Biosolids may be defined as organic wastewater solids that can be reused after suitable sewage sludge treatment processes leading to sludge stabilization, such as anaerobic digestion and composting.
Alternatively, the definition of biosolids may be restricted by local regulations to wastewater solids only after those solids have completed a specified treatment sequence and have concentrations of pathogens and toxic chemicals below specified levels.
The United States Environmental Protection Agency (EPA) defines the two terms – sewage sludge and biosolids – in the Code of Federal Regulations (CFR), Title 40, Part 503 as follows: Sewage sludge refers to the solids separated during the treatment of municipal wastewater (including domestic septage). In contrast, biosolids refers to treated sewage sludge that meets the EPA pollutant and pathogen requirements for land application and surface disposal. A similar definition has been used internationally, for example, in Australia.
Use of the term "biosolids" may officially be subject to government regulations. However, informal use describes a broad range of semi-solid organic products from sewage or sewage sludge. This could include any solids, slime solids or liquid slurry residue generated during the treatment of domestic wastewater including, scum and solids removed during primary, secondary or advanced treatment processes. Materials that do not conform to the regulatory definition of "biosolids" can be given alternative terms like "wastewater solids.