Synergy Effect during Water Treatment by Electric Discharge and Chlorination

: Water treatment, such as disinfection, is an integral stage of its use for human life. The use of plasma technology with high-voltage electric discharge in a liquid for obtaining a bactericidal effect is discussed. It has been experimentally shown that among the factors accompanying a high-voltage electric discharge in a liquid and affecting the viability of bacteria, cavitation is the main one. Simultaneous use of electric discharge in the special cavitation mode and oxidizing agents makes it possible to achieve stable disinfection of water. At the same time, bactericidal doses of the oxidizing agent are reduced by a factor of 10 relative to existing standards, and the energy costs for electric discharge exposure, enhanced by small doses of an oxidizing agent, are reduced by 6 times compared to the costs of disinfection by only an electric discharge.


Introduction
The growth of the world's population and the associated increase in the consumption of water for drinking and technical purposes, as well as the negative human impact on the environment, led to the fact that by the beginning of the XXI century, fresh water has become one of the most essential and demanded resources for humankind. More than 1 billion people are currently constantly experiencing a lack of clean fresh water [1][2][3]. According to forecasts, by 2025, this problem will become even more acute in half of the countries of the world, and by the end of the century, two-thirds of the population of our planet will find themselves in a difficult situation due to the lack of drinking water [4]. The US Environmental Protection Agency estimates that nearly 35% of deaths in developing countries are attributable to contaminated water [5]. Pollution of rivers and streams with chemical and aggressive biological objects is one of the most pressing environmental problems. Polluted water entering natural reservoirs causes a chain of devastating consequences [6][7][8]. Then there is also an obvious need for domestic wastewater to be treated before it is returned, with the main goal being to avoid unacceptable damage to the environment. A modern method of wastewater treatment should include the choice of the desired level of disinfection and have long-term efficiency. The ultimate goal of wastewater treatment is the efficient management of water resources that is economically and environmentally justified. composition of the cell wall, the age of the culture, and the intensity and frequency of sonication [40,41].
By changing the duration and frequency of sounding, it is possible to influence almost all types of microorganisms; in the field of ultrasonic waves, Gram-positive and Gramnegative bacteria, aerobic and anaerobic bacteria, and rod-shaped, coccal, and other forms are disintegrated; sounding affects both vegetative and spore forms [42][43][44]. To date, the main limiting factor for adapting the ultrasonic method to treat large volumes of water is the limited power of ultrasonic sources. This limitation, in turn, is caused by the fact that with an increase in the power of the installation and the intensity of ultrasound, under the influence of cavitation, the elements of the vibration source itself are quickly destroyed.
Plasma technologies now occupy a prominent place among the tools for water purification from harmful and dangerous micropollutants [9] and are used in the treatment of domestic wastewater [45][46][47], recycling of wastewater from industrial enterprises [48][49][50][51], and bringing natural water from surface sources to the level of suitability for drinking [52][53][54][55][56]. The tool for influencing a substance in the implementation of such technologies is a nonequilibrium plasma-an ionized gas consisting of electrons, ions, and neutral particles. It is known that an electric discharge in gases and liquids is not only one of the most relatively simple ways to create plasma states of matter, but itself, in turn, has several types: glow, spark, arc, corona, and high-voltage breakdown in a liquid. The implementation of each type of electric discharge leads to the emergence of plasma chemical and plasma physical processes of varying intensity, which can play a significant role in solving water purification problems.
Recognizing the predominant use of chlorination technology for the disinfection of wastewater at present and the inertia of the processes of introducing the latest water treatment methods, one should use the important advantage of this method, namely a prolonged effect on pathogenic microorganisms, and, supplementing it with other methods of disinfection, reduce the harmful effects of chlorination on the environment.
One of the options for solving this problem can be a synergistic technology, the subject of our research, including the complex effect on water of a high-voltage electric discharge simultaneously with chlorination, and the goal of the work is a significant decrease in the concentration of chemicals necessary in this case compared to traditional chlorination and a simultaneous decrease in the energy of high-voltage electric discharge required to achieve the effect of disinfection.
In our studies, we proceeded from the hypothesis that the main mechanism of action on objects (bacteria) is oxidizing particles that appear in the liquid as a result of sonolysis in the entire volume of the discharge chamber. In this case, we assumed that the cause of sonolysis is cavitation; the source of cavitation, under certain conditions, can be an electric discharge. The most well-known consequence of high-voltage breakdown of liquids is shock waves. However, when shock waves act on objects with dimensions smaller than the width of the wavefront (e.g., bacteria), the objects are simply exposed to quasi-static all-around compression. Thus, the direct role of a shock wave in a liquid when it acts on small objects cannot be overestimated.
