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Hydrochar from sludge replaces peat in soil

10 May 2022, 11:06

When sludge becomes soil for plants, waste is transformed into value. In a joint R&D Council activity, the companies Econova and C-Green demonstrate that the foundation is in place for using hydrochar to replace peat in today’s soil products.

C-Green has developed a process for managing wet sludge and residual streams from sources like the paper industry and wastewater treatment plants. Through so-called hydrothermal carbonization, or HTC, a new material is extracted: hydrochar. At the same time, the soil and gardening company Econova is looking for alternatives to peat, mineral fertilisers and sand in its production of soil. Both companies actively focus on sustainability and circularity.

Recipes for soil

Using biochar in soil products is nothing new. But thanks to HTC technology, wet residues can also be transformed into a carbon-rich raw material. Econova wanted to test whether hydrochar could act as a substitute for peat while adding helpful properties to the soil in order to improve plant growth.

Many people are surprised that soil needs to be made – they think that it’s just taken out of the ground and packaged,” says Elsa Bertils, sustainability integrator at Econova. “We mix soils from different raw materials so that it obtains the right properties. The soil’s main purpose is to be a place where roots can stay put and where water and nutrients are retained. We have ‘recipes’ consisting of peat, carbon, sand, lime and much more, depending on what the soil is used for, says Elsa Bertils, Sustainability integrator at Econova

Hydrochar from sludge

C-Green’s idea is to convert sludge from sources like forestry into hydrochar, in a more sustainable HTC process that does not require heat supplied by the treatment of separated water with oxygen generated from air.

You can compare the process with a pressure cooker,” says Peter Axegård, chief technology officer at C-Green. “We treat sludge at 200 degrees for an hour and then get a carbonization of sludge that can originate from different sources. Carbonization results in the formation of a solid product, hydrochar. By allowing oxygen to react with the organic matter in the separated water, all the heat we need is generated through wet oxidation. So in addition to hydrochar, we get both heat and water purification in the process, says Peter Axegård, Chief Technology Officer på C-Green.

Hydrochar, a carbon-rich material, looks much like brewed ground coffee.

- Today hydrochar is mainly used as fuel in biofuel boilers, but we are now looking at new uses. We believe that there is commercial potential to replace peat in different products since peat is a primary raw material. So, we discovered a common interest with Econova to evaluate whether it’s possible to replace peat with hydrochar in their soil products, says Peter Axegård,

The R&D Council

With assistance from RISE Processum’s R&D Council, which is an activity within the framework of IB:ACCEL, C-Green and Econova were able to examine the potential of hydrochar as a substitute for peat in soil.

With this support, we’ve mainly been able to cover the costs of the cultivation experiments done by researchers at the Swedish University of Agricultural Sciences, SLU, in Alnarp in Skåne. We also got a lot of help from RISE Processum with planning and administration. This collaboration has been informal and smooth, in a way that I really appreciate.

Potential exists

The Swedish University of Agricultural Sciences has tested germination rates for basil seeds and the replanting of pak choi that had already germinated.

The results show that hydrochar from the HTC process has potential to be used as a raw material, so there’s a reason to proceed, and that there are differences between different types of sludge. Now the project is completed, and we’ve shown that it works – the seeds germinate and the seedlings grow in the new soil. So it planted the seed of a continued collaboration that allowed us to finally create commercial products, says Elsa Bertils.