About the project

Municipal wastewater treatment generates sludge, which contains almost all the phosphorus found in the sewage as well as a large part of the organic material. A ban on sludge spreading on farmland is being discussed in Sweden, but regardless of a ban or not, municipal wastewater works recognise a need for complements/alternatives to today’s sludge management.

Pyrolysis with sludge biochar spreading on farmland

Pyrolysis with spreading of sludge biochar on farmland has in system studies (von Bahr et al., 2017; Grundestam et al., 2020) proven to be better than sludge spreading with regard to climate impact, acidification, eutrophication and dispersion of heavy metals such as cadmium and mercury. The energy requirement is greater but the treatment plant can still produce an excess of electricity, heat and/or biogas. However, the need for resources is greater and the maturity of technology is lower.

Important benefits of adding sludge biochar to agricultural land are improved soil properties and the fact that carbon is more stable in sludge biochar than in ordinary sludge and thus acts as an carbon sink. In order to be able to do a fair system study, extended system and sustainability analyses are needed, but also pilot studies. However, pyrolysis has been shown, in terms of most parameters, to be better than mono-combustion of sludge. For example, pyrolysis plants do not have to be as large-scale and a large part of the carbon is found in the sludge biochar, while in combustion the carbon is emitted as carbon dioxide.

Drying of sludge

Drying of the sludge is a prerequisite for being able to apply pyrolysis, but the drying itself can be interesting to make current sludge attractive to agriculture as the quantities that need to be spread will be smaller and the handling of the sludge will be easier. Drying also provides a hygienisation of the sludge and stops the emissions of nitrous oxide and methane. However, drying is very energy intensive. But in combination with pyrolysis and biogas production, external energy does not need to be imported.

Refining sludge

Refining sludge, to make it an even more attractive product, is expected to be easier with a sludge biochar or a dried sludge. For example, it would be good to be able to increase the nitrogen and potassium content of these products through additives. Also, pelleting of the sludge product would also be attractive and is much easier with dry substrates.

Project structure

The project consists of three parts; design and establishment, test bed operation and a final synthesis. Test bed operation includes four different parts: operation of sludge dryer and pyrolysis, attraction of testers, development of business models for processes and products, and system analysis of the entire chain from infrastructure to final use of the products. During the last project year, a final synthesis is performed that describes what the test bed has achieved, and how the test bed can live on.