The role of operating parameters and oxidative damage mechanisms of advanced chemical oxidation processes in the combat against antibiotic-resistant bacteria and resistance genes present in urban wastewater

An upsurge in the study of antibiotic resistance in the environment has been observed in the last decade.
Nowadays, it is becoming increasingly clear that urban wastewater is a key source of antibiotic resistance
determinants, i.e. antibiotic-resistant bacteria and antibiotic resistance genes (ARB&ARGs). Urban
wastewater reuse has arisen as an important component of water resources management in the European
Union and worldwide to address prolonged water scarcity issues. Especially, biological wastewater
treatment processes (i.e. conventional activated sludge), which are widely applied in urban wastewater
treatment plants, have been shown to provide an ideal environment for the evolution and spread of
antibiotic resistance. The ability of advanced chemical oxidation processes (AOPs), e.g. light-driven
oxidation in the presence of H2O2, ozonation, homogeneous and heterogeneous photocatalysis, to
inactivate ARB and remove ARGs inwastewater effluents has not been yet evaluated through a systematic
and integrated approach. Consequently, this review seeks to provide an extensive and critical appraisal
on the assessment of the efficiency of these processes in inactivating ARB and removing ARGs in
wastewater effluents, based on recent available scientific literature. It tries to elucidate how the key
operating conditions may affect the process efficiency, while pinpointing potential areas for further
research and major knowledge gaps which need to be addressed. Also, this review aims at shedding light
on the main oxidative damage pathways involved in the inactivation of ARB and removal of ARGs by
these processes. In general, the lack and/or heterogeneity of the available scientific data, as well as the
different methodological approaches applied in the various studies, make difficult the accurate evaluation
of the efficiency of the processes applied. Besides the operating conditions, the variable behavior
observed by the various examined genetic constituents of the microbial community, may be directed by
the process distinct oxidative damage mechanisms in place during the application of each treatment
technology. For example, it was shown in various studies that the majority of cellular damage by
advanced chemical oxidation may be on cell wall and membrane structures of the targeted bacteria,
leaving the internal components of the cells relatively intact/able to repair damage. As a result, further
in-depth mechanistic studies are required, to establish the optimum operating conditions under which
oxidative mechanisms target internal cell components such as genetic material and ribosomal structures
more intensively, thus conferring permanent damage and/or death and preventing potential posttreatment
re-growth.