Electrochemical disinfection of secondary wastewater treatment plant (WWTP) effluent

The occurrence, ecological risk, and control of disinfection by-products from intensified wastewater disinfection during the COVID-19 pandemic

Increased disinfection of wastewater to preserve its microbiological quality during the coronavirus infectious disease-2019 (COVID-19) pandemic have inevitably led to increased production of toxic disinfection by-products (DBPs). However, there is limited information on such DBPs (i.e., trihalomethanes, haloacetic acids, nitrosamines, and haloacetonitriles). This review focused on the upsurge of chlorine-based disinfectants (such as chlorine, chloramine and chlorine dioxide) in wastewater treatment plants (WWTPs) in the global response to COVID-19. The formation and distribution of DBPs in wastewater were then analyzed to understand the impacts of these large-scale usage of disinfectants in WWTPs. In addition, potential ecological risks associated with DBPs derived from wastewater disinfection and its receiving water bodies were summarized. Finally, various approaches for mitigating DBP levels in wastewater and suggestions for further research into the environmental risks of increased wastewater disinfection were provided. Overall, this study presented a comprehensive overview of the formation, distribution, potential ecological risks, and mitigating approaches of DBPs derived from wastewater disinfection that will facilitate appropriate wastewater disinfection techniques selection, potential ecological risk assessment, and removal approaches and regulations consideration.

Sustainable application of electrocatalytic and photo-electrocatalytic oxidation systems for water and wastewater treatment: a review

Wastewater treatment and reuse have risen as a solution to the water crisis plaguing the world. Global warming-induced climate change, population explosion and fast depletion of groundwater resources are going to exacerbate the present global water problems for the forthcoming future. In this scenario, advanced electrochemical oxidation process (EAOP) utilising electrocatalytic (EC) and photoelectrocatalytic (PEC) technologies have caught hold of the interest of the scientific community. The interest stems from the global water management plans to scale down centralised water and wastewater treatment systems to decentralised and semicentralised treatment systems for better usage efficiency and less resource wastage. In an age of rising water pollution caused by contaminants of emerging concern (CECs), EC and PEC systems were found to be capable of optimal mineralisation of these pollutants rendering them environmentally benign. The present review treads into the conventional electrochemical treatment systems to identify their drawbacks and analyses the scope of the EC and PEC to mitigate them. Probable electrode materials, potential catalysts and optimal operational conditions for such applications were also examined. The review also discusses the possible retrospective application of EC and PEC as point-of-use and point-of-entry treatment systems during the transition from conventional centralised systems to decentralised and semi-centralised water and wastewater treatment systems.

Chloride-Mediated Enhancement in Heat-Induced Activation of Peroxymonosulfate: New Reaction Pathways for Oxidizing Radical Production

This study is the first to demonstrate the capability of Cl^- to markedly accelerate organic oxidation using thermally activated peroxymonosulfate (PMS) under acidic conditions. The treatment efficiency gain allowed heat-activated PMS to surpass heat-activated peroxydisulfate (PDS). During thermal PMS activation at excess Cl^-, accelerated oxidation of 4-chlorophenol (susceptible to oxidation by hypochlorous acid (HOCl)) was observed along with significant degradation of benzoic acid and ClO₃^- occurrence, which involved oxidants with low substrate specificity. This indicated that heat facilitated HOCl formation via nucleophilic Cl^- addition to PMS and enabled free chlorine conversion into less selective oxidizing radicals. HOCl acted as a key intermediate in the major oxidant transition based on temperature-dependent variation in HOCl concentration profiles, kinetically retarded organic oxidation upon NH₄⁺ addition, and enabled rapid organic oxidation in heated PMS/HOCl mixtures. Chlorine atom that formed via the one-electron oxidation of Cl^- by the sulfate radical served as the primary oxidant and was involved in hydroxyl radical production. This was corroborated by the quenching effects of alcohols and bicarbonates, reactivity toward multiple organics, and electron paramagnetic resonance spectral features. PMS outperformed PDS in degrading benzoic acid during thermal activation operated in reverse osmosis concentrate, which was in conflict with the well-established superiority of heat-activated PDS.

Effect of microbial fuel cell operation time on the disinfection efficacy of electrochemically synthesised catholyte from urine

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The MFC with the thickest ceramic membrane produced the best quality catholyte.

