Evaluation of the Biological Efficiency of Water Disinfection Using High-Frequency Electrical Discharge

Abstract

The object of this research is the process of water disinfection by means of high-frequency electrical discharge. The study addresses the problem of achieving high biological efficiency while reducing energy consumption and avoiding harmful by-products typical of traditional methods such as chlorination or UV irradiation. As a result, a comprehensive theoretical and experimental investigation was conducted, demonstrating that within 20 s of plasma exposure, E. coli, S. aureus, and P. aeruginosa bacteria were inactivated by 99.2–99.9%. The observed efficiency is explained by the synergistic action of reactive oxygen and nitrogen species (•OH, O₃, H₂O₂, NO₂⁻, NO₃⁻) formed in the plasma–water interface. The distinctive features of the obtained results include the establishment of optimal operating parameters-voltage U = 12–18 kV, frequency f ≈ 35 kHz, and gap distance d = 15 mm—under which the normalized specific energy input (SEI) was 6–9 kWh per cubic meter of water. This value represents the standard normalization used for plasma-based treatment systems, where the electrical energy delivered to the reactor is divided by the treated volume (1.0 L in our setup) and scaled to m³ for comparison with other studies, 30–40% lower than in previously reported plasma systems. The validated physicochemical model (Poisson, Navier–Stokes, and continuity equations) matched experimental data with R² ≥ 0.95, confirming its predictive capability for further scale-up. The practical significance of the results lies in the potential application of this method for decentralized and industrial water treatment systems. The reagent-free, energy-efficient, and environmentally safe nature of the proposed approach makes it suitable for sustainable water purification under real operating conditions.

1. Introduction

Ensuring the microbiological safety of water resources is one of the most pressing scientific challenges of the modern era. According to the World Health Organization, approximately 2.2 billion people worldwide lack access to safe drinking water, and nearly 485,000 deaths per year are caused by diarrheal infections resulting from the consumption of contaminated water [1]. UNICEF reports that 74% of rural areas in developing countries have insufficient water treatment infrastructure [2,3]. Furthermore, over the past decade, the prevalence of antibiotic-resistant pathogens has increased by 35% [4], highlighting the limitations of traditional disinfection methods.

1.1. Mechanisms and Limitations of Chlorination and UV Disinfection

Conventional chlorination and ultraviolet (UV) irradiation technologies remain widely used for water disinfection; however, their efficiency is constrained by fundamental physicochemical mechanisms. During chlorination, natural organic matter (NOM) reacts with chlorine molecules, forming carcinogenic disinfection by-products (DBPs) such as trihalomethanes (THMs) and haloacetonitriles (HANs):

$$\mathrm{N}\mathrm{O}\mathrm{M}+\mathrm{C}{\mathrm{l}}_2\to \mathrm{T}\mathrm{H}\mathrm{M}\mathrm{s}+\mathrm{H}\mathrm{A}\mathrm{N}\mathrm{s}$$

(1)

In some cases, the concentration of these by-products in treated water can reach ≈ 0.1 mg/L [5,6,7,8]. This formation pathway is further influenced by chlorine hydrolysis–dissociation equilibria:

$$\mathrm{C}{\mathrm{l}}_2+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}+{\mathrm{H}}^{+}+\mathrm{C}{\mathrm{l}}^{-}$$

(2)

At elevated pH values, hypochlorous acid (HOCl) partially transforms into hypochlorite ions, altering its reactivity and increasing DBP production potential, despite retaining biocidal activity.

UV disinfection performance is governed by the Beer–Lambert law, which determines the light penetration in water:

$$\mathrm{I}={\mathrm{I}}_0{\mathrm{e}}^{-\mathsf{\alpha }\mathrm{L}}$$

(3)

where $I_0$ and $\mathrm{I}$ are the incident and transmitted UV intensities, α is the absorption coefficient, and L is the optical path length. In waters with high turbidity or complex organic composition, α increases, thereby reducing photon penetration depth and lowering UV efficiency by 20–40% [9,10]. Microbial inactivation during UV irradiation can also be described by first-order decay kinetics:

$$\mathrm{N}={\mathrm{N}}_0{\mathrm{e}}^{-\mathrm{k}\mathrm{t}}$$

(4)

where $N_0$ and $\mathrm{N}$ are the initial and remaining microorganism concentrations, k is the inactivation rate constant, and t is the exposure time. As turbidity increases, the value of k decreases, requiring higher UV doses (mJ/cm²) to achieve equivalent disinfection performance.

1.2. Scientific Novelty of the Research

The scientific novelty of the research lies in the development of a physicochemical model of water disinfection using high-frequency electrical discharge and its experimental validation. An optimal operating regime (U = 12–18 kV, f = 35 kHz, d = 15 mm) was identified, reducing energy consumption to 6–9 kWh/m³. In addition, it was found that within 20 s of treatment, E. coli, S. aureus, and P. aeruginosa bacteria were inactivated by 99.2–99.9%.

1.3. The Aim and Objectives of the Study

The aim of the study is to comprehensively evaluate the biological efficiency of water disinfection using high-frequency electrical discharge and to identify ways to improve the process’s energy efficiency and stability.

To achieve this aim, the following objectives are set:

• To investigate the mechanisms of microorganism inactivation by high-frequency plasma discharge and the specific resistance characteristics of various species;
• To analyze the influence of reactor parameters (frequency, voltage, electrode ge-ometry, and gap distance) on disinfection efficiency and energy consumption (5–20 kWh/m³);
• To model the physicochemical processes at the plasma–water interface and com-pare the obtained simulation results with experimental data.

1.4. Literature Review and Problem Statement

In recent years, water disinfection technologies based on high-frequency electrical discharge have been actively studied. The relevance of these studies is determined by the need to develop new environmentally safe approaches due to the formation of by-products and the low energy efficiency of traditional methods (chlorination and ultraviolet irradiation). In general,

Table 1. Comparative indicators of water disinfection technologies based on chlorination, ultraviolet irradiation, and high-frequency electrical discharge.

