Electrochemical disinfection using boron-doped diamond electrode--the synergetic effects of in situ ozone and free chlorine generation

Current status of advancement in remediation technologies for the toxic metal mercury in the environment: A critical review

Currently, pollution due to heavy metals, in particular dissolved mercury, is a major concern for society and the environment. This work aims to evaluate the current scenario regarding the removal/elimination of mercury. Mercury removal through adsorption is mainly done through artificial resins and metallic-organic frameworks. In the case of the zinc organic framework, it was able to adsorb Hg2+, reaching an adsorption capacity of 802 mg g-1. As for the Hg(0) the coconut husk was found to have the lowest equilibrium time, 30 min, and the highest adsorption capacity of 956.2 mg g-1. Experimental reports and molecular simulation indicate that the adsorption of mercury and other chemical forms occurs due to electrostatic interactions, ion exchange, precipitation, complexation, chelation, and covalent bonds, according to the material nature. The reported thermodynamic results show that, in most cases, the mercury adsorption has an endothermic nature with enthalpy levels below 40 kJ mol-1. Thermal and chemical regeneration methods lead to a similar number of 5 cycles for different materials. The presence of other ions, in particular cadmium, lead, and copper, generates an antagonistic effect for mercury adsorption. Regarding the other current technologies, it was found that mercury removal is feasible through precipitation, phytoremediation, and marine microalgae; all these methods require constant chemicals or a slow rate of removal according to the conditions. Advanced oxidative processes have noteworthy removal of Hg(0); however, Fenton processes lead to mineralization, which leads to Fe2+ and Fe3+ in solution; sonochemical processes are impossible to scale up at the current technology level; and electrochemical processes consume more energy and require constant changes of the anode and cathode. Overall, it is possible to conclude that the adsorption process remains a more friendly, economical, and greener process in comparison with other processes.

Design, Validation, and Fabrication of a Tailored Electrochemical Reactor Using 3D Printing for Studies of Commercial Boron-Doped Diamond Electrodes

Boron-doped diamond (BDD) electrodes are the most effective and resistant electrodic materials to perform advanced oxidation processes. Having a reactor that can provide adequate hydrodynamic conditions is mandatory to use these electrodes effectively. In this work, the diamond anode electrochemical reactor (E3L-DAER) is designed to fulfill this necessity. Several features are included to improve its efficiency, like conic inlet/outlet, flow enhancers, and a reduced interelectrode gap. The fluid dynamic validation has been performed using computer fluid dynamics (CFD) calculations, residence time distribution (RDT) curves, and mass transfer analysis. The reactor has been made using a three-dimensional (3D) printing stereolithography (SLA) technique, which allows us to build chemical-resistant reactors with nonstandard and tailored features in a cheap and fast way. The obtained results demonstrate that the designed reactor has the required fluid dynamics properties to perform reliable BDD electrode studies and applications. Finally, a BDD electrode was used to test the production of different oxidants such as persulfate, peroxophosphate, and chlorine-derived species.

Electrochemical arsenite oxidation for drinking water treatment: Mechanisms, by-product formation and energy consumption

The mechanisms and by-product formation of electrochemical oxidation (EO) for As(III) oxidation in drinking water treatment using groundwater was investigated. Experiments were carried out using a flowthrough system, with an RuO2/IrO2 MMO Ti anode electrode, fed with synthetic and natural groundwater containing As(III) concentrations in a range of around 75 and 2 µg/L, respectively. Oxidation was dependent on charge dosage (CD) [C/L] and current density [A/m2], with the latter showing plateau behaviour for increasing intensity. As(III) concentrations of <0.3 µg/L were obtained, indicating oxidation of 99.9 % of influent As(III). Achieving this required a higher charge dosage for the natural groundwater (>40 C/L) compared to the oxidation in the synthetic water matrix (20 C/L), indicating reaction with natural organic matter or other compounds. As(III) oxidation in groundwater required an energy consumption of 0.09 and 0.21 kWh/m3, for current densities of 20 and 60 A/m2, respectively. At EO settings relevant for As(III) oxidation, in the 30-100 C/L CD range, the formation of anodic by-products, as trihalomethanes (THMs) (0.11-0.75 µg/L) and bromate (<0.2 µg/L) was investigated. Interestingly, concentrations of the formed by-products did not exceed strictest regulatory standards of 1 µg/L, applicable to Dutch tap water. This study showed the promising perspective of EO as electrochemical advanced oxidation process (eAOP) in drinking water treatment as alternative for the conventional use of strong oxidizing chemicals.

Bibliometric analysis of electrochemical disinfection: current status and development trend from 2002 to 2022

The removal of waterborne pathogens from water is critical in preventing the spread of waterborne diseases. Electrochemical methods have been extensively researched and implemented for disinfection, primarily owing to their simplicity, efficiency, and eco-friendliness. Thus, it is essential to conduct a review about the research progress and hotspots on this promising technique. In this paper, we provided a comprehensive bibliometric analysis to systematically study and analyze the current status, hotspots, and trends in electrochemical disinfection research from 2002 to 2022. This study analyzed literature related to electrochemical disinfection or electrochemical sterilization published in the Web of Science database from 2002 to 2022 using CiteSpace and Biblioshiny R language software packages. The analysis focused on the visualization and assessment of annual publication volume, discipline and journal distribution, collaborative networks, highly cited papers, and keywords to systematically understand the current status and trends of electrochemical disinfection. The results showed that between 2002 and 2022, 1171 publications related to electrochemical disinfection were published, with an exponential increase in the cumulative number of publications (y=17.518e0.2147x, R2= 0.9788). The publications covered 76 disciplines with many articles published in high-impact journals. However, the research power was characterized by a large number of scattered research efforts and insufficient cooperation, indicating the need for further innovative collaboration. The citation analysis and keyword analysis suggest that future development in this field may focus on optimizing electrode materials, investigating the disinfection performance of ·OH based systems, optimizing conditions for actual wastewater treatment, and reducing energy consumption to promote practical applications.

