Optimization of a Compact Corona Discharge Ozone Generator for Emergency Water Treatment in Brazil

Abstract

The growing demand for effective water treatment solutions, particularly in smaller communities in Brazil, highlights the potential of ozonation. However, implementing this technology at a smaller scale presents challenges, including the need to adapt it for compact systems and optimize processes for both efficiency and feasibility. This study investigates the use of a corona discharge ozone generator operating at 60 Hz in compact systems. Experiments evaluated ozone production at different gas flow rates (0.2 to 1.0 L of ozone-containing gas per minute), with the total flow divided between two lines, A (60%) and C (40%), for simultaneous treatment applications. Mass balance tests were performed using caffeine (CAF) and atenolol (ATL) as model compounds to assess molecular interactions. The results highlight the need to stabilize ozone generation to ensure consistent production and process efficiency, confirming ozone’s effectiveness in degrading emerging compounds (ECs), CAF and ATL, by approximately 80%, after process optimization using the compact ozonation unit. Key factors such as the position and diameter of the flow divider, diffuser type, and pollutant characteristics were shown to affect gas distribution, head loss, and ozone transfer efficiency. Thus, this work underscores the critical role of system configuration in optimizing ozonation, offering insights to enhance its feasibility for providing safe potable water during water crises and emergencies in Brazil.

1. Introduction

One of the main concerns for water treatment plants is to treat their water matrix with maximum efficiency and at the lowest possible cost, in order to deliver water with reduced pollution potential that meets current environmental regulations [1]. The demand for effective water treatment solutions becomes even more critical in smaller communities and emergency situations, such as floods, where water quality can be compromised by resistant pollutants and various pathogens [2].

In these scenarios, it is essential to have treatment technologies that are not only efficient but also fast and compact, capable of addressing the urgent need for potable water supply. However, attention should be given to the performance of specific operational processes. Generally, the effectiveness of the treatment heavily depends on the types of pollutants present in the water matrix and their resistance to the applied treatment technologies [3,4].

Among the various methods for treating contaminated waters, advanced oxidation processes (AOPs) using ozone (O₃) have been highlighted due to their ability to remove a wide range of organic and inorganic pollutants [5,6,7]. These processes can occur via two main mechanistic pathways: direct oxidation by molecular O₃ and indirect oxidation through highly reactive hydroxyl radicals (·OH) generated from ozone decomposition. The predominance of each pathway is influenced by factors such as pH, the presence of radical scavengers, and the composition of the water matrix, which affect the formation and interaction of radicals with the compounds [6,8].

However, the application of ozone faces challenges related to high investment and operational costs. These challenges must be overcome to enable its broader application, especially in small communities and emergency situations. Among the main limitations are the efficiency of gas-to-liquid mass transfer, the minimization of pressure loss, and the need for better understanding and control of the process [9,10]. To become a viable and scalable alternative under such conditions, ozonation systems must be specifically designed for low ozone flows and doses, technologies that remain scarce in Brazil. Moreover, the equipment should operate in a compact and rapid manner to meet the urgent demands for potable water supply during crises [5,11].

A key factor influencing the efficiency of ozonation is the design and configuration of the equipment. Mass transfer efficiency and head loss are directly affected by components such as the type of reactor, gas injector (e.g., diffusers), and piping systems [12]. Studies made by Pathapati et al. (2019) [13] and Tashtoush et al. (2015) [14] demonstrated that these structural elements significantly impact the overall process performance. Additionally, the quality of the feed gas and the consistency of ozone production must be carefully controlled. In corona discharge systems, for instance, the feed gas must be dry to avoid the formation of nitrogen-based byproducts such as nitric acid, which can cause corrosion of the generator’s internal parts and reduce process efficiency [15].

While ozone is highly effective for degrading contaminants, its use can also lead to the formation of potentially harmful byproducts, especially in the presence of bromide or natural organic matter (NOM) in the water. For example, ozonation may produce bromate (BrO₃⁻), a regulated carcinogenic compound, when bromide ions are oxidized under certain conditions [16]. In addition, when reacting with pharmaceutical compounds, ozonation can generate various intermediate oxidation byproducts, such as aldehydes, ketones, and carboxylic acids. These byproducts may increase the biological activity of the treated water or interfere with subsequent treatment steps [17]. Therefore, understanding and minimizing byproduct formation is crucial to ensure both the safety and efficacy of ozonation-based treatment systems.

To enhance ozone generation efficiency and reduce energy consumption, several parameters related to corona discharge have been investigated in the literature [18], including applied voltage [19], discharge frequency [20], duty cycle [21], gap length [22], gas flow rate [23], dielectric material [24], and gas temperature [25]. However, few studies have specifically addressed the practical challenges of applying compact ozonation units to matrices contaminated with emerging pollutants in emergency situations, highlighting a gap that still needs to be explored.

In this context, this study investigates the feasibility of a compact, rapidly deployable ozonation unit using a corona discharge ozone generator operating at 60 Hz, 220 V, and 2.0 A. It focuses on identifying and overcoming challenges related to ozone generation efficiency and mass transfer in smaller-scale systems. By targeting the removal of persistent pollutants like caffeine and atenolol, the research aims to provide practical insights into adapting ozonation technology for smaller communities and emergency situations in Brazil. Additionally, the study examines the simultaneous application of the ozonation process in different water matrices and at various gas concentrations, with the goal of optimizing its effectiveness in diverse contexts.

2. Materials and Methods

2.1. Description of Pilot Ozonation Systems

The ozonation process was evaluated in two systems: one based on the complete gas flow (Figure 1) into one reactor, and another based on the divided gas flow (Figure 2) between three reactors. Both systems are composed of an oxygen concentrator (Philips^®, Everflo 5Lpm with OPI, Murrysville, PA, USA); an ozone generator device operating at 60 Hz, 220 V, and 2.0 A by the corona discharge method (DegradaTox/AquaOz^®, 3.0 E, São Paulo, Brazil); a flow meter (rotameter); and off-gas bottles, each with a capacity of 500 mL, for ozone destruction and operational safety.

