Polychaetes and black soldier fly larvae can grow on sludge from aquaculture farms and become protein-rich feed for fish and livestock. However, toxic substances can enter the loop. One of the risks is that the sludge can contain various unwanted substances, such as heavy metals and environmental toxins that can be taken up by the animals.
A research team from SINTEF, the Norwegian Veterinary Institute and the Institute of Marine Research has investigated whether such substances can enter the food chain through the project SecureFeed. Researchers have also looked at the potential for transmitting prion diseases and whether polychaetes and black soldier fly larvae can pass on bacteria and viruses from the fish sludge.
Metals are a challenge
The Institute of Marine Research has been responsible for analyzing heavy metals and other unwanted substances such as PCBs and dioxins. The project analyzed approximately 50 fish sludge samples from various facilities along the Norwegian coast. Researchers compared the levels of these substances in the fish sludge with the levels typically seen in fish feed. These are pollutants that already exist in the environment, and will often accompany the marine and fatty feed ingredients such as fish oil.
“We found that the levels in sludge and feed are at roughly the same levels. Some of the sludge samples contained levels close to the limit value,” said Veronika Sele, researcher at the Institute of Marine Research.
When it comes to heavy metals such as cadmium and arsenic, several of the sludge samples contained levels above the limit set for feed. In addition, the researchers found other metals, such as copper, iron, zinc, and aluminum with varying amounts in the different sludge samples.
“We found elevated levels of some unwanted substances in some samples. This should be investigated further,” said Sele. She suggested that fish sludge should be monitored for these metals to get a better data basis and get control of what goes further in a possible value chain.
Uncertainty about how much is taken up by the animals
But how much heavy metals and organic pollutants are taken up by polychaetes and black soldier fly larvae when they eat fish sludge?
“We see that some of the metals, PCBs, and dioxins can increase in black soldier fly larvae. The levels here were right up to or exceeded the limit values set for feed ingredients,” said Sele. “There is generally little knowledge about the level of organic pollutants in sludge and their potential for transfer to insects. The project is therefore in its infancy in this regard. There are several organic pollutants that we have not investigated, where there is also a need for more research.”
Lack of regulations
The research has provided new knowledge about what sludge consists of, both in terms of nutritional value and what may be a risk with regard to further use.
“The goal has been to find out if it is safe to use sludge in a bio-circular production of feed with polychaetes and insects,” said SINTEF researcher Andreas Hagemann. “It will require analyses from several different plants and over a longer period. We find that most plants can deliver sludge of good quality, but we also see that there are some outliers.”
“In terms of limit values, most sludge samples could have been given the green light to be used as feed for low-trophic animals. Authorities must get involved so that the regulations are adapted to this solution,” Hagemann said.
References:
Sele V, Ali A, Liland N, Lundebye AK, Tibon J, Araujo P, Sindre H, Nilsen H, Hagemann A, Belghit I. Characterization of nutrients and contaminants in fish sludge from Atlantic salmon (Salmo salar L.) production sites - A future resource. Journal of Environmental Management, Volume 360, 2024, 121103, ISSN 0301-4797, https://doi.org/10.1016/j.jenvman.2024.121103
Belghit I, Liland NS, Lundebye AK, Tibon J, Sindre H, Nilsen H, Hagemann A, Sele V. Aquaculture sludge as feed for black soldier fly: Transfer of chemical and biological contaminants and nutrients. Waste Management, Volume 187, 2024, Pages 39-49, ISSN 0956-053X, https://doi.org/10.1016/j.wasman.2024.07.005
Malzahn AM, Sele V, Belghit I, Tibon J, Nilsen H, Sindre H, Liland NS, Hagemann A. Transfer and bioaccumulation of chemical and biological contaminants in the marine polychaete Hediste diversicolor (OF müller 1776) when reared on salmon aquaculture sludge. Journal of Environmental Management, Volume 367, 2024, 122073, ISSN 0301-4797, https://doi.org/10.1016/j.jenvman.2024.122073
Pettersen KS, Sele V, Araujo P, Belghit I, Benestad SL, Bernhoft A, Booth AM, Eriksen GS, Farkas, J, Handå AH, Hansen BH, Helgesen KO, Holst-Jensen A, Johannessen GS, Liland NS, Lundebye AK, Malzahn AM, Nilsen H, Nordtvedt TS, Norström M, Owczarek-Kościelniak MM, Øines Ø, Patel SJ, Sindre H, Standal IB and Hagemann A. (2025), Fish Sludge as Feed in Circular Bioproduction: Overview of Biological and Chemical Hazards in Fish Sludge and Their Potential Fate via Ingestion by Invertebrates. Rev Aquac, 17: e12996. https://doi.org/10.1111/raq.12996