New diamond coatings for a safer electrolytic disinfection

In this work, a new coating of boron-doped diamond ultra-nanocrystalline (U-NBDD), tailored to prevent massive formation of perchlorates during disinfection, is evaluated as electrode for the reclaiming of treated secondary wastewater by the electrochemically assisted disinfection process. Results obtained are compared to those obtained by using a standard electrode (STD) that was evaluated as a standard in previous research showing outstanding performance for this application. First tests were carried out to evaluate the chlorine speciation obtained after the electrolysis of synthetic chloride solutions at two different ranges of current densities. Concentrations of hypochlorite obtained using the U-NBDD anode at 25 mA cm-2 were 1.5-fold higher, outperforming STD anode; however, at 300 mA cm-2, an overturn on the behavior of anodes occurs where the amount of hypochlorite produced on STD anode was 1.5-fold higher. Importantly, at low current density the formation of chlorates and perchlorates is null using U-NBDD. Then, the disinfection of the real effluent of the secondary clarifier of a municipal wastewater treatment facility is assessed, where inactivation of Escherichia coli is achieved at low charge applied per volume electrolyzed (0.08 A h L-1) at 25 mA cm-2 using the U-NBDD. These findings demonstrate the appropriateness of the strategy followed in this work to obtain safer electro-disinfection technologies for the reclaiming of treated wastewater.

Water treatment and reclamation by implementing electrochemical systems with constructed wetlands

Seasonal or permanent water scarcity in off-grid communities can be alleviated by recycling water in decentralized wastewater treatment systems. Nature-based solutions, such as constructed wetlands (CWs), have become popular solutions for sanitation in remote locations. Although typical CWs can efficiently remove solids and organics to meet water reuse standards, polishing remains necessary for other parameters, such as pathogens, nutrients, and recalcitrant pollutants. Different CW designs and CWs coupled with electrochemical technologies have been proposed to improve treatment efficiency. Electrochemical systems (ECs) have been either implemented within the CW bed (ECin-CW) or as a stage in a sequential treatment (CW + EC). A large body of literature has focused on ECin-CW, and multiple scaled-up systems have recently been successfully implemented, primarily to remove recalcitrant organics. Conversely, only a few reports have explored the opportunity to polish CW effluents in a downstream electrochemical module for the electro-oxidation of micropollutants or electro-disinfection of pathogens to meet more stringent water reuse standards. This paper aims to critically review the opportunities, challenges, and future research directions of the different couplings of CW with EC as a decentralized technology for water treatment and recovery.

Comparison of MBR and MBBR followed by UV or electrochemical disinfection for decentralized greywater treatment

Greywater is an attractive source for water reuse at the household or building level, particularly for non-potable applications. Two greywater treatment approaches are membrane bioreactors (MBR) and moving bed biofilm reactors (MBBR), yet, their performance has not been compared so far within their respective treatment flowsheets, including post-disinfection. Two lab-scale treatment trains were operated on synthetic greywater: a) MBR with either polymeric (chlorinated polyethylene, C-PE, 165 days) or ceramic (silicon carbide, SiC, 199 days) membranes coupled with UV disinfection; and b) single-stage (66 days) or two-stage (124 days) MBBR coupled with an electrochemical cell (EC) for in-situ disinfectant generation. Water quality was constantly monitored, and Escherichia coli log removals were assessed through spike tests. Under low-flux operation of the MBR (<8 L·m ^- 2·h ^- 1), the SiC membranes delayed the onset of membrane fouling and needed less frequent cleaning compared to C-PE membranes. Both treatment systems met most water quality requirements for unrestricted greywater reuse, at a 10-fold lower reactor volume for the MBR than the MBBR. However, neither the MBR nor the two-staged MBBR allowed adequate nitrogen removal, and the MBBR did not consistently meet effluent chemical oxygen demand and turbidity requirements. Both EC and UV provided non-detectable E. coli concentrations in the effluent. Although the EC provided residual disinfection, scaling and fouling decreased its energetic and disinfection performance over time, making it less efficient than UV disinfection. Several outlines to improve the performance of both treatment trains and disinfection processes are proposed, thus, allowing a fit-for-use approach that leverages the advantages of the respective treatment trains. Results from this investigation will assist in elucidating the most efficient, robust, and low-maintenance technology and configurations for small-scale greywater treatment for reuse.