Let us consider the essence of high-voltage liquid breakdown. If an electric voltage above the critical value is applied to a pair of electrodes immersed in a liquid (weak electrolyte), a breakdown occurs: a plasma channel appears, through which an electric current flows. However, before an electric discharge channel appears in the electrode's gap, some time passes-the static delay time. During this short period of time, a current already appears in the discharge gap, because the processes of liquid ionization start, the formation of plasma begins, and initial plasma formations (streamers) are moving from the anode to the cathode. Then, some more time passes-the stage of breakdown's preparation. As the brush of streamers develops, some of them slow down and disappear, and only one of them reaches the counter electrode. The stage of discharge formation is characterized by an additional slow increase in current and a slight drop in voltage. It is important that energy losses occur at the stage of breakdown's preparation. This energy is expended Environments 2023, 10, 93 4 of 13 on the heating of liquid and the formation of vapor bubbles. The delay and breakdown stages together last from a few microseconds to hundreds of microseconds in total and are collectively combined into the so-called "pre-breakdown stage".
The pre-breakdown stage ends with the formation of a highly conductive electric discharge channel (breakdown)-the electric current increases sharply, and the voltage drops. The energy accumulated by the capacitor bank is rapidly injects into the formed plasma channel of the electric discharge. The substance in the electric discharge channel is heated up to a temperature of 20,000 . . . 40,000 K, and the pressure rises to 300 . . . 1000 MPa. Under the action of this pressure, the discharge channel expands, and the channel walls receive high velocities (as a rule, much more than the speed of sound in the liquid). The stage of active energy conversion in the electric discharge channel takes from several tens to several hundreds of microseconds. Then the accelerated motion of the walls of the discharge channel stops, and they continue to move more slowly, forming a vapor-gas cavity. After reaching equilibrium, the cavity closes to a minimum size, expands again, and continues to oscillate with decreasing amplitude and periods. In short periods of time, when the cavity closes to a minimum size, secondary compression waves are formed in the liquid. This is the stage, which lasts several milliseconds after the electric discharge channel disappearance, that is very important for creating cavitation in the entire volume of the chamber. The point is that multiple cavitation breaks in a liquid can occur only on cavitation "germs" after the excitation of a negative (tensile) pressure in the liquid. That is, a necessary condition for the development of cavitation in the entire volume of the chamber is the presence of germinal bubbles in the liquid, and the cavitation phenomenon strongly depends on the initial amount of the gas phase in the treated liquid.
In ordinary water, there are relatively few bubbles, i.e., cavitation germs, and in order to cause cavitation in the entire volume, it is necessary to ensure their occurrence before tensile stresses are excited in the liquid. We use the phenomena occurring at the prebreakdown stage as a way to create multiple cavitation germs in working liquids. If a mode of high-voltage breakdown of the liquid is provided, which will give active gas formation at the pre-breakdown stage, then all the cavitation germ bubbles will shrink-stretch after the main vapor-gas cavity.
Post-discharge cavitation processes are observed in this case in the far (from a few milliseconds to hundreds of milliseconds) time interval in the entire volume of the working chamber. It is very possible to create conditions for the formation of numerous bubbles (germs): with a relatively long voltage pulse, gas formation is facilitated due to boiling and electrolysis. Nevertheless, the condition of the breakdown of the discharge gap is mandatory for the powerful electric discharge cavitation to arise. Liquid breakdown causes the appearance of compression-tension waves in the primary gas-vapor cavity; in addition, the shock wave destroys the gas phase formed at the pre-breakdown stage, and a cloud of microscopic bubbles (germs) is formed. As a result, conditions for the occurrence of cavitation in the region of each bubble are created. Thus, for the occurrence of powerful electric discharge cavitation, high gas generation at the pre-breakdown stage and breakdown of the liquid are necessary. The issues of the origin and development of volumetric electric discharge cavitation in a liquid and its control are considered in detail in [57][58][59][60][61][62].

