Survival of <i>Salmonella</i> Typhimurium, <i>Listeria monocytogenes</i>, and ESBL Carrying <i>Escherichia coli</i> in Stored Anaerobic Biogas Digestates in Relation to Different Biogas Input Materials and Storage Temperatures

Anaerobic digestates derived from agricultural mesophilic biogas plants are mainly used as organic fertilizers. However, animal derived pathogens could persist in the anaerobic digestates (ADs) posing an epidemiological risk. The present study investigated whether storage of ADs could reduce <i>Salmonella</i> Typhimurium, <i>Listeria monocytogenes</i>, and ESBL carrying <i>Escherichia coli</i> and whether reduction rates are dependent on temperature and substrate. Quantified bacterial suspensions were used to inoculate ADs derived from five biogas plants using different input materials to investigate the substrate dependence of the pathogen reduction. ADs were stored over six months with four different temperature profiles each representing six consecutive months, and, thus, the four seasons. Pathogen reduction during storage was shown to be strongly dependent on the temperature but also on the type of AD. This influence was higher at low temperatures. At higher temperatures (spring and summer profiles), a 5-log reduction was achieved after twelve weeks for <i>S.</i> Typhimurium, after twenty weeks for <i>E. coli</i> (ESBL) and after twenty-four weeks for <i>L. monocytogenes</i> in all ADs, respectively. In contrast at lower temperatures (autumn and winter profiles), a 5-log reduction was reached after twenty-four weeks for <i>S</i>. Typhimurium and not reached for ESBL-<i>E. coli</i> and <i>L. monocytogenes</i>. In conclusion, storing the ADs after the biogas process improves the hygienic quality and reduce the risk of introducing pathogens to the environment, but each case should be evaluated individually considering the composition of the ADs and the storage temperatures.