Biosolids is a term used by the water treatment industry that refers to treated sludge. Sludge, or “biosolids,” is the byproduct of the treatment of domestic and commercial wastewater or sewage in a wastewater treatment plant. During waste water treatment, bacteria and other microorganisms break down components in wastewater into simpler and more stable forms of organic matter. Non-organic matter also settles into sludge. Biosolids in their liquid form look like muddy water and contain 1-10% solids. Biosolids may be dewatered in a second step of the treatment process, which turns it into a “cake” with the texture of a wet sponge. In this stage it contains 11-40% solids.
According to US EPA, biosolids that meet treatment and pollutant content criteria “can be safely recycled and applied as fertilizer to sustainably improve and maintain productive soils and stimulate plant growth.” The US EPA’s promulgated regulation is 40 CFR Part 503.