Materials and Methods
The study of the possibility of reducing the concentration of the chlorine agent during complex use with an electric discharge was carried out on a laboratory electric discharge installation [63] (Figure 1). The laboratory setup provided an operating voltage (U) of 15 to 25 kV, the capacitance of the capacitor bank (C) varied from 0.1 to 1 µF, and the discharge frequency (f ) was from 1 to 5 Hz. The energy in one pulse (W) changed both due to the voltage and due to the capacitance of the capacitor bank according to the well-known expression W = CU necessary to determine the combination of electrical parameters for the electric discharge mode with large gas formation at the pre-breakdown stage and the obligatory liquid breakdown-a special cavitation mode. The frequency of the discharges was changed from 1 to 5 Hz, because the effect of volumetric cavitation that is of interest to us during an electric discharge occurs within milliseconds after the breakdown of the liquid itself.
to 25 kV, the capacitance of the capacitor bank (C) varied from 0.1 to 1 μF, a charge frequency (f) was from 1 to 5 Hz. The energy in one pulse (W) change to the voltage and due to the capacitance of the capacitor bank according t known expression = . Such wide possibilities for changing the energy were necessary to determine the combination of electrical parameters for the e charge mode with large gas formation at the pre-breakdown stage and the obl uid breakdown-a special cavitation mode. The frequency of the discharges w from 1 to 5 Hz, because the effect of volumetric cavitation that is of interest to an electric discharge occurs within milliseconds after the breakdown of the liq The high-voltage pulse current generator was connected to the industrial t main supply. The generator contained three identical inductors 1, three iden voltage capacitors 2, the three-phase high-voltage rectifier 3, the capacitive ene device 4, the high-voltage switch 5, the working chamber and electrode system protection system 7. The high-voltage pulse current generator was equippe three-phase frequency converter 8 and the dividing capacitor 9.
Cylindrical working chambers with volumes of 1, 2, and 3 dm 3 were mad plastic; the main electrode system of the "point-plane" type was made of titan the diameter of the electrode point was 4 mm, and the interelectrode gap wa from 4 to 15 mm, because the interposition and ratio of the dimensions of th chamber and the electrode system are also important for obtaining the cavitatio the electric discharge. The choice of titanium as an electrode material made it avoid intense electroerosion and to exclude the influence of additional contam fecting oxidative processes in the aquatic environment. For a targeted study o of electrode erosion processes on water disinfection, steel, brass, and aluminum of the same design were used.
As shown above, one of the conditions for the occurrence of powerful cavi ing an electric discharge is the presence of as many gas bubbles as possible in The effect of water boiling at temperatures below 100 °C if the pressure is standard atmospheric one is widely known; for example, in the mountains at of approx. 5000 m (pressure about 0.5 atm.), water boils at 80 °C. In this work, was made to intensify the formation of cavitation germs by reducing the externa To create reduced pressure, the working chamber was placed in a hermetic container, from which air was pumped out with a vacuum pump.
Bacteriological tests were carried out in the laboratory of the Inhulets wate facilities of the Myikolaivvodokanal enterprise. Since the concentration of Esc (E. coli) is the main sanitary indicator microorganism, the bactericidal effect w using this test object. Water for treatment was taken from the wastewater ma Inhulets treatment facilities; the initial content of E. coli varied from 10 4 to 10 The high-voltage pulse current generator was connected to the industrial three-phase main supply. The generator contained three identical inductors 1, three identical highvoltage capacitors 2, the three-phase high-voltage rectifier 3, the capacitive energy storage device 4, the high-voltage switch 5, the working chamber and electrode system 6, and the protection system 7. The high-voltage pulse current generator was equipped with the three-phase frequency converter 8 and the dividing capacitor 9.
Cylindrical working chambers with volumes of 1, 2, and 3 dm 3 were made of fluoroplastic; the main electrode system of the "point-plane" type was made of titanium alloy, the diameter of the electrode point was 4 mm, and the interelectrode gap was changed from 4 to 15 mm, because the interposition and ratio of the dimensions of the working chamber and the electrode system are also important for obtaining the cavitation mode of the electric discharge. The choice of titanium as an electrode material made it possible to avoid intense electroerosion and to exclude the influence of additional contaminants affecting oxidative processes in the aquatic environment. For a targeted study of the effect of electrode erosion processes on water disinfection, steel, brass, and aluminum electrodes of the same design were used.