Parameter	Chlorination	UV Irradiation	High-Frequency Electrical Discharge	Reference
Energy consumption, kWh/m3	0.2–0.5	1–3	5–20	[11,12,13]
Microorganism removal efficiency, %	90–99	95–99	98–99	[11,12,13]
Virus removal efficiency, %	80–90	85–95	90–95	[11,12,13]
Residual reagent concentration, mg/L	0.3–0.5	0	0	[5,6,7,8,11,12,13]
By-product concentration (THMs, HANs), mg/L	0.05–0.10	0	≤0.005	[5,6,7,8,11,12,13]
Operating voltage, kV	–	–	5–20	[11,12,13]
Water turbidity limit (NTU)	≤10	≤5	≤100	[11,12,13]
Treatment time, s	600–1800	60–120	10–30	[11,12,13]
Service life (equipment), h	5000–7000	8000–10,000	15,000–20,000	[11,12,13]
Maintenance interval, months	1–2	6	12	[11,12,13]
Residual odor and taste change	present	none	none	[11,12,13]
Environmental risk index (0–10, lower is better)	7–8	3–4	1–2	[11,12,13]
Average system cost, thousand USD	0.5–1.0	2–3	5–10	[11,12,13]

presents the main technical parameters of water microorganism inactivation methods—chlorination, UV irradiation, and plasma discharge [11,12,13].

The table shows that high-frequency electrical discharge demonstrates the highest microbial and viral inactivation efficiency (98–99% and 90–95%) compared to chlorination and UV irradiation.

Although its energy consumption (5–20 kWh/m³) is higher, the method eliminates chemical residues and by-products, offering superior ecological safety and faster treatment times.

1.5. Plasma-Based Water Disinfection

The first comprehensive analysis of plasma technologies applied to water purification was presented in the work of Zeghioud H. et al. [14]. The authors theoretically described the microbiological effects of reactive oxygen and nitrogen species (ROS/RNS) generated by high-voltage electrical discharges, as well as their interactions with organic pollutants in water. During plasma discharge, collisions between high-energy electrons and water molecules lead to molecular excitation and subsequent dissociation according to the reaction:

$${\mathrm{e}}^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{e}}^{-}+{\mathrm{H}}_2\mathrm{O}*\to \mathrm{O}{\mathrm{H}}^{\bullet }+{\mathrm{H}}^{\bullet }$$

(5)

The hydroxyl ($\mathrm{O}{H}^{\bullet }$) and hydrogen ($H^{\bullet }$) radicals formed in this process are highly reactive oxidants capable of damaging microbial cell membranes and DNA structures, resulting in effective microbial inactivation.

In addition, the interaction of nitrogen and oxygen molecules within the plasma phase produces secondary reactive species:

$${\mathrm{N}}_2+{\mathrm{O}}_2\stackrel{\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{g}\mathrm{e}}{\to }2\mathrm{N}\mathrm{O}, \mathrm{N}\mathrm{O}+{\mathrm{O}}_3\to \mathrm{N}{\mathrm{O}}_2+{\mathrm{O}}_2$$

(6)

These reactions generate nitrogen oxides (NO and NO₂), which dissolve in water to form nitrite (NO₂⁻) and nitrate (NO₃⁻) ions. Simultaneously, long-lived oxidizing agents such as ozone (O₃) and hydrogen peroxide (H₂O₂) are produced, enhancing the overall disinfection efficiency of the plasma-treated water.

Furthermore, the reactive species generated by plasma interact with dissolved organic contaminants, initiating oxidative degradation reactions:

$$\mathrm{R}\mathrm{H}+•\mathrm{O}\mathrm{H}\to {\mathrm{R}}^{\bullet }+{\mathrm{H}}_2\mathrm{O},{\mathrm{R}}^{\bullet }+{\mathrm{O}}_2\to \mathrm{R}\mathrm{O}{\mathrm{O}}^{\bullet }$$

(7)

This chain of reactions leads to the progressive oxidation of organic compounds, eventually converting them into harmless end products such as carbon dioxide (CO₂) and water (H₂O). Consequently, plasma discharges can simultaneously remove both biological and chemical pollutants from water.

Overall, the theoretical models described by Zeghioud H. and colleagues established the scientific foundation of plasma-chemical disinfection methods. The combined action of short- and long-lived reactive species (•OH, O₃, NO₂⁻, H₂O₂) generated in plasma promotes efficient microbial inactivation and complete mineralization of organic compounds. Therefore, high-voltage plasma discharge technology is considered an environmentally safe, reagent-free, and effective approach for advanced water disinfection and purification.

In the works of Barjasteh et al. (“Recent Progress in Applications of Non-Thermal Plasma for Water Purification, Bio-Sterilization, and Decontamination”, 2021) and Foster et al. (“Perspectives on the Interaction of Plasmas with Liquid Water for Water Purification”, 2012), the multifactorial effects of plasma discharge on the inactivation of bacteria and viruses in water are comprehensively discussed [15,16]. The studies demonstrated that plasma discharge acts not only through the generation of chemically active species (•OH, O₃, H₂O₂) but also via the combined influence of acoustic shocks and electromagnetic fields that disrupt the structural integrity of microorganisms. Overall,

Table 2. Quantitative Evaluation of Bacterial and Viral Inactivation Efficiency in Water under Plasma Discharge Conditions.

№	Microorganism	Plasma Power (W)	Exposure Time (s)	Inactivation Level (%)	Inactivation (log)
1	E. coli	140	60	99.999	6.0
2	S. aureus	45,000	90	98.7	5.0
3	P. aeruginosa	200	120	99.5	5.3
4	MS2 virus	80	180	96.2	4.0
5	B. subtilis	150	240	92.5	3.7

presents the dependence of microbial inactivation on plasma power and exposure time in an aqueous medium.