For the study of the complete gas flow (100%—Figure 1), the reaction takes place in a single liquid/gas contact reactor, with a capacity of 300 mL, an internal diameter of 3.5 cm, and height of 30 cm (Vidrolab^®, Porto Alegre, Brazil). The ozone is transferred through a circular sintered stone diffuser (1.2 cm in diameter and 0.5 cm in height), which ensures the formation of fine and homogeneous gas bubbles throughout the reaction medium.

For the divided gas flow system (Figure 2), ozone is distributed among three reactors (A, B, and C), each with a capacity of 500 mL (Pyrex^®, Downers Grove, IL, USA). Reactor A receives the full incoming ozone flow (100%), but due to the outlet connector diameter (~0.5 cm), it is estimated that approximately 60% of the ozone passes through this reactor. The remaining ozone exiting Reactor A is directed to Reactor B, which operates with a lower ozone concentration. This step was introduced both to enhance safety and to investigate the use of residual ozone. In parallel, Reactor C is supplied directly from the main ozone line. Due to the smaller diameter of its inlet connector (~0.4 cm), it receives approximately 40% of the total ozone flow. This dual setup allows for the evaluation of different ozone utilization strategies, including full and partial gas flow applications, promoting both process safety and operational efficiency.

A volume of 200 mL of 2% potassium iodide solution (KI) was added in the reactors and the off-gas to quantify the production of ozone following the iodometric method [26]. The gas injection was performed through a porous diffuser to increase the mass transfer coefficient of the ozone gas of the solution. In the complete flow, the residual portion of the ozone, which is not retained in the liquid, leaves through the top of the reactor, and is sent to the off-gas.

2.2. Determination of Ozone Production and Dosage by the Generator

The best way to determine the ozone dose that will be produced and/or applied is by carrying out experimental validation tests, with repetitions, to increase the reliability of the results. The method used to determine the ozone generator production (in grams of ozone-containing gas per hour—g_O3·h⁻¹) was the iodometric method, a widespread method for quantifying ozone concentration described in Standard Methods for Examination of Water and Wastewater [26].

The principle of the method is based on the ability of ozone to release iodine in a 2% potassium iodide solution, making it alkaline. After the ozonation process, the solution is acidified and, consequently, iodine is released; thus, titration with a standard solution of sodium thiosulphate (Na₂S₂O₃) using the starch solution as an indicator is possible. From the calibration of the generator, the percentage of consumed and transferred production of ozone to the solution is established, this being necessary to obtain the required doses in the ozonation tests.

The production of ozone (ozone feed rate) in the ozonation column and in the off-gas (portion of ozone that is not transferred to the liquid solution) can be determined by Equations (1) and (2), respectively.

P_column = (N_thio × (V_thio − V_b) × V_KI × 1440)/(V_sample × t)

(1)

P_off-gas = (N_thio × (V_thio − V_b) × V_KI × 1440)/(V_sample × t)

(2)

where P_column is the ozone production in the reactor (g_O3·h⁻¹); P_off-gas is the off-gas ozone production (g_O3·h⁻¹); N is the normality of sodium thiosulphate (N_thio) used for the titration process; V_thio corresponds to the volume of sodium thiosulphate used in the sample titration (mL); V_b is the volume of sodium thiosulphate used in the blank titration (mL); V_KI is the volume of 2% potassium iodide solution added to the ozonation column (L); V_sample is the sample volume collected for titration (mL); t is the ozonation time (min) and 1440 corresponds to the conversion factor.

The total ozone production is given by the sum of the individual portions of the ozonation column and the off-gas. From these data, a graph was constructed that correlates the ozone production (in g_O3·h⁻¹) to the gas flow (in liters of ozone-containing gas per minute—L_O3·min⁻¹) applied.

The ozone dosage applied to the reactor is related to the total ozone production of the generator, as shown in Equation (3).

D = (P_column × t × 1000)/(V_total × 60)

(3)

where D is the ozone dosage applied in the reactor (mg_O3·L⁻¹); P_column is the ozone production (g_O3·h⁻¹); t is the contact time (min); V_total is the total sample ozonated volume (L); and 1000 and 60 are the conversion factors.

Measurements of ozone concentration in the off-gas and its residual concentration in the liquid solution were performed using the iodometric method and the LineLab aqueous ozone meter, model O₃ Eco, respectively.

2.3. Mass Balance of the Ozonation Process

To ensure the quantification of the mass and concentration of ozone transferred (amount of total ozone transferred from the gas to the aqueous matrix present in the reactor) and consumed (amount of ozone that reacted in the aqueous matrix present in the reactor) during the tests, for mass balance calculations a working flow rate (Q_O3) of 0.5 L_O3·min⁻¹ applied to the rotameter (complete gas flow) was considered during contact times of 5 and 10 min, using an aqueous solution containing the pharmaceutical compounds caffeine and atenolol as model pollutants (single and multicomponent). Caffeine is considered an anthropogenic marker, and both caffeine and atenolol are widely detected in various water matrices [27,28,29].

An ozone mass (M_A) was applied to the volume of aqueous matrix inserted in the ozonation column. To calculate the ozone mass (M_A, mg) in the oxidized volume in the ozonation column, the ozone dosage applied in the reactor was multiplied by the total sample ozonized volume (V_total). During the test, the total mass supplied by the generator is composed ofthe mass of ozone consumed (M_C, mg) by the liquid solution and the mass of ozone not consumed in the reaction, quantified in the off-gas (M_off-gas, mg). The masses can be calculated, respectively, by Equations (4), (5), and (6).

M_A = D × V_total

(4)

M_off-gasv = (N_thio × (V_thio − V_b) × V_off-gas × 24)/V_sample

(5)

M_C = M_A − M_off-gas

(6)

where D is the ozone dosage (mg_O3·L⁻¹); V_total is the total sample ozonized volume (L); N_thio is the normality of sodium thiosulphate (N) used in the titration process; V_thio is the volume of sodium thiosulphate used in the sample titration (mL); V_b is the volume of sodium thiosulphate used in the blank titration (mL); V_off-gas is the volume of 2% potassium iodide solution added to the gas washing bottle (L); V_sample is the titration sample volume (L); and 24 is the conversion factor.

