As well-known as barrier discharges or silent discharges, dielectric-barrier discharges (DBDs) have for a long period been seen as the ozonizer discharge [1]. In 1932, Buss [2] discovered that in a plane parallel gap with insulated electrodes, air breakdown takes place in a number of individual tiny breakdown channels. Lately, it was seen that plasma could be affected, modeled and optimized for a specific utilization via micro-discharges [3] [4] [5]. The most remarkable feature of DBDs remains that non-equilibrium plasma circumstances could be furnished in a much easier manner than with other options such as low-pressure discharges, fast pulsed high-pressure discharges or electron beam injection. Further, the DBDs could breakdown most of the gases at around atmospheric pressure in a big number of independent current filaments or micro-discharges. The dielectric barrier restricts the quantity of charge and energy deposited in a micro-discharge and distributes the micro-discharges across the full electrode surface. At what time a DBD is run in rare gases or a rare gas halogen mixture, plasma circumstances in a micro-discharge channel are identical to those in pulsed excimer lasers. As a result, each micro-discharge could operate like an intense source of ultraviolet (UV) or vacuum ultraviolet (VUV) radiation. The absorption coefficient of most substances augments at shorter wavelengths. Thus, in several conditions, the UV radiation is absorbed in a very thin surface layer. The xenon excimer lamp can form photo-cleavage of water and oxygen [1] [4].

In 1979, Donohoe and Wydeven [6] acquired a uniform glow discharge with pulsed excitation in a helium/ethylene mixture from which the term atmospheric pressure glow (APG) was originated. In 1956, for producing uniform glow discharges at atmospheric pressure in helium, air, argon, oxygen, and nitrogen via 50 Hz power source, Gambling and Edels [7] employed an electrode arrangement comprising two metal foils masked with a particular metal mesh and ceramic plates [1]. Electret dielectrics could collect charges on the surface, and are trapped uniformly on the surface through the applied voltage. The charge carriers are ejected spontaneously from the surface conducting to homogeneous discharge because of the alteration in the polarity of the electric field surpassing the threshold value [1].

Using gas plasmas efficiently demobilized microbes caught the attention of several scientists [1]. It is established that there are abundant charged particles, chemically reactive species, and UV photons in the plasma discharge, all of which could provoke harm to cells, so attaining demobilization or alteration [3] [8]. Biologically, biochemical effects are noticed at what time microbes are revealed to plasmas, for instance, 1) protein denaturalization, 2) enzyme deactivation and 3) deoxyribonucleic acid (DNA) mutation [1].

It is known that atmospheric pressure plasma discharge possesses a huge influence on Escherichia coli thanks to the deterioration of the cell membrane that conducts to cell lysis [7]. Until 2015, Paunikar et al. [1] affirmed that they are incapable to assess the impact of plasma treatment on the bio-macromolecules such as cell wall and membrane, made up of polysaccharides and membrane-bound proteins. In order to comprehend the pathway of plasma treatment, it is crucial to study the phenomenon of cell lysis and death, and also the mutation [1].

This work examines the chronicle of DBDs, kinds of disinfection techniques, the necessity for substitutional disinfection process (thermal plasma and non-thermal plasma), proposed theories for adequate clarification of pathways supporting such a discharge process for the induction of corona discharges in water, impact of sterilization employing DBD setup and impacts of DBD plasma, water purification via electrical discharges, ozone for treating water, and arc discharge-mediated disassembly for killing viruses.

In 1857, Siemens [9] first proposed DBDs for the objective of ozone production in the air; however, in 1778, Lichtenberg [10] performed its first test. At the commencement of the 20^th Century, investigations were conducted leading to a better comprehension of the DBD and diverse utilizations [1].

Relating to public health issues (like multi- and extensive drug-resistant microbes, bioterrorism, etc.), there is a demanding necessity for reinforced endeavors to avoid transmission of infections employing ecological controls [1] [11] [12] [13]. Because of this, ultraviolet germicidal irradiation (UVGI) has attracted large regard. Some interrogations concerning performance and security restrict the implementation of UV founded techniques for disinfecting, even with that UVGI could be secure and greatly efficient in disinfecting the air, water, and surfaces that way avoiding transmission of a set of infections. Right now, low-pressure mercury (Hg) discharge lamps are utilized in UVGI implementations and release shortwave ultraviolet-C (UV-C, 100 - 280 nm) [14] radiation, mostly at 254 nm. Nevertheless, mercury-based lamps may play a part in heavy metal contamination and linked health risks [1].

