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Electrochemical water treatment is gaining increasing popularity due to its wide range of potential applications, its decreasing costs, and its suitability as a decentralized treatment alternative, but mainly due to it being considered a ”green technology”. In the field of municipal wastewater treatment, the use of iron electrocoagulation (Fe-EC) has been marginal and although disinfection has been reported, its underlying mechanisms are not fully understood, for which significant controversy remains. In this study, microbial inactivation during Fe-EC was evaluated as a two-component process, namely, physical removal by microbial floc sorption/entrapment, and inactivation by reactive oxygen species (ROS) produced by (semi)­Fenton reactions. Using the fecal indicators Escherichia coli WR1 and somatic coliphage ØX174 suspended in a synthetic water matrix, the role of ROS and the role of flocculation were quantitatively evaluated. Fenton inhibitor TEMPOL was used to quench ROS production during Fe-EC. At circumneutral pH, ROS were found to be highly detrimental to E. coli, yet only mildly damaging for phage ØX174 (≈3.9 log10 and ≈0.8 log10 inactivation, respectively). Inactivation for both indicators increased under acidic conditions (pH 5.5), likely due to the formation of hydroxyl radicals (•OH), exceeding 5.1 log10 for E. coli and 1.5 log10 for phage ØX174. The ROS inactivation pathway is linked to the oxidation of ferrous iron (Fe2+), being independent of flocculation settings. Experiments involving different flocculation settings demonstrated that there is a strong positive correlation between orthokinetic-like flocculation conditions, floc sedimentation, and microbial removal, meaning that floc entrapment is a major removal pathway following Fe-EC. When compared to control experiments in which no proper flocculation stage was introduced, orthokinetic flocculation produced additional 3.1 log10 and 4.4 log10 removal for E. coli and phage ØX174, respectively. We conclude that ROS production is not a prerequisite for removal of E. coli and phage ØX174, however, it does offer an additional disinfection barrier, which increases the robustness of Fe-EC for water treatment.

At wastewater treatment plants (WWTPs), additional steps are introduced for removal of organic micropollutants (OMPs) from the treated effluents, especially pharmaceutical residues. At the same time, a new concern is emerging: antibiotic resistance (AR). This research studied the effect of ozonation, coagulation and granular activated carbon (GAC) filtration applied as tertiary treatment for the removal of OMPs and nutrients, on AR removal. Bacterial culture methods in selective media were used to screen for four different microorganisms: two faecal indicators (Escherichia coli and Enterococci) as antibiotic sensitive bacteria (ASB), and a resistant strain of each of these bacteria, namely Extended-Spectrum Beta-lactamase producing E. coli (ESBL-E.coli) and Vancomycin Resistant Enterococci (VRE) as antibiotic resistant bacteria (ARB). At laboratory scale, ozonation experiments (ozone dose 0.4–0.6 g O3/g DOC) and coagulation experiments using Polyaluminum chloride (PAX-214) and FeCl3 (coagulant dose 0.004–1 mM/L) were performed using secondary effluent from two municipal WWTPs. In addition in a pilot plant and full-scale plant ozonation (ozone dose 0.4 g O3/g DOC) and GAC filtration (empty bed contact time 15 min) were studied for AR removal. No significant differences were found between ARB and ASB removal for coagulation and ozonation which could indicate that ASB can be used as an initial proxy for ARB removal for these technologies. In the laboratory experiments, ozonation and coagulation showed a good removal of both ARB and ASB. However, the doses needed to reach 2–3 log removal were a factor 2.5–4 (ozonation) and 250 (coagulation) higher than applied for OMP removal (by ozonation) and phosphorus (P) removal (by coagulation). In the GAC filters, the risk of ARB enhancement occurred, especially in filters with a matured biology. Although these bacteria are not necessarily directly harmful, they can pass down their resistance to pathogenic bacteria via horizontal gene transfer.