On the other hand, the residual ozone mass (M_R), unreacted by the solution, which is present in the liquid solution, is given by multiplying the residual ozone concentration in the liquid by the volume of the total sample ozonized, according to Equation (7).

M_R = [O₃]_residual × V_total

(7)

Thus, the mass of ozone transferred (M_T) to the middle of the solution corresponds to the sum of the residual mass (M_R) and the consumed mass (M_C), as shown in Equation (8).

M_T = M_R + M_C

(8)

The total mass (M_P, mg) of ozone produced by the generator can be obtained by adding the mass applied to the volume of aqueous matrix, inserted in the ozonation column (M_A), to the mass not consumed in the column, which was quantified in the off-gas (M_off-gas), and the residual mass of ozone in the liquid medium (M_R), as shown in Equation (9).

M_P = M_A + M_off-gas + M_R

(9)

Finally, the consumed ozone ([O₃]_C), in terms of concentration, is estimated by Equation (10).

[O₃]_C = D − ([O₃]_off-gas + [O₃]_residual)

(10)

where [O₃]_C is the concentration of consumed ozone (mg_O3·L⁻¹); D is the ozone dosage (mg_O3·L⁻¹); [O₃]_off-gas is the ozone concentration in the off-gas (mg_O3·L⁻¹); and [O₃]_residual is the ozone concentration in the residual (mg_O3·L⁻¹).

In order to avoid significant amounts of residual ozone in the samples, O₂ was bubbled for 5 min after turning off the ozone generator. This procedure guarantees that all the applied ozone that did not react was captured and consequently measured by the method used in the present work. Furthermore, to evaluate the relationship between time and pollutant concentration, a kinetic analysis through the observed rate constant (k_obs) for each pollutant was calculated, according to Equation (11) [30].

ln (C_t/C₀) = − k_obs × t

(11)

where C_t is the pollutant concentration at time t (mg·L⁻¹); C₀ is the initial pollutant concentration (mg·L⁻¹); k_obs is the observed kinetic constant (min⁻¹); and t is the reaction time (min).

3. Results and Discussions

3.1. Determination of Generator’s Stabilization Time

The ozone generator used in this work operates with variable oxygen flows (Q_O2), between 0 and 5.0 L_O2·min⁻¹, which are adjusted through a rotameter coupled to the equipment. As the generator has a fixed operating frequency, around 60 Hz, it was necessary to carry out preliminary tests to determine the gas flow variation and the ideal contact time for its stabilization with the flow to be adopted during the experiments. Information about the power of the equipment was obtained from the manufacturer.

From the results, it was observed that the minimum flow produced amounts of ozone not measurable by the volumetric titration (in this condition, there was no color variation in the solution) and that, at high flow rates, it causes a high gas production, higher than the value provided by the manufacturer. Thus, a variation in the ozone flow, between 0.2 and 1 L_O3·min⁻¹, was adopted and was able to obtain an average gas production by the generator, making it possible to use it in the treatment of various contaminated waters.

Once the flow variation was defined, the contact time in the column was varied (1–60 min) in order to identify the time in which the ozone production by the generator stabilizes. This step is extremely important because the amount of production and the type of ozone molecule formed is directly related to the generator technology. Furthermore, it is reported that a corona discharge generator, as used in the present study, can assume many shapes, sizes, and ozone outputs [13,14].

In all tests, the volumes of sodium thiosulphate used in the sample titration were recorded, thus making it possible to calculate the ozone production in the ozonation column through Equation (1). Figure 3 shows the results of these experiments.

As shown in Figure 3, it can be observed that there were significant oscillations during the production of ozone gas in the first 5 min of operation, in all tested flows. After this period, an average production of ozone in the gaseous phase of 1.3 ± 0.05 g_O3·h⁻¹ was obtained. The differences in ozone production between the first 5 min and the subsequent operation times can be attributed to the need for system stabilization. Factors such as temperature, pressure, and flow rate may fluctuate during the initial period [31,32]. Additionally, the reaction kinetics involved in ozone generation can vary during startup, resulting in rapid initial production followed by stabilization as the reactions reach equilibrium, which was achieved after 15 min of operation, as could be seen in Figure 3. Ozone absorption may also contribute to these fluctuations, as the ozonated water absorbs ozone more rapidly at first, leading to inconsistencies that stabilize over time [33,34]. As the system continues to operate, these dynamics tend to stabilize, leading to more consistent ozone production over time.

The importance of stabilizing the ozone generator cannot be overstated, as the consistent production of ozone is crucial for maintaining effective treatment over time. During the stabilization phase, ensuring that the ozone generator operates at its optimal conditions is essential for achieving reliable results. As the system stabilizes, the process efficiency improves, and ozone production becomes more predictable, ultimately enhancing the overall performance of the ozonation process. The value obtained by the generator studied in the present work is considerably satisfactory, since, when compared with other data obtained in the literature, the production was relatively high in relation to the applied gas flow. This result can be explained by several reasons, among them the system power supply, the high gas flow obtained, and the high generation capacity of the ozonator.

Trevizani (2019) [35] obtained an average ozone production in the gaseous phase of 1.27 g_O3·h⁻¹, using a flow rate of 10 L_O3·min⁻¹. Trevizani et al. (2018) [36] obtained ozone production of 0.70 g_O3·h⁻¹ in the oxidation of aqueous solutions with the azo dye Corafix Red BR. Furthermore, Scandelai et al. (2018) [37] verified ozone production of 14.60 g_O3·h⁻¹ in the leachate treatment in a system composed of an oxygen generator (maintained at 4.0 L_O2·min⁻¹). Based on the productions found in the literature, it is possible to state that the quantities of ozone produced by the same type of generators, evidencing the generation capacity with less gas loss, highly depend on each device and the operational conditions applied in the process. Therefore, the capacity of each generator must be evaluated for each case studied.

3.2. Determination of Ozone Production Considering the Complete Ozone Flow

After calibrating the equipment through the stabilization of the ozonation system, as previously carried out in this work, a fixed time of 15 min was stipulated for the stabilization of the generator at the applied flow rates (according to the position of the rotameter) in all subsequent tests. The determination of ozone production (g_O3·h⁻¹) of the generator in the ozonation column, as a function of the different flow rates, was made and the results obtained can be seen in

Table 1. Ozone production and performance by the generator at complete ozone flow applied in the present work.

