Sewage sludge

Wastewater treatment plants perform an essential function: cleaning urban and industrial water before returning it to the environment. That process produces a high-volume by-product known as sewage sludge (in Spanish, lodos de depuradora).

Far from being only a problem, sludge is also an opportunity for the circular economy, because it can become a resource for agriculture or a source of renewable energy through processes such as composting or anaerobic digestion.

What is sewage sludge?

Sewage sludge is the semi-solid residue generated during wastewater treatment. It typically contains:

• Suspended organic matter.
• Nutrients (nitrogen, phosphorus and potassium).
• Water: up to 80-90% in sludge that has not been dewatered.
• Heavy metals and emerging contaminants such as microplastics and pharmaceutical residues.
• Microorganisms, including potential pathogens.

It is produced at different stages: primary sludge from initial settling, secondary sludge from biological treatment, and tertiary sludge from advanced treatment.

Applicable regulation

• Directive 86/278/EEC on the protection of the environment when sewage sludge is used in agriculture.
• Royal Decree 1310/1990, which regulates the use of sludge in the agricultural sector in Spain.
• Law 7/2022 on waste and contaminated soil.
• Regulation (EC) 1069/2009 on animal by-products, relevant for some sludge.
• Regional legislation in autonomous communities such as Catalonia and the Basque Country.

Treatment methods

1. Thickening and dewatering: reducing water content using centrifuges, filter presses or drying beds.
2. Stabilisation: biological processes (aerobic or anaerobic digestion) or chemical treatment with lime.
3. Sanitisation: reducing pathogens for safe agricultural use.
4. Final recovery or disposal: composting, anaerobic digestion with biogas production, incineration with energy recovery, or controlled application to soils.

Uses of sewage sludge

Agriculture

• Direct application as a fertiliser or as compost, providing nitrogen, phosphorus and organic matter.

Energy

• Production of biogas in anaerobic digesters.
• Combined heat and power generation, and thermal use in cement kilns after drying.

Civil works

• Use of ash from incinerated sludge in cement manufacturing, and as fill and soil-stabilisation material.

Associated risks

• Chemical contamination: heavy metals and persistent organic compounds.
• Biological contamination: pathogens if sanitisation is inadequate.
• Odour and local nuisance.
• Soil and groundwater impacts if sludge is applied without proper control.

Relationship with the circular economy

Sewage sludge is a clear example of how a waste can become a resource: as a circular fertiliser replacing mineral fertilisers, as renewable energy through biogas, and as a source of construction materials from ash and by-products. Closing the loop, however, requires overcoming contamination challenges and earning social acceptance.

Sustainable management of sewage sludge is key to advancing towards a circular economy of water and waste in which nothing is lost and everything is turned into a resource. At Manglai we help companies measure their environmental footprint and prepare their sustainability reporting. Discover how Manglai can help you.