The table shows that as plasma discharge power and exposure time increase, the inactivation level of microorganisms also rises significantly. The highest efficiency was observed for E. coli at 140 W and 60 s exposure, achieving 99.999% inactivation, which demonstrates the strong microbiological effectiveness of plasma-based disinfection.

In 2021, Rathore et al. in their work “Investigation of Physicochemical Properties of Plasma Activated Water and Its Bactericidal Efficacy” comprehensively examined the physicochemical characteristics and bactericidal performance of plasma-activated water (PAW) [17]. Meanwhile, Zhang et al. (2024) demonstrated the strong antibacterial effect of PAW against Vibrio parahaemolyticus and elucidated its oxidative damage mechanism on bacterial cell membranes [18]. These findings indicate that plasma-activated water can serve as an eco-friendly and highly effective alternative to conventional reagent-based disinfection methods. Overall,

Table 3. Characteristic Parameters of Reactive Species in Plasma Activated Water (PAW).

№	Reactive Species	Lifetime	Concentration Range (mg/L)	Membrane Damage Efficiency (%)	Oxidation Potential (V)
1	OH	10−9–10−6 s	0.01–0.1	95–99	+2.80
2	O3	1–60 s	0.2–2.5	90–97	+2.07
3	H2O2	103–104 s	1–15	80–92	+1.78
4	NO2−	104–105 s	0.5–5	60–75	+0.98
5	ONOO−	10−3–10−1 s	0.05–0.2	85–93	+1.40

presents the characteristic parameters of reactive species in plasma-activated water (PAW).

The table shows that the reactive species generated in plasma-activated water (PAW) differ significantly in their lifetimes and oxidation potentials. Short-lived radicals such as •OH and O₃ exhibit high oxidative potentials (2.07–2.80 V) and can damage bacterial cell membranes with an efficiency of up to 95–99%.

Malyushevskaya et al. (2024), in their study “Hybrid Water Disinfection Process Using Electrical Discharges”, investigated a hybrid water disinfection method based on high-voltage underwater electrical discharge and identified the key physicochemical factors responsible for microbial inactivation [19]. The research highlighted the critical role of cavitation, shock waves, and ultrasonic effects in enhancing disinfection efficiency. However, the authors did not provide a detailed analysis of the influence of energy consumption and reactor operating parameters (such as frequency and voltage). Overall, Figure 1 illustrates the relative contribution of physicochemical factors to microbial inactivation under high-voltage underwater electrical discharge. The relative contribution of cavitation, shock waves, ultrasound, and reactive species shown in Figure 1 is adapted from Malyushevskaya et al. (2024) [19].

The figure shows that during high-voltage underwater discharge, cavitation and shock waves have the highest contribution to microbial inactivation, accounting for 35% and 25%, respectively. Ultrasound and reactive species contribute between 15–20%, indicating their secondary yet significant role in the overall disinfection process.

Mai-Prochnow et al. (2021), in their study “Interactions of Plasma-Activated Water with Biofilms: Inactivation, Dispersal Effects and Mechanisms of Action”, investigated in detail the effects of plasma-activated water (PAW) on biofilms and microorganisms [20]. The authors identified the mechanisms by which PAW disrupts biofilm structures, inactivates microbial cells, and inhibits their regrowth—including oxidative stress, disruption of intercellular communication, and degradation of the biopolymeric matrix. However, the results were mainly limited to laboratory-scale experiments and have not yet been fully adapted for practical engineering applications. Overall,

Table 4. Quantitative Evaluation of Plasma-Activated Water (PAW) Effects on Biofilms and Microbial Biosafety.

№	Sample/Parameter	Exposure Time (min)	Reactive Species Concentration (mg/L)	Biofilm Thickness Reduction (%)	Cell Viability Decrease (log)	Regrowth Inhibition (%)
1	E. coli biofilm	10	1.8	70	4.8	80
2	S. aureus biofilm	15	2.1	65	4.3	78
3	P. aeruginosa biofilm	12	1.5	60	3.9	72
4	Mixed culture biofilm	20	2.5	80	5.2	85

presents the antibiofilm and antimicrobial efficacy of plasma-activated water (PAW) under laboratory conditions.

The table shows that plasma-activated water (PAW) reduces biofilm thickness by 60–80% and decreases microbial cell viability by an average of 4–5 log units. These results demonstrate the high efficacy of PAW against complex microbiological structures and confirm its potential as an alternative to conventional disinfection methods.

The ecological and sanitary efficiency of plasma was comprehensively studied by Hamza I.A. et al. (2023) [21]. The researchers demonstrated that cold atmospheric plasma exhibits significantly higher effectiveness in the inactivation of viruses, bacteria, and protozoa in water and wastewater compared to conventional chlorination and ultraviolet methods (Figure 2). Moreover, the plasma technology was found to be capable of degrading organic pollutants; however, high energy consumption and electrode erosion remain technical limitations that need further improvement.

According to Figure 2, the plasma discharge method demonstrated the highest efficiency: 99% of viruses and 98% of bacteria were eliminated. In comparison, ultraviolet and chlorination methods showed 90–88% and 85–82% efficiency, respectively, clearly highlighting the superiority of plasma technology.

Research on modeling the interaction between plasma and water has been actively developing in recent years. In the study by Nahrani N.M. et al. (2022), the process of discharge formation inside a gas bubble in water was analyzed through numerical modeling [22]. The authors described the propagation of the electric field, the dynamics of charged particles, and the energy exchange at the bubble boundary, thereby analyzing the complex physico-chemical phenomena occurring at the plasma–water interface. Meanwhile, Lu X. et al. (2022) investigated various discharge modes of plasma bubbles and their interaction mechanisms with liquids using both experimental and theoretical approaches [23]. Although these studies have laid the scientific foundation for understanding the plasma–water interface, the developed models and obtained results still require further extensive experimental validation. In general, Figure 3 below presents the time-dependent variation in plasma discharge parameters inside a gas bubble, illustrating the interrelation between the dynamics of the electric field, temperature, and pressure.