Electrochemical disinfection may increase the spread of antibiotic resistance genes by promoting conjugal plasmid transfer

Current in the milliampere range can be used for electrochemical inactivation of bacteria. Yet, bacteria-including antibiotic resistant bacteria (ARB) may be subjected to sublethal conditions due to imperfect mixing or energy savings measures during electrochemical disinfection. It is not known whether such sublethal current intensities have the potential to stimulate plasmid transfer from ARB. In this study, conjugal transfer of plasmid pKJK5 was investigated between Pseudomonas putida strains under conditions reflecting electrochemical disinfection. Although the abundance of culturable and membrane-intact donor and recipient cells decreased with applied current (0-60 mA), both transconjugant density and transconjugant frequency increased. Both active chlorine and superoxide radicals were generated electrolytically, and ROS generation was induced. In addition, we detected significant over expression of a core oxidative stress defense gene (ahpCF) with current. Expression of selected conjugation related genes (traE, traI, trbJ, and trbL) also significantly correlated with current intensity. ROS accumulation, SOS response and subsequent derepression of conjugation are therefore the plausible consequence of sublethal current exposure. These findings suggest that sublethal intensities of current can enhance conjugal plasmid transfer, and that it is essential that conditions of electrochemical disinfection (applied voltage, current density, time and mixing) are carefully controlled to avoid conjugal ARG transmission.

Optimization and assessment of an electrochemical advanced oxidation system for synthetic stormwater treatment

Electrochemical advanced oxidation processes (eAOPs) such as the current advanced oxidation system (AOS) are a type of electrochemical wastewater treatment that creates oxidative species, such as iodide species, chloride species, and hydroxyl radicals, that can treat even the most recalcitrant contaminants. It is important to determine the concentrations and locations of oxidative species in eAOPs for optimization of the wastewater treatment process. In this study, a spectrophotometric methodology was used to determine concentrations of iodide and chloride oxidative species (starting at 10, 25, and 50 ppm) within an AOS under various input voltages (6, 12, and 24 V). Overall, it was found that iodate and chlorite were the dominant species created in their respective treatments. Additionally, the concentration of iodide oxidative species increased with increasing voltage, whereas the chloride species decreased with increasing voltage. The optimal conditions for the efficient creation of AOS oxidative species were 12 V and 10 ppm potassium iodide and 6 V and 10 ppm sodium chloride, respectively. In addition, the use of iodide is recommended for wastewater treatment using the AOS to effectively create oxidative species. Following optimization, the AOS performance was tested for synthetic stormwater. Results indicated that the AOS performed well for reduction of Escherichia coli; however, reduction of other contaminants was inconsistent as would be expected given the AOS was optimized for disinfection, not decontamination. Further AOS optimization for decontamination would be expected to result in improved decontamination performance.

Toward real applicability of electro-ozonizers: Paying attention to the gas phase using actual commercial PEM electrolyzers technology

This work focuses on increasing the TRL of electro-ozonizer technology by evaluating the effect of electrolyte composition and operation conditions on the production of ozone, using an actual commercial cell, CONDIAPURE®, in conditions similar to what could be expected in a real application. Not only is attention paid to the changes in the concentration of ozone in the liquid phase, but also to those observed in the gas phase. The electrolyte and its recirculation flowrate, as well as operation temperatures and pressures are found to have significant influence on production rates. The most efficient way to produce ozone is operating at low temperatures and high pressures. In this work, 0.25 and 0.21 mg O₃/min were obtained operating at 10 A in electrolytes consisting of aqueous solutions of perchloric and sulfuric acid, respectively, in tests carried out at 13 °C and 2 bars of gauge pressure. The negative effect of scavengers that appear electrochemically along the production of ozone is very important and seems to be partially compensated when organics are present in the solution due to the competition between the reaction of these scavengers with ozone or organics.

Simultaneous electrochemical detection of ozone and free chlorine with a boron-doped diamond electrode

O3 and free chlorine play significant roles in disinfection and organic degradation.

Stormwater treatment for reuse: Current practice and future development - A review

Stormwater harvesting is an effective measure to mitigate flooding risk and pollutant migration in our urban environment with the continuously increasing impermeable faction. Treatment of harvested stormwater also provides the fit-for-purpose water sources as an alternative to potable water supply ensuring the reliability and sustainability of the water management in the living complex. In order to provide the water management decision-maker with a broad range of related technology database and to facilitate the implementation of stormwater harvesting in the future, a comprehensive review was undertaken to understand the corresponding treatment performance, the applicable circumstances of current stormwater treatment and harvesting technologies. Technologies with promising potential for stormwater treatment were also reviewed to investigate the feasibility of being used in an integrated process. The raw stormwater quality and the required quality for different levels of stormwater reuses (irrigation, recreational, and potable) were reviewed and compared. The required level of treatment is defined for different 'fit-for-purpose' uses of harvested stormwater. Stormwater biofilter and constructed wetland as the two most advanced and widely used stormwater harvesting and treatment technologies, their main functionality, treatment performance and adequate scale of the application were reviewed based on published peer-reviewed articles and case studies. Excessive microbial effluent that exists in stormwater treated using these two technologies has restricted the stormwater reuse in most cases. Water disinfection technologies developed for wastewater and surface water treatment but with high potential to be used for stormwater treatment have been reviewed. Their feasibility and limitation for stormwater treatment are presented with respect to different levels of fit-for-purpose reuses. Implications for future implementation of stormwater treatment are made on proposing treatment trains that are suitable for different fit-for-purpose stormwater reuses.

Adapting the low-cost pre-disinfection column PREDICO for simultaneous softening and disinfection of pore water

In previous works, a low-cost predisinfection column that combined coagulation-flocculation and GAC filtration was proposed for combination with electrodisinfection in the successful treatment of highly faecal polluted surface water. In this work, this column is adapted for the treatment of pore water by transforming the coagulation chamber into a chemical reactor with lime and replacing the GAC of the filter with ion exchange resins. This adapted system can soften water, remove nitrate and condition water for very efficient electrochemical disinfection, where 4 logs and 3 logs in the removal of E. coli and P. aeruginosa, respectively, were reached using commercial electrochemical cells, i.e., CabECO ® or MIKROZON®. The availability and low cost of the technology are strong points for usage in poor areas of developing countries.

