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 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.

Persistence of SARS-CoV-2 in the environment and COVID-19 transmission risk from environmental matrices and surfaces.

The Coronavirus disease 2019 (COVID-19) is spreading around the world, representing a global pandemic, counting, as of June 5th, 2020, over 6,600,000 confirmed cases and more than 390,000 deaths, with exponentially increasing numbers. In the first half of 2020, because of the widespread of the COVID-19, researches were focused on the monitoring of SARS-CoV-2 in water, wastewater, sludge, air, and on surfaces, in order to assess the risk of contracting the viral infection from contaminated environments. So far, the survival of the novel Coronavirus out of the human body has been reported for short time periods (from hours to few days, in optimized in vitro conditions), mainly because of the need of an host organism which could consent the viral attack, and due to the weak external membrane of the virus. SARS-CoV-2 viral shedding strategies in the environment, either through animate and unanimate matrices, or exploiting the organic matter in water, wastewater, and waste in general, have been discussed in the present article. We concluded that, besides the high infectuousness of the novel Coronavirus, the transmission of the pathogen may be efficiently contained applying the adequate preventive measures (e.g., personal protection equipments, and disinfecting agents), indicated by national and international health authories.