As shown above, one of the conditions for the occurrence of powerful cavitation during an electric discharge is the presence of as many gas bubbles as possible in the liquid. The effect of water boiling at temperatures below 100 • C if the pressure is lower than standard atmospheric one is widely known; for example, in the mountains at an altitude of approx. 5000 m (pressure about 0.5 atm.), water boils at 80 • C. In this work, an attempt was made to intensify the formation of cavitation germs by reducing the external pressure. To create reduced pressure, the working chamber was placed in a hermetically sealed container, from which air was pumped out with a vacuum pump.
Bacteriological tests were carried out in the laboratory of the Inhulets water treatment facilities of the Myikolaivvodokanal enterprise. Since the concentration of Escherichia coli (E. coli) is the main sanitary indicator microorganism, the bactericidal effect was studied using this test object. Water for treatment was taken from the wastewater masses of the Inhulets treatment facilities; the initial content of E. coli varied from 10 4 to 10 6 CFU/dm 3 depending on the time of water intake. To avoid additional contamination of the water after treatment, all used glassware was subjected to a preliminary heat treatment at 433 K for 2 h, and the containers for sampling treated with an electric discharge were sterilized by flambéing. Determination of E. coli was carried out by the method of membrane filters. For indirect quantitative determination of the intensity of the formation of oxidants during the electric discharge treatment of water, the iodometric titration method was used, adapted to study the consequences of electric discharge processes in water [64]. The method is based on the oxidation of KI solutions, which is accompanied by the release of I 2 ; its content can, in turn, be quantified by titration: where: [Ox] particle is the oxidizing agent and [Red] is the reduced form of the oxidizing particle.
The isolated iodine was titrated with a solution of sodium thiosulfate until the disappearance of the blue color of the solution, which is formed when starch is used as an indicator. The process is described by the following reaction equation: It can be seen that the described processes in the system occur without the participation of hydrogen ions; this allows iodometric determinations to be made in a wide range of solution acidity (pH = 2 . . . 10). For example, the content of active chlorine (chlorine radicals, hypochlorite ions, hypochlorous acid, etc.) [Cl*] in mg/dm 3 was calculated using Equation (1): where: a-volume of 0.01 N sodium thiosulfate solution used for sample titration, mL; b-volume of 0.01 N sodium thiosulfate solution used for titration in a blank determination, mL; k-correction factor to bring the concentration of sodium thiosulfate solution exactly to 0.01 N; V-volume of the analyzed sample, mL; 35.45-atomic weight of chlorine; 0.01-normality of sodium thiosulfate solution.

Results and Discussion
The bactericidal effect of the specific energy of an electric discharge introduced into the volume of disinfected water without the addition of any chemical oxidizing agents is shown in Figure 2.
during the electric discharge treatment of water, the iodometric titration me used, adapted to study the consequences of electric discharge processes in water method is based on the oxidation of KI solutions, which is accompanied by the I2; its content can, in turn, be quantified by titration: where: [Ox] particle is the oxidizing agent and [Red] is the reduced form of the particle.
The isolated iodine was titrated with a solution of sodium thiosulfate until t pearance of the blue color of the solution, which is formed when starch is used a cator. The process is described by the following reaction equation: It can be seen that the described processes in the system occur without the p tion of hydrogen ions; this allows iodometric determinations to be made in a w of solution acidity (рН = 2…10). For example, the content of active chlorine (chlo icals, hypochlorite ions, hypochlorous acid, etc.) [Cl*] in mg/dm 3 was calculat Equation (1)

Results and Discussion
The bactericidal effect of the specific energy of an electric discharge introd the volume of disinfected water without the addition of any chemical oxidizing shown in Figure 2.   It can be seen that with an increase in specific energy, the concentration of bacteria decreases; at an initial concentration of E. coli 10 5 CFU/dm 3 and E sp = 150 kJ/dm 3 (41 kWh/m 3 ), a good, but not 100%, bactericidal effect is observed. However, with such specific energy consumption, it is meaningless to talk about the practical application of electric discharge installations. Therefore, the main ideology of our research, as stated, was based on the discovery of a synergistic effect of chemical and electrical discharge treatment.