Hospital Wastewater-Source of Specific Micropollutants, Antibiotic-Resistant Microorganisms, Viruses, and Their Elimination

Municipal wastewaters can generally provide real-time information on drug consumption, the incidence of specific diseases, or establish exposure to certain agents and determine some lifestyle consequences. From this point of view, wastewater-based epidemiology represents a modern diagnostic tool for describing the health status of a certain part of the population in a specific region. Hospital wastewater is a complex mixture of pharmaceuticals, illegal drugs, and their metabolites as well as different susceptible and antibiotic-resistant microorganisms, including viruses. Many studies pointed out that wastewater from healthcare facilities (including hospital wastewater), significantly contributes to higher loads of micropollutants, including bacteria and viruses, in municipal wastewater. In addition, such a mixture can increase the selective pressure on bacteria, thus contributing to the development and dissemination of antimicrobial resistance. Because many pharmaceuticals, drugs, and microorganisms can pass through wastewater treatment plants without any significant change in their structure and toxicity and enter surface waters, treatment technologies need to be improved. This short review summarizes the recent knowledge from studies on micropollutants, pathogens, antibiotic-resistant bacteria, and viruses (including SARS-CoV-2) in wastewater from healthcare facilities. It also proposes several possibilities for improving the wastewater treatment process in terms of efficiency as well as economy.

Electrochemical Disinfection in Water and Wastewater Treatment: Identifying Impacts of Water Quality and Operating Conditions on Performance

Electrochemical disinfection-a method in which chemical oxidants are generated in situ via redox reactions on the surface of an electrode-has attracted increased attention in recent years as an alternative to traditional chemical dosing disinfection methods. Because electrochemical disinfection does not entail the transport and storage of hazardous materials and can be scaled across centralized and distributed treatment contexts, it shows promise for use both in resource limited settings and as a supplement for aging centralized systems. In this Critical Review, we explore the significance of treatment context, oxidant selection, and operating practice on electrochemical disinfection system performance. We analyze the impacts of water composition on oxidant demand and required disinfectant dose across drinking water, centralized wastewater, and distributed wastewater treatment contexts for both free chlorine- and hydroxyl-radical-based systems. Drivers of energy consumption during oxidant generation are identified, and the energetic performance of experimentally reported electrochemical disinfection systems are evaluated against optimal modeled performance. We also highlight promising applications and operational strategies for electrochemical disinfection and propose reporting standards for future work.

Inhibition of staphylococci and S. aureus in wastewater by ferrates and electrochemical methods

Increasing concentration of antibiotics in environment and their subinhibitory concentrations in wastewater may result in increased antibiotic resistance of present bacteria. Therefore, this study was aimed to analyze the efficiency of coagulase-positive staphylococci and Staphylococcus aureus inhibition in wastewater by electrochemical methods and addition of ferrates. Advanced electrochemical oxidation by boron doped diamond electrodes in anode; cathode and anode-cathode connection were used for wastewater disinfection. Results showed that the most effective connection was the anodic one, as complete inhibition of coagulase-positive staphylococci as well as of S. aureus was observed after 40 min. Energy consumption was 3.69 kWh/m3 for effluent wastewater disinfection. The second studied method of wastewater disinfection was the application of powdered ferrates. Addition of 100 mg of ferrates resulted in the inhibition of 84—96 % of coagulase-positive staphylococci and 97—99 % of S. aureus in influent water, while the inhibition of coagulase-positive staphylococci and S. aureus was 61—83 % and 83—86 %, respectively, in effluent wastewater.

Electrodisinfection of bacteria-laden in surface water using modified Ti electrode by antimony-and nickel-doped tin oxide composite

Providing clean and safe drinking water by point of use (POU) disinfection methods has become a critical issue, especially in crises and epidemics. In this study, antimony-and nickel-doped tin oxide electrode (Ni-Sb-SnO₂) was employed as an electrode for electro-catalytic disinfection of surface water. The synthetized electrodes were characterized using scanning electron microscope, linear sweep voltammetry and X-Ray diffraction techniques. The results revealed that the highest electrochemical disinfection efficiency was achieved by the Ni-Sb-SnO₂ electrode under weak acidic conditions and its performance decreased with increasing pH towards alkaline environment. Based on the results, total coliform (TC) and fecal coliform (FC) were completely removed at current density of 0.67 mA cm^-2. Moreover, the electrochemical disinfection of microorganisms showed that the process efficiency was directly proportional to increasing time and at 0.6 C cm^-2 of charge passed, 3-log removal of the both indicators occurred after 15 min. The highest removal efficiency of TC and FC was also achieved at 8 mmol of NaCl concentration at <10 min of detention time. The results of this study depicted that the Ti/Ni-Sb-SnO₂ electrode provides higher disinfection efficiency for the removal of TC and FC compared with Ti and SS/PbO₂ electrodes. Moreover, the proposed system was able to completely eliminate heterotrophic, Streptococcus faecalis and Pseudomonas aerogenes indicators under optimal conditions. Therefore, it can be concluded that the proposed electrochemical system can be efficiency applied as a POU disinfection system for disinfection of water contaminated with microbial indicators, especially for crises and epidemics.

