Disinfection during Iron Electrocoagulation: Differentiating between Inactivation and Floc Entrapment for Escherichia coli and Somatic Coliphage Øx174

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.