	Ozone Flow Rate (LO3·min−1)
	0.2	0.4	0.6	0.8	1.0
Ozone production (gO3·h−1)	0.86	1.27	1.65	1.70	1.90
Generator performance (gO3·kWh−1)	1.95	2.89	3.75	3.87	4.32

.

For the resulting productions and their respective gas flows applied to the system, the equipment calibration curve was compiled considering the complete gas flow. This stage of the process is important, since the calibration curve of the equipment describes the response of the ozone production capacity of the equipment over a certain range of gas flow, minimizing errors in its application. From the results obtained from the calibration curve it can be seen that the maximum ozone production reached a value of 1.90 g_O3·h⁻¹, with a maximum energy efficiency of 4.32 g_O3·kWh⁻¹, a concentration higher than the value reported by the manufacturer of 1.0 g_O3·h⁻¹. Then, the linear regression equation and the correlation coefficient for the graph presented were determined in order to verify if the calibration curve was adequate.

The ozone production in the ozonation column, in g_O3·h⁻¹, can be obtained by Equation (12).

P(g_O3/h₎ = 2.241 × Q_O3

(12)

where P is the ozone production in the ozonation column (g_O3·h⁻¹); and Q_O3 is the ozone flow rate (L_O3·min⁻¹).

Through the calibration curve it is possible to conclude that the ozone production, in g_O3·h⁻¹, is maximized as the ozonator flow rate increases, as expected. In addition, the correlation coefficient (R²), obtained by the curve is equal to 0.95, indicating that the production of O₃ (P) can be well explained by the equation that relates the ozone flow rate applied to the rotameter (Q_O3), keeping the contact time process variable.

3.3. Determination of Ozone Production Considering the Divided Ozone Flow

Due to the high ozone production capacity of the generator used in the present study, an evaluation of the division of the process flow into three lines was suggested, after calibrating the equipment, at different ozone flows of 0.2 to 1.0 L_O3·min⁻¹ (also depending on the position of the rotameter). The ozone productions (g_O3·h⁻¹) in each reactor, depending on the different flows applied, can be seen in

Table 2. Production of the ozone generator dividing the flow according to the applied gas flow rate.

		Ozone Flow Rate (LO3·min−1)
	Reactor	0.2	0.4	0.6	0.8	1.0
Ozone production (gO3·h−1)	A	0.44	0.86	1.14	1.48	1.51
	B	0.0065	0.0014	0.0019	0.022	0.061
	C	0.16	0.15	0.14	0.085	0.10

. The load loss associated with the flow division for each ozone gas flow rate in L_O3·min⁻¹, in terms of g_O3·h⁻¹, can be seen in

Table 3. Load loss of ozone production with the flow division as a function of the ozone flow.

		Ozone Flow Rate (LO3·min−1)
		0.2	0.4	0.6	0.8	1.0
Ozone production (gO3·h−1)	Total without flow division	0.86	1.27	1.65	1.70	1.90
	Total diving the flow	0.60	1.02	1.29	1.58	1.68
Load loss (gO3·h−1)		0.26	0.25	0.36	0.12	0.22
Load loss (%)		30.12	19.87	21.97	6.99	11.66

.

In this work, flow division plays a crucial role in optimizing the treatment of smaller volumes of aqueous matrices simultaneously. By replicating, on a smaller scale, the processes of larger systems, the goal is to achieve the same efficiency in pollutant removal and compound mineralization, using fewer resources and time. A pilot ozonation system with flow division allows for more precise control of the process, optimizing the ozone dosage, treating different volumes of water simultaneously, and minimizing waste. Additionally, flow division helps reduce ozone losses, which is essential given the limitations in gas production of ozone generators. This approach is particularly relevant in emergency situations, such as floods, where a rapid response is crucial. Flow division enables the use of a compact and efficient technology capable of providing potable water with the necessary quality, making the process more sustainable and feasible, while also allowing the treatment to be adapted to different scales and volumes of effluents.

From Table 2 it can be seen that the highest ozone flow, in all evaluated flow rates, is directed to Reactor A. In addition, it is possible to notice that the percentage of gas flow to Reactor A increases as the applied flow rate also increases; in this way, the proportion between the vials does not remain constant (e.g., for 0.2 g_O3·h⁻¹, 60 and 40%; for 0.6 g_O3·h⁻¹, 80 and 20%; and for 1.0 g_O3·h⁻¹, 85 and 15% for Reactors A and C, respectively). These behaviors can be explained due to the position of the connector in relation to the gas supply flow, which is parallel to Reactor A, providing a greater pressure to Reactor A, and in this way directing most of the gas flow to the reactor. Also, the diameter of the hoses added to the splitter connector is larger in Reactor A than in Reactor C. Duan et al. (2019) [38] also found that the gas pumping loss decreases with a larger opening for the flow. In other words, the authors concluded that the gas flow was favored in the direction of the largest diameter of the system.

Considering Table 3, the three gas flow lines provided a head loss in the process, probably caused by using additional connectors and hoses, when compared to the complete gas flow. The addition of these connectors can limit the flow of gas along the pipe, causing the loss of dynamic energy due to the friction of the fluid particles against each other and against the walls of the pipe. Also, the lower flow of gas along the tube may also be due to the deceleration of the fluid in the smaller diameter section [14].

In addition, from Table 3, it is observed that the lowest head loss, around 6.99%, occurs at a flow rate of 0.8 L_O3·min⁻¹ and that the greatest loss, around 30.12%, occurs at the lowest flow rate, of 0.2 L_O3·min⁻¹. With this, the head loss of the current process is inversely proportional to the workflow used in the system. This can be explained by the pressure that the gas exerts on the flow, since the increase in flow provides an increase in system pressure. Consequently, due to the lower friction of the fluid and the lower loss of pressure distributed along the length of the pipe, there is a decrease in the head loss of the system [13]. Li et al. (2021) [39] also observed a lower gas head loss when sufficient gas is supplied to the system, in this way, at higher flow rates. Also, it is observed that the percentage of head loss is not constant in the process as the inlet flow rate changes; this can be explained due to the difference between the gas feed ratios, since they are not the same in all ozone flow rates applied in the process; thus, altering the inlet flow the head loss also changes, as expected.