Data on the bactericidal effect during the successive action of an electric discharge and chlorination on water are presented in Figure 3. According to one scheme, water was first treated with an electric discharge, reducing the specific energy of treatment to 50 kJ/dm 3 (approximately 13 kWh/m 3 ), and then the treated water was chlorinated, bringing the indicators of epidemic safety of drinking water according to E. coli [Cl*] = 5 mg/dm 3 (the total exposure time was 2 h) ( Figure 3a); in the other scheme, water, previously chlorinated (exposure time 10 min), was subjected to electric discharge treatment for 20 s with a specific energy of 50 kJ/dm 3 , and in this case, the concentration of active chlorine was reduced to [Cl*] = 1 mg/dm 3 (Figure 3b). decreases; at an initial concentration of E. coli 10 5 CFU/dm 3 and Esp = 150 kJ/dm 3 (41 kWh/m 3 ), a good, but not 100%, bactericidal effect is observed. However, with such specific energy consumption, it is meaningless to talk about the practical application of electric discharge installations. Therefore, the main ideology of our research, as stated, was based on the discovery of a synergistic effect of chemical and electrical discharge treatment.
Data on the bactericidal effect during the successive action of an electric discharge and chlorination on water are presented in Figure 3. According to one scheme, water was first treated with an electric discharge, reducing the specific energy of treatment to 50 kJ/dm 3 (approximately 13 kWh/m 3 ), and then the treated water was chlorinated, bringing the indicators of epidemic safety of drinking water according to E. coli [Cl*] = 5 mg/dm 3 (the total exposure time was 2 h) ( Figure 3a); in the other scheme, water, previously chlorinated (exposure time 10 min), was subjected to electric discharge treatment for 20 s with a specific energy of 50 kJ/dm 3 , and in this case, the concentration of active chlorine was reduced to [Cl*] = 1 mg/dm 3 (Figure 3b).
It is important to note that in both cases, an important advantage of the chlorination method was preserved-a prolonged action. If in the first treatment scheme, this can be attributed to an excess of active chlorine in the treated water, then the second scheme, at a concentration of active chlorine 5 times lower, also made it possible to obtain water that remained sterile even after settling for 48 h after treatment. The next stage of our work was aimed at further reducing energy consumption and reducing the concentration of the chlorine agent by combining electric discharge and chemical treatment. The stage of preliminary chlorination was excluded, and the electrical discharge treatment started immediately after the addition of the chlorine agent at a concentration of [Cl*] = 1, 0.8, 0.6, 0.4, 0.2 mg/dm 3 , but the pulse frequency was reduced from 4 to 1 Hz.
During the treatment, water samples were taken with an interval of 5 kJ/dm 3 of input energy, and the initial concentration of E. coli was 10 6 CFU/dm 3 ; the results are shown in Figure 4.
As follows from the presented histogram, the addition of chlorine in the amount of 0.4 mg/dm 3 or more and simultaneous electric discharge treatment give a good bactericidal effect-after the introduction of a specific energy of 25 kJ/dm 3 into the aqueous solution of the chlorine by the electric discharge method, no colony-forming units of E. coli It is important to note that in both cases, an important advantage of the chlorination method was preserved-a prolonged action. If in the first treatment scheme, this can be attributed to an excess of active chlorine in the treated water, then the second scheme, at a concentration of active chlorine 5 times lower, also made it possible to obtain water that remained sterile even after settling for 48 h after treatment.
The next stage of our work was aimed at further reducing energy consumption and reducing the concentration of the chlorine agent by combining electric discharge and chemical treatment. The stage of preliminary chlorination was excluded, and the electrical discharge treatment started immediately after the addition of the chlorine agent at a concentration of [Cl*] = 1, 0.8, 0.6, 0.4, 0.2 mg/dm 3 , but the pulse frequency was reduced from 4 to 1 Hz.
During the treatment, water samples were taken with an interval of 5 kJ/dm 3 of input energy, and the initial concentration of E. coli was 10 6 CFU/dm 3 ; the results are shown in Figure 4. fields (considering the wavelength of their action) can be neglected. The m sources of intensification of redox processes in this case can be photolysis ( visible part of the electric discharge spectrum), electrolysis (pulse flow of elec sonolysis (ultrasonic component of the electric discharge spectrum), and vol itation due to the occurrence of powerful hydroacoustic flows. Using KI as an indicator of redox processes in tap water during treatm electric discharge, the following was shown: (1) When only the light component of the discharge was exposed to the (a transparent polyethylene container with 2 g/dm 3 KI was placed in the disc ber), no formation of I2 was observed.
This indicates the absence of a photocatalytic reaction H2O → OH − + H + quent oxidation according to the following reactions: KI + OH − → I − + KOH Under the described conditions, there is no bactericidal effect, because the int light flux and the range of wavelengths of UV radiation generated by an electr in water are not lethal to bacteria.
