What Is sewage sludge and what can it be used for?
Sewage sludge is a high–water-content by-product generated during wastewater treatment. It contains organic and inorganic matter, microorganisms, and various contaminants. Although it was long regarded solely as waste, it is increasingly being treated as a resource that must be managed within environmental protection frameworks.
Wastewater treatment consists of several stages, and sludge can be generated at each of them.
Types of sewage sludge
1. Primary sludge
Formed from settleable solids during primary sedimentation. It has a high organic content, is poorly stabilised, and typically odorous.
2. Secondary sludge
A by-product of biological treatment, consisting of living and dead microorganisms (activated sludge). Due to its very high water content and dilute structure, it must be dewatered—typically using special centrifuges—before utilisation or transport.
3. Mixed sludge
A mixture of primary and secondary sludge. This is the most common sludge type subjected to further treatment at municipal wastewater treatment plants.
Main characteristics of sewage sludge
Water content: 95–99% (before treatment).
Organic matter content: significant.
Nutrients: nitrogen, phosphorus.
Risk components: pathogens, heavy metals, micropollutants.
Because of these risks, sewage sludge treatment and disposal are strictly regulated.
Main steps of sewage sludge processing
1. Thickening
The objective is to reduce water content—and thus volume. This is achieved using gravity thickeners or flotation processes.
2. Dewatering
For transport and/or further treatment (e.g. biogas production), sludge must be dewatered. The most common technologies are:
- centrifuges,
- belt filter presses,
- chamber filter presses.
After dewatering, the dry solids content typically reaches 20–35%.
3. Stabilisation
To reduce biological activity and odour formation, sludge must be stabilised. In anaerobic digestion for biogas production, stabilisation occurs inherently: part of the organic matter is degraded, biogas (methane) is produced, and the sludge becomes more stable.
An alternative method is lime stabilisation, applied when sludge is not intended for biogas production.
4. Post-treatment / disposal
Depending on final use, residual sludge may be:
- dried,
- composted,
- subjected to thermal treatment,
- disposed of at designated sites (an increasingly discouraged and unsustainable option in Europe).
Utilisation options for sewage sludge
1. Agricultural use
Possible only if quality requirements, continuous monitoring, and regulatory permits are met. Sludge may be applied as a soil conditioner or fertiliser on non-food-producing land. Dewatered sludge is often used for land reclamation, although digested sludge generally has relatively low nutrient content.
2. Energy recovery
The most common route is biogas production via anaerobic digestion. The methane produced can be combusted in gas engines to generate electricity, significantly contributing to the energy self-sufficiency of wastewater treatment plants and reducing greenhouse gas emissions. Methane can also be upgraded and injected into the natural gas grid.
3. Material recovery
Phosphorus recovery from sewage sludge is possible, and research is ongoing into its use as an additive in construction materials.