Effective treatment of greywater via green wall biofiltration and electrochemical disinfection

Low energy and cost solutions are needed to combat raising water needs in urbanised areas and produce high quality recycled water. In this study, we investigated key processes that drive a unique greywater treatment train consisting of a passive green wall biofiltration system followed by disinfection using a Boron-doped diamond (BDD) electrode with a solid polymer electrolyte (SPE). In both systems, the treatment was performed without any additional chemicals and pollutants of concern were monitored for process evaluation. The green wall system removed over 90% of turbidity, apparent colour, chemical oxygen demand, total organic carbon, and biological oxygen demand, and 1 log of E. coli and total coliforms, mostly through biological processes. The green wall effluent met several proposed greywater reuse guidelines, except for E. coli and total coliform treatment (below 10 MPN/100 mL). Further disinfection of treated greywater (contained 28 mg/L Cl¯ and electrical conductivity (EC) of 181.3 µS/cm) by electrolysis at current density 25 mA/cm² inactivated over 3.5 logs of both E. coli and total coliforms, in 10 - 15 min of electrolysis, resulting in recycled water with less than 2 MPN/100 mL. A synergistic effect between electrochemically-generated free chlorines and reactive oxygen species contributed to the inactivation process. Although the treated water contained diluted chloride and had low EC, estimated energy consumption was just 0.63 - 0.83 kWh/m³. This is the first study to show the effectiveness of a low energy and a low cost greywater treatment train that combines green urban infrastructure with BDD electrochemical treatment process with SPE, offering a reliable and an environmentally-friendly method for greywater reuse.

Testing the use of cells equipped with solid polymer electrolytes for electro-disinfection

This work focuses on disinfection of water using electrolysis with boron doped diamond (BDD) coatings and faces this challenge by comparing the performance of two different cells manufactured by CONDIAS GmbH (Izehoe, Germany): CONDIACELL® ECWP and CabECO cells. They are both equipped with diamond electrodes, but the mechanical design is completely different, varying not only by geometry but also by the flow conditions. ECWP is a flow-through cell with perforated electrodes while the CabECO cell is a zero-gap cell with a proton exchange membrane as a solid polymer electrolyte (SPE) separating the anode and cathode. At 0.02 Ah dm^-3 both cells attain around 3-5 logs pathogen removal, but design and sizing parameters give an advantage to the CabECO: it can minimize the production of chlorates and perchlorates when operating in a single-pass mode, which becomes a really remarkable point. In this paper, we report tests in which we demonstrate this outstanding performance and we also explain the differences observed in the two cells operating with the same water.

How to avoid the formation of hazardous chlorates and perchlorates during electro-disinfection with diamond anodes?

This work focuses on disinfection of water using electrolysis with diamond coatings avoiding or minimizing the formation of hazardous chlorates and perchlorates using a special type of commercial cells designed by CONDIAS (Itzehoe, Germany) in two different sizes: the CabECO and the MIKROZON cells. In these cells, the electrolyte that separates the anode and cathode is a proton exchange membrane. This helps to minimize the production of perchlorate and this behavior is enhanced in the smallest cell for which the very low contact times between the electrodes and the water allows to avoid the production of perchlorates when operating in a single-pass mode, which becomes a really remarkable point. In this paper, we report tests in which we demonstrate this outstanding performance and we also explain the differences observed in the two cells operating with the same water.

Sequential electrocoagulation-electrooxidation for virus mitigation in drinking water

Electrochemical water treatment is a promising alternative for small-scale and remote water systems that lack operational capacity or convenient access to reagents for chemical coagulation and disinfection. In this study, the mitigation of viruses was investigated using electrocoagulation as a pretreatment prior to electrooxidation treatment using boron-doped diamond electrodes. This research is the first to investigate a sequential electrocoagulation-electrooxidation treatment system for virus removal. Bench-scale, batch reactors were used to evaluate mitigation of viruses in variable water quality via: a) electrooxidation, and b) a sequential electrocoagulation-electrooxidation treatment train. Electrooxidation of two bacteriophages, MS2 and ΦX174, was inhibited by natural organic matter and turbidity, indicating the probable need for pretreatment. However, the electrocoagulation-electrooxidation treatment train was beneficial only in the model surface waters employed. In model groundwaters, electrocoagulation alone was as good or better than the combined electrocoagulation-electrooxidation treatment train. Reduction of human echovirus was significantly lower than one or both bacteriophages in all model waters, though bacteriophage ΦX174 was a more representative surrogate than MS2 in the presence of natural organic matter and turbidity. Compared to conventional treatment by ferric salt coagulant and free chlorine disinfection, the electrocoagulation-electrooxidation system was less effective in model surface waters but more effective in model groundwaters. Sequential electrocoagulation-electrooxidation was beneficial for some applications, though practical considerations may currently outweigh the benefits.

Activated Carbon as a Cathode for Water Disinfection through the Electro-Fenton Process

Unlike many other water disinfection methods, hydroxyl radicals (HO•) produced by the Fenton reaction (Fe2+/H2O2) can inactivate pathogens regardless of taxonomic identity of genetic potential and do not generate halogenated disinfection by-products. Hydrogen peroxide (H2O2) required for the process is typically electrogenerated using various carbonaceous materials as cathodes. However, high costs and necessary modifications to the cathodes still present a challenge to large-scale implementation. In this work, we use granular activated carbon (GAC) as a cathode to generate H2O2 for water disinfection through the electro-Fenton process. GAC is a low-cost amorphous carbon with abundant oxygen- and carbon-containing groups that are favored for oxygen reduction into H2O2. Results indicate that H2O2 production at the GAC cathode is higher with more GAC, lower pH, and smaller reactor volume. Through the addition of iron ions, the electrogenerated H2O2 is transformed into HO• that efficiently inactivated model pathogen (Escherichia coli) under various water chemistry conditions. Chick-Watson modeling results further showed the strong lethality of produced HO• from the electro-Fenton process. This inactivation coupled with high H2O2 yield, excellent reusability, and relatively low cost of GAC proves that GAC is a promising cathodic material for large-scale water disinfection.

