Page 29 - Green knowledge 2024

Photo: RENEVO
Anaerobic digestate from marine resources
Many farmers are sceptical about digestate from marine residual resources, but research
shows that it can be good fertiliser as long as the nutrient content is adjusted according to
the plants’ needs.
There is increasing interest in using fish sludge and
fish silage for biogas production. With the growing
national focus on biogas, the amount of digestate
produced will also increase. Anaerobic digestate con-
tains many nutrients and can replace mineral fertil-
iser.
However, many farmers are sceptical about diges-
tate, particularly from marine residual resources,
because they are unfamiliar with the quality.
Researchers have now investigated this. Analyses
show that the nutrient content in digestate increases
when marine resources are used. The challenge is
that the balance of nutrients in the digestate is not
optimal.
Dr Eva Brod explains that fish sludge contains a par-
ticularly high amount of phosphorus relative to
nitrogen. With the new fertiliser regulation, over-fer-
tilisation with phosphorus will no longer be allowed.
One solution is for biogas plants to treat the diges-
tate to produce good fertiliser products.
“Phosphorus can be extracted from the digestate in a
form that can be transported to areas in need of
phosphorus fertiliser. What the biogas plants are left
with is a phosphorus-reduced digestate that is no
longer problematic to spread locally,” says Dr Brod.
She mentions that there are technologies to reduce
phosphorus content, but these are costly.
“Therefore, it will be important to have financial sup-
port schemes. The new fertiliser regulations will
likely also force new solutions,” she says.
Fish sludge increases the heavy metal content in
digestate, but compared to the high nutritional value,
the levels are not critical. However, Dr Brod is con-
cerned that the aquaculture industry needs to take
greater responsibility.
“The goal must be to avoid heavy metals ending up in
food and the environment,” she says.
Purpose: Collaboration: Funding: Contact: Contribute to increased knowledge about how the use of marine residual resources (fish sludge and
fish silage) affects the quality of anaerobic digestate as fertiliser, and to communicate this
knowledge to agriculture.
NORSUS and the Norwegian Farmers’ Association. Biokraft, Renevo, and Liholmen Biogass have
been industry partners in the project
Norwegian Agricultural Agency (Climate and Environment Programme)
Research Scientist Eva Brod, Division of Environment and Natural Resources.
Email: eva.brod@nibio.no | Phone: +47 902 77 760
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