Roles and fates of antibiotics in anaerobic digestion of waste activated sludge: Insights to pro- and re-duction of antibiotic resistance genes

Anaerobic digestion (AD) of waste activated sludge (WAS) is considered as a promising alternative to realize sludge stabilization and bioenergy recovery, thereby driving carbon neutrality in wastewater treatment plants. However, most of antibiotics are adsorbed by sludge flocs rather than bio-degraded in wastewater treatment process. Regarding that the accumulation of antibiotics in WAS can not only affect performance of AD but also induce the production of antibiotic resistance genes (ARGs), presenting great ecological risks, this review systematically highlighted the recent progress on the fates and roles of antibiotics in AD of WAS, the production and spread of ARGs related to antibiotics and co-contaminants, and the reduction and control strategies of antibiotics and ARGs. The effects of antibiotics on AD stages like solubilization, hydrolysis, acidogenesis, and methanogenesis are type- and dose-dependent. The co-existence of antibiotics and other contaminants enhances the spread of ARGs via enriching potential antibiotic resistance bacteria and promoting horizontal gene transfer, and the control of intracellular ARGs is the key to improve ARGs removal in AD. To mitigate the ecological risks posed by antibiotics and ARGs, the enhanced strategies like pretreatment, conductive materials, anaerobic codigestion, and microbial electrolysis cell-assisted anaerobic digestion have been explored. In the future study, the interaction mechanisms between the transfer of ARGs and key metabolic genes in microorganisms responded to co-pollutants should be verified to reveal the potential relationships among co-pollutants, ARGs transfer, and bacterial metabolism. After that, the green and economical strategies for simultaneous resource recovery and pollutants removal enhancement should be explored. Moreover, machine learning technique are urgently needed to quantitatively assess the ecological and human health risks caused by residual antibiotics and ARGs.