Biosolids, a product derived from wastewater treatment, are increasingly utilized in agriculture as a soil amendment. As their use becomes more widespread, questions arise regarding their safety. Understanding their benefits, potential risks, and governing regulations is important for informed discussions.
What Are Biosolids?
Biosolids are treated solid organic materials resulting from the wastewater treatment process. During this process, liquids are separated from solids, which then undergo further treatment to stabilize the material. This treatment significantly reduces disease-causing pathogens and volatile organic matter. While raw sewage sludge contains around 3% solids, treated biosolids typically have a solids content ranging from 15% to 90%.
Wastewater treatment facilities process used water from homes, including kitchens, laundries, and bathrooms. Industrial discharges into the sewer system are also regulated to limit potentially harmful compounds. Microorganisms in treatment plants break down organic solids, and mechanical processes then reduce the water content. This transforms raw solids into biosolids.
Agricultural Applications and Benefits
Biosolids are widely applied in agriculture as a soil amendment and fertilizer, with approximately 70% of biosolids in the United States being land-applied to agricultural land, pastures, and rangelands. They provide essential macro and micronutrients necessary for plant growth. These nutrients include nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, copper, zinc, iron, boron, and molybdenum.
The application of biosolids can enhance soil fertility by adding organic matter, which improves soil structure and increases its capacity to retain water and nutrients. Nitrogen in biosolids is released slowly over time, providing a sustained nutrient supply for crops throughout their growing season. This slow-release property can reduce the need for synthetic fertilizers, contributing to nutrient recycling. Studies have shown that crops like corn, wheat, and soybeans can achieve yields similar to or exceeding those grown with chemical fertilizers when biosolids are applied.
Examining Potential Concerns
Despite their benefits, biosolids can contain various contaminants that raise concerns regarding their agricultural use. Heavy metals like arsenic, cadmium, lead, mercury, and nickel may be present, originating from industrial and household discharges. While regulations set limits, their potential for plant uptake and soil accumulation remains a consideration.
Pathogens, including bacteria, viruses, protozoa, and helminths, can also be found in biosolids, though treatment significantly reduces their numbers. These disease-causing microorganisms could pose a public health risk if transferred to food crops, or if they migrate into water bodies through runoff or leachate. Pathogen survival depends on environmental conditions, with some, like viruses and parasites, potentially persisting longer than bacteria.
Emerging contaminants represent another area of concern. Pharmaceuticals and personal care products (PPCPs), such as triclocarban and triclosan, are detected in biosolids and can enter the environment through land application. These compounds are designed to be biologically active, and their long-term effects on non-target organisms and ecosystems are under study.
Microplastics, tiny plastic particles, are also present in biosolids as they are removed from wastewater during treatment. While biosolids contribute to microplastic presence in agricultural soils, other sources also exist, and research is ongoing regarding their environmental fate and potential impacts.
Per- and polyfluoroalkyl substances (PFAS), known as “forever chemicals” due to their persistence, also accumulate in biosolids. These substances can leach from land-applied biosolids into groundwater and surface water. Concerns exist about their potential for bioaccumulation in organisms and entry into the food chain, impacting both wildlife and agricultural products.
Regulatory Oversight and Monitoring
To ensure the safe use of biosolids in agriculture, regulatory bodies establish comprehensive standards and monitoring requirements. In the United States, the Environmental Protection Agency (EPA) sets federal regulations under 40 CFR Part 503, which address pollutant limits, pathogen reduction, and management practices. These regulations aim to protect public health and the environment from potential adverse effects.
Biosolids are categorized into different classes based on their treatment levels and associated usage restrictions. Class A biosolids undergo processes that further reduce pathogens, leading to virtually undetectable levels of disease-causing organisms. These highly treated biosolids have fewer restrictions on their use and can be applied in areas with public contact, such as home gardens and parks.
Class B biosolids undergo a process to significantly reduce pathogens, meaning some pathogens may still be present. Their agricultural application requires additional management practices, including site restrictions, limitations on public access, and harvesting restrictions to allow for natural pathogen die-off.
Beyond pathogen and pollutant limits, regulations also address vector attraction reduction, minimizing the potential for insects or rodents to spread contaminants. Monitoring involves regular testing of biosolids for metals, nutrients, and pathogens. Best management practices, such as applying biosolids at agronomic rates to match crop nutrient needs and considering phosphorus content to prevent buildup, are also emphasized to minimize environmental risks. Many states and local jurisdictions implement additional, often more stringent, requirements beyond federal guidelines to further ensure safety.