Downes and Blunt [15] established that the potential of sunlight to demobilize microbes was a function of intensity, time, and wavelength, with the shorter wavelengths of the solar spectrum being the most performant [1]. UV-C radiation demobilizes microorganisms via destroying their DNA. Many types of research were dedicated to locating the wavelength dependence of the germicidal work of light [16]. The main manner of demobilization happens at the time that the absorption of a photon generates pyrimidine dimers among adjacent thymine bases and makes the microorganism unable of replicating [1].

Paunikar et al. [1] presented a list of the main steps throughout history for developing VUV/UV DBDs and their corresponding efficiencies in killing pathogens (Table 1).

In terms of water disinfection engineering, numerous conventional sterilization techniques (like thermal sterilization, chemical sterilization, and irradiation of UV and gamma rays) become currently utilized on a large scale. However, throughout recent years, plasma-founded apparatuses have been valued for biological sterilization [1]. Such devices possess a large extent for utilization relative to traditional techniques. Irradiation of UV and gamma rays produces energetic photons which could occasion grave harm to DNA and are toxic for human being [1]. Nevertheless, most of the classical sterilization manners are time-consuming and are run in closed space. The plasma-founded setups could be implemented to open space, possess a very short sterilization period (1 min), produce several reactive species (such as ozone, hydroxyl radical, and oxygen atom) and possess a great benefit over different sterilization devices [43]. The physical technique performs via positive and negative ions in the discharge’s streamer, and the chemical process is realized through ozone, atomic oxygen, hydroxyl radical, etc. [1] [44] [45] [46].

Some of the classically applied sterilization techniques for water treatment involve [47] [48] [49] [50] : boiling, chlorine [47] [51] [52] [53] [54], chlorine dioxide, dry heat sterilization, ozone [55], UV light [14], etc. [1].

Recently, the engineering of killing pathogens employing non-thermal plasma formed via gas discharge at atmospheric pressure has attracted considerable interest [56]. Conventional disinfection and sterilization technologies possess numerous disadvantages [56]. Such techniques require extended treatment periods. Numerous polymer-founded tools and several very expensive equipments (like endoscopes) could not be disinfected via autoclaving. Virulent substances like formaldehyde, ethylene oxide, and glutaric dialdehyde not only hurt the human body but as well as contaminate nature [1].

In the 1960s, surveys on plasma sterilization begun. Following this period, large investigations have been performed on plasma sterilization. As juxtaposed to incineration and thermal plasma treatment, the merit of utilizing a non-equilibrium discharge is that most of the discharge energy could be employed to speed electrons and produce free radicals [43]. Moreover, DBDs possess benefits over classical techniques if numerous contaminants should be handled at the same instant. DBDs are adopted particularly when contaminant levels

are small, i.e., in the 10 - 1000 ppm span. Utilizing DBD sources, the degradation of greenhouse gases (CO and CH) below diverse running circumstances is examined across a large temperature and pressure extent. Further, the DBD remediation of big parts is made possible at atmospheric pressure. Plasma could be categorized in capacitively coupled plasmas (CCPs) and inductively coupled plasmas (ICP) because of the power input. Two sorts of atmospheric plasma utilized for biological sterilization comprise thermal and non-thermal plasma [1].

For adequate clarification of pathways supporting such a discharge process for the induction of corona discharges in water, principles have been suggested [1].

Numerous pathways have been suggested to interpret the production of diffuse DBDs. Those comprise gas pre-ionization by electrons or metastable from previous discharges and interaction among the plasma and the dielectric surfaces. Because atmospheric pressure circumstances are most appropriate for several DBD utilizations, the study of the features of the numerous discharge modes has concentrated mostly on atmospheric pressure circumstances rather than on the low-pressure regime. The bactericidal agents formed through DBD plasma can involve UV radiation, charged particles, ROSs, etc. The kinetics of cell decease throughout plasma subjection is not symptomatic of UV radiation excitation. Both plant and animal hosts embrace defense action plans that employ the ROSs in opposition to the invaded microbes. The DBD could form such ROSs since oxygen atoms, ozone, metastable oxygen molecules, peroxide, superoxide, and hydroxyl radicals [91] [92] [93] [94], and all of them are germicidal. These ROSs possess a powerful oxidizing capacity and are apt to take action with the bacteria cells [95] [96] [97]. The ROSs could oxidize the cell membrane and could provoke the infiltration of cytoplasm [98] [99] [100] [101] [102]. With plasma treatment, in the first some seconds, because of the laceration of the cell membrane, the cytoplasm progressively leaks out, leading to the concentrations of K⁺, protein [103], and nucleic acid in bacterial suspension augmenting to a higher level. Further, this is compatible with the rapid doom of cells during the first some seconds. Nevertheless, with the plasma subjection period prolonged, the escaped protein and nucleic acid will be progressively oxidized by ROSs [103] [104] [105] [106] [107], conducting to the diminution of their level; however, for K⁺, it cannot be oxidized, and so, its level turns saturated. Following this mechanism, the bacteria cells are murdered. Consequently, the ROSs [108] [109] could possess a major contribution to the demobilization phenomenon. A greater discharge power correlates to more ROSs formation and better demobilization impact [1].