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MFC operation time contributes to the catholyte quality and killing properties.

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Catholyte from ceramic MFC (10 mm) reached pH 11 at day 42 and eradicated E. coli.

Microbial fuel cells (MFCs) offer an excellent solution to tackle some of the major challenges currently faced by humankind: sustainable energy sources, waste management and water stress. Besides treating wastewater and producing useful electricity from urine, ceramic MFCs can also generate biocidal catholyte in-situ. It has been proved that the electricity generation from the MFCs has a high impact in the catholyte composition. Therefore, the catholyte composition constantly changes while electricity is generated. However, these changes in catholyte composition with time has not yet been studied and that could highly contribute to the disinfection efficacy. In this work, the evolution of the catholyte generation and composition with the MFC operation time has been chemically and microbiologically evaluated, during 42 days. The results show an increase in pH and conductivity with the operation time, reaching pH 11.5. Flow cytometry and luminometer analyses of bioluminescent pathogenic E. coli exposed to the synthesised catholyte revealed killing properties against bacterial cells. A bio-electrochemical system, capable of electricity generation and simultaneous production of bactericidal catholyte from human urine is presented. The possibility to electrochemically generate in-situ a bacterial killing agent from urine, offers a great opportunity for water reuse and resource recovery for practical implementations.

Micro-structured copper and nickel metal foams for wastewater disinfection: proof-of-concept and scale-up

It is necessary to disinfect treated wastewater prior to discharge to reduce exposure risks to humans and the environment. The currently practiced wastewater disinfection technologies are challenged by toxic by-products, chemicals and energy demand, a range of effectiveness limitations, among other concerns. An effective, eco-friendly, and energy-efficient alternative disinfection technique is desirable to modernize and enhance wastewater treatment operations. Copper and nickel micro-structured metal foams, and a conventional copper mesh, were evaluated as disinfecting surfaces for treating secondary-treated wastewater contaminated with coliform bacteria. The micro-structured copper foam was adopted for scale-up study, due to its stable and satisfactory bactericidal performance obtained over a wide range of bacterial concentrations and metal-to-liquid ratios. Three scales of experiments, using two types of reactor designs, were performed using municipal wastewater to determine the optimal scale-up factors: small lab-scale batch reactor, intermediate lab-scale batch reactor, and pilot-scale continuous tubular reactor experiments. The performance was evaluated with the aim of minimizing metal material requirement with respect to bactericidal efficiency and leaching risks at all scales. Copper foam, at or above optimal conditions, consistently inactivated over 95 % of total coliforms, fecal coliforms and E.coli in wastewater at various scales, and leachate copper concentrations were determined to be below Canadian guideline values for outfall. This study successfully implemented the "structure" strategy of process intensification, and opens up the possibility to apply micro-structured copper foam in a range of other water disinfection systems, from pre-treatment to point-of-use, and should thus become a topic of further research.

Electrooxidation of short and long chain perfluorocarboxylic acids using boron doped diamond electrodes

This study investigates electrooxidation of short (C3-C6) and long (C7-C-18) chain perfluorocarboxylic acids (PFCAs) including perfluorooctane sulfonate (PFOA) using Si/BDD electrode. The effect of operational parameters (supporting electrolyte type, applied current density, and initial pH) were explored for PFOA removal. At the optimized conditions, 74% TOC removal and 37% defluorination ratio were gained for 10 mg L^-1 of PFOA solution which evidences that the shorter chain PFCAs were formed. The PFOA degradation pathway followed one direct electron transfer from PFOA molecule to anode surface. Then two different degradation pathways were proposed. The first proposed degradation mechanism involved the reaction of perfluoroheptyl radical and hydroxyl radical, the release of HF and hydrolysis. The second mechanism involved the reaction between perfluoroheptyl radical and O₂, formation of C₇F₁₅O and perfluorohexyl radical with releasing COF₂. The removal of short- (C3-C6) and long-chain PFCAs (C7-C18) was also characterized. More than 95% of removal efficiency was gained for all long-chain PFCAs, excluding C7. The removal ratios of short-chain PFCAs (C3-C6) were 39%, 41%, 66% and 70% for C3, C4, C5 and C6, respectively. Contrary to long-chain PFCAs, chain-length dependence for short-chain PFCAs were observed. Defluorination ratio of short-chain PFCAs was only 45% signifying that defluorination partially occurred. Water matrix did not significantly affect the degradation of short-chain PFCAs in deionized water (DI), river water and secondary effluent of a wastewater treatment plant (WWTP). In contrast, defluorination ratio of long-chain PFCAs was noticeably affected by water matrix with the order of DI water > WWTP effluent > river 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.

