Biogas can increase the capacity of pulp and paper mills

The wastewater streams from kraft mills have been considered to be too difficult to digest for biogas production, despite containing large quantities of organic substances. Eva-Maria Ekstrand of the Department of Thematic Studies – Environmental Change reveals a solution in her doctoral thesis.

Universitetsadjunkt Eva-Maria Ekstrand. Eva-Maria forskar kring hur man kan få ut biogas från pappers- och massabrukens avloppsströmmar. Eva-Maria Ekstrand Anna Nilsen

The number of pulp and paper mills in Europe has fallen by half in recent years, while the manufacture of paper has increased by 12% in the same period. Many mills are working at the limit of their capacity, where the main limiting factor is the water purification stage. Eva-Maria Ekstrand, newly promoted doctor at the Department of Thematic Studies – Environmental Change, presents a solution in her thesis: biogas.

“This is an industry that in addition to producing pulp and paper also produces large quantities of wastewater and sludge with a high content of organic substances. It requires a lot of money and effort from the mill to get rid of the wastewater sludges, while it is a large potential source of methane. My research has looked at how we can use that material for biogas production, since this is an important part of the puzzle in the transition to a fossil-free society”, says Eva-Maria Ekstrand.

Several mills in Europe treat their wastewaters for use in biogas production.
“But they deal with the easiest water to treat, mainly at mechanical mills and recycled paper mills. The wastewater streams are more concentrated at these mills, and have low levels of substances that can cause problems in biogas production”, she says.

Kraft method

Biogasproduktion på Rockhammar och Frövi brukWood chips at Billerud Korsnäs Photo credit Charlotte Perhammar“We have also been able to demonstrate that the flows from mills that manufacture mechanical pulp have the highest potential for biogas production.”

However, most mills, both in Sweden and around the world, manufacture chemical pulp using the kraft method. Not only does the water in such mills contain chemicals added during cooking and bleaching, it contains also organic substances from the wood, such as resin acids and other substances dissolved during the kraft process.

Eva-Maria Ekstrand decided to concentrate her research on the contents of the fibre sludge. She collected large amounts of fibre sludge from many mills that used different pulping processes. During the initial work, she discovered that the fibre sludge from kraft mills had a high potential for methane production.

The fibre sludge is formed during a primary clarification stage in which the fibres are separated in sedimentation ponds. Eva-Maria Ekstrand’s results show that fibre sludge is easy to digest, but that it lacks many of the nutrients that are required for biogas production.

Active sludge and fibre sludge

After the first step, the remaining water is passed to huge aeration ponds, where oxygen and nutrients are added. Over a considerable period, the sludge degrades: larger microorganisms consume the smaller ones, and the volume of sludge falls.

“Somewhat simplified, you could say that the sludge consumes itself. This gives an activated sludge that is difficult to digest, but that contains important nutrients”, she explains.

The next step in the research, therefore, was to mix the activated sludge with fibre sludge. It turned out that the mixture gave a more stable process in the biogas digester, but it was still difficult to break down the organic material in the activated sludge. Thus, a further step was added in which the age of the activated sludge was lowered by adding more wastewater.

“We tested this in a pilot plant, and it became clear that the mill could treat water four times as rapidly, which makes it possible either to reduce the active volume of the aeration pond or increase the production of pulp or paper.”

The results presented in her thesis make it perfectly clear that anaerobic digestion, which proceeds in the absence of oxygen, of the waste streams from the mill is not only possible but also gives huge advantages such as high methane production, higher wastewater treatment capacity and savings in both energy and nutrients.

“The mills no longer need to worry about the increase in the volume of sludge that follows from reducing its age, since we can convert the organic material to biogas. We have Universitetsadjunkt Eva-Maria Ekstrand. Eva-Maria forskar kring hur man kan få ut biogas från pappers- och massabrukens avloppsströmmar. Photo credit Anna Nilsenshown that this works well in kraft mills, which previously were considered to be a serious challenge. But the method works in a similar manner in mills that work with mechanical pulping”, she says.

Large amounts of biogas

By reducing the age of the activated sludge and mixing it with fibre sludge in a biogas digester, large amounts of biogas can be produced, while the mill at the same has the opportunity to use its production capacity more efficiently.

“This means in practice that the mill can significantly increase the production of pulp and paper, without having to build more aeration ponds, which require a lot of space and cost a lot of money.”

Part of Eva-Maria Ekstrand’s research has been carried out at the Biogas Research Center, BRC, a national competence centre at Linköping University.

The thesis: Anaerobic digestion in the kraft pulp and paper industry - Challenges and possibilities for implementation, Eva-Maria Ekstrand, Department of Thematic Studies – Environmental Change, Linköping University 2019. Principal supervisors have been Professor Emeritus Bo Svensson and Senior Lecturer Annika Björn.

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