Figure 3 shows the time-dependent variation in plasma discharge parameters inside a gas bubble. Over the 0–10 s measurement interval, the electric field gradually decreases from approximately 25 kV/cm to 20 kV/cm. The temperature remains within the range of 10–15 °C, exhibiting sinusoidal oscillations that indicate the rhythmic nature of energy exchange in the plasma process. Meanwhile, the pressure fluctuates slightly within 0.8–1.2 atm, reflecting the relative stability of the system and the dynamic equilibrium at the plasma–water interface.

As seen from the reviewed domestic and international studies, several challenges remain in water disinfection using high-frequency electric discharge. Firstly, the influence of reactor parameters on microbiological efficiency has not been fully determined, while the energy consumption of about 5–20 kWh/m³ limits the large-scale application of this technology. In addition, the resistance of various microorganisms and their ability to recover after plasma exposure are still insufficiently studied. Therefore, comprehensive research aimed at improving energy efficiency and accurately evaluating biological effects is of great scientific and practical relevance today.

2. Material and Methods

The research was conducted at the Department of Electronics, Telecommunications and Space Technologies of Satbayev University, Almaty, Kazakhstan. The study focused on investigating the biological efficiency of water disinfection using high-frequency electrical discharge through theoretical and experimental methods. A model describing plasma formation and reactive species interaction was developed, and key parameters were measured in a laboratory plasma reactor.

2.1. Multifactor Antimicrobial Action of High-Frequency Plasma Discharge

In recent years, plasma-chemical disinfection methods based on high-frequency electrical discharge have attracted significant scientific interest. This type of discharge generates reactive oxygen and nitrogen species (ROS/RNS), ultrasound, local electromagnetic fields, and radicals, providing a multifactor antimicrobial effect. Studies show that this technology can inactivate 98–99% of bacteria and 90–95% of viruses [24,25]. In addition, plasma treatment is considered an environmentally safe “green” technology, as it does not significantly alter the physicochemical properties of water. In general, the schematic of plasma water disinfection is presented in Figure 4 below.

The figure illustrates water disinfection using plasma discharge. Reactive species inactivate microorganisms, resulting in purified water.

2.2. Current Challenges and Prospects for Advancing High-Frequency Plasma Discharge Systems

However, the influence of reactor parameters, discharge frequency and voltage on biological effects, microbial species-specific resistance, and criteria for optimizing energy consumption have not yet been fully investigated. Current scientific evidence indicates that biochemical modeling of the plasma–water interface remains incomplete, and most experimental results still predominantly correspond to laboratory-scale studies [26,27]. Additionally, reported energy consumption values range from 5 to 20 kWh/m³, and research aimed at reducing this energy demand remains insufficient [28,29,30]. In general, Figure 5 presents the overall view of the laboratory plasma reactor designed for high-voltage plasma discharge in water.

This image shows a laboratory plasma reactor generating a high-voltage discharge directly in water. The setup includes a cylindrical reaction chamber and submerged electrodes connected to a high-voltage power supply, producing a visible plasma glow in the liquid.

A comprehensive assessment of the microbiological efficiency of high-frequency electrical discharge—based water disinfection methods is important not only from a scientific perspective but also within the framework of national and international programs aimed at improving water safety. Thus, in-depth investigation of the biological efficiency of water disinfection using high-frequency electrical discharge represents a necessary and timely research objective.

Theoretical modeling was carried out by taking into account the hydrodynamic and electrical field parameters of the high-frequency plasma discharge. The calculations were based on the Poisson, Navier–Stokes, and continuity equations, complemented by a system of differential expressions describing the transport processes of charged and neutral particles.

The spatial distribution of the electric field was determined by the Poisson equation:

$${\nabla }^2\phi =-{\displaystyle \frac{\mathsf{\rho }}{{\mathsf{\epsilon }}_0}}$$

(8)

where φ is the electric potential, ρ is the space charge density, and ε₀ is the permittivity of free space.

The conservation of mass in the liquid medium was expressed by the continuity equation:

$${\displaystyle \frac{\partial \mathsf{\rho }}{\partial \mathrm{t}}}+\nabla ·(\mathsf{\rho }\overrightarrow{\mathrm{v}})=0$$

(9)

The conservation of momentum was described by the Navier–Stokes equation:

$$\mathsf{\rho }\left({\displaystyle \frac{\partial \overrightarrow{\mathrm{v}}}{\partial \mathrm{t}}}+\left(\overrightarrow{\mathrm{v}}·\nabla \right)\overrightarrow{\mathrm{v}}\right)=-\nabla \mathrm{p}+\mathsf{\mu }{\nabla }^2\overrightarrow{\mathrm{v}}+\mathsf{\rho }\overrightarrow{\mathrm{E}}$$

(10)

where $\overrightarrow{\mathrm{v}}$ is the velocity vector, $\rho$ is the pressure, μ is the viscosity coefficient, and $\overrightarrow{E}$ is the electric field.

The evolution of electron and ion concentrations was modeled by transport equations:

$${\displaystyle \frac{{\partial \mathrm{n}}_{\mathrm{e}}}{\partial \mathrm{t}}}+\nabla ·({\mathrm{n}}_{\mathrm{e}}{\overrightarrow{\mathrm{v}}}_{\mathrm{e}})={\mathrm{S}}_{\mathrm{i}}-{\mathrm{S}}_{\mathrm{r}}$$

(11)

$${\displaystyle \frac{{\partial \mathrm{n}}_{\mathrm{i}}}{\partial \mathrm{t}}}+\nabla ·({\mathrm{n}}_{\mathrm{i}}{\overrightarrow{\mathrm{v}}}_{\mathrm{i}})={\mathrm{S}}_{\mathrm{i}}-{\mathrm{S}}_{\mathrm{r}}$$

(12)

where S_i and S_r are the ionization and recombination rates, respectively.

The temperature field was calculated using the energy conservation equation:

𝜌𝑐𝑝𝜕𝑇𝜕𝑡 + 𝑣·∇T = ∇·k∇T + Q

(13)

where Q represents the energy source in the plasma (e.g., Joule heating and reaction heat).

