Anaerobic digestate from marine resources

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 contains many nutrients and can replace mineral fertiliser.

However, many farmers are sceptical about digestate, 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 particularly high amount of phosphorus relative to nitrogen. With the new fertiliser regulation, over-fertilisation with phosphorus will no longer be allowed. One solution is for biogas plants to treat the digestate 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 support 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 concerned 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.