What are Biosolids

Sewage Sludge that is specially treated and meets the most stringent pollutant, pathogen, and vector attraction reduction set by EPA requirements may be purchased by the public from hardware stores, home, and garden centers, or their local wastewater treatment plant. What are Biosolids?

Biosolids are a product of the wastewater treatment process. During wastewater treatment, the liquids are separated from the solids. The solids are then treated physically & chemically to produce a nutrient-rich product known as biosolids. Biosolids and Sewage Sludge are often used interchangeably.

Raw Sewage Sludge refers to the solids separated during the treatment of municipal wastewater. The definition includes domestic septage. Terms Biosolids refer to treated sewage sludge that meets the EPA pollutant and pathogen requirements for land application and surface disposal.

What are Biosolids


Biosolids are nutrient-rich organic materials resulting from the treatment of domestic sewage in a wastewater treatment facility. Biosolids are a beneficial resource,
containing essential plant nutrients and organic matter, and are recycled as a fertilizer and soil amendment. Biosolids are created through the treatment of domestic wastewater generated from sewage treatment facilities.


The treatment of biosolids can actually begin before the wastewater reaches the sewage treatment plant. In many wastewater treatment systems, regulations require that industrial facilities pre-treat their wastewater to remove hazardous chemical contaminants before sending it to a wastewater treatment plant.

Wastewater treatment facilities monitor incoming wastewater streams to ensure their recyclability and compatibility with the treatment plant process. Once the wastewater reaches the plant, the sewage goes through a biological process that cleans the wastewater and removes the solids. The excess biological solids are then digested or stabilized through other processes to reduce or eliminate pathogens.

Through decades of research, the scientific and agricultural communities have come to understand that municipal sludge or “biosolids” contain critical nutrients and organic matter that improve the makeup of the soil in a way similar to animal manures.

It is important to understand that biosolids are not raw sewage. Biosolids are organic solids that have been treated to stabilize organic matter and reduce disease-causing organisms or pathogens.

The use of biosolids in agriculture is controlled by what is known as the 503 regulations (40 CFR Part 503), which took effect in 1993. These federal regulations were developed by the U.S. Environmental Protection Agency after nearly 10 years of research on the possible risks due to diseases, heavy metals, or other components of the biosolids.

The regulations set limits on the amount of metals and pathogens that can be involved in biosolids; they also have requirements to lessen the possibility that the biosolids will attract flies or other pests that might spread disease. The law requires that specific conditions be met before biosolids can be applied to the land.

After treatment and processing, these residuals can be recycled and applied as fertilizer to improve and maintain productive soils and stimulate plant and crop growth. Farmers and gardeners have been recycling biosolids for ages, reducing the need for chemical fertilizers.

Biosolids are used to promote the growth of agricultural crops, fertilize gardens and parks, and reclaim mining sites. When applied to crops application the amount of biosolids is restricted to the nutrient needs of the particular crop.

The plant nutrients are slowly released throughout the growing season enabling the crop to absorb these nutrients slowly as the crops grow. This efficiency lessens the likelihood of groundwater pollution of nitrogen and phosphorus.

Biosolids are one of the most studied materials that have ever been regulated by the U.S. Environmental Protection Agency (USEPA). Decades of studies have demonstrated that biosolids can be safely used for the production of crops that are consumed by the public.


What is the Difference Between Biosolids and Sewage Sludge


Sludge is created in a solid or semi-solid state and is a by-product of wastewater treatment, sewage treatment, water treatment, or on-site sanitation systems.

Sewage sludge is produced from the treatment of wastewater in sewage treatment plants and consists of two basic forms 

  1. Raw primary sludge
  2. Secondary sludge is also known as activated sludge in the case of the activated sludge process.

Sewage Sludge” refers to the solids separated during the treatment of municipal wastewater. The definition includes domestic septage. “Biosolids” refers to treated sewage sludge that meets the EPA pollutant and pathogen requirements for land application and surface disposal.

The most common treatment of sewage sludge is by anaerobic digestion to pathogen reduction levels. Methane that is generated during the digestion process is used for co-generation or heating. About 1/3 of the biosolids receive further treatment to a higher class of treatment by lowering pathogen reduction levels, by means such as composting, solar air-drying, alkali treatment, thermophilic digestion, pasteurization, or heat drying.

EPA collects annual biosolids reports from roughly 2,500 larger facilities in the U.S. These annual biosolids reports are required by Part 503 for the larger public facilities that land apply, incinerate, or dispose of their sewage sludge via surface disposal.

Many small treatment plants use methods of treatment other than anaerobic digestion, such as air drying, aerobic digestion, or lime treatment. Once treated, sewage sludge is then dried and added to a landfill, applied to agricultural cropland as fertilizer, or bagged with other materials and marketed as “biosolid compost” for use in agriculture and landscaping.



Bio-solids are the left-over stabilized product from sewage treatment that looks like topsoil or peat that is rich in nutrients and can be applied to soil to promote the growth of agricultural crops and fertilize gardens.



JimGalloway Author/Editor



References: EPA- Sewage Sludge/Biosolids Program 


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