Energy-efficient recovery of the resource sewage sludge by pyrolysis

Sewage sludge is a bio-based resource that contains phosphorus, potassium and carbon. Currently, the sludge is largely spread on fields, thereby recovering these substances. The sludge can contain contaminants such as cadmium, pharmaceuticals, infectious agents, etc. An alternative is to make biochar from the sludge through pyrolysis. This eliminates most of the contaminants while the desirable substances remain. The biochar becomes a valuable fertilizer raw material.

The sludge contains 75 percent water that must be dried, which is energy-intensive. This feasibility study aimed to investigate the energy benefits of combining it with an incineration plant from which heat is taken for drying and to which moisture-saturated gases and flue gases from the pyrolysis can be returned for energy recovery and gas purification.

All stakeholders in this new value chain have participated in the project: wastewater treatment, energy company, technology supplier and recycling company.

One purpose of the feasibility study has also been to develop a basis for a subsequent pilot-scale project.

The project results show that pyrolysis of sewage sludge and production of district heating from waste can generate a synergy where:

• The total district heating yield increases
• The cost level becomes reasonable in relation to today’s sludge disposal costs
• The climate impact from sludge management is estimated to be lower

In this value chain, sludge biochar is intended to be used as a phosphorus resource for agriculture. The challenge for the future lies primarily in the fact that agricultural demand for sludge biochar is not known. At the same time, the proposed technical integration requires further in-depth investigation.