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What Are Biosolids

Biosolids are mostly organic solids resulting from the treatment of wastewater that have undergone additional treatment to kill pathogens.

During wastewater treatment, bacteria and other tiny organisms break sewage down into simpler, harmless organic matter. The organic matter combined with bacterial cell masses, settles out to form biosolids. Biosolids, which are rich in nutrients, have been approved by the U.S. Environmental Protection Agency (EPA) for land application as a fertilizer and soil amendment and for composted products.

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 wastewater treatment facilities are modernized with the newest and best technology, the amount of biosolids produced in Virginia and 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 undergo additional 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 can then be recycled on farms and forests, or composted.