Electrochemical disinfection and removal of ammonia nitrogen for the reclamation of wastewater treatment plant effluent

Residual ammonia and pathogenic microorganisms restrict the reclamation and reuse of wastewater treatment plant (WWTP) effluent. An electrochemical system was developed for the simultaneous removal of ammonia and disinfection of actual WWTP effluent. The performance of the electrochemical process on synthetic wastewater at different chloride ion concentrations was also investigated. Results demonstrated that the optimal chloride concentration for ammonia and Escherichia coli (E. coli) removal was 250 mg/L. Successful disinfection of E. coli in actual effluent was achieved at 0.072 Ah/L, but the inverse S-type inactivation curve indicated that there was a competitive consumption of strong oxidants and chloramines working as another disinfectant. A higher electric charge (0.58 Ah/L) was required to simultaneously reduce E. coli and ammonia to levels that meet the reclamation requirements for WWTP effluent. At this electric charge, no trihalomethane, chlorate, or perchlorate in the system was observed, indicating the biological safety of this process. These results demonstrate the potential of this electrochemical process as a tertiary wastewater treatment process for WWTP effluent reclamation purposes.

Electrochemical treatment of water containing Microcystis aeruginosa in a fixed bed reactor with three-dimensional conductive diamond anodes

An electrochemical treatment was investigated to remove Microcystis aeruginosa from water. A fixed bed reactor in flow was tested, which was equipped with electrodes constituted by stacks of grids electrically connected in parallel, with the electric field parallel to the fluid flow. Conductive diamond were used as anodes, platinised Ti as cathode. Electrolyses were performed in continuous and in batch recirculated mode with flow rates corresponding to Re from 10 to 160, current densities in the range 10-60Am(-2) and Cl(-) concentrations up to 600gm(-3). The absorbance of chlorophyll-a pigment and the concentration of products and by-products of electrolysis were measured. In continuous experiments without algae in the inlet stream, total oxidants concentrations as equivalent Cl2, of about 0.7gCl2m(-3) were measured; the maximum values were obtained at Re=10 and i=25Am(-2), with values strongly dependent on the concentration of Cl(-). The highest algae inactivation was obtained under the operative conditions of maximum generation of oxidants; in the presence of microalgae the oxidants concentrations were generally below the detection limit. Results indicated that most of the bulk oxidants electrogenerated is constituted by active chlorine. The prevailing mechanism of M. aeruginosa inactivation is the disinfection by bulk oxidants. The experimental data were quantitatively interpreted through a simple plug flow model, in which the axial dispersion accounts for the non-ideal flow behaviour of the system; the model was successfully used to simulate the performances of the reactor in the single-stack configuration used for the experiments and in multi-stack configurations.

Electrochemical disinfection of toilet wastewater using wastewater electrolysis cell

The paucity of proper sanitation facilities has contributed to the spread of waterborne diseases in many developing countries. The primary goal of this study was to demonstrate the feasibility of using a wastewater electrolysis cell (WEC) for toilet wastewater disinfection. The treated wastewater was designed to reuse for toilet flushing and agricultural irrigation. Laboratory-scale electrochemical (EC) disinfection experiments were performed to investigate the disinfection efficiency of the WEC with four seeded microorganisms (Escherichia coli, Enterococcus, recombinant adenovirus serotype 5, and bacteriophage MS2). In addition, the formation of organic disinfection byproducts (DBPs) trihalomethanes (THMs) and haloacetic acids (HAA5) at the end of the EC treatment was also investigated. The results showed that at an applied cell voltage of +4 V, the WEC achieved 5-log10 reductions of all four seeded microorganisms in real toilet wastewater within 60 min. In contrast, chemical chlorination (CC) disinfection using hypochlorite [NaClO] was only effective for the inactivation of bacteria. Due to the rapid formation of chloramines, less than 0.5-log10 reduction of MS2 was observed in toilet wastewater even at the highest [NaClO] dosage (36 mg/L, as Cl2) over a 1 h reaction. Experiments using laboratory model waters showed that free reactive chlorine generated in situ during EC disinfection process was the main disinfectant responsible for the inactivation of microorganisms. However, the production of hydroxyl radicals [OH], and other reactive oxygen species by the active bismuth-doped TiO2 anode were negligible under the same electrolytic conditions. The formation of THMs and HAA5 were found to increase with higher applied cell voltage. Based on the energy consumption estimates, the WEC system can be operated using solar energy stored in a DC battery as the sole power source.

