Inactivation of Antibiotic-Resistant Bacteria in Wastewater by Advanced Oxidation Processes

Abstract

The emergence of antimicrobial-resistant bacteria in treated effluents presents a growing public health and environmental concern, particularly in the context of water reuse and discharge. This PhD research explores the effectiveness of ultraviolet- and cast iron-based advanced oxidation processes (AOPs) for enhancing microbial inactivation and minimizing harmful by-product formation. The work is presented in two major phases. In the first phase, the disinfection efficacy of 280 nm ultraviolet light-emitting diodes (UV LEDs) was evaluated against Escherichia coli ST131-A, a high-risk, multidrug-resistant uropathogen known to persist in treated wastewaters. Bench-scale experiments demonstrated that E. coli ST131-A exhibited significant ultraviolet tolerance, requiring a fluence of 35 mJ/cm² to achieve a 6-log₁₀ inactivation, compared to only 12.5 mJ/cm² for the reference strain E. coli ATCC 25922. Moreover, photoreactivation was observed in approximately 1% of the E. coli ST131-A population even at doses up to 90 mJ/cm², suggesting a strong repair capacity. The addition of hydrogen peroxide at concentrations ranging from 10 to 200 mg/L substantially enhanced bacterial inactivation—achieving up to a 6000- and 300,000-fold reduction compared to ultraviolet treatment alone and H2O2 (200 mg/L) alone, respectively. This enhancement was attributed to hydroxyl radical generation via ultraviolet-induced photolysis of hydrogen peroxide and residual hydrogen peroxide–mediated bacteriostasis that suppressed regrowth. Supporting results from a genetically modified Pseudomonas aeruginosa model confirmed the role of photolyase in reactivation, emphasizing the need for combined disinfection strategies against resilient strains. The second phase focused on a cast iron-based photo-Fenton-like process operating at near-neutral pH. This system combined micro-sized cast iron-B particles with persulfate and UVA irradiation (λ = 365 nm) and achieved complete inactivation of E. coli ATCC 25922 within 15 minutes under optimized conditions (100 mg/L cast iron-B, 0.42 mM persulfate, UVA intensity of 2.1 mW/cm²). The process effectiveness was attributed to enhanced reactive oxygen species generation and favorable electrostatic adsorption between the positively charged cast iron-B surface and negatively charged bacterial cells, particularly under acidic conditions. A clear correlation was observed between pH, surface charge, and inactivation efficiency, underscoring the importance of adsorption-mediated contact in radical-driven disinfection systems. Recognizing the potential for by-product formation in sulfate radical–based AOPs, the third component of this research investigated bromate formation in bromide-containing waters treated with cast iron, persulfate, and UVA. Time-resolved analysis confirmed the formation of bromate under these conditions. To address this risk, nanoscale zero-valent iron (nZVI) was applied as a post-treatment strategy for bromate reduction. Experimental results showed that higher concentrations of nZVI significantly improved bromate reduction, with nearly complete conversion to bromide. The use of continuous sonication further enhanced performance by improving nZVI dispersion and promoting cavitation-induced temperature increases that accelerated reaction kinetics. Mass balance analysis confirmed a near-stoichiometric conversion of bromate to bromide, with minimal deviation due to potential oxidation during handling. Overall, this dissertation provides new insights into ultraviolet- and iron-based advanced oxidation processes as dual-purpose strategies for bacterial inactivation and disinfection by-product control. The findings highlight the importance of understanding microbial tolerance mechanisms, process optimization, and the integration of reduction technologies for safe and sustainable water treatment practices.