In general, the articles in the literature usually do not use flow division for the ozonation system. When this factor is considered, it suggests that the technique’s applicability is enhanced, such as for kinetic ozonation studies, since a lower gas dosage is necessary to evaluate the route of degradation/mineralization rates of the compounds, and for multiple treatments at the same time with the same system, avoiding losses of the ozone that is produced by the generator [2,40,41,42,43,44].

Wang et al. (2023) [44], when evaluating the degradation kinetics of metribuzin and metamiron compounds, used ozone dosages of 1, 3, and 5 mg_O3·L⁻¹. Similarly, Anjali and Shanthakumar (2022) [2] applied a dosage of 1–5 mg_O3·L⁻¹ to evaluate the simultaneous degradation kinetics of amoxicillin, acetaminophen, and ciprofloxacin through the ozonation process. For advanced oxidation of benzalkonium chloride, Yu et al. (2021) [45] used an O₃ dosage of 0·37 g_O3·h⁻¹ for 20 min. Previously, Lopes (2016) [46] quantified a maximum ozone production of approximately 0.4 g_O3·h⁻¹, in an oxygen flow of 0.06 L_O2·min⁻¹, at the maximum power of the equipment of 80 W. Therefore, lower ozone dosages are necessary for the kinetic study of the reaction of each aqueous matrix to be ozonated, which is often not possible on a larger scale. Dividing the total gas flow can be an effective and simple approach to circumvent this problem.

3.4. Determination of Residual Ozone in the Ozonation Process Without Flow Division

It was verified that the residual ozone concentrations in the off-gas were low and, in all the cases analyzed, represent the same value, i.e., a concentration of 0.0058 mg_O3·L⁻¹. The residual concentration of the gas was considered in the calculation of the generator production for each flow of ozone applied, depending on the rotameter, from 0.2 to 1.0 L_O3·min⁻¹.

Comparing the results obtained from the ozone concentration in the off-gas in relation to those found in the literature, it is suggested that the system used in the present work presents a good efficiency, since the residual ozone concentration in the reactor is small. This behavior reports a good mass transfer of the process, suggesting the efficient transport of chemical species from ozone gas to the reaction medium. Lopes (2016) [46] determined that the off-gas volumes could not be quantified in any of the analyzed conditions, a similar result as found in the present work. Previously, Souza (2009) [47] quantified an average concentration of ozone in the off-gas, during 45 s of experiment, of 19.15 mg_O3·L⁻¹. Lourenção (2009) [41] obtained an average residual off-gas concentration of 0.516 mg_O3·L⁻¹, approximately one hundred times more than the value obtained in this study.

3.5. Mass Balance of Dissolved Ozone in the Liquid Phase

Table 4. Ozone mass balance for caffeine and atenolol.

	Caffeine		Atenolol
Variables	Time (min)
	5	10	5	10
P (gO3·h−1)	1.23	1.23	1.23	1.23
D (mgO3·L−1)	512.5	1025.0	512.5	1025.0
MA (mg)	102.5	205.0	102.5	205.0
Moff-gas (mg)	89.28	188.64	92.07	192.28
MC (mg)	13.22	16.36	10.43	12.72
MR (mg)	0.25	0.52	0.213	0.32
MT (mg)	13.47	16.88	10.64	13.04
MP (mg)	192.03	394.16	194.78	397.60
%T	7.01	4.28	5.46	3.28
%C	6.88	4.15	5.36	3.20
[O3]C (mgO3·L−1)	66.10	81.8	52.15	63.6
[O3]R (mgO3·L−1)	1.24	2.61	1.07	1.58
[O3]off-gas (mgO3·L−1)	446.4	943.20	460.35	961.40

Footer: P: ozone production; D is the applied ozone dosage; M_A: applied mass of ozone; M_T: transferred mass of ozone; M_off-gas: mass of ozone in off-gas; M_R: residual ozone mass; M_C: mass of ozone consumed; [O₃]_C: concentration of consumed ozone; [O₃]_R: residual ozone concentration; [O₃]_off-gas: ozone concentration in the off-gas; %T: percentage of transferred ozone and %C: percentage of consumed ozone.

presents the results of the mass balance of each compound, caffeine and atenolol, in order to better evaluate the mass transfer efficiency of the system, at an intermediate flow rate of Q_O3 = 0.5 L_O3·min⁻¹, for the application in contaminated waters. The mass and concentration of the oxidant transferred and consumed during the ozonation tests were quantified in order to associate these results with the degradation efficiency of each pollutant.

In the data presented in Table 4, the gas production remains constant as the contact time increases, reaching a value of 1.23 g_O3·h⁻¹. This result enhances the importance of the equipment stabilization period, since it promotes the effectiveness of the process through, in this case, a uniform concentration of gas production. It is also observed that the ozone dosage increases as the time also increases, being proportional to the time of the process (5 min: 512.5 mg_O3·L⁻¹; 10 min: 1025 mg_O3·L⁻¹), as expected. In addition, from the residual ozone concentration ([O₃]_R), the loss values between the two applied ozonation times are also proportional to each other, confirming once again that the constancy in the experiments resulted from the stabilization of the generator under study.

In Table 4, it is also observed that the percentage of ozone transferred to the middle of the solution was very low when compared to reports in the literature [11,30,48], reaching average values of 6.54 and 4.98%, for 5 and 10 min of process time. This low mass transfer efficiency can be explained by the type of gas–liquid diffuser/counter used in the system, responsible for the formation and definition of the size of the gas bubbles.

From the results, practical and low-cost interventions, to increase gas–liquid transfer in Reactor B, can include replacement of coarse diffusers by microbubble or sintered-porous diffusers to reduce mean bubble diameter and increase interfacial area by cylindrical ones; mild mechanical mixing (low-speed impeller or gas-induced circulation) to increase contact time; recirculation loops that increase cumulative gas–liquid contact; and geometric modification of the gas splitter to equalize gas distribution between branches. These modifications are expected to increase % transferred and reduce off-gas, at the expense of modest increases in hydraulic complexity or electrical consumption [6,11].