(2) Treatment of a solution containing 2 g/dm 3 NaCl, unlike solutions of lead to the release of Cl2, although in the usual electrolysis of table salt, the p tion at the anode is the decomposition of chloride ions with the release of gaseo As follows from the presented histogram, the addition of chlorine in the amount of 0.4 mg/dm 3 or more and simultaneous electric discharge treatment give a good bactericidal effect-after the introduction of a specific energy of 25 kJ/dm 3 into the aqueous solution of the chlorine by the electric discharge method, no colony-forming units of E. coli were observed in the samples. Such treatment also retains a prolonged action-after settling for 48 h after treatment, water samples with the content of [Cl*] = 1, 0.8, 0.6, 0.4 mg/dm 3 remained sterile, and in the sample with [Cl*] = 0.2 mg/dm 3 , the number of colony-forming units increased from 3 to 200.
Thus, a synergistic effect is observed in the combined treatment of contaminated water by chlorination and electric discharge: the bactericidal effect of such a combination is more powerful than each method separately, taking into account a significant reduction in the energy and chemical reagents spent. The following mode of processing can be preliminarily recommended: electrical parameters: operating voltage 25 kV, energy storage capacity 0.25 µF, pulse frequency 1 Hz-such a combination [65] is in the range of high-voltage electric discharge modes that allow generating post-discharge cavitation; processing time 20 . . . 30 s, specific energy 25 kJ/dm 3 , [Cl*] = 0.4 . . . 0.5 mg/dm 3 .
Of great interest is the mechanism of the bactericidal action of an electric discharge. To date, the question of the influence of numerous factors that occur during an electric discharge in water on microorganisms remains open and is discussed at the level of hypotheses [46,55,56].
Considering that the object of treatment is rather small (E. coli dimensions are 0.4 . . . 0.8 × 1 . . . 3 µm), it can be assumed that the mechanisms of action are at the molecular level, that is, such factors of the electric discharge as a shock wave and magnetic and electric fields (considering the wavelength of their action) can be neglected. The most probable sources of intensification of redox processes in this case can be photolysis (UV and the visible part of the electric discharge spectrum), electrolysis (pulse flow of electric current), sonolysis (ultrasonic component of the electric discharge spectrum), and volumetric cavitation due to the occurrence of powerful hydroacoustic flows.
Using KI as an indicator of redox processes in tap water during treatment with an electric discharge, the following was shown: (1) When only the light component of the discharge was exposed to the KI solution (a transparent polyethylene container with 2 g/dm 3 KI was placed in the discharge chamber), no formation of I 2 was observed.
This indicates the absence of a photocatalytic reaction H 2 O → OH − + H + with subsequent oxidation according to the following reactions: KI + OH − → I − + KOH; 2I − → I 2 ↓. Under the described conditions, there is no bactericidal effect, because the intensity of the light flux and the range of wavelengths of UV radiation generated by an electric discharge in water are not lethal to bacteria.
(2) Treatment of a solution containing 2 g/dm 3 NaCl, unlike solutions of KI, does not lead to the release of Cl 2 , although in the usual electrolysis of table salt, the primary reaction at the anode is the decomposition of chloride ions with the release of gaseous chlorine. This allows us to conclude that pulsed electrolysis is not the cause of the observed formation of I 2 during the electric discharge KI treatment: 2Cl − -2e − → Cl 2 ↑.
(3) KI treatment by solutions with ultrasound at a frequency of 22 kHz and an intensity of about 0.5 W/cm 2 leads to a slow accumulation of I 2 (a blue color appears in the KI + starch system). In the work of Ku and others [66], it was shown that during ultrasonic treatment of solutions, the main factor of the sonochemical effect is the process of "collapse" of cavitation bubbles. Obviously, under the action of ultrasound, the intensity of cavitation is not high, and, accordingly, the process of I 2 accumulation is slow.
Thus, it can be assumed that in the case of electric discharge treatment of aqueous solutions, the main factor influencing the appearance of oxidizing particles is volumetric cavitation.