The electric field direction was determined from the potential gradient according to:

$$\overrightarrow{\mathrm{E}}=-\nabla \phi$$

(14)

The reaction rates and transport balance at the plasma–water interface were defined using Fick’s law and chemical kinetic equations:

$${\mathrm{J}}_{\mathrm{i}}=-{\mathrm{D}}_{\mathrm{i}}\nabla {\mathrm{C}}_{\mathrm{i}},$$

(15)

$${\displaystyle \frac{\mathrm{d}{\mathrm{C}}_{\mathrm{i}}}{\mathrm{d}\mathrm{t}}}={\mathrm{k}}_{\mathrm{f}}{\mathrm{C}}_{\mathrm{A}}{\mathrm{C}}_{\mathrm{B}}-{\mathrm{k}}_{\mathrm{r}}{\mathrm{C}}_{\mathrm{C}}$$

(16)

where $D_i$ is the diffusion coefficient, $C_i$ is the concentration of species i, and $k_f$ and $k_r$ are the forward and reverse rate constants.

All equations were combined into a chemical-kinetic model that included more than 20 elementary reactions (formation of O₃, •OH, H₂O₂, NO₂⁻, and NO₃⁻). Numerical simulations were performed in the Python v3.13 environment, and the results were analyzed using Mathcad v11. As a result, the spatial distributions of the electric field, temperature gradient, and ozone concentration were determined.

The experiments were carried out in a quartz reactor with an inner diameter of 50 mm and a height of 200 mm. The reactor was equipped with two tungsten electrodes with a gap of 10–20 mm. The generator operated within a voltage range of 5–20 kV and a frequency range of 15–50 kHz. The setup included an oscilloscope, a spectrophotometer, and a microbiological testing system. During the experiments, the temperature was maintained at 22 ± 1 °C and the pH at 7.0 ± 0.2, while the initial microbial concentration was approximately 10⁶ CFU/mL. The temperature was maintained at 22 ± 1 °C to ensure stable and reproducible physicochemical conditions during the experiments. Temperature strongly affects the solubility of oxygen in water, the reaction kinetics of plasma-generated reactive species (•OH, O₃, H₂O₂, NO₂⁻/NO₃⁻), and the survival characteristics of microorganisms. Maintaining constant temperature eliminates thermal effects and ensures that the observed disinfection is solely due to non-thermal plasma chemistry rather than heating of the liquid. The overall structural diagram of the plasma reactor setup is shown in Figure 6.

The voltage and current parameters were continuously monitored, and the instantaneous power was calculated using the relation P(t) = U(t) · I(t). To describe the temporal evolution of plasma-generated reactive species, a coupled system of differential equations was formulated as follows:

$$\left\{\begin{array}{c}{\displaystyle \frac{\mathrm{d}\left[{\mathrm{O}}_3\right]}{\mathrm{d}\mathrm{t}}}={\mathrm{k}}_1\left[{\mathrm{O}}_2\right]\left[\mathrm{O}\right]-{\mathrm{k}}_2\left[{\mathrm{O}}_3\right]\\ {\displaystyle \frac{\mathrm{d}\left[{\mathrm{H}}_2{\mathrm{O}}_2\right]}{\mathrm{d}\mathrm{t}}}={\mathrm{k}}_3{\left[\mathrm{O}\mathrm{H}\right]}^2-{\mathrm{k}}_4\left[{{\mathrm{H}}_2\mathrm{O}}_2\right]\\ {\displaystyle \frac{\mathrm{d}\left[\mathrm{N}{\mathrm{O}}_{\mathrm{x}}\right]}{\mathrm{d}\mathrm{t}}}={\mathrm{k}}_5\left[{\mathrm{N}}_2\right]\left[\mathrm{O}\right]-{\mathrm{k}}_6\left[{\mathrm{N}\mathrm{O}}_{\mathrm{x}}\right]\end{array}\right.$$

(17)

Here: [O₃], [H₂O₂], [NO_x] represent the concentrations of ozone, hydrogen peroxide, and nitrogen oxides, respectively; k₁–k₆ are reaction rate constants determined experimentally; The production and decay of reactive species depend on the instantaneous discharge power P(t) governed by the applied voltage and current waveforms.

The concentrations of ozone, hydrogen peroxide, and nitrogen compounds were determined using standard photometric methods, which provided experimental validation of the modeled differential system. The validity of the model was verified by comparison with experimental data. The difference between the calculated and measured values did not exceed 10%, while the correlation coefficient was at the level of R² ≥ 0.95. These results demonstrate that the developed model accurately describes the processes occurring in the plasma–water system and is suitable for predicting the efficiency of water disinfection.

3. Results and Discussion

For the scientific research, a high-frequency laboratory setup was developed to conduct specialized experimental studies at the department. The system allows voltage regulation within the range of 15–30 kV and frequency control from 13 kHz to 50 kHz, providing flexible adjustment of discharge parameters for precise investigation of plasma–liquid interaction processes. The general view of the setup is presented in Figure 7.

Figure 7 shows the general view of the laboratory setup designed to evaluate the biological efficiency of water disinfection using high-frequency electrical discharge. The system includes a high-voltage power supply, discharge chamber, water circulation unit, and bubble aeration module, which together enable the investigation of the microbiological purification efficiency of water.

3.1. Mechanisms of Microorganism Inactivation by High-Frequency Plasma Discharge

Experimental studies have shown that the destruction of microorganisms in plasma-treated water occurs through the combined effects of chemical oxidation, physical disruption, and electromagnetic influence. At a voltage of 15–20 kV and a frequency of 30–40 kHz, intense emission within the 280–310 nm range was recorded from the plasma region. This range confirms the formation of hydroxyl radicals (•OH) and ozone (O₃). The optical emission spectrum obtained by spectrophotometric analysis revealed distinct peaks at 309 nm (OH A²Σ⁺ → X²Π) and 777 nm, indicating the active generation of oxidizing species. In general, the spectral peaks and energy characteristics observed in the plasma–water system are summarized in

Table 5. Spectral Peaks and Energy Characteristics of Plasma–Water Interaction.

