Fertilization with digestate and digestate products – availability and demonstration experiments within the project Botnia nutrient recycling

To increase our food security in Västerbotten we will need to become more self-sufficient of both energy, feed and nutrients that are now imported to the region. Biogas production from different waste streams is one solution to this. Biogas is produced using biowaste or sewage sludge as substrate in the major cities Umeå and Skellefteå. Biogas systems offer a range of benefits to society. Biogas production is currently prized for its climate benefits when replacing fossil fuels for the production of heat, electricity and vehicle gas, but at Bothnia Nutrient Recycling we have studied how to use the digestate, i.e. the residual product of production, as fertilizer in agriculture. We have been working to improve profitability for biogas producers and develop sustainable products from recycled nutrients, like phosphorus and nitrogen. Improving the uses for digestate increases self-sufficiency in agriculture and contributes to a circular economy. We conducted three agricultural demonstration experiments in collaboration with agricultural high schools in Finland and Sweden to introduce digestate and digestate products to the future farmers in the regions. We found that it may be possible to replace cattle slurry with compost when growing maize despite the low levels of nitrogen, N, available to plants in the compost. In barley, NPK fertilizers gave the highest yield. Digestate from HEMAB and sludge biochar supplemented with recycled ammonium sulphate gave a smaller yield but higher than unfertilized crop. Digestate from a dry digestion biogas plant in Härnösand was better suited to barley than to grass because in an experiment on grass ley the viscous fertilizer did not penetrate the grass and did not increase the growth of the grass. Fertilizer effects on crop quality were small. There was no increased uptake of heavy metals in barley after fertilization with digestate or digestate products compared to NPK fertilization. These demonstration experiments show that more thorough scientific experimentation is needed as a foundation for recommendations to farmers. The amounts of nitrogen and phosphorous in digestate from Västerbotten that could become used as fertilizer were modelled. It showed that if sewage sludge digestate is used to make sludge biochar and ammonium sulphate and the other available digestates are used directly in agriculture, the entire phosphorous demand but only a small part of the nitrogen demand in the county, could be covered. Thus, to achieve a true circular food production, development and increase of both the waste handling sector and agriculture is needed