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Article

Sterilization of Water-Based Cutting Fluids Using Compact Air-Cooled Coaxial Dielectric Barrier Discharge Reactor with Bubbler

by
Se Min Chun
1,2,†,
Geum Ran Ahn
1,†,
Geon Woo Yang
1,3,
Hee Jae Lee
1,2 and
Yong Cheol Hong
1,4,*
1
Institute of Plasma Technology, Korea Institute of Fusion Energy (KFE), 37 Dongjansan-ro, Gunsan 54004, Republic of Korea
2
Department of Applied Plasma & Quantum Beam Engineering, Jeonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju 54896, Republic of Korea
3
Department of Bio-Nano System Engineering, Jeonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju 54896, Republic of Korea
4
KFE-School, University of Science and Technology (UST), 217, Gajeong-ro, Yuseong-gu, Deajeon 34113, Republic of Korea
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Appl. Sci. 2023, 13(22), 12302; https://doi.org/10.3390/app132212302
Submission received: 28 September 2023 / Revised: 7 November 2023 / Accepted: 10 November 2023 / Published: 14 November 2023
Browse Figures
Versions Notes

Abstract

:
Odor and discoloration in water-based cutting fluids are caused by the growth of microorganisms and putrefying bacteria. This significantly reduces cutting performance, prevents rust, and deteriorates the working environment. To overcome these drawbacks, we developed a compact air-cooled coaxial dielectric barrier discharge (DBD) with a bubbler. Bacteria and microorganisms living in waste cutting fluids were sterilized by the high concentration of ozone produced under the optimized conditions of the compact air-cooled coaxial DBD. Moreover, it was confirmed that 99.99% of bacteria and microorganisms were completely removed. Ozone was found to not affect property changes such as the composition, concentration, and pH of the water-based cutting fluids. The chromaticity and complex odor of waste cutting liquids were thus found to have been improved by the effects of microorganism and bacterial sterilization. We conclude that the proposed a compact air-cooled coaxial DBD with a bubbler is an efficient method for sterilizing water-based cutting fluids.

