What Does Biosolids Mean?
Biosolids are nutrient-rich organic materials that are a byproduct of wastewater treatment. If treated and managed properly, biosolids can be used as a substitute for commercial fertilizers, as a soil amendment in farming, and as a component of topsoil or compost.
Safeopedia Explains Biosolids
When wastewater undergoes a treatment process, it results in three separate types of output:
- Effluent that can be safely released into the environment
- Sewage sludge that contains pathogens and is not considered safe for use
- Biosolids that have a higher concentration of solids that sewage sludge and contains fewer pathogens as well
To achieve that state, biosolids must go through a stabilization process to eliminate pathogens, such as chemical dosing, digestion, or drying.
Contents of Biosolids
Biosolids are derived from wastewater sludge, which is a mixture of water and organic material. Biosolids may contain:
- Macronutrients such as phosphorus, potassium, sulfur, and nitrogen
- Micronutrients like calcium, magnesium, and manganese
- Metals such as arsenic, lead, cadmium, selenium, nickel, and chromium
- Traces of synthetic organic compounds
Application of Biosolids
Biosolids are used in a variety of applications, including:
- Improving soil productivity
- Stimulating plant growth
- Fertilizing gardens and parks
- Rehabilitating mining sites
- Land rehabilitation
- Landscaping and topsoil
Categories of Biosolids
In the US, biosolids are categorized as Class A and Class B, as defined under the national regulation USEPA 40 CFR Part 503. Individual states may also impose more stringent requirements or additional criteria.
Class A biosolids must have non-detectable pathogen levels (especially fecal coliforms and salmonella) and levels of odor, metals, and vector attraction reduction (VAR) within stipulated thresholds. Stabilization processes used to achieve this state include anaerobic digestion, aerobic digestion, alkaline stabilization, and thermal hydrolysis.
Class B biosolids contain higher pathogen levels, restricting their application to those that have limited risk of human exposure.