Electrochemical Oxidation/Disinfection of Urine Wastewaters with Different Anode Materials

In the present work, electrochemical technology was used simultaneously for the deactivation of microorganisms and the destruction of micro-pollutants contained in synthetic urine wastewaters. Microorganisms (E. coli) were added to synthetic urine wastewaters to mimic secondary treated sewage wastewaters. Different anode materials were employed including boron-doped diamond (BDD), dimensionally stable anode (DSA: IrO₂ and RuO₂) and platinum (Pt). The results showed that for the different anode materials, a complete deactivation of E. coli microorganisms at low applied electric charge (1.34 Ah dm⁻³) was obtained. The complete deactivation of microorganisms in wastewater seems to be directly related to active chlorine and oxygen species electrochemically produced at the surface of the anode material. Complete depletion of COD and TOC can be attained during electrolyses with BDD anode after the consumption of specific electric charges of 4.0 and 8.0 Ah dm⁻³, respectively. Higher specific electric charges (>25 Ah dm⁻³) were consumed to removal completely COD and about 75% of TOC during electrolyses with DSA anodes (IrO₂ and RuO₂). However, the electrolysis using Pt anode can partially remove and even after the consumption of high specific electric charges (>40 Ah dm⁻³) COD and TOC did not exceed 50 and 25%, respectively. Active chlorine species including hypochlorite ions and chloramines formed during electrolysis contribute not only to deactivate microorganisms but also to degrade organics compounds. High conversion yields of organic nitrogen into nitrates and ammonium were achieved during electrolysis BDD and DSA anodes. The results have confirmed that BDD anode is more efficient than with IrO₂, RuO₂ and Pt electrodes in terms of COD and TOC removals. However, higher amounts of perchlorates were measured at the end of the electrolysis using BDD anode.

Carbon Fiber-Based Flow-Through Electrode System (FES) for Water Disinfection via Direct Oxidation Mechanism with a Sequential Reduction-Oxidation Process

Flow-through configuration for electrochemical disinfection is considered as a promising approach to minimize the formation of toxic byproducts and energy consumption via the enhanced convective mass transport as compared with conventional flow-by one. Under this hydrodynamic condition, it is essential to ascertain the effect of sequential electro-redox processes with the cathode/anode then anode/cathode arrangements on disinfection performance. Here, carbon fiber felt (CFF) was utilized to construct two flow-through electrode systems (FESs) with sequential reduction-oxidation (cathode-anode) or oxidation-reduction (anode-cathode) processes to systematically compare their disinfection performance toward a model Escherichia coli ( E. coli) pathogen. In-situ sampling and live/dead backlight staining experiments revealed that E. coli inactivation mainly occurred on anode via an adsorption-inactivation-desorption process. In reduction-oxidation system, after the cathode-pretreatment, bulk solution pH increased significantly, leading to the negative charge of E. coli cells. Hence, E. coli cells were adsorbed and inactivated easily on the subsequent anode, finally resulting in its much better disinfection performance and energy efficiency than the oxidation-reduction system. Application of 3.0 V resulted in ∼6.5 log E. coli removal at 1500 L m^-2 h^-1 (50 mL min^-1), suggesting that portable devices can be designed from CFF-based FES with potential application for point-of-use water disinfection.

Elimination of bacterial contaminations by treatment of water with boron-doped diamond electrodes

Boron-doped diamond electrodes can be used to generate reactive oxygen species directly at the electrode's surface. This property was used in this study for in-situ electrochemical oxidation to eliminate different bacteria, i.e. Escherichia coli, Pseudomonas fluorescens and Pseudomonas aeruginosa, as well as Bacillus subtilis spores from water samples. Application of low voltages in the rage from 4 to 10 V and short incubation times in the range of minutes allowed a complete disinfection of water contaminated with enterobacteria and freshwater microbes including nosocomial pathogens as well as a significant reduction of spores. A pilot reactor was constructed, which allowed to decrease microbial contamination of sewage plant effluent drastically. Boron-doped diamond electrodes allow efficient reduction of bacterial contaminations in water samples.

Electrochemical oxidation disinfects urban stormwater: Major disinfection mechanisms and longevity tests

Although electrochemical oxidation (ECO) has shown excellent potential for disinfecting wastewater and surface waters, its application on urban stormwater has been rarely tested. In order to improve stormwater ECO design, this paper explores the major inactivation processes using Boron Doped Diamond (BDD) and titanium Dimensional Stable Anodes (DSA). Both BDD and DSA showed comparable disinfection rates. The mechanism study suggested that BDD relied on hydroxyl radical and the presence of chloride ions, while DSA disinfected stormwater mainly via the production of free‑chlorine. A deterioration study carried out at a catchment in Melbourne, showed a steady performance for BDD and revealed that DSA's performance degraded with time, likely linked to the high operational voltage required for specific chemistry of stormwater. Scanning Electron Microscopes and an Energy Dispersive X-ray Detector tests confirmed elemental losses occurred on the DSA surface, together with an aluminium/silicon coating layer potentially sourced from the stormwater clayish sediments. Furthermore, disinfection by-products in electrochemical disinfected stormwater using either BDD or DSA were at least one order of magnitude lower than the Australia Drinking Water Guidelines limits. The mechanism and long-term study demonstrated that careful anode selection is required as some anodes will deteriorate in stormwater matrices faster than others.