1. Introduction

Metal cutting fluids are classified into three main categories: straight oil (insoluble), water-based, and synthetic cutting fluids (grinding fluids). Among the various cutting oils, water-based cutting fluids, which are mainly composed of water, have always played an important role in metal processing and are widely used. They overcome the risks associated with straight-cutting oils and provide excellent cooling effects [1]. However, water-based cutting fluids may decay due to the breeding of numerous bacteria or fungi that may decompose the additives in the cutting fluids, causing the cutting fluid to fail and even endangering the health of workers [2]. They absorb nutrients such as hydrocarbons, SO4 ions, and sulfur compounds from soluble cutting oil, forming compounds such as hydrogen sulfide, thus generating odors [3]. This consequently results in the discoloration and complex odor of water-based cutting fluids because of the growth of microorganisms and putrefying bacteria, which significantly reduce cutting performance, prevent rust, and deteriorate the environment for workers [4]. Therefore, water-based cutting fluids are replaced frequently [5,6]. Exposure to water-based cutting fluids has been reported to cause health problems among workers; the methodologies used to assess the health risks from its exposure were discussed in the 2002 National Safety Council report. According to the previous investigations, it has been reported that exposed workers had elevated respiratory and digestive issues, along with cough and phlegm. Several case studies have indicated that occupational asthma can be caused by odor exposure to the fluids [7]. Ozone is a strong oxidizing agent that efficiently sterilizes microorganisms [8]. Recently, ozone treatment technologies have been developed to purify polluted water [9,10]. Compared with chlorine, ozone is a very effective chemical species that readily oxidizes organic matter, pesticides, diverse microbes, and chemical residuals at low concentrations and contact times. Moreover, it rapidly disintegrates into oxygen molecules in water without creating secondary contaminants [5]. Therefore, ozone treatment is an appropriate technique for sterilizing water-based cutting fluids [5]. However, the ozone treatment methods used in industry suffer from technical and economic limitations, such as high installation costs, high energy dissipation, inconsistent treatment efficiency, and the need for a large space. To overcome these disadvantages, ozone generators have been studied in various industrial fields [11]. Recently, corona discharge and UV radiation have been used in many studies to generate ozone [12]. Corona discharge has the disadvantages of high energy consumption and a bulky power supply. UV irradiation is inefficient in generating ozone in terms of energy consumption. To overcome these drawbacks, a compact air-cooled coaxial dielectric barrier discharge (DBD) was developed. An alternating current (AC) high-voltage power supply to the electrodes in the DBD system provides a strong electric field between the electrodes, and atmospheric air is broken down to fire the discharge. The proposed system, with its compact structure, features a low energy density and low manufacturing cost, making it easy to generate high concentrations of ozone and install anywhere else. In addition, this system uses the air generated in the blower as the discharge and cooling gases rather than compressed air or oxygen gas, which decreases the running cost. In this system, the most significant reaction for ozone production is as follows [13,14,15,16]:
O + O2 + M → O3 +M
where M represents the nitrogen and oxygen molecules in air as third collision partners. Ozone is produced in a compact air-cooled coaxial DBD via a three-body reaction, as shown in Equation (1). A general disadvantage of DBDs is that the temperature of the high-voltage electrode is increased by Joule heating, and the insulator is easily damaged. In this system, the discharge gas injected into the high-voltage electrode tube is heated by Joule heating generated from the electrode, so that discharge can occur in the discharge gap. In addition, it serves to cool the high-voltage electrode. Because ozone is easily decomposed by heat, it is important to cool the electrode. The coaxial structure of this system, in which ambient air is injected into the high-voltage electrode as a third collision partner, decreases the temperature of the electrode. The electrode can be maintained at a low temperature owing to the self-cooling effect of the discharge gas and can generate a high concentration of ozone as ozone decomposes rapidly at high temperatures. To prove this, we measured the optical and electrical properties of compact air-cooled coaxial DBD plasma. The optimized discharge conditions were derived according to the flow rate of the discharge gas and the change in power consumption. In contrast, in a previous study, plasma was generated using water-based cutting fluids and sterilized microorganisms [5]. Underwater plasma has an advantage in that the active species and radicals of the plasma directly react with water-based cutting fluids. However, the electrodes and components of the underwater plasma devices are damaged when operated for a long period in water-based cutting fluids containing approximately 10% oil. Thus, long-term plasma generation in water-based cutting fluids and equipment maintenance are difficult. To overcome these limitations, high concentrations of ozone were efficiently dissolved in water-based cutting fluids. A rectangular bubbler was added to the gas outlet of the compact air-cooled coaxial DBD to dissolve the ozone in the water-based cutting fluids. Ozone and bubbles dissolved in water-based cutting fluids can effectively react with impurities and microorganisms. Consequently, a high concentration of ozone and numerous bubbles containing ozone can effectively treat water-based cutting fluids. We performed sterilization experiments with various bacteria in waste-cutting fluids using a compact air-cooled coaxial DBD plasma with a bubbler under optimized conditions. Additionally, we analyzed the changes in the concentration, pH, and composition of water-based cutting fluids after ozone treatment.