Potential contribution of inorganic ions to whole effluent acute toxicity and genotoxicity during sewage tertiary treatment

Two acute toxicity tests (luminescent bacteria assay and cladoceran assay) and one genotoxicity test (broad bean assay) were used to evaluate whole effluent toxicity during the standard anion exchange resin-based pilot-scale sewage tertiary treatment that stably achieved significant dissolved organic carbon and inorganic ions reduction. The effect of six representative inorganic ions (i.e., Cl(-), SO4(2-), NO3(-)-N, NO2(-)-N, NH4(+)-N and PO4(3-)-P) on the acute toxicity and genotoxicity was further investigated. Significant whole effluent genotoxicity reduction was observed as an ∼ 57% micronucleated cell frequency reduction and ∼ 46% mitotic index increment during the pilot-scale periods, which should be attributed to significant organic removal since no significant (p ≥ 0.116) increase in genotoxicity was observed with the increase in these ionic concentrations. However, no significant (p ≥ 0.14) reductions were observed for whole effluent acute toxicity using two acute toxicity assays during the pilot-scale periods, and these inorganic ions, especially NH4(+)-N, contributed considerably to the acute toxicity. Based on Pearson correlation coefficients, whole effluent acute toxicity showed significant positive (p < 0.001, r ≥ 0.758) correlations with the NH4(+)-N concentration. Two optimal models were finally developed using step-wise multiple linear regression to predict the whole effluent acute toxicity via NH4(+)-N concentrations.

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

This work investigated the capability of using a boron-doped diamond (BDD) electrode for bacterial disinfection in different water matrices containing varying amounts of chloride. The feed water containing Pseudomonas aeruginosa was electrochemically treated while applying different electrode conditions. Depending on the applied current density and the exposure time, inactivation between 4- and 8-log of the targeted microorganisms could be achieved. The disinfection efficiency was driven by the generation of free chlorine as a function of chloride concentration in the water. A synergetic effect of generating both free chlorine and ozone in situ during the disinfection process resulted in an effective bactericidal impact. The formation of the undesired by-products chlorate and perchlorate depended on the water matrix, the applied current density and the desired target disinfection level. In case of synthetic water with a low chloride concentration (20 mg L(-1)) and an applied current density of 167 mA cm(-2), a 6-log inactivation of Pseudomonas aeruginosa could be achieved after 5 min of exposure. The overall energy consumption ranged between 0.3 and 0.6 kW h m(-3) depending on the applied current density and water chemistry. Electrochemical water disinfection represents a suitable and efficient process for producing pathogen-free water without the use of any chemicals.

Reduction of pollutants and disinfection of industrial wastewater by an integrated system of copper electrocoagulation and electrochemically generated hydrogen peroxide

The objective of this study was to evaluate the effect of copper electrocoagulation and hydrogen peroxide on COD, color, turbidity, and bacterial activity in a mixed industry wastewater. The integrated system of copper electrocoagulation and hydrogen peroxide is effective at reducing the organic and bacterial content of industrial wastewater. The copper electrocoagulation alone reduces COD by 56% in 30 min at pH 2.8, but the combined system reduces COD by 78%, biochemical oxygen demand (BOD5) by 81%, and color by 97% under the same conditions. Colloidal particles are flocculated effectively, as shown by the reduction of zeta potential and the 84% reduction in turbidity and 99% reduction in total solids. Additionally, the total coliforms, fecal coliforms, and bacteria are all reduced by 99%. The integrated system is effective and practical for the reduction of both organic and bacterial content in industrial wastewater.

