Diversity and fate of human pathogenic bacteria, fungi, protozoa, and viruses in full-scale sludge treatment plants

Sewage sludge, as the by-product of urban sewage biological treatment, is a potential reservoir and source of pathogens. However, the distribution and fate of different pathogens during sludge treatment and disposal remain unclear. Here, we investigated the diversity and fate of human pathogenic bacteria, fungi, protozoa, and viruses in sludge from four full-scale sludge treatment plants (STPs) using high-throughput sequencing ap-proaches. The results indicated that the diversity of pathogens in different STPs was different, which may be related to sludge characteristics and local epidemics. Sludge treatment steps differentially affected pathogens reduction, and anaerobic digestion (AD) reduced the relative abundance and richness of pathogens. AD com-bined with thermal hydrolysis pretreatment effectively reduced most pathogenic bacteria except for Clostridium perfringens. Sanitary performance of AD was also varied by substrate characteristics (high-solid or low-solid sludge) and compositions (mono-digestion or co-digestion). Resistance of pathogens to sludge treatment was type-dependent, and pathogenic protozoa and fungi showed greater resistance to AD. Meanwhile, some patho-genic bacteria and viruses (Adenovirus and Herpesvirus) were widespread in digestate, revealing a biological risk in disseminating pathogenicity. These findings reveal the diversity and fate of human pathogens during sludge treatment and disposal, which would be crucial in monitoring and controlling pathogen proliferation.