From a theoretical perspective, the most efficient way to increase the ozone transport rate for an aqueous solution is to increase the interface area available for transport, reduce the dimensions of the small gas bubbles dispersed in the solution, and prolong their permanence [49]. However, the average bubble size is influenced not only by the characteristics of the gas and liquid, but also to a large extent by the mechanisms that drive the fragmentation and union of the bubbles, that is, the fluid dynamics of the reactor [50].

The devices that promote the interaction between gas and liquid in water and sewage treatment systems cover several options, such as submersible devices with openings, resembling sprinklers, rubber membranes, microbubble generators through electrostatic spraying, and nozzles, in addition to plates composed of sintered metal or sintered glass [51,52,53]. In order to optimize the configuration and size of the bubbles formed in the system, it is recommended that mechanical agitation be used together with the gas–liquid contactor, aiming to increase the interfacial contact between the phases [50,54,55]. This strategic union can impart kinetic energy to the fluid in a large amount, effectively promoting the dispersion of bubbles more effectively.

Hsu and Huang (1996) [54] developed a gas-powered reactor consisting of two turbines with blades inclined at 45 degrees in series, installed inside a suction pipe. The controlled introduction of gas and the redirection of the bubbles around the aspiration tube immersed in the liquid contributed to the extension of the period in which the bubbles remain in the solution, consequently expanding the interaction surface between gas and liquid. More contemporary research led by the same authors [55] revealed that, by employing appropriate agitation rates, the efficiency in the use of ozone during the decomposition process of textile dyes can exceed 90%.

Geppert (2018) [56] obtained 51.74% ozone transfer efficiency by applying a percentage of maximum voltage of one hundred percent, in 5 min of ozonation. However, the author used a different shape of sintered stone diffuser, this being cylindrical, suggesting a greater contact area with the solution, and reached a gas production of 4.6 g_O3·h⁻¹ for a period of 5 min, a value higher than that obtained in the present study.

The values obtained for the concentration of ozone consumed in the process, that is, the amount of ozone reacted in the aqueous matrix present in the reactor, during the time of 5 and 10 min, were, respectively, 13.22 mg_O3·L⁻¹ and 16.36 mg_O3·L⁻¹, which increased in a longer process, as expected. However, it is possible to notice that the consumption was not proportional, indicating that as the time process increases the resistance in the solution for the solubilization of the gas is enhanced. In addition, there is a residual mass with considerably low values when compared to data obtained in the literature applied to the degradation of pharmaceuticals, which infers an effective mass consumption and reinforcing the importance of the diffuser type for better ozone transfer [6,46,47].

Furthermore, from the results obtained for the calibration of the ozone generator, for the total flow and for the division of the flow system, it can be inferred that the potassium iodide solution (KI 2%) is extremely reactive with the gas, since the residual ozone concentration in the off-gas of the total production remained constant and very low, at all applied gas flows. On the other hand, when applying the mass balance with the caffeine indicator solution, the residual ozone in the off-gas was relatively high, directly affecting the efficiency of the ozone transfer process, suggesting that, according to the water matrix applied in the ozonation system, the mass transfer efficiency changes depending on the structure of the molecules of the pollutants and its concentrations. In the divided system, the greatest head loss was about 10% of the total ozone production, showing a very high transfer from the gas to the solution.

3.6. Compact Ozonation System and Engineering Implications

The primary objective of this study was to evaluate the feasibility and suggest a compact ozonation system that ensures effective degradation of contaminants while maintaining operational efficiency. Figure 4 shows the designed compact ozonation system developed in this work: (a) front view, (b) back view, (c) top view, and (d) three-dimensional view.

The proposed compact ozonation unit is designed for treating small volumes of water in emergency situations, such as floods, where rapid and effective treatment is crucial. This project aims to optimize contact time and pollutant degradation through precise control of water and ozone gas flow. Splitting the flow allows for simultaneous treatment of different matrices, maintaining water quality and minimizing the formation of unwanted byproducts. It consists of 10 L matrix storage tanks, an ozone generator with an oxygen concentrator, and two 3 L ozonation reactors—B and C (Figure 2), each capable of treating approximately 0.5 m³·h⁻¹. Additionally, all off-gas exiting the reactors B and C is routed to an integrated catalytic ozone destructor prior to release to the ambient environment. The destructor consists of a stainless-steel housing packed with a manganese dioxide-based catalyst, which decomposes residual ozone into oxygen at room temperature. This configuration effectively minimizes occupational exposure to ozone, ensures compliance with workplace safety standards, and prevents ozone release to the surrounding air during operation. The catalytic unit requires no external power, has negligible pressure drop, and is sized to handle the maximum expected gas flow from the contactor under the operational conditions of this study, which is approximately 0.0058 mg_O3·L⁻¹, corresponding to 2.96 ppm at 25 °C and 1 atm.

Stabilization tests of the generator and continuous ozone production were essential to ensure the process’s effectiveness, and the flow division allowed for the simultaneous treatment of different matrices, maintaining water quality and minimizing the formation of by-products, while using a reduced ozone dosage. The compact design facilitates transportation and quick installation, making it ideal for hard-to-reach areas or in emergency scenarios. Operation is made possible by valves and measurement systems that adjust the ozone dosage as needed, ensuring efficiency and reducing reagent consumption.

Ozonation tests were carried out using the compact system to assess the effectiveness and feasibility of the designed unit. Water matrixes with caffeine and atenolol (model pollutants), each at an initial concentration of 60 mg_POL·L⁻¹, were oxidized under two different pH conditions (3.0 and 12.0) over a 30 min reaction period following a 15 min stabilization phase (Figure 3). The experiments were performed with an ozone dosage of D_O3 = 0.022 and 0.085 g_O3·h⁻¹ (Table 2) for caffeine and atenolol, respectively. Figure 5 illustrates the concentration profiles of the compounds over time, and Figure 6 and Figure 7 present the degradation efficiency of caffeine (%D_CAF) and atenolol (%D_ATL) as a function of time for both pH conditions evaluated in this study. These results provide insights into the performance of the compact ozonation system under varying operational parameters.