Electrochemical oxidation for stormwater disinfection: How does real stormwater chemistry impact on pathogen removal and disinfection by-products level?

Preliminary laboratory work has shown that electrochemical oxidation (ECO) is a promising technology for disinfection of harvested stormwater. This paper focuses on understanding how stormwater chemistry (e.g. pH, chloride, bicarbonate, ammonia and total organic carbon - that can vary substantially between sites) impacts the disinfection performance of ECO. Real stormwater samples from four different urban catchments were collected and tested for ECO performance in disinfecting stormwater pathogens using a boron doped diamond anode under the current density of 4.2 mA/cm². Results showed that total disinfection of indigenous Escherichia coli (E. coli), as well as three different stormwater pathogens (Enterococci, Campylobacter and C. perfringens) was achievable for all four tested stormwater within 30 min. Compared to the synthetic stormwater, lower disinfection rates were observed in real stormwater which has more complex chemistry. Stormwater chloride concentration was the only tested parameter that had significant impact on the treatment performance, with higher initial stormwater chloride concentration leading to an increased disinfection rate. Disinfection by-products in the treated stormwater were well below the Australian Drinking Water Guideline value for health, with its production level positively correlated to the pH values of stormwater.

Electrochemical inactivation of bacteria with a titanium sub-oxide reactive membrane

A reactive electrochemical membrane (REM) system was developed with titanium suboxide microfiltration membrane serving as the filter and the anode, and was examined to inactivate Escherichia coli (E. coli) in water at various current densities. After passing through the membrane filter, the concentration of E. coli decreased from 6.46 log CFU/mL to 0.18 log CFU/mL. The REM operation and effects, including membrane pressure, anode potential, protein leakage, and cell morphology, were characterized under different treatment conditions. It was found that several mechanisms, including membrane filtration, external electrical field influence, and direct oxidation, functioned in concert to lead to bacteria removal and inactivation, and direct oxidation likely played the major role. As revealed by scanning electron microscope and extracellular protein analysis, high current density and voltage caused severe cell damage that resulted in partial or complete cell disintegration. The removal of a model virus, bacteriophage MS2, was also investigated at the current density of 10 mA cm^-2 and achieved 6.74 log reduction compared to the original concentration (10¹¹ PFU/mL). In addition to illustration of mechanisms, this study may provide a potentially promising approach that is suitable for decentralized treatment to meet dispersed water disinfection needs.

Can CabECO® technology be used for the disinfection of highly faecal-polluted surface water?

In this work, the disinfection of highly faecal-polluted surface water was studied using a new electrochemical cell (CabECO^® cell, manufactured by CONDIAS) specifically designed to produce ozone in water with very low conductivity. The disinfection tests were carried out in a discontinuous mode to evaluate the influence of the electrode current charge passed. The effect of the current density was also studied in order to optimize the disinfection conditions and to simultaneously prevent the formation of undesirable by-products (chlorates and perchlorates) during the electrolysis. The results demonstrate that this technology is robust and efficient, and it can suitably disinfect water. During electrolysis, the chloride contained in the water was oxidized to hypochlorite, and this compound was combined with ammonia to form chloramines. Both hypochlorite and chloramines (formed by the well-known break point reaction) promoted persistent disinfection and seemed to be mainly responsible for the disinfection attained during the electrochemical process. Chlorate and perchlorate could also be produced, although the low concentrations of chloride in the tested water made them irrelevant. The removal of the total organic carbon under the applied operating conditions was not very efficient (although it reached 50% in 2 h) and the production of trihalomethanes was very low, below 100 ppb for all tests.

Pre-disinfection columns to improve the performance of the direct electro-disinfection of highly faecal-polluted surface water

This work presents the design and evaluation of a new concept of pre-disinfection treatment that is especially suited for highly polluted surface water and is based on the combination of coagulation-flocculation, lamellar sedimentation and filtration into a single-column unit, in which the interconnection between treatments is an important part of the overall process. The new system, the so-called PREDICO (PRE-DIsinfection Column) system, was built with low-cost consumables from hardware stores (in order to promote in-house construction of the system in poor countries) and was tested with a mixture of 20% raw wastewater and 80% surface water (in order to simulate an extremely bad situation). The results confirmed that the PREDICO system helps to avoid fouling in later electro-disinfection processes and attains a remarkable degree of disinfection (3-4 log units), which supplements the removal of pathogens attained by the electrolytic cell (more than 4 log units). The most important sizing parameters for the PREDICO system are the surface loading rate (SLR) and the hydraulic residence time (HRT); SLR values under 20 cm min^-1 and HRT values over 13.6 min in the PREDICO system are suitable to warrant efficient performance of the system.

