Impact of emerging contaminants on inhibited steady-state anaerobic digestion: a review of the fate and degradation characteristics of antibiotics, PFAS, and microplastics

Abstract

Anaerobic digestion (AD) is widely applied to stabilize wastewater sludge and recover energy, but it increasingly operates in the presence of emerging contaminants such as antibiotics, per- and polyfluoroalkyl substances (PFAS), and plastic particles including microplastics (MPs, < 5 mm) and nanoplastics (NPs, < 1 & micro;m), collectively referred to as micro- and nano-plastics (MNPs) in the substrates. These highly persistent pollutants are routinely detected in wastewater treatment plants and often resist conventional removal, raising concerns about their long-term impacts on public health, ecosystems, and AD performance. On the other hand, full-scale digesters rarely operate under optimal conditions. Inhibitory levels of ammonia, sulfite, heavy metals, and toxic organics frequently induce systems into an inhibited steady-state, characterized by reduced biogas production and microbial activity, but without complete system failure. This review explores current knowledge on the fate, transformation, and removal of antibiotics, PFAS, and plastic particles (MPs and NPs) in AD, with a specific focus on these inhibited steady-state regimes. The impact of inhibition on microbial community structure and function, alterations in contaminant sorption and degradation pathways, and its influence on process stability are comprehensively discussed. Particular attention is given to prevention and mitigation strategies, including process optimization, pretreatment, the addition of sorbents and conductive materials, and combined treatment options. By explicitly accounting for realistic, non-ideal operating conditions, this work provides a framework for more accurate risk assessment and for designing robust, economically viable AD systems that can simultaneously manage complex contaminant mixtures while maintaining high treatment performance.