The results presented in Figure 5, Figure 6 and Figure 7 demonstrate the effectiveness of ozonation in degrading caffeine and atenolol under different conditions using the compact system. As shown in Figure 5, both compounds exhibit a progressive decrease in concentration over time, confirming ozone’s capability to degrade emerging compounds (ECs). Oliveira et al. (2025) [8] emphasized that the degradation of CAF and ATL occurs via both direct oxidation by molecular ozone and indirect oxidation through hydroxyl radicals (·OH), with reported kinetic constants of k_O3,CAF = 1.19 M⁻¹·s⁻¹ and k_OH,CAF = 8.94 × 10⁶ M⁻¹·s⁻¹ for caffeine, and k_O3,ATL = 0.019 M⁻¹·s⁻¹ and k_OH,ATL = 4.20 × 10⁶ M⁻¹·s⁻¹ for atenolol. The contribution of indirect oxidation becomes increasingly relevant as reactive intermediates accumulate throughout the process.

In the present study, caffeine degradation under acidic conditions (pH 3.0) using a compact ozonation unit and low ozone dose (D_O3 = 0.022 g_O3·h⁻¹), showed a rapid decrease in concentration (Figure 5a) and high degradation efficiency over time (Figure 6a). At higher pH (Figure 6b), degradation remained effective, likely due to the enhanced formation of ·OH via ozone decomposition. However, the higher standard deviation observed at pH 12.0 suggests variability related to intermediate reactivity, ozone instability, or gas–liquid mass transfer limitations under alkaline conditions.

In contrast, slower degradation kinetics were shown, as observed in Figure 5b and Figure 7. Even with a higher ozone dose (D_O3 = 0.085 g_O3·h⁻¹), its removal efficiency was lower than that of CAF. This behavior aligns with the findings of Oliveira et al. (2025) [8], who reported ATL’s low reactivity toward direct ozone attack and a greater dependence on the ·OH-mediated pathway. These pathways are more susceptible to changes in operational conditions, particularly pH and ozone transfer efficiency. Additionally, the higher variability in degradation at pH 12.0 (Figure 7b) further reflects the instability of ozone and the complexity of radical formation in alkaline media.

These differences arise primarily from the distinct chemical structures and functional groups of the two compounds, which influence their reactivity with ozone. Ozone typically reacts with electron-rich moieties, such as double bonds (C=C), hydroxyl (OH), methyl (CH₃), methoxy (OCH₃) groups, and negatively charged atoms, such as nitrogen (N), phosphorus (P), oxygen (O), and sulfur (S). Caffeine contains a higher abundance of these reactive groups compared to atenolol, which enhances its degradation rate [57]. These findings highlight the importance of tailoring ozonation parameters to the specific oxidative mechanisms of each compound and reinforce the relevance of compact systems that enable fine control over ozone flow and contact time, especially in decentralized or emergency water treatment application. In addition, by calculating k_obs (Equation (11)) of both compounds [30], through direct and indirect oxidation, the rapid degradation of caffeine by ozone (k_obs,CAF = 0.314 and 0.0445 min⁻¹ at pH = 3.0 and 12.0, respectively) and the faster degradation of atenolol by ·OH (k_obs,ATL = 0.0245 and 0.0957 min⁻¹ at pH = 3.0 and 12.0, respectively) can be seen.

However, ozonation’s overall effectiveness is limited by the formation of oxidation byproducts such as aldehydes and carboxylic acids, which are less reactive toward ozone and may persist in treated water. This incomplete mineralization necessitates the use of complementary treatment technologies to ensure full removal of organic contaminants [57]. As shown in Figure 6a, caffeine degradation is most rapid during the initial stages of ozonation but slows considerably after approximately 20 min, likely due to the accumulation of stable intermediates. In contrast, Figure 6b and Figure 7b demonstrate that ·OH-driven degradation is more sustained over time due to the non-selective nature of hydroxyl radicals. Thus, a basic pH is preferable in wastewater or drinking water treatment applications, as it facilitates effective action by both oxidizing agents and supports faster degradation, reducing their environmental impact.

Costa et al. (2024) [17] observed the formation of up to 15 transformation products after ozonation of caffeine, including compounds such as N,N-dimethylparabanic acid, 1,3,7-trimethyluric acid, and 1,3-dimethyluric acid. The authors also report that toxicity predictions indicate caffeine and several of its transformation products (TP2–TP8) may present high toxicological potential, including low biodegradability, mutagenicity alerts, and skin irritation risks. Some byproducts were also associated with altered gene expression due to reactive functional groups, reinforcing the importance of evaluating the risks linked to ozonation byproducts. Similar profiles have been reported in other studies exploring ozone reactivity with pharmaceuticals under different pH conditions [58].

Regarding atenolol, previous studies have shown that ozonation leads to a variety of transformation products resulting from oxidative cleavage, hydroxylation, and dealkylation reactions. Tay et al. (2011) [59] identified several major products, including hydroxylated derivatives of atenolol, likely formed by hydroxyl radical attack on the aromatic ring or side chain, and carbonyl-containing species resulting from oxidative cleavage of the secondary alcohol group. The authors also reported products corresponding to loss of the amide side chain and formation of hydroxyphenylacetamide derivatives, suggesting ring-opening and further oxidation pathways. Quaresma et al. (2019) [60] confirmed the formation of hydroxylated atenolol derivatives and proposed additional products arising from cleavage of the ether linkage and removal of the isopropylamino group, leading to smaller, more polar compounds such as hydroquinone- and catechol-type structures. They also reported short-chain carboxylic acids as ultimate oxidation products, consistent with progressive mineralization.

The development of compact ozonation units for smaller-scale and emergency applications presents several significant engineering implications and perspectives, particularly given the unique challenges faced in Brazil. Water treatment facilities in Brazil often face the dual challenge of achieving high treatment efficiency while managing operational costs. This challenge becomes especially acute in smaller communities and during emergency situations, such as floods, where water quality can be severely compromised by resistant pollutants and pathogens. In such scenarios, the need for rapid and effective treatment solutions is critical.