№	Wavelength (nm)	Emission Intensity (a.u.)	Frequency (THz)	Energy (eV)	Species	Physical Process
1	280	0.42	1070.0	4.43	–	Excitation of water molecules
2	290	0.55	1034.0	4.27	•OH	Initial hydroxyl radical formation
3	309	1.00	970.0	4.01	•OH	Electronic transition of hydroxyl radical (A2Σ+ → X2Π)
4	400	0.47	749.0	3.10	–	Electron–ion collision region
5	500	0.32	600.0	2.48	–	Background emission spectrum
6	777	0.68	386.0	1.59	OI/O3	Ozone and oxygen atomic emission zone
7	850	0.35	353.0	1.46	–	Energy decay phase
8	900	0.28	333.0	1.38	–	Plasma weakening stage

below.

According to Table 5, the emission intensity reaches its maximum value of 1.00 a.u. at 309 nm, confirming the active formation of hydroxyl radicals (•OH). Additionally, at 777 nm, the intensity of 0.68 a.u. indicates ozone and atomic oxygen emission, representing the key energy transition zone of oxidative plasma processes.

According to microbiological analysis, E. coli, S. aureus, and P. aeruginosa bacteria were inactivated by 99.2–99.9% (≈5 log units) after 20 s of plasma exposure. Figure 8 shows the time-dependent change in the viable concentration of microorganisms. The obtained data follow a first-order kinetic law, expressed as N = N₀ e^(−kt), where the average inactivation rate constant is k = 0.23 s⁻¹.

Figure 8 illustrates the exponential decrease in microorganism concentration over time during plasma treatment. After 20 s of exposure, the viable cell count decreased from approximately 10⁶ to 10⁴ CFU/mL, corresponding to an inactivation constant of k = 0.23 s⁻¹ and an overall removal efficiency of 99.2–99.9%.

The inactivation rate showed a strong correlation with the concentration of reactive oxygen and nitrogen species (R² ≥ 0.96). Short-lived radicals (•OH, O₃) rapidly oxidized and disrupted the cell membrane, while long-lived species (H₂O₂, NO₂⁻, NO₃⁻) inhibited the regrowth of residual microorganisms. Microscopic observations revealed membrane rupture, cytoplasmic leakage, and DNA structural damage—all in full agreement with the literature data [21,22,23,24,25,26,27]. Overall,

Table 6. Microscopic evaluation of cell membrane integrity and DNA degradation induced by plasma-generated reactive species.

№	Parameter	Control Group	After Plasma Exposure	Change, %	Main Reactive Species
1	Intact cell membranes, %	100 ± 2.3	2.1 ± 0.4	−97.9	•OH, O3
2	Cells with ruptured membranes, %	0 ± 0.0	95.6 ± 1.2	+95.6	•OH, O3
3	Cells with cytoplasmic leakage, %	0 ± 0.0	92.3 ± 1.8	+92.3	•OH, H2O2
4	DNA damage index (F/F0)	1.00 ± 0.03	0.18 ± 0.02	−82.0	H2O2, NO2−, NO3−
5	Morphologically deformed cells, %	3.2 ± 0.5	88.9 ± 2.1	+85.7	O3, •OH
6	Residual viability, %	100 ± 1.5	0.25 ± 0.05	−99.75	All species
7	Correlation coefficient (R2)	-	0.96 ± 0.01	-	-

illustrates the microscopic evaluation of cell membrane and DNA damage induced by plasma-generated reactive species.

Table 6 shows that plasma exposure caused extensive bacterial cell damage: the proportion of ruptured membranes and cytoplasmic leakage exceeded 90%, while DNA integrity decreased by over 80%. These findings indicate a strong correlation (R² ≥ 0.96) between the concentration of reactive oxygen and nitrogen species and the rate of microbial inactivation, confirming the dominant oxidative destruction mechanism.

3.2. Influence of Reactor Parameters on Disinfection Efficiency and Energy Consumption

Forty experimental series were conducted to determine the effect of reactor operating parameters-voltage, frequency, electrode geometry, and gap distance-on biological efficiency. The discharge voltage was varied from 5 to 20 kV, frequency from 15 to 50 kHz, and electrode gap from 10 to 20 mm.

Figure 9 shows the dependence of microbial disinfection efficiency on voltage. At voltages below 8 kV, the plasma discharge was unstable, and microorganisms were not completely inactivated (<90%). In the range of 12–18 kV, a stable luminous plasma was formed, ensuring more than 98% inactivation. Further increase in voltage slightly improved the efficiency (<1%) but led to higher energy consumption and accelerated electrode wear.

In Figure 9, the disinfection efficiency increases notably with the applied voltage. Within the 12–18 kV range, a stable plasma discharge forms, achieving over 98% microbial inactivation, while further voltage growth only raises energy consumption and electrode wear.

The energy consumption was calculated using the equation $\mathrm{E}={\displaystyle \frac{P·t}{V_{\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}}}}$, where $P=U·I$ represents the instantaneous power. The specific energy input (SEI) was calculated as the ratio of the electrical energy supplied to the plasma generator to the effective water volume treated during each batch experiment (1.0 L). For correct comparison with published plasma-based systems, the obtained values were normalized to 1 m³, resulting in an SEI of 6–9 kWh/m³. This normalization does not imply that one cubic meter was physically treated but represents a standard metric widely used to compare plasma reactor efficiency, which is approximately 30–40% lower than those reported in previous studies (5–20 kWh/m³ [29,30]). This improvement was attributed to the optimized electrode geometry and the use of a 35 kHz operating frequency.

The electrode configuration also played an important role: needle-shaped cylindrical electrodes created a high field concentration, while flat electrodes produced a wider but weaker discharge zone. An electrode gap of 15 mm provided the most stable plasma formation and the highest energy efficiency (Figure 10). The temperature variation did not exceed 3 °C, indicating that the process was non-thermal in nature. Thus, the high-frequency discharge effectively inactivated microorganisms in water while preserving its physicochemical properties.