Removal of organic contaminants from secondary effluent by anodic oxidation with a boron-doped diamond anode as tertiary treatment

Electrochemical advanced oxidation processes (EAOPs) have been widely investigated as promising technologies to remove trace organic contaminants from water, but have rarely been used for the treatment of real waste streams. Anodic oxidation with a boron-doped diamond (BDD) anode was applied for the treatment of secondary effluent from a municipal sewage treatment plant containing 29 target pharmaceuticals and pesticides. The effectiveness of the treatment was assessed from the contaminants decay, dissolved organic carbon and chemical oxygen demand removal. The effect of applied current and pH was evaluated. Almost complete mineralization of effluent organic matter and trace contaminants can be obtained by this EAOP primarily due to the action of hydroxyl radicals formed at the BDD surface. The oxidation of Cl(-) ions present in the wastewater at the BDD anode gave rise to active chlorine species (Cl2/HClO/ClO(-)), which are competitive oxidizing agents yielding chloramines and organohalogen byproducts, quantified as adsorbable organic halogen. However, further anodic oxidation of HClO/ClO(-) species led to the production of ClO3(-) and ClO4(-) ions. The formation of these species hampers the application as a single-stage tertiary treatment, but posterior cathodic reduction of chlorate and perchlorate species may reduce the risks associated to their presence in the environment.

Effect of bipolar electrode material on the reclamation of urban wastewater by an integrated electrodisinfection/electrocoagulation process

This work presents an integrated electrodisinfection/electrocoagulation (ED-EC) process for urban wastewater reuse that employs iron bipolar electrodes. Boron doped diamond (BDD) was used as the anode and stainless steel (SS) as the cathode. A perforated iron plate was introduced between the anode and cathode to function as a bipolar electrode. This ED-EC combined cell makes it possible to conduct the simultaneous removal of microbiological content and elimination of turbidity from urban wastewater. The results show that current densities greater than or equal to 6.70 A m(-2) enable complete disinfection of the effluent and the removal of more than 90% of its initial turbidity. Hypochlorite and chloramines formed during the ED-EC process were found to be the main compounds responsible for the disinfection process. Furthermore, a cell configuration of cathode (inlet)-anode (outlet) improves the process performance by enhancing turbidity removal. Finally, the influence of the bipolar electrode material (iron or aluminium) was assessed. The results indicate that the efficiency of the electrodisinfection process depends mainly on the anodic material and is not influenced by the material of the bipolar electrode. In contrast, the removal of turbidity is more efficient when using iron as a bipolar electrode, especially at low current densities, due to the formation of a passive layer on the aluminium that hinders the dissolution of the bipolar electrode.

Assessment of the formation of inorganic oxidation by-products during the electrocatalytic treatment of ammonium from landfill leachates

This work investigates the formation of oxidation by-products during the electrochemical removal of ammonium using BDD electrodes from wastewaters containing chlorides. The influence of the initial chloride concentration has been experimentally analyzed first, working with model solutions with variable ammonium concentration and second, with municipal landfill leachates. Two different levels of chloride concentration were studied, i) low chloride concentrations ranging between 0 and 2000 mg/L and, ii) high chloride concentrations ranging between 5000 and 20,000 mg/L. Ammonium removal took place mainly via indirect oxidation leading to the formation of nitrogen gas and nitrate as the main oxidation products; at high chloride concentration the formation of nitrogen gas and the rate of ammonium removal were both favored. However, chloride was also oxidized during the electrochemical treatment leading to the formation of free chlorine responsible of the ammonium oxidation, together with undesirable products such as chloramines, chlorate and perchlorate. Chloramines appeared during the treatment but they reached a maximum and then started decreasing, being totally removed when high chloride concentrations were used. With regard to the formation of chlorate and perchlorate once again the concentration of chloride exerted a strong influence on the formation kinetics of the oxidation by-products and whereas at low chloride concentrations, chlorate appeared like an intermediate compound leading to the formation of perchlorate, at high chloride concentrations chlorate formation was delayed significantly and perchlorate was not detected during the experimental time. Thus this work contributes first to the knowledge of the potential hazards of applying the electro-oxidation technology as an environmental technology to deal with ammonium oxidation under the presence of chloride and second it reports efficient conditions that minimize or even avoid the formation of undesirable by-products.