Electrochemical ferrous iron (Fe²⁺) wastewater treatment is gaining momentum for treating municipal wastewater due to its decreasing costs, environmental friendliness and capacity for removal of a wide range of contaminants. Disinfection by iron electrocoagulation (Fe-EC) has been occasionally reported in full scale industrial applications, yet controversy remains regarding its underlying elimination mechanisms and kinetics. In this study, it was demonstrated that substantial inactivation can be achieved for Escherichia coli WR1 (5 log₁₀) and somatic coliphage ΦX174 (2–3 log₁₀). Electrochemically produced Fe²⁺ yielded similar inactivation as chemical Fe²⁺. Reactive oxygen species (ROS)-quenching experiments with TEMPOL confirmed that E. coli inactivation was related to the production of Fenton-like intermediates during Fe²⁺ oxidation. The observed E. coli disinfection kinetics could be mathematically related to Fe-EC current intensity using a Chick-Watson-like expression, in which the amperage is surrogate for the disinfectant's concentration. We hereby show that it is possible to mathematically predict disinfection based on applied Fe dosage and dosage speed. Phage ΦX174 inactivation could not be described in a similar way because at higher Fe dosages (>20 mg/l), little additional inactivation was observed. Also, ROS-quencher TEMPOL did not completely inhibit phage ΦX174 removal, suggesting that additional pathways are relevant for its elimination.

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 O ₃/g DOC) and coagulation experiments using Polyaluminum chloride (PAX-214) and FeCl ₃ (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 O ₃/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.

In this paper we analyse the feasibility of low voltage iron electrocoagulation as a means of municipal secondary effluent treatment with a focus on removal of microbial indicators, Antibiotic Resistant Bacteria (ARB) and nutrients. A laboratory scale batch unit equipped with iron electrodes was used on synthetic and real secondary effluent from a municipal wastewater treatment plant. Synthetic secondary effluent was separately assayed with spiked Escherichia coli WR1 and with bacteriophage ΦX174, while real effluent samples were screened before and after treatment for E. coli, Extended Spectrum Betalactamase-producing E. coli, Enterococci, Vancomycin Resistant Enterococci, Clostridium perfringens spores and somatic coliphages. Charge dosage (CD) and charge dosage rate (CDR) were used as the main process control parameters. Experiments with synthetic secondary effluent showed >4log₁₀ and >5log₁₀ removal for phage ΦX174 and for E. coli WR1, respectively. In real effluents, bacterial indicator removal exceeded 3.5log₁₀, ARB were removed below detection limit (≥2.5log₁₀), virus removal reached 2.3log₁₀ and C. perfringens spore removal exceeded 2.5log₁₀. Experiments in both real and synthetic wastewater showed that bacterial removal increased with increasing CD and decreasing CDR. Virus removal increased with increasing CD but was irresponsive to CDR. C. perfringens spore removal increased with increasing CD yet reached a removal plateau, being also irresponsive to CDR. Phosphate removal exceeded 99%, while total nitrogen and chemical oxygen demand removal were below 15% and 58%, respectively. Operational cost estimates were made for power and iron plate consumption, and were found to be in the range of 0.01 to 0.24€/m³ for the different assayed configurations. In conclusion, low voltage Fe-EC is a promising technology for pathogen reduction of secondary municipal effluents, with log₁₀ removals comparable to those achieved by conventional disinfection methods such as chlorination, UV or ozonation.