According to the data presented, when the electrode gap was 10 mm, the plasma stability reached a relative value of 0.7, while the energy efficiency was 0.75. Both parameters increased with distance and peaked at 1.0 when the gap was 15 mm, indicating the most stable and energy-efficient plasma formation. However, further widening of the gap to 20 mm caused a decline-stability dropped to 0.6 and efficiency to 0.7, confirming that excessive spacing weakens the electric field concentration and reduces plasma uniformity.

3.3. Modeling and Validation of Physicochemical Processes at the Plasma–Water Interface

The processes describing the interaction between plasma and water (Equations (8)–(17)) were numerically modeled. The Poisson, Navier–Stokes, and continuity equations were solved to determine the spatial distributions of the electric field, charge density, and temperature. According to the model, the electric field intensity at the electrode tip reached (2.0–2.5) × 10⁴ V/cm, which is in full agreement with the experimental observations. The results of the study are presented in

Table 7. Main parameters and characteristics of the plasma–water interaction model.

№	Parameter	Model Value	Experimental Value	Deviation, %	Unit	Note
1	Electric field intensity (E)	2.3 × 104	2.2 × 104	4.3	V/cm	Maximum near the electrode tip
2	Charge density (ρ)	8.5 × 10−5	9.0 × 10−5	5.6	C/m3	Observed in the discharge region
3	Temperature difference (ΔT)	4.8	4.6	4.2	°C	Confirms non-thermal plasma behavior
4	Ozone concentration (Cₒ3)	2.1	2.0	5.0	mg/L	Highest near the anode zone
5	Electrode gap (d)	15	15	–	mm	Optimal configuration for stable plasma
6	Reactor voltage (U)	18	18	–	kV	At 35 kHz discharge frequency
7	Energy consumption (Eₑ)	6.5	6.8	4.4	kWh/m3	30–40% lower than previous studies
8	Microbial inactivation rate	99.7	99.6	0.1	%	E. coli and S. aureus samples
9	Correlation coefficient (R2)	0.958	–	–	–	High agreement between model and experiment

below.

The table demonstrates that the physical processes in the plasma–water system are quantitatively consistent: the electric field intensity near the electrode tip reached (2.0–2.5) × 10⁴ V/cm, the charge density ranged between 10⁻⁶–10⁻⁴ C/m³, and the temperature difference did not exceed 5 °C. These results confirm the formation of a stable non-thermal plasma, while the ozone concentration near the anode (≈2.1 mg/L) indicates effective oxidation and water disinfection performance.

To verify the accuracy of the model, the calculated and experimental data were compared. The differences between the concentrations of ozone, hydrogen peroxide, and nitrogen oxides did not exceed 10%, while the correlation coefficient was R² ≥ 0.95. Overall,

Table 8. Comparative analysis of model and experimental results in plasma–water interaction processes.

Parameter	Model Value	Experimental Value	Difference (%)
Ozone (O3), mg/L	2.05	2.0	5.0
Hydrogen peroxide (H2O2), mg/L	8.40	8.90	5.6
Nitrite + Nitrate (NO2− + NO3−), mg/L	4.70	5.00	6.4
Energy consumption, kWh/m3	6.5	6.8	4.4

presents a comparative analysis of the modeled and experimental results for the plasma–water interaction process.

The table presents the correspondence between the modeled and experimental results in the plasma–water system, with deviations ranging only from 4.9% to 6.4%. These findings confirm the reliability of the developed model and demonstrate a high level of agreement for key parameters such as ozone (O₃), hydrogen peroxide (H₂O₂), nitrogen compounds (NO₂⁻ + NO₃⁻), and energy consumption.

This correspondence confirms that the model accurately describes the processes occurring in the plasma–water system and can reliably predict the efficiency of water disinfection. Overall, the results of theoretical modeling and experimental validation demonstrate that the high-frequency electrical discharge is an environmentally friendly, reagent-free, and highly efficient method. This approach enables rapid inactivation of microorganisms in water while improving water quality and ensuring energy efficiency.

To highlight the novelty and practical improvement of the proposed system, a comparative analysis between previous studies and the present work was conducted.

Table 9. Comparison of microbial inactivation efficiency and energy consumption between previous studies and the present work.

Study	Microorganisms	Voltage/Frequency	Inactivation (%)	Exposure Time (s)	Energy Consumption (kWh/m3)
Zeghioud et al., 2020 [14]	E. coli	10–20 kV	90–99%	30–120	8–20
Barjasteh et al., 2021 [15]	Mixed bacteria	12–25 kV	95–99%	60–180	10–25
Foster et al., 2012 [16]	E. coli, viruses	15–30 kV	90–98%	60–180	~12–22
Malyushevskaya et al., 2024 [19]	S. aureus	15–30 kV	90–98%	90–240	7–15
This study (2025)	E. coli, S. aureus, P. aeruginosa	12–18 kV, 35 kHz	99.2–99.9%	20 s	6–9

summarizes the key differences in microbial inactivation efficiency, required exposure time, and energy consumption between state-of-the-art plasma-based water disinfection systems and the results obtained in this study.

As shown in Table 9, the present study demonstrates superior microbial inactivation efficiency (99.2–99.9% within only 20 s) compared with previously reported plasma systems, which typically require 60–240 s to achieve comparable results. In addition, the normalized specific energy input (6–9 kWh/m³) is 30–40% lower than many reported systems, indicating that the optimized electrode geometry and discharge frequency significantly improve overall energy performance.

