Occurrence and reduction of viruses in sludge from full-scale wastewater and organic waste treatment plants during anaerobic digestion

Anaerobic digestion (AD) is a green biotechnology feed with various materials, including wastewater sludge and organic fraction of municipal solid waste (OFMSW). Even if a viral contamination of the organic materials is present, the persistence of the viruses in the effluent after the anaerobic treatment is not yet well known. This study aims to assess viral contamination in the influents and effluents of AD process combining innovative methods—digital polymerase chain reaction (ddPCR) and transmission electron microscope (TEM) observations—in a real-scale context. The research activity involved 2 wastewater treatment plants (WWTP) and 2 OFMSW treatment plants with an anaerobic digestion (AD) step. After a screening on 12 viral targets, including known pathogens and indicators, the 7 most present—adenovirus, norovirus genotype II (GII), SARS-CoV-2 and GII-GIII coliphages, pepper mild mottle virus (PMMoV), tomato mosaic virus (ToMV)—were investigated in 72 samples. Adenoviruses and the viral indicators were present in higher concentrations (> 5 log gene copies/kg sludge), lower values were observed forthe other viruses. SARS-CoV-2 RNA positivity was detected with a different prevalence in the samples (84% WWTP vs 36% OFMSW; p < 0.01). AD produced limited viral decrease (≤ 1 Log) especially in WWTP sludges. ToMV was always observed when at least one viral pathogen was noted in the samples, supporting that it may be a promising viral marker. The innovative methods applied have produced useful evidence on the persistence of viruses in the sludges, valuable for the management and improvement of current waste treatments.