Innovation of digestate – an important issue for husbandry in the County of Rogaland, Norway

12 January 2022 - Published by Elisabeth Krey Jenssen

Innovation of digestate – an important issue for husbandry in the County of Rogaland, Norway

Rogaland biogas network wants to bring forth knowledge on how, through research and innovation, digestate can become a resource, and how to utilize organic waste in a sustainable way.

Rogaland biogas network was established in February 2021. The network has members from the entire value chain and arranges regular meetings about specific topics. Between 50 – 100 biogas enthusiasts use to attend the digital meetings.

The County of Rogaland, Norway, has a high density of animal husbandry, as climatic conditions are well suited for grass and roughage production. The huge amount of livestock manure forces the county to transform digestate from biogas production into new products, for instance organic fertilizer, soil improvers or cultivators, to obtain better water conditions and less greenhouse gas emissions and still ensure food production. Therefore, biogas stakeholders, agriculture and local authorities are collaborating to innovate new methods for handling digestate.

Written by the project leader of Rogaland biogas network, Helga Hellesø (landbruksparken@gmail.com)

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Manure spreading

Large amounts of organic resources

The county has a great potential for biogas production from various types of organic waste such as livestock manure, fish sludge, food waste and sewage sludge. These are valuable organic resources that should be treated professionally. When large amounts of organic waste enter the value chain, the amounts of nutrients also increase. This must be taken care of to prevent nutrient leaks to water and air.

As phosphorus content of digestate depends on the substrates, agriculture should be aware of the consequences of blending different substrates into biogas production. Phosphorus-rich substrates should be avoided and the use of phosphorus in feeding should be optimized. N/P ratio in digestate is to a small extent adapted to individual farmer's needs. Different mechanical separations can be used to get a more suitable N/P ratio. New technology makes it possible to analyze the nutrient content of digestate delivered to the farmer. Therefore, fertilization can be adapted to farmer's needs to a greater extent. But farmers must be aware that the number of heavy metals that may adhere to certain substrates, and contaminants as plastics and glass, can make digestate unsuited for usage as an organic fertilizer.

To succeed, we need to consider the entire resource management of all the organic resources. The whole community is responsible to find sustainable solutions, and agriculture can’t be responsible to solve this alone.

Changes of the livestock manure

The livestock manure changes character after the fermentation process. The fiber is decomposed, and the fertilizer becomes more fluid, foams more and smells different (more "soil"). It changes color from yellow brown to more greenish, and probably infiltrates soil better. Higher pH-value of the digestate increases the risk of ammonia loss. High ratio of mineral nitrogen to total N also increases risk of nitrogen losses. The content of sulfur is probably reduced, while phosphorus concentration remains unaffected.

Fertilizer products

The availability of large amounts of nutrients in the County of Rogaland, demands developing high-quality organic fertilizer products to transport organic substances in concentrated form to regions in need of organic material and nutrition for the soil. This can be done by making various fertilizer products produced from the dewatered digestate from fish sludge, livestock manure, food waste, sewage sludge, and various combinations of these.

It turns out that fertilizer products from digestate alone must be enriched with nitrogen and potassium to become a full-fledged fertilizer product.

Soil improvers

Use of digestate as soil improvers and cultivators is meant to replace the use of peat and subsequently reduce greenhouse gas emissions. The physical and chemical qualities differ from the quality of peat by having less structure and high nutrition concentration. Therefore, it can only be used in small amount in soil improvers and cultivators. Digestate must be enriched to prevent nutrient deficiencies on plants, and pH-value must be adapted to plants' needs.

Digestate used as soil improver is tested enriched with various components such as biochar and composted bark. These are tested in growth experiments for water storage capacity and pore volume. Digestate as soil improver and cultivators is challenged by its high content of nutrients and easily metabolized organic material.

Research and innovation

Research and innovation on digestate are taken place on several locations in Norway:

Jæren Biogass, Rogaland. A local founder having his own farm is testing new applications and technical methods for handling the digestate.

NIBIO Særheim, Norwegian Institute of Bioeconomy Research. The MAFIGOLD project (Manure, fish sludge, food wastes, from problem to farmers gold: MAFIGOLD) is an interdisciplinary collaboration in which the business community from both the green and blue sectors participate to develop new products from various organic resources.

NORSØK, center for organic farming in Norway, has researched within digestate since 2011.

Cattle grazing in Orre

Cattle grazing in Orre, photo by Ane Harestad