Plasma sterilization can be categorized into three routes: the hydroxyl radical could fix to unsaturated fatty acids and provoke lipid peroxidation, oxygen radicals could give rise to DNA oxidation, and oxidation of amino acids could happen pursued by protein oxidation [1].

Fatty acid peroxide could be produced via plasma and could modify the membrane lipids. However, sterilization did not perform by a single chemical impact. The charge aggregation on the cell membrane caused electrostatic stress that was in charge of cell laceration. Using plasma, the sterilization pathway has been broadly investigated; however, it remains ambiguous. Indeed, the route of sterilization through DBD influence stays not yet fully comprehended. Electric field and reactive species are fundamental parameters for bacterial demobilization [65] [91]. Active species comprise UV or visible photons, charged particles (like electrons, ions, and free radicals), highly reactive neutral species like reactive atoms (oxygen, fluorine, ozone, nitrogen oxides, etc.), exited states atoms, and reactive molecular fragments. The collision of heavy ions with microbial cells could ruin their membrane. The collision of energetic electrons with some atoms and molecules could fracture some molecular bonds and form the excited and active particles like radicals and metastable atoms [1].

Disinfecting water and wastewater via UV radiation looks to be a prospective option for chlorine [110] [111] [112] [113] [114]. Small levels of chlorine remainders are poisonous to aquatic life, and numerous of the DBPs of chlorination are mutagenic [91] [115] [116] [117]. Germicidal UV radiation does not form unwanted DBPs and it is efficacious in demobilizing a set of pathogens [1] [14] [118] [119].

Moreover, the impact of electrical sterilization could be efficient if the current flowing through spores augmented via adding water. Throughout the subjection time to DBD, the cell membrane can be breakdown thanks to the elevated electric field across the membrane. The impact of DBD treatment begun to be softer with augmenting the discharge gap. Adding NaCl augmented the impact of sterilization and attained the maximum at 4 g/L of NaCl [94]. Nevertheless, sterilizing with 8 g/L and 10 g/L NaCl solution was smaller than that of pure water. If the spores are wrapped with conductive liquid, they depicted to be electrically protected so that the electrical membrane breakdown is eliminated [1].

The tests performed to examine the influence of sterilization on Bacillus subtilis by the dry method show that almost all B. subtilis spores were neutralized by the dry method with a D-value of around 40 s as juxtaposed to the wet method, which was approximatively 7 s. Such findings propose that adding water improved the sterilization influence. It is suggested that ^●OH were formed from H₂O, and the sterilization was efficiently realized via the ^●OH generated in the spore’s neighborhood. Oxidative decomposition pursuing VUV photolysis of water, thanks to its ease, has quickly turn into an interesting option to else advanced oxidation processes (AOPs) [1] [48] [76] [82] [83] [120].

Bactericidal impact of UV light for the bacteria E. coli, Salmonella typhi, Shigella sonnei, Streptococcus faecalis, S. aureus, and B. subtilis spores was estimated for a 99.9% demobilization of the cultured vegetative bacteria, total coliforms, and standard plate count microorganisms. Nevertheless, the viruses, the bacterial spores, and the amoebic cysts requested around 3 - 4 times, 9 times, and 15 times, respectively, than the injection needed for E. coli. Such ratios covered a narrower relative injection span than that already mentioned for chlorine disinfection of E. coli, viruses, spores, and cysts [1] [80].

Consequently, the injections of UV light needed to neutralize pathogenic microorganisms, involving viruses, bacteria spores, and protozoa, are much more similar to the injections of UV light indispensable to demobilize indicator bacteria than is the instance for chlorine [80]. Therefore, the UV degrees requested to satisfy coliform standards could be comparatively more performant than chlorination in neutralizing pathogenic microbes. In the main, the VUV method is so easy and possesses special merit that no chemical products request to be introduced. The technique constitutes a dare to different photochemical water treatment methods [1] [16].

The impact of the DBD device on sterilization has been well investigated on vegetative bacteria, viruses, bacterial spores, and protozoa [1].

