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Biosolids are the nutrient-rich organic materials resulting from the additional treatment of sewage sludge in a municipal wastewater treatment plant. When treated and processed to meet specific quality standards, sewage sludge becomes biosolids, which can be safely recycled and applied as fertilizer and soil conditioner to stimulate plant growth and to improve and maintain productive soils.

Thirty years ago, many American cities dumped their raw sewage directly into our nation's oceans, rivers, lakes and bays. Today, because of improved wastewater treatment, our waterways are cleaner and safer for recreation and seafood harvest. As new and improved wastewater treatment facilities have come on line, the amount of biosolids produced nationwide has increased.

Municipal wastewater treatment uses the same processes that nature uses to clean the environment. In streams and lakes, natural aeration helps to purify the water, while microorganisms break down solids. Wastewater treatment plants use settling basins, aeration tanks and digestion or lime to reduce the pathogens—organisms that cause disease—and break down solids.

Municipal treatment facilities require pretreatment measures from businesses and industries to reduce contaminants from wastewater before it enters the sewer. Once wastewater is conveyed to a typical treatment facility, “grit materials” (such as sticks, rags and pebbles) are removed. The wastewater is next allowed to sit in large sedimentation tanks, where some solids settle to the bottom and are collected. The wastewater then continues on for digestion in large tanks where natural beneficial microbes are used to consume and break down wastes and nutrients. These microbes and other solid particles settle to the bottom of the tanks where they are collected. The cleaned wastewater may then receive additional treatment before it is disinfected and returned to waterways.

The solids that are collected from the primary and secondary treatment processes, called sludge, are collected and may undergo further biological treatment or lime stabilization to further decompose the material and destroy any potential disease organisms. These treatment processes reduce odors and destroy most of the potentially harmful pathogens contained in the solids. These treated, stabilized solids are now called biosolids and are mostly organic matter and minerals.