Evaluation of advanced oxidation processes for water and wastewater treatment - A critical review

This study provides an overview of established processes as well as recent progress in emerging technologies for advanced oxidation processes (AOPs). In addition to a discussion of major reaction mechanisms and formation of by-products, data on energy efficiency were collected in an extensive analysis of studies reported in the peer-reviewed literature enabling a critical comparison of various established and emerging AOPs based on electrical energy per order (E_EO) values. Despite strong variations within reviewed E_EO values, significant differences could be observed between three groups of AOPs: (1) O₃ (often considered as AOP-like process), O₃/H₂O₂, O₃/UV, UV/H₂O₂, UV/persulfate, UV/chlorine, and electron beam represent median E_EO values of <1 kWh/m³, while median energy consumption by (2) photo-Fenton, plasma, and electrolytic AOPs were significantly higher (E_EO values in the range of 1-100 kWh/m³). (3) UV-based photocatalysis, ultrasound, and microwave-based AOPs are characterized by median values of >100 kWh/m³ and were therefore considered as not (yet) energy efficient AOPs. Specific evaluation of 147 data points for the UV/H₂O₂ process revealed strong effects of operational conditions on reported E_EO values. Besides water type and quality, a major influence was observed for process capacity (lab-vs. pilot-vs. full-scale applications) and, in case of UV-based processes, of the lamp type. However, due to the contribution of other factors, correlation of E_EO values with specific water quality parameters such as UV absorbance and dissolved organic carbon were not substantial. Also, correlations between E_EO and compound reactivity with OH-radicals were not significant (photolytically active compounds were not considered). Based on these findings, recommendations regarding the use of the E_EO concept, including the upscaling of laboratory results, were derived.

Optimization of disinfection by-product analysis methods for IMO G9 approval

This study assessed the methods for analyzing disinfection by-products (DBPs) to determine which were most suitable for ballast water in an approval test according to the Procedure for Approval of Ballast Water Management Systems that make use of Active Substances (G9). The existing analysis methods are optimized for drinking water. Therefore, it is necessary to assess the characteristics of ballast water, which has high levels of total residual oxidants (TROs) and salinity, prior to applying the existing methods. Sample preservation, pre-treatment and instrumental analysis methods were summarized based on certified test methods and the G9 final approval reports. Following the assessment, applicable methods were selected in consideration of the matrix effect arising from the high levels of TROs and salinity. The applicability was assessed using seawater and brackish water. The results are expected to be applied to the G9 test as well as in investigations of DBPs in ballast water.

Inactivation, lysis and degradation by-products of Saccharomyces cerevisiae by electrooxidation using DSA

The yeast Saccharomyces cerevisiae, a microorganism with cell walls resistant to many types of treatments, was chosen as a model to study electrochemical disinfection process using dimensionally stable anodes (DSA). DSA electrodes with nominal composition of Ti/RuO₂TiO₂ and Ti/RuO₂TiO₂IrO₂ were evaluated in 0.05 mol L^-1 Na₂SO₄ containing yeast. The results showed inactivation about of 100 % of the microorganisms at Ti/RuO₂TiO₂ by applying 20 and 60 mA cm^-2 after 120 min of electrolysis, while a complete inactivation at Ti/RuO₂IrO₂TiO₂ electrode was achieved after 180 min at 60 mA cm^-2. When chloride ions were added in the electrolyte solution, 100 % of the yeast was inactivated at 20 mA cm^-2 after 120 min of electrolysis, independent of the anode used. In the absence of chloride, the energy consumption (EC) was of 34.80 kWh m^-3, at 20 mA cm^-2 by using Ti/RuO₂TiO₂ anode. Meanwhile, in the presence of chloride, EC was reduced, requiring 30.24 and 30.99 kWh m^-3 at 20 mA cm^-2, for Ti/RuO₂TiO₂ and Ti/RuO₂IrO₂TiO₂ electrodes, respectively, The best performance for cell lysis was obtained in the presence of chloride with EC of 88.80 kWh m^-3 (Ti/RuO₂TiO₂) and 91.85 kWh m^-3 (Ti/RuO₂IrO₂TiO₂) to remove, respectively, 92 and 95 % of density yeast. The results clearly showed that yeast, as a model adopted, was efficiently inactivated and lysed by electrolysis disinfection using DSA-type electrodes.

The ability of electrochemical oxidation with a BDD anode to inactivate Gram-negative and Gram-positive bacteria in low conductivity sulfate medium

The disinfection of 100 mL of synthetic water containing 7 mM Na2SO4 with 10(6) CFU mL(-1) of either Gram-negative or Gram-positive bacteria has been studied by electrochemical oxidation. The electrolytic cell was a stirred tank reactor equipped with a boron-doped diamond (BDD) anode and a stainless steel cathode and the trials were performed at acidic and neutral pH, at 33.3 mA cm(-2) and 25 °C. Reactive oxygen species, pre-eminently hydroxyl radicals, were efficiently produced in both media from water oxidation at the BDD anode and the bacteria concentration was reduced by ≥ 5 log units after 60 min of electrolysis, thus constituting a good chlorine-free disinfection treatment. All the inactivation kinetics were described by a logistic model, with no significant statistical differences between acidic and neutral suspensions. The electrochemical disinfection with BDD was very effective for Gram-negative bacilli like Escherichia coli and Pseudomonas aeruginosa and Gram-positive ones like Bacillus atrophaeus, whereas the Gram-positive cocci Staphylococcus aureus and Enterococcus hirae were more resistant. Thus, the latter organisms are a better choice than E. coli as process indicators. Scanning electron microscopy highlighted a transition from initial cells with standard morphology supported on clean filters to inactivated cells with a highly altered morphology lying on dirty filters with plenty of cellular debris. Larger damage was observed for Gram-negative cells compared to Gram-positive ones. The inactivation effect could then be related to the chemical composition of the outer layers of the cell structure along with the modification of the transmembrane potentials upon current passage.