) is established on a case-by-case basis by DNR.

Table 2
Biosolids low metal concentration.1 (40 CFR 503 and Missouri Department of Natural Resources, Permit Standard Conditions Part IV, June 1993.)

PollutantMilligrams per kilogram dry weight
1You may apply low metal biosolids without tracking cumulative metal limits, provided the cumulative application of biosolids does not exceed 500 dry tons per acre.
* The February 25, 1994 Part 503 Rule Amendment deleted the molybdenum limits but retained the molybdenum CCL.

Pounds per acre limits

Each pollutant in Table 3 has an annual and a total cumulative loading limit, based on the allowable pounds per acre for various soil categories. See Table 4 for pollutants that do not have a published EPA standard.

Table 3
Pounds per acre limits by soil Cation Exchange Capacity (CEC). (Missouri Department of Natural Resources, Permit Standard Conditions Part IV, June 1993)

PollutantCEC 15+CEC 5 to 15CEC 0 to 5
1Total cumulative loading limits for soils with equal or greater than 6.0 pH (salt-based test).

Table 4
Guidelines for land application of other trace substances.1

Cumulative loading
PollutantPounds per acre
Dioxin(10 ppt in soil)3
Other substance4
1Design of Land Treatment Systems for Industrial Waste, 1979. Michael Ray Overcash, North Carolina State University and Land Treatment of Municipal Wastewater, EPA, 1981.)
2This applies for a soil with a pH between 6.0 and 7.0. Case-by- case review is required for higher pH soils. Leachable aluminum in biosolids mixture must not exceed 0.2 milligram per liter based on ASTM method D-3987.
3Total Dioxin Toxicity Equivalents (TEQ) in soils, based on a risk assessment under 40 CFR 744, May 1991.
4Case-by-case review. Concentrations in sludge should not exceed the 95th percentile of the National Sewage Sludge Survey, EPA, October 1989.
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