2. Materials and Methods

2.1. Structure of Compact Air-Cooled Coaxial DBD Reactor with Bubbler

Figure 1 shows a diagram of the compact air-cooled coaxial DBD reactor with a bubbler configuration. This system consisted of a self-manufactured power supply (100 W 25 kHz transformer, NT Electronics, Seoul, Republic of Korea), which was connected to a high-voltage and a ground electrode, a batch reactor (200 × 80 × 250 mm, acrylic) with a capacity of 4 L (of which we used only 2 L in this study), and a DBD reactor with a bubbler. The reactor includes a high-voltage electrode (1/4 inch, SUS tube), an internal insulator (inner diameter of 7.1 mm; outer diameter of 10 mm; alumina tube, purity of 99.8%, density of 3.9 g/cm3, dielectric strength of 30 kV/mm, Coma Technology, Gumi-si, Republic of Korea) surrounding the high-voltage electrode, and a ground electrode (inner diameter, 13 mm; outer diameter, 27 mm; anodized aluminum rod). The interior and exterior of the ground electrode were anodized for insulation. As shown in Figure 1a, the high-voltage electrode is surrounded by an inner insulator, which has a structure separated by a discharge gap of 1.5 mm from the anodized ground electrode. The plasma generated in the discharge gap between the inner insulator and the anodized ground electrode contributes to the production of high-concentration ozone. We added a rectangle bubbler (width length, 150 mm; vertical length, 50 mm; depth, 50 mm; pore size, about 100 μm, silicon oxide 99.8%, Sam Heung Machinery, Hanam, Republic of Korea) to the gas outlet of the compact air-cooled coaxial DBD. The ozone generated in the compact air-cooled coaxial DBD reactor at atmospheric pressure was injected into the bubbler to generate a large number of bubbles and efficiently reduce the bacteria in water-based cutting fluids.

2.2. Analysis Equipment and Methods

The current and voltage were measured using current (6585, Pearson Electronics, Palo Alto, CA, USA) and high-voltage probes (P6015A, Tektronix, Beaverton, OR, USA) with a digital storage oscilloscope (DPO 4054 B, Tektronix, Beaverton, OR, USA). The optical emission spectrum of the plasma was obtained using an optical emission UV-NIR spectrometer (HR4000CG-UV-NIR; Ocean Optics, Inc., Orlando, FL, USA). The temperature and concentration of ozone were measured at the gas outlet point of the DBD reactor using thermometer (905-T1, Testo, Titisee-Neustadt, Germany) and ozone monitor (OM-1500B, OZONETECH, Daejeon, Republic of Korea), with a measurement error of ± 0.2%. Water-based cutting fluids (CIMPERIAL 1311D; Cimcool Industrial Products Inc., Cincinnati, OH, USA) were diluted with water to a concentration of 10% to form fresh cutting fluids. Water-based cutting fluids (Aryung Machinery Ind. Co., Ltd., Damyang, Republic of Korea) aged > six months were used in this study. We measured the changes in water-based cutting fluids’ concentration, pH, and composition by ozone treatment using a cutting fluid refractometer (PAL-102S, Atago, Tokyo, Japan) with a measurement error of ±0.2%, a pH meter (HI 98194, HANNA Instruments, Carrollton, TX, USA) with errors of pH (±0.02%), and a gas chromatograph/mass spectrometer (GC/MS), respectively. The complex odor was measured using an Odor Mater (OMX-SRM, Shinyei Technology, Kobe, Japan), and the concentration of the complex odor was acquired from values averaged over triplicate measurements.

2.3. Analysis of Bacteria

The experiment of bacterial concentration was performed by serial dilutions of water-based cutting fluids, and 100 μL was spread on a LB agar plate (BD, Franklin Lakes, NJ, USA) three times and incubated at 37 °C for 48 h, and the number of colonies formed was counted for concentration assessment. Following this, a single colony transferred to the new LB agar plate was employed with the three streaking methods and cultured at 37 °C for 48 h. Single colonies were incubated, and their nucleotide sequences were determined by Macrogen Corp. (Seoul, Republic of Korea). Nucleotide sequences were searched for bacterial sequences using the EzBioCloud website [17]. A bacterial sterilization experiment was performed via ozone treatment using a compact air-cooled coaxial DBD reactor with a bubbler to confirm the sterilization value of bacteria in waste cutting fluids. The samples were analyzed for 20 min each; they were serially diluted, and 100 μL was spread on each LB agar plate thrice and incubated at 37 °C for 48 h, and the number of colonies formed was counted for concentration assessment.

