Sludge

Sludge is semi-solid material produced by the treatment of liquids, comprising water, organic matter and solids. It forms as a byproduct of wastewater treatment, industrial processes, mining and agriculture. The term is used for settled or thickened suspensions that require further handling before disposal or reuse.

Municipal wastewater sludge includes primary sludge from sedimentation and secondary or biological sludge from microbial treatment. Industrial sludges arise from chemical production, metal finishing, pulp and paper, and food processing. Biosolids is a term commonly used for properly treated municipal sludge approved for beneficial reuse, such as land application.

Composition varies widely but typically contains water (often 80–99%), organic matter, nutrients, inorganic solids, and contaminants such as metals, salts, and trace organics. Pathogens are a concern in untreated sludges; heavy metals and emerging contaminants may require monitoring and limits for reuse.

Management generally follows sequence: thickening, stabilization (for example anaerobic digestion or lime stabilization), and dewatering to reduce volume. Treated sludges may be incinerated, landfilled, composted, or beneficially reused as biosolids or soil amendments, subject to regulatory limits and environmental risk assessments. Energy can be recovered from digestion.

Regulation governs permissible handling, testing, storage, and disposal. In the United States, biosolids are regulated under EPA Part 503; in the European Union, sludge use in agriculture is controlled by directives with quality criteria. Public health and environmental protection guide sludge policy worldwide.