Numerous scientists have noted several empirical findings and data founded on their watching of the influence of DBD plasma on bacterial survival, level of membrane-bound proteins, and intracellular proteins and polysaccharides of the cell membrane (Figure 4) [42]. Such results provide a new understanding of the action of DBD plasma throughout microbial disinfection techniques [1].

Electrical discharges in association with strong electric fields, shock waves, UV radiation, O₃, H₂O₂, etc., all of which could participate for microbial disinfection and are an efficient sterilizing agent. It has been suggested that high-intensity pulsed electric fields without corona or spark discharge activities could be viewed as a promising technique for sterilizing food products since they neutralize microbes without ruining food constituents and its nourishing level [64]. Diverse parameters like the period and the maximum level of the voltage, the shape of the electrodes, etc. dictate the sterilization performance of a pulsed electric field. Germicides like O₃ or H₂O₂ enhance its performance on a small scale. It is suggested that corona or spark discharges below the powerful electric field ameliorate the sterilization phenomenon [1].

Electrical discharges occurring in an air or oxygen medium transform oxygen into ozone [1]. In addition to ozone, electrical discharges in air generate a collection of chemically active species, such as ^●O, ^●OH, ^●N, O³⁻, N²⁻, N⁻, OH”, O”, O²⁺, N²⁺, N⁺, O⁺, etc. Such species are short-lived and disintegrate before ozone enriched air/oxygen enters into the water. Further, electrical discharges in aerated water are likely and they form ^●OH, ^●H, ^●O, O₃, H₂O₂, etc. The in situ electrical discharges for ozone generation in water can furnish a tool to employ most of such chemically active species for water purifying. The interactive impact of powerful electric fields requested for electrical discharges in integration with traditional disinfectants like O₃ and H₂O₂ are fatal to numerous pathogens detected in the water. In water, the electrical discharges could generate UV radiation and shock waves that are useful in ruining contaminants. Electrical discharges are the best and environmentally-friendly next-generation techniques for water treatment and they can look far more performant than traditional oxidants and disinfectants.

As mentioned above, in a DBD device, the electrical discharges occur among electrodes where at least one of the electrodes is enveloped with a fine film of dielectric material, like glass or quartz [1]. Upon inelastic collision, the free electron may ionize an ambient gas molecule, so forming more free electrons that may reiterate the phenomenon and then generate an electron avalanche (streamer).

In both instances of contact glow discharge electrolysis and DBD setups, the electrical discharges happen in the gas phase in adjacent proximity to the water surface. They need a strong electric field (>1 MV/cm) for the electrical discharge to occur in water. These elevated electric fields are probable via implementing high-voltage pulses (15 - 100 kV) in pulsed corona discharge and are employed as efficacious disinfectants [1]. For this reason, most of the researches on water treatment are realized utilizing pulsed corona discharge devices and the obtainable industrial-scale units are also founded on such process. The pathway of the induction of corona discharges in the water stays not completely grasped and more investigations are being dedicated to examining such techniques.

The ozone level can be augmented by augmenting the ozone production reactions and/or via diminishing the ozone demolition reactions. Using a double discharge surfaces' reactor or a hybrid of silent and surface discharges could augment the number of sites for ozone generation reactions inside the given discharge volume. Further, porous silica gel packing could efficiently augment the ozone production performance via the spreading of an active plasma zone through micro-discharges. The pore size of alumina packing is a crucial parameter in ozone formation activity, where the dissolved ozone dissociates into ^●OH via a cyclic chain pathway that is also in charge of the oxidation of aqueous pollutants. It is known that ^●OH (10⁷ - 10⁹ M⁻¹∙s⁻¹) is much more performant than O₃ (10¹ - 10⁷ M⁻¹∙s⁻¹) for organic pollutants decomposition. Consequently, a quicker transformation rate of ozone into hydroxyl radicals dictates the effectiveness of pollutant ruin. In AOPs, UV radiation, H₂O₂, activated carbon, etc., catalyze the O₃ to ^●OH transformation. Besides, the catalytic transformation of ozone into ^●O could ameliorate the performance of ozonation [1].

Throughout ozonation, pulsed corona discharge possesses a small number of benefits; where distributed ozone enriched air/oxygen augments the rate of ozone dissolution in water and produces extra free radicals like ^●OH and ^●O [1]. The technology of pulsed corona discharge in water throughout ozonation has to be more studied because the electrical discharge could form free radicals and neutral active species and can avoid the necessity of a distinct device for producing ozone. In the main, the density of the chemically active species augments with an elevation of the applied voltage. The pathway of ozone transformation to free radicals throughout the ozonation technique is attributed to, negative polarity of direct current (DC) voltage, bubbling some gas throughout the discharge, and utilizing argon instead of oxygen for gas bubbling [1].

