Effects of thermal treatment and anaerobic digestion on pathogen and ARG removal in bio-solids from a co-treatment plant for sewage and fecal sludge

Abstract

This study assesses the bacterial pathogen load in secondary sludge from a sewage treatment plant that co-processes fecal sludge with municipal wastewater over 1 year. It also examines how anaerobic digestion and temperature treatment affect pathogens (including bacteria and nematodes) and antibiotic-resistance genes in secondary sludge. Furthermore, it assesses the impact of bacterial pathogen survival during co-digestion of secondary sludge with food waste, bakery waste, and flower waste. A fluctuating viable pathogenic bacterial count with no consistent trend was observed during the 1-year period. Staphylococcus spp. remained consistently abundant with log values of 6.2–7.4 CFU/g TS. Meanwhile, E. coli and Salmonella spp. ranged from 1.3 to 4.6 and 2.7 to 4.6 log CFU/g TS, respectively, significantly exceeding the Class A biosolid limits. Mesophilic anaerobic digestion for 21 days showed limited pathogen removal, requiring 55 days for complete elimination of Gram-negative bacteria, while Staphylococcus spp. exhibited only a 0.4-log reduction. Treatment at 70 °C for 60 min reduced 2–4 log units (p < 0.05) of all Gram-negative bacteria tested, whereas 100 °C for 30 min was required to eliminate Staphylococcus spp. Complete eradication was not achieved for ARGs or nematodes after anaerobic digestion or treatment at 100 °C for up to 90 min. Co-digestion of secondary sludge with food, bakery, and flower waste over 21 days effectively eliminated most bacterial pathogens, achieving complete removal (up to 100%) of Salmonella spp., E. coli, Vibrio spp., and Klebsiella spp. Meanwhile, Staphylococcus spp. persisted during co-digestion, where the extent of reduction varied with the co-substrate used.