New research reveals that recycled food waste may be contaminated with pharmaceutical residues. The good news is that fungi grown on biogas digestate show minimal absorption of these contaminants. On February 16, Astrid Solvag will defend her PhD dissertation at the Norwegian University of Life Sciences (NMBU) in Ness.
When food waste or sewage sludge is processed in a biogas plant, it produces energy-rich biogas and a byproduct known as biogas digestate or slurry. This digest contains many nutrients and can be used as fertilizer in agriculture. Additionally, it can act as food for soil microorganisms, improving soil structure and contributing to overall soil health.
The challenge in using biogas digestate as fertilizer is that it may contain substances that can be contaminants, such as residues of pharmaceuticals and impregnating agents. These substances can be taken up by plants or leach into rivers and lakes, potentially harming organisms living in and out of the soil.
Although this problem is well known and partially researched for sewage sludge, there has been relatively little research on pollutants present in food waste in Norway.
In her doctoral thesis, NIBIO researcher Astrid Solvåg Nesse collected and tested biogas digestates of food waste and sewage sludge from all public biogas plants in Norway.
Recycled organic material in the form of biogas digestate can be effectively used as fertilizer, as long as it does not contain substances that we prefer not to put in the soil. What I have done is to investigate the types of pollutants that can be found in food waste digestate and compare it to sewage sludge digestate. I also examined the risks of using contaminated biogas digestate to produce edible fungi.”
Astrid Solvåg Nesse, NIBIO researcher
Digestion analysis for various substances
Through detailed analysis, Nesse found that several biogas digestate samples of processed food waste contained approximately the same amount of pharmaceutical residues as sewage sludge digestate.
“It was quite surprising,” the researcher said. “I don’t know why this happens, but one theory is that it could be related to poor source selection. For example, people who have incorrectly disposed of pharmaceutical residues with their food waste.”
The researchers also analyzed the digestate for per- and polyfluoroalkyl substances (PFAS). PFAS is a group of man-made chemicals widely used in products such as Teflon pans and water-resistant textiles.
“PFAS are extremely stable in the environment, and some are toxic and can accumulate in humans and animals,” Nesse explains. “Our analysis showed that for most PFASs, there were higher concentrations in biogas digestate from sewage sludge than from food waste.”
Contaminated digestate for fungal cultivation
In addition to analyzing the biogas digestate for contaminants, Nesse conducted several growth experiments with fungi cultivating the digestate. The objective was to investigate whether mushroom cultivation could be a way to use contaminated recycled organic waste.
“Initially, biogas digestate is suitable for growing mushrooms and other edible fungi. However, it is important to control how much of the digestate’s contaminants end up in the mushrooms,” Nesse explains.
The results showed that the fungus absorbed very little of both pharmaceuticals and perfluorinated substances. Instead, these substances remain through growth.
“However, we found that the amount of contaminants through growth decreased significantly over time,” the researchers said. “Therefore it is possible to grow edible mushrooms in contaminated digestate, while at the same time reducing contamination in the digestate. The growth medium used may be suitable for further use as fertilizer, for example, in agriculture.”