3.4. Discussion of the Results of the Study

The obtained results confirm the high efficiency of water disinfection using high-frequency electrical discharge. This phenomenon is explained by the combined action of reactive oxygen and nitrogen species (•OH, O₃, H₂O₂, NOx) generated in the plasma region. As shown in Table 5 and Figure 8, the spectral lines at 309 nm and 777 nm confirm the active formation of these species. They oxidize and destroy the microbial cell membrane and DNA structure, leading to 99.2–99.9% inactivation within just 20 s. The decrease in microbial concentration follows the first-order kinetic law (N = N₀ e^(−kt)), where the inactivation rate constant is k = 0.23 s⁻¹. The agreement between the model and experimental data (Table 7 and Table 8, R² ≥ 0.95) indicates that the physicochemical processes in the plasma–water system are accurately described.

Compared to traditional methods such as chlorination and ultraviolet (UV) irradiation (Table 1), the proposed method offers several advantages. Firstly, it operates without chemical reagents and does not generate harmful by-products (THMs, HANs), making it environmentally safe. Secondly, the treatment time is much shorter—10–30 s, which is about 30 times faster than chlorination and 3–5 times faster than UV disinfection. The energy consumption ranges between 6–9 kWh/m³, which is 30–40% lower than values reported in previous plasma studies ([29,30]). This improvement is due to the optimized operating parameters (U = 12–18 kV, f ≈ 35 kHz, d = 15 mm; Figure 10). Thus, the method ensures high biological efficiency and energy savings while providing reagent-free, “green” water purification.

The main limitations of this study include its laboratory scale, a limited set of microorganisms (E. coli, S. aureus, P. aeruginosa), and experiments performed only in clear water under constant temperature and pH conditions. The reproducibility of the results under conditions of high turbidity, varying organic content, or ionic composition has not yet been examined. Additionally, long-term electrode degradation and system stability under prolonged operation were not fully evaluated.

The disadvantages of this study include the absence of large-scale industrial testing and the lack of assessment of long-term operational performance. In future work, it is necessary to scale the technology for flow-through reactors, test it on complex microbiological environments, and develop wear-resistant electrode materials.

The future development of this research involves mathematical modeling of plasma discharge dynamics in turbulent water flows, implementation of real-time monitoring and automatic control systems, and detailed identification of plasma-generated species using advanced analytical techniques (e.g., LC–MS or HRMS). Experimentally, during pilot-scale implementation, challenges such as bubble stability and uniform energy distribution may arise. Addressing these issues will make this technology energy-efficient, environmentally friendly, and suitable for industrial-scale water treatment applications.

Although high-frequency electrical discharge requires a higher specific energy input compared with conventional chlorination and UV-based disinfection, its practical applicability remains promising under several realistic conditions. First, the method operates without chemical reagents and does not produce harmful by-products, making it suitable for decentralized drinking water systems, point-of-use devices, emergency water sanitation, and applications in hospitals, food processing, and rural or remote regions where chemical supply is limited.

Furthermore, recent studies show that the energy demand of plasma reactors decreases significantly when transitioning from small batch volumes to flow-through configurations due to improved energy transfer efficiency, larger electrode surfaces, and optimized discharge distribution. Engineering improvements such as pulsed-power excitation, advanced electrode materials, and hydrodynamic control have demonstrated a 20–50% reduction in specific energy consumption at pilot scale.

Therefore, despite the relatively high energy input in laboratory conditions, the technology has realistic potential for practical implementation in real drinking water treatment systems, particularly where environmentally safe, reagent-free, and rapid disinfection is required.

4. Conclusions

The main objective of this research was to comprehensively evaluate the biological efficiency of water disinfection using high-frequency electrical discharge and to identify ways to improve the process’s energy efficiency and stability. Based on this objective, the following results were obtained:

(1)

Mechanisms of microorganism inactivation were identified. Experimental studies confirmed that E. coli, S. aureus, and P. aeruginosa bacteria were inactivated by 99.2–99.9% (≈5 log units) within 20 s of plasma exposure. This effect is explained by the combined action of short-lived (•OH, O₃) and long-lived (H₂O₂, NO₂⁻, NO₃⁻) reactive species generated in the plasma region. Spectral analysis revealed intense emission at 309 nm and 777 nm, corresponding to hydroxyl radical and ozone formation. The inactivation process followed a first-order kinetic law (N = N₀ e^(−kt)) with a rate constant of k = 0.23 s⁻¹, while the correlation coefficient between reactive species concentration and microbial reduction reached R² ≥ 0.96. These findings confirm that oxidative reactions are the dominant mechanism of microbiological inactivation;

(2)

Optimal operating parameters and energy efficiency were established. The most efficient operating regime was determined as U = 12–18 kV, f ≈ 35 kHz, and d = 15 mm. Under these optimal conditions, the normalized specific energy input (SEI) was 6–9 kWh/m³, corresponding to the conventional normalization of batch-reactor energy consumption to one cubic meter of treated water, which is 30–40% lower than reported in previous plasma-based studies. A stable discharge was achieved in the 12–18 kV range, providing over 98% inactivation, while further voltage increase led to higher power losses and accelerated electrode wear. The process remained non-thermal (ΔT ≤ 3 °C), ensuring that the physicochemical properties of water were preserved. These results demonstrate the environmental and energetic advantages of the proposed method.

(3)

Consistency between theoretical modeling and experimental results was verified. The developed physicochemical model, based on the Poisson, Navier–Stokes, and continuity equations, accurately described the processes occurring at the plasma–water interface. The deviation between the modeled and experimental data did not exceed 10%, with a correlation coefficient of R² ≥ 0.95. The electric field intensity near the electrode tip reached (2.0–2.5)·10⁴ V/cm, ozone concentration was ≈ 2.0 mg/L, and hydrogen peroxide ≈ 8.9 mg/L. This validated model provides a reliable predictive framework for describing plasma-induced reactions and optimizing reactor parameters.

General Conclusion: The research demonstrated that high-frequency electrical discharge is an effective, reagent-free, and environmentally friendly technology for water disinfection. The method ensures over 99% microbial removal, reduces energy consumption by 30–40%, and maintains the natural composition of water. The strong agreement between theoretical modeling and experimental validation confirms the feasibility of scaling this technology for industrial water treatment applications.