Thermophilic anaerobic digestion enables simultaneous waste activated sludge reduction and metronidazole degradation

Metronidazole (MNZ), a drug targeting anaerobic infections, disrupts anaerobic digestion (AD) more severely than other broad-spectrum antibiotics. Activated by reduction, it resists aerobic degradation, leading to its discharge from wastewater treatment plants. This study investigated the fate and behavior of MNZ in thermophilic (TD, 55 °C) and mesophilic digestion (MD, 37°C) of waste activated sludge (WAS). TD exhibited superior resilience compared to MD. Specifically, MNZ exposure in TD increased methane yield from 252.23 ± 14.74 to 346.00 ± 11.81 mL/g VSS and soluble total organic carbon (STOC) from 2414.18 ± 399.68 to 3092.06 ± 247.08 mg/L. Conversely, MD suffered severe inhibition, with methane yield dropping from 188.60 ± 7.41 to 92.07 ± 9.62 mL/g VSS alongside suppressed WAS solubilization. While biodegradation was the primary removal route in both systems, TD uniquely facilitated a reductive inactivation pathway where nitro groups were reduced to non-toxic forms, avoiding the toxic oxidative intermediates (e.g., 5-hydroxymethyl) generated in MD. Additionally, the higher initial concentration of tryptophan-like substances in extracellular polymeric substances potentially acted as a protective barrier in TD. Microbial analysis revealed fundamentally distinct community structures between the TD and MD groups. Notably, the dominant genera in TD (Pseudomonas and Tepidimicrobium) maintained high abundance under MNZ stress, whereas MD underwent significant succession with the suppression of key genera like Advenella. These findings establish TD as a robust strategy for simultaneous MNZ degradation and bioenergy recovery.