Employing a submerged plasma reactor of arc discharge (underwater arc) (Figure 6) that formed a shockwave, UV light, ROSs, and reactive nitrogen species, Lee et al. [126] studied its demobilization impacts on murine norovirus (MNV-1) with/without purification in water. Underwater arc treatments of 3

and 6 Hz at 12 kV conducted to 2.6- and 4.2-log removals in the virus titer of non-purified MNV-1 after 1 min of treatment, respectively. The removal of purified MNV-1 was bigger than that of non-purified MNV-1 after underwater arc treatment for all applied conditions (12 or 15 kV and 3 or 6 Hz). One of the viral capsid proteins (VP1) was not observable after underwater arc treatment, when its integrity was assessed by western blot analysis. Further, transmission electron microscopy (TEM) analysis showed that MNV-1 particles were fully dissembled by the treatment (Figure 7). Such research proves that underwater arc treatment, which was apt to decaying the MNV-1 virion structure and the viral capsid protein, could be a performant disinfection technique for killing water-borne noroviruses.

These excellent results are encouraging for COVID-19 elimination from both water and wastewater. Systematically testing such outstanding technologies (Figure 8) could lead to defining the best configurations and optimizations for removing COVID-19 and other pathogens from water and wastewater [122].

Concerning the design of these highly-efficient processes, the focus would be accorded to intensify the reactors in terms of residence time and close contact opportunities between water pollutants and electrodes area [75]. Further, combining plasma discharge, electrocoagulation (EC) [127] [128], and magnetic field application [127] [129] as a hybrid process would lead to better efficiencies in removing pathogens and organic matters (OMs) [130] [131] [132] [133]. The last stages could contain activated carbon adsorption assisted by a membrane process [134] - [139] to remove the remaining OM especially released from the cellular cytoplasm during oxidation and disinfection processes.

From this work, the main conclusions emerge:

1) More efficient, cheaper, and environmentally-friendly than traditional water treatment methods, electrical discharge technologies are confirmed as. UV

emission from plasma dispositions and the impacts of irradiation on microorganisms become broadly studied. In the field of treating water via electrical discharges, more expansions are, however, requested. Especially, novel and more performant materials that can be employed as catalysts for producing ozone are required. More importantly, the catalyst materials’ physicochemical characteristics contribution should be more highlighted. Throughout ozonation, implementing pulsed high-voltages can lead to better diffusion of ozone in water and quicker transformation of ozone into free radicals that could reduce the ozonation price. Via direct electrical discharges, purifying water has trends to be examined on a large-scale. In this context, the demolition mechanisms of water contaminants, comprising pathogens and poisonous OMs, have to be deeply investigated. Further, defining the demolition by-products has to be performed to illustrate the route of plasma chemical responses implied [1].

2) Both in water and above water level, the electrical discharges possess their advantages and disadvantages. Above water level, which is in the gas phase, electrical discharges need less energy for the discharge to occur; however, in water, electrical discharges need an easier setup and form the chemically active species that could immediately bombard the aqueous contaminants. One of the kinds of electrical discharges, pulsed corona discharge remains the most tried and looks to be the most encouraging for treating water. Minutely set the UV injection needed to kill pathogenic microorganisms, comprising bacteria, viruses, spores, and cysts, stays to be worked on it for better UV disinfection performance.

3) Through this work, the examined techniques, especially plasma discharge, show good results in dealing with viruses’ removal. Such methods could be methodically experimented with determining the optimal circumstances for killing COVID-19 and different pathogenic microbes from water. The attention can be dedicated to enhancing the devices in a matter of residence period and approaching contact among microorganisms and electrode surfaces [75]. Merging plasma discharge, EC, and magnetic field implementation can lead to better performances in eliminating viruses and OMs. As a secure physical separation, the final step has to involve activated carbon adsorption pursued by a membrane process to retain OM liberated from the cellular cytoplasm throughout disinfection methods.

This research has been funded by the Research Deanship of University of Ha’il, Saudi Arabia, through the Project RG-191190.

The authors declare no conflicts of interest regarding the publication of this paper.

Ghernaout, D. and Elboughdiri, N. (2020) Disinfecting Water: Plasma Discharge for Removing Coronaviruses. Open Access Library Journal, 7: e6314. https://doi.org/10.4236/oalib.1106314