Publicly owned water treatment works and onsite wastewater systems generate by-products known as biosolids, leftover organic materials that can serve as valuable soil amendments for agricultural production. In Kansas, approximately 80% of wastewater treatment facilities recycle biosolids through land application rather than disposing of them in landfills or incinerating them. When handled properly, biosolids can provide nutrients and improve soil properties, offering both agronomic and environmental benefits.
Figure 1. Land application of biosolids is being done on cropland, which will then be incorporated with tillage. Photo by DeAnn Presley, K-State Extension.
What Are Biosolids?
Biosolids, sometimes called sewage sludge, are the organic solids separated from wastewater during treatment. They typically contain 2–15% solids and are most often applied as a liquid or semi-solid material. Depending on the treatment process, biosolids can also be dewatered, composted, or dried and sold as a soil amendment (though not for certified organic production).
Nutrient Value
The main agronomic advantage of biosolids is their nutrient and organic matter content. They supply nitrogen (N), phosphorus (P), and smaller amounts of potassium (K), along with organic carbon that enhances soil microbial activity and water-holding capacity. Biosolids also contain small amounts of secondary nutrients (e.g., sulfur, calcium, magnesium) and micronutrients (e.g., zinc and copper) that can contribute to overall soil fertility. On average, one dry ton of biosolids contains roughly 80 pounds of N, 200 pounds of P₂O₅, and 10 pounds of K₂O. However, much of the N is in an organic form that becomes plant-available slowly over time, a process in soil called mineralization. Proper application rates must account for mineralization, ammonia volatilization losses, and nitrate leaching. To minimize ammonia volatilization, avoid surface applications when temperatures exceed 50°F. Research shows that, depending on the treatment used to produce the sludge, about 10 to 50 percent of the organic N will be available in the first year after application. On the other hand, usually all of the P and K found in biosolids is considered plant-available during the year of application.
Regulations Governing Biosolids Application
Land application of biosolids is regulated under the EPA 503 rules (40 CFR Part 503), which set limits on pathogens, metals, and nutrient application rates to protect public health and the environment. In Kansas, these regulations are administered by the Kansas Department of Health and Environment (KDHE).
Pathogens. EPA 503 rules classify biosolids into two categories based on pathogen reduction:
- Class A biosolids are essentially pathogen-free and can be used on lawns, gardens, and crops for direct human consumption.
- Class B biosolids meet a lower level of pathogen reduction and are typically applied to cropland or pasture, with restrictions on harvest and public access.
Vector attractions (i.e., rodents and insects). Specific standards and treatments are required to reduce the tendency of biosolids to attract rodents, flies, mosquitoes, or other vectors that may carry diseases. Class A biosolids must incorporate multiple factors that reduce vector attraction to a minimum. Class B biosolids have less stringent requirements for treatment because they are often used in less populated areas.
Metals. Biosolids applied to land must meet limits for nine metals, including cadmium, copper, lead, and zinc. Kansas biosolids rarely exceed these thresholds.
Nutrients. Application rates must not exceed the agronomic rate (the N rate required for the intended crop). Over-application can increase the risk of nitrate leaching to groundwater or phosphorus runoff to surface water. KDHE provides specific forms and guidance to help treatment facilities and haulers determine proper application rates and maintain compliance.
Emerging Contaminants in Biosolids
In addition to valuable nutrients, biosolids can also contain emerging contaminants such as microplastics, pharmaceutical residues, and per- and polyfluoroalkyl substances (PFAS). These materials can persist in soil or water after land application, and researchers continue to study their long-term effects on human health and the environment. According to the U.S. EPA, certain PFAS may lead to adverse health outcomes, and ongoing efforts are focused on improving wastewater treatment and reducing PFAS sources before they reach biosolids.
Environmental and Safety Considerations
The EPA 503 standards are designed to minimize risks to human health and the environment, but additional precautions are necessary. Workers handling biosolids should wear protective gear, avoid inhaling or ingesting particles, and follow safety training protocols. Vaccinations for potential disease exposure may be recommended for frequent handlers.
To prevent soil compaction, biosolids should not be applied when soils are wet. Heavy application vehicles can negate soil health benefits provided by the organic matter. Proper timing, traffic management, and suitable access roads reduce these risks.
Nutrient losses, particularly phosphorus runoff, can harm water quality. Producers are encouraged to evaluate fields using the Phosphorus Index and adopt Best Management Practices (BMPs) such as buffer strips, incorporation of biosolids, and erosion control. Odor management is also important. Anaerobically treated sludge should be injected or incorporated into the soil, and applications near residences should maintain adequate setbacks (typically 500 feet or more).
Site Restrictions and Management Practices
The EPA 503 regulations outline specific management practices to ensure biosolids are applied safely and responsibly.
Biosolids must not be applied:
- If an adverse effect on threatened or endangered species is likely.
- To flooded, frozen, or snow-covered land where solids could enter a wetland or body of water.
- Within 33 feet (10 meters) of any body of water.
- At rates exceeding the agronomic nitrogen rate, except on permitted reclamation sites.
Class B biosolids have additional restrictions:
- Harvest delays:
- Food, feed, or fiber crops whose edible parts do not touch the soil (e.g., corn, wheat, soybeans, apples, peaches) may not be harvested for 30 days after application.
- Crops with edible parts that may contact the soil (e.g., melons, tomatoes, lettuce) require a 14-month waiting period.
- Root crops (e.g., potatoes, beets, carrots) require at least 38 months if biosolids are incorporated within 4 months, or 20 months if they remain on the surface longer.
- Grazing restriction: Animals must not graze on treated land for 30 days following application.
- Turf restriction: Turf intended for high human-use areas cannot be harvested for 1 year unless specifically authorized by KDHE.
- Public access: Sites with potential human exposure must remain closed for 1 year, while areas with low exposure risk must restrict access for 30 days (not including operators or employees familiar with safety precautions).
Determining Agronomic Nitrogen Rates
To ensure proper application, KDHE developed the LA-ANR form, based on K-State soil fertility research. This procedure helps calculate the nitrogen availability from biosolids, credits existing soil nitrogen and previous applications, and determines a safe and effective application rate. Producers are required to maintain application records, including soil test results and field maps, for at least five years.
Summary
When properly managed, biosolids provide Kansas farmers with a cost-effective source of nutrients and organic matter. However, successful use depends on understanding nutrient content, following EPA and KDHE regulations, and adopting sound management practices to protect human health and the environment.
Producers interested in using biosolids can contact their local wastewater treatment plant or the Kansas Department of Health and Environment, Bureau of Water, for more information on availability, permitting, and application guidelines.
This article is a summarized version of the KSRE publication “Use of Biosolids on Agricultural Land: Agronomic, Environmental, and Safety Considerations” (MF2878). Read the full publication online at: https://bookstore.ksre.ksu.edu/pubs/MF2878.pdf
DeAnn Presley, Soil Management Specialist
deann@ksu.edu
Tags: biosolids land application