Although no direct pathogen assays were performed in this study, ozone is widely recognized as a broad-spectrum antimicrobial agent. It effectively inactivates viruses (particularly enveloped viruses such as SARS-CoV-2), bacteria, fungi, protozoa, and spores by oxidizing critical cellular and viral components, including membranes, proteins, and nucleic acids [61]. Costa & Féris (2022) [61] highlight that ozonation can achieve over 99% inactivation of enveloped viruses by targeting their lipid envelopes and RNA structures, with effective exposures ranging from 0.1 to 4 mg·L⁻¹.min. Furthermore, Costa et al. (2024) [16], using a plug-flow reactor, demonstrated significant log-scale inactivation of bacteria and viruses in aqueous matrices under optimized ozonation conditions. These findings strongly support ozone’s potent disinfection capability in water treatment applications, indicating its potential effectiveness for pathogen control in systems such as the one proposed in this work.

Designing compact ozonation units that can be quickly deployed and effectively treat water under these conditions requires addressing specific engineering challenges. For example, as suggested, the technology must be optimized to handle lower ozone doses and flow rates, making it suitable for smaller-scale applications. Innovations in system design are essential to ensure these units are both space-efficient and capable of delivering effective treatment in crisis situations.

In emergency situations, such as floods that compromise water quality, these systems can treat specific volumes and ensure the supply of drinking water, reducing risks to public health. In addition, by eliminating contaminants in the short term, they help to mitigate environmental impacts on nearby bodies of water. Thus, the proposed design showed an efficient ozone generation to treat simultaneous water matrix with different compounds, a custom-designed reaction chamber, for optimal gas–liquid contact, and advanced monitoring tools to ensure precise control of operational parameters minimizing load loss and stabilization of the system. The results show that operational control, especially of pH, is essential to maximize process efficiency and reduce environmental impacts. This design balanced simplicity and functionality, making it suitable for a wide range of applications.

Costa & Féris (2025) [62] evaluated operating parameters to mitigate pollution during flooding scenarios, using the Dilúvio stream in Porto Alegre, Rio Grande do Sul, as a case study. Similarly, Costa et al. (2024) [16] investigated the optimization of pilot-scale ozonation processes for pathogen removal and disinfection byproduct control, aiming for potable water reuse in a realistic context. Based on these studies, our study’s central purpose was to develop modular and adaptable equipment, specifically designed for use in emergency situations, combining practicality and robustness. Thus, by focusing on both operational optimization and constructive feasibility, our manuscript aimed to fill a methodological gap that goes beyond theoretical modeling and provides a straightforward and applicable solution for emergency situations in environments with limited infrastructure.

However, the high cost associated with large-scale ozonation systems also presents a significant barrier to widespread adoption, particularly in resource-limited areas. Therefore, it is necessary to focus on developing cost-effective solutions that reduce both initial investments and ongoing operational expenses. Modular and scalable designs could provide a means to lower costs while maintaining treatment effectiveness.

Operational control is another critical aspect. In emergency situations, real-time monitoring and adjustment of treatment parameters are essential to ensure consistent performance and adherence to water quality standards. Developing robust control systems that can adapt to varying water qualities and treatment needs is necessary for maintaining effective treatment. Moreover, the environmental and health impacts of ozonation must be carefully managed. The ozonation process should not produce harmful byproducts or residual ozone that could compromise water quality or public health. Ensuring compliance with environmental regulations and health standards is crucial for the successful implementation of ozonation technology.

Looking forward, there is potential for further advancements in ozonation technology to enhance its effectiveness and affordability. Innovations in ozone generation methods, better integration with other treatment technologies, and advancements in materials science could improve the performance and cost-effectiveness of compact ozonation units. Expanding their use in remote and underserved areas, particularly during water crises such as floods, could significantly improve access to safe drinking water and contribute to better public health outcomes.

Thus, the unit designed for rapid deployment in emergencies results in compactness and deployability. Also, the ability to split flow across multiple reactors (A, B, C) allows simultaneous treatment of different water volumes/qualities and lower ozone dosing for water matrices with lower organic matters. This also reduces waste when treating small volumes, and few components and straightforward controls (valves, rotameters) allow easier maintenance and lower capital cost vs. full-scale DBD/industrial O₃ systems, especially in emergency situations.

4. Conclusions

This study identified key challenges in implementing the ozonation process and highlighted the importance of a comprehensive understanding of this technique for treating contaminated water. The results underscored the critical need for stabilizing the equipment to ensure process effectiveness and consistent ozone production throughout the reaction medium. Calibration of the corona discharge generator was crucial for minimizing errors related to gas production capacity across various flow rates, achieving a maximum ozone production rate of 1.90 g_O3·h⁻¹. The characteristics of the gas flow divider, such as its position and opening diameter, significantly affect gas distribution and head loss. Increasing the flow rate reduced head loss, which is beneficial for enhancing the system’s versatility, including applications in kinetic ozonation studies and simultaneous multiple treatments, thus minimizing ozone losses. Mass balance calculations indicated very low ozone transfer and consumption efficiencies: average efficiencies of 7.16% and 5.46% for transfer, and 7.40% and 3.28% for consumption over 5 and 10 min, respectively, for different pollutant combinations (CAF and ATL). These findings suggest that the mass transfer efficiency is influenced by the water matrix, pollutant molecular structure, and concentrations. The results also emphasized the importance of diffuser type and demonstrated a strong affinity between ozone and potassium iodide solution. Despite low residual mass values (0.25–0.52 mg), which indicate effective transport of ozone to the reaction medium, further optimization is required. Considering these results, this study provides valuable insights into configuring and controlling ozonation systems to enhance their effectiveness and scalability for large-scale applications. The unit’s compact design allows for easy transport and rapid deployment, making it ideal for regions with limited access or emergencies, confirming ozone’s effectiveness in degrading emerging compounds (ECs), CAF, ATL, and propranolol (PRO), by approximately 80%, after process optimization using the compact ozonation unit. This innovation ensures a timely and reliable supply of potable water during critical situations. By addressing key challenges, this research enhances decision-making for implementing ozonation technology, improving responses to contamination crises and ensuring safe water supply in emergency scenarios across Brazil.