Chemical and microbiological hazards associated with recycling of anaerobic digested residue intended for agricultural use

In the present study, three full-scale biogas plants (BGP) were investigated for the concentration of heavy metals, organic pollutants, pesticides and the pathogenic bacteria Bacillus cereus and Escherichia coli in the anaerobically digested residues (ADR). The BGPs mainly utilize source-separated organic wastes and industrial food waste as energy sources and separate the ADR into an ADR-liquid and an ADR-solid fraction by centrifugation at the BGP. According to the Norwegian standard for organic fertilizers, the ADR were classified as quality 1 mainly because of high zinc (132–422 mg kg−1 DM) and copper (23–93 mg kg−1 DM) concentrations, but also because of high cadmium (0.21–0.60 mg kg−1 DM) concentrations in the liquid-ADR. In the screening of organic pollutants, only DEHP (9.7–62.1 mg kg−1) and ∑ PAH 16 (0.2–1.98 mg kg−1 DM) were detected in high concentrations according to international regulations. Of the 250 pesticides analyzed, 11 were detected, but only imazalil (<0.30–5.77 mg kg−1 DM) and thiabendazol (<0.14–0.73 mg kg−1 DM) were frequently detected in the ADR-fiber. Concentrations of imazalil and thiabendazol were highest during the winter months, due to a high consumption of citrus fruits in Norway in this period. Ten percent of the ADR-liquid samples contained cereulide-producing B. cereus, whereas no verotoxigenic E. coli was detected. The authors conclude that the risk of chemical and bacterial contamination of the food chain or the environment from agricultural use of ADR seems low.