Testbed Ellinge – drying, pyrolysis and productification of sewage sludge

Five water service companies are collaborating to run a testbed for drying, pyrolysis and productification of different types of sewage sludge.

Traktor sprider slam på åker

Traktor sprider slam på åker

Testbed for sustainable sludge management and disposal

The pilot tests with sewage sludge from different wastewater treatment plants (WWTPs) will be performed at Ellinge WWTP in Eslöv. A competence team consisting of people from universities, research institutes and companies are involved are also involved in the project. The project is co-financed by Sweden`s Innovation Agency (Vinnova). VA SYD is the coordinator partner.

The purpose of Testbädd Ellinge is to create a basis for Swedish sewage works path choice for sustainable sludge management and disposal.

The project is expected to result in that relatively unexplored alternatives for sewage sludge management and plant nutrient recycling are elucidated so that water service companies can invest in technologies without performing own pilot studies. In addition to effects at the system level regarding the infrastructure for sludge management, new non-toxic plant nutrient products adapted to current and future agriculture can reduce imports of mineral fertilisers and make Sweden more self-sustaining of resources required for sustainable and more self-sufficient food production.

Municipal wastewater treatment generates residual product 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 arable land is being discussed, but regardless of a ban or not, water service companies see a need for complements/alternatives to today’s sludge management.

Pyrolysis, drying and refining sludge

Pyrolysis with arable land spreading of sludge biochar has in system studies (von Bahr et al., 2017; Grundestam et al., 2020) proven to be better than sludge spreading in terms of climate impact, acidification, eutrophication and dispersion of heavy metals such as cadmium and mercury. The energy requirement is greater, due to the fact that drying of the sludge is required before the pyrolysis, 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 excellent 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-incineration 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.

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 become smaller and the handling of the sludge becomes easier. Drying also provides a hygienisation of the sludge and stops the production 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, 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. A pelletisation would also be attractive and much easier with dry substrates.

About the project

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. Finally, a synthesis is performed that describes what the test bed has achieved, and how the test bed can live on.

The main goals of the test bed are to:

  • Obtain operating experiences and optimise operating conditions for drying and pyrolysis of different types of sewage sludge
  • Create new, better market-adapted fertiliser products from sewage sludge.
  • Create an innovation platform for future, alternative sludge management.