Uncovering effects of anaerobic digestion process on viral communities in activated sludges using viromic approaches

Viruses in disposed sludges may pose potential risks to both environmental ecosystems and humans. In this study, sludges were collected along the entire anaerobic sludge digestion process including anaerobic digestion and 3 related treatment steps (i.e., primary dewatering process, thermal hydrolysis, and final dewatering process after anaerobic digestion) and analyzed using viromic approaches, aiming to evaluate the effectiveness of anaerobic digestion in removing viruses from the sludges. Unclassified viruses were consistently predominant in all sludges, followed by prokaryotic and eukaryotic viral families. The total levels of prokaryotic viruses in sludges ranged from 1.3 x 10 7 to 5.3 x 10 10 virus-like particles per gram wet sludge (VLPs/g), with Siphoviridae being preponderant. By contrast, the total levels of eukaryotic viruses were lower (3.7 x 10 5 to 1.0 x 10 10 VLPs/g), and Circoviridae were much more abundant than others. Undergoing all treatment steps, the levels of eukaryotic viruses were reduced by half from the sludges whereas that of prokaryotic viruses was increased by approximately 2.3 times. Thermal hydrolysis step dramatically reduced the level of viruses in the sludge, but the following anaerobic digestion step was an important process for the reconstruction of viral communities. Viruses in the sludges could infect a wide taxonomic range of microbes, mainly including Firmicutes, Bacteroidetes, Proteobacteria, etc. A large variety of auxiliary metabolic genes carried by viruses in the sludges were relevant to the cycling of carbon, sulfur, phosphorus and nitrogen. Overall, it is suggested that the sludge treatment process must be improved for effectively eliminating viruses in the sludges after anaerobic digestion step.