Residuals from the mill feed the forest

Published 28 August 2020
Our Skoghall Mill in Sweden is part of a research project that investigates if residual products from the forest industry could be used as a recycled fertilizer for the forest. The project is a way of contributing to circular bioeconomy, where waste and side streams are recycled and reused, and to sustainable forestry, which both are important topics for Stora Enso.

“Stora Enso wants to do good things both for the environment and the economy,” says Margareta Sandström, Environmental Manager at Skoghall Mill. "A sustainability focus is a must for a modern company with environmentally conscious customers."

Trees in forests store energy through photosynthesis. In addition to carbon dioxide and water, growing trees need nutrients. When trees are harvested and forest residues, such as branches and treetops, are left in the woods, some of the nutrients are returned to the soil, but when both the wood and the forest residues are removed, less nutrition remains in the forest. 

“We want to find a balance where we can use branches and treetops for energy production while ensuring that the forest stays healthy and growing trees receive the nutrients they need,” says Sandström. “This is why we are finding ways to turn side streams from our production into a forest fertilizer. Another key target for the project is to find ways to combat soil acidification in the forest, which can occur when more if too much of the forest residues are removed.”

Sludge and ash used for nutrition

Since 2019, Stora Enso's Skoghall Mill has been part of a research project with Karlstad University and other organisations. The project investigates the possibility to use sludge from the mill's wastewater treatment plant and ash from the biofuel boiler to produce a sustainable fertilizer for the forest. Both the sludge and ash contain the original nutrients from the harvested trees. The sludge can be made into char that, together with the ash, can be pressed into nutritional pellets that can be returned to the forest. The pellets are planned to be returned in connection with replanting after harvesting.

“When we take advantage of the residual products during pulp production and return them to the forest, we close the cycle,” says Sandström. “We are also hoping to see increased tree growth as a result of this project.”

Cultivation trials are ongoing. Spruce and pine seedlings were planted during spring 2020, also at Stora Enso's Sjögränd nursery. The nursery’s role is to guide Karlstad University in early cultivation experiments in a laboratory environment and to provide cultivation materials, such as seeds and fertilizers. A slightly larger cultivation trial is being conducted in the forest where different degrees of biochar and ash are incorporated into the soil and different fertilizers are tested.

“This is an important project because the goal is to return residuals from the forest industry back to the forest in an environmentally friendly way,” says Margareta Persson, Assistant Nursery Manager at Sjögränd nursery. “This is done partly in the nursery where the biochar binds nitrogen from our ordinary fertilizer, reducing the risk of nutrient loss in the nursery environment, and partly in the forest where biochar and ash are used as fertilizer and to improve the soil.”

In the autumn of 2020, the seedlings will be planted in the forest. The research project will be ongoing 2019-2022 and is a collaboration between Stora Enso Skoghall Mill, Stora Enso Sjögränd nursery, Stora Enso Biomaterials division, Karlstad University, Paper Province, Karlstad Municipality, Econova, Mellanskog, the Swedish Forest Agency and Umeå Energi.