To test this assumption, the effect of simultaneous electric discharge treatment and chlorination on the disinfection process at a reduced relative to atmospheric pressure was studied. As shown earlier [67][68][69], powerful volumetric cavitation occurs in the electric discharge reactor. Treatment of water with the initial content of E. coli = 10 6 CFU/dm 3 was carried out at a pressure reduced to 0.08, 0.06, and 0.04 MPa, an operating voltage of 18 kV, a storage capacity of 0.25 µF, a pulse frequency of 2.5 Hz, the [Cl*] content of 0.3 mg/dm 3 , and the treatment time of 6 s, and the specific energy introduced into the liquid was only 5 kJ/dm 3 . In all experiments with reduced pressure, a 100% bactericidal effect was observed at the end of the treatment, and the water content of E. coli satisfied the requirements for drinking. This once again confirms that the main disinfecting factor in the process of electric discharge treatment in this case is volumetric cavitation.
Some researchers in their works attribute the bactericidal effect of water treatment with an electric discharge to the erosion of electrodes, which is inevitable with this kind of treatment [70,71]. The destruction of the electrodes during the passage of high discharge currents leads to the appearance of metal ions in the liquid, some of which have an oligodynamic effect. Water with small amounts of positive metal ions (for example, copper and silver) is a fatal environment for microorganisms; a study of the survival of E. coli bacteria in water treated with so-called diaphragm electric discharges confirmed the oligodynamic effect of the electrode material [72,73]. Experiments were carried out to identify the dependence of the degree of electrode erosion on various electrical parameters [74]. Obviously, the erosion of electrodes is greatly influenced by the energy released in the electrode gap. The concentrations of iron, aluminum, and copper in water increase in proportion to the increase in the input energy, and the erosion of a brass electrode, for example, at the same energy density, is approximately 2 times higher than that of a steel one.
More important for us was the study of the dependence of the number of oxidizers formed in the course of an electric discharge on the electrode material. During the implementation of an electric discharge with an operating voltage of 25 kV, an energy storage capacity of 0.25 µF, a pulse frequency of 1 Hz, a processing time of 20 . . . 30 s, and a specific energy of 150 kJ/dm 3 in distilled water, it was found that the yield of oxidants detected by the iodometric method weakly depends on the electrode material and increases with increasing energy density. Carrying out the treatment of contaminated water with an initial content of E. coli = 10 4 CFU/dm 3 , similar in terms of electrical parameters, with a specific input energy of 25 kJ/dm 3 without the addition of chemical reagents showed a greater efficiency of electrode systems made of aluminum and brass in terms of disinfection ( Figure 5). Nevertheless, taking into account the high electroerosive ability of copper an minum, and, consequently, the high wear rate of the electrodes and the poorly cont contamination of the treated water with erosion products, the maximum allowable centrations of which can be quickly exceeded during processing, electrodes are prefe for the practical application of the electric discharge cavitation processing method titanium.

Conclusions
Thus, a synergistic effect was shown in the joint treatment of water with a ch agent and an electric discharge. The use of this effect will reduce the bactericidal do chlorine to 0.4…0.5 mg/dm 3 (10 times less than the existing standards), with energy for the electric discharge effect of about 25 kJ/dm 3 , 6 times less than without the intr tion of oxidizing agents, and significantly reduce the time of the chlorination pr while providing a prolonged bactericidal effect.
The decisive influence of volumetric cavitation on the process of disinfection b electric discharge method has been experimentally confirmed. The development method should be continued in terms of creating a phenomenological model of el discharge cavitation and clarifying the mechanism of its bactericidal action.

Data Availability Statement:
The authors confirm that the data supporting the findings study are available within the article. Raw data that support the findings of this study are av from the corresponding authors, upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest. Nevertheless, taking into account the high electroerosive ability of copper and aluminum, and, consequently, the high wear rate of the electrodes and the poorly controlled contamination of the treated water with erosion products, the maximum allowable concentrations of which can be quickly exceeded during processing, electrodes are preferable for the practical application of the electric discharge cavitation processing method from titanium.

Conclusions
Thus, a synergistic effect was shown in the joint treatment of water with a chlorine agent and an electric discharge. The use of this effect will reduce the bactericidal doses of chlorine to 0.4 . . . 0.5 mg/dm 3 (10 times less than the existing standards), with energy costs for the electric discharge effect of about 25 kJ/dm 3 , 6 times less than without the introduction of oxidizing agents, and significantly reduce the time of the chlorination process, while providing a prolonged bactericidal effect.
The decisive influence of volumetric cavitation on the process of disinfection by the electric discharge method has been experimentally confirmed. The development of the method should be continued in terms of creating a phenomenological model of electric discharge cavitation and clarifying the mechanism of its bactericidal action.