Hygienic assessment of digestate from a high solids anaerobic co-digestion of sewage sludge with biowaste by testing Salmonella Typhimurium, Escherichia coli and SARS-CoV-2

Anaerobic digestion is a consolidated technology to convert sewage sludge and other organic wastes into biogas and a nutrient-rich fertilizer (i.e. digestate). The origin of sewage sludge does not exclude the potential presence of pathogens (e.g. Salmonella spp. and SARS-CoV-2) in mature digestate that hence could represent a source of sanitary concerns when it is spread on soil for agriculture purpose. Therefore, an experimental study aimed at proving the sanitizing effect of a full scale thermophilic high solids anaerobic digestion process was conducted by monitoring the hygienic characteristics of mature digestate. Although Salmonella spp. was detected and SARS-CoV-2 was presumably present in the sewage sludge fed to the full scale plant, the anaerobic digestion treatment demonstrated sanitization capacity since the monitored pathogens were never found in the mature digestate over the entire duration of the monitoring survey. Furthermore, tests on the regrowth of Salmonella Typhimurium and Escherichia coli, artificially inoculated on mature digestate, were also conducted under both anaerobic and aerobic conditions with the aim to assess the effectiveness of mature digestate as microbial growth medium. Concentrations of Salmonella Typhimurium and Escherichia coli were drastically reduced after a short time of incubation under anaerobic process and the two microorganisms already resulted undetectable after 24-48 h, whereas, under aerobic conditions, two microorganisms' concentrations were stably high for longer than 10 days. The combination of no free oxygen, high temperature, anaerobic metabolites (e.g. total ammonium nitrogen, and volatile fatty acids) production, bacteria competition and lack of nutritional elements in mature digestate considerably reduced in 24-48 h the sanitary risks associated to accidently contaminated digestate. Furthermore, a SARS-CoV-2 monitoring survey on mature digestate during 13 months, resulted in the absence of the virus RNA in the analyzed digestate.