3. Results

3.1. Discharge of Compact Air-Cooled Coaxial DBD Plasma at Atmospheric Pressure

The proposed compact air-cooled coaxial DBD can generate a high ozone concentration with a reasonable power consumption using a self-manufactured power supply. A bubbler was installed at the bottom of the batch reactor to bubble micro-sized ozone gas. The ozone generated from the coaxial DBD plasma was injected into the bubbler. Figure 1b shows stable plasma generation at an applied power of 60 W and an air flow rate of 20 L min–1. The discharge gas passing through the gap between the inner insulator and the anodized ground electrode was broken down by a strong electric field. As shown in Figure 1b, plasma was generated inside a space of 1.5 mm between the insulator and the electrodes. The stable generation of plasma requires optimization of the voltage, frequency, cooling electrode, and flow rate of the discharge gas. We confirmed the optimal conditions for the compact air-cooled coaxial DBD plasma to generate a high concentration of ozone and maintain a stable state.

3.2. Analysis of Voltage and Current Waveform

As shown in Figure 2, the waveform of the discharge current is similar to that of a common DBD, as it takes the shape of a pointed current spike for every half cycle [18,19,20]. The total time span of the current oscillation was a few tens of sub-microseconds. We measured the consumption of electrical energy using a power detector (SJPM-C16, Seojun Electric, Republic of Korea) connected to the input line of the power supply. As shown in Figure 2a–e, the energies consumed by the power supply of the compact air-cooled coaxial DBD system were measured to be 40, 50, 60, 70, and 80 W. The energy for discharge of the plasma was calculated using the electric current method for high accuracy and simplicity [21]. In this study, the discharge power was estimated by integrating the instantaneous current and voltage, as shown in Equation (2) [14,22].
P W = f × ∫ 0 T ( V t I t d t ) ,
where f is the frequency, t is the period of the cycle, and V(t) and I(t) are the data recorded using an oscilloscope. At applied power consumptions of 40, 50, 60, 70, and 80 W, the measured sinusoidal voltages (Vrms) were 3.56, 3.6, 3.74, 3.9, and 4.1 kV, respectively; in addition, the discharge currents (Arms) were 10.95, 12.45, 13.90, 16.20, and 16.80 mA, respectively (Figure 2). The power used for the plasma discharge can calculated based on the sinusoidal voltage and current. The calculated discharge powers at the abovementioned applied powers were 38.98, 44.82, 51.99, 63.18, and 68.88 W, respectively. The difference between the measured and calculated power comes from the efficiency of the power supply used in the test. An FET (field effect transistor) in parts of the power supply discharges heat. The heat also increases as the power increases. Accordingly, the heat becomes power loss. We expect that the difference between the measured power and the calculated discharge power stems from the efficiency of the power supply used in this test, revealing an electrical efficiency of more or less 90%.

3.3. Optical Emission Spectrum

Optical emission spectrometry (OES) is an important technique used in plasma diagnostics. The distribution and internal states of various species within the plasma can be determined by observing the plasma-emitted light. Figure 1b shows the compact air-cooled coaxial DBD plasma generated at an applied power of 60 W and an air flow rate of 20 L min–1. A collimating lens was mounted under the emission hole of the compact air-cooled coaxial DBD. Emission analyses were performed at the center of the discharge gap. The excited nitrogen and oxygen species are expected to be measured in the air at the DBD [22]. Figure 3 shows the optical spectrum in the range 200–900 nm for the coaxial DBD plasma. The spectrum was found to be dominated by N2 (C3Πu − B3Πg) second positive system emissions at 300–450 nm as well as by N2+ (B2Σu − X2Σg) first negative system emissions 300–450 nm as a result of the direct or step-wise electron impact excitations and the pooling reaction of the nitrogen metastable state [23]. In addition, the OH band was not observed in the range 306–312 nm, along with the absence of lines associated with oxygen atoms in the air at the DBD. Ozone generated in a compact air-cooled coaxial DBD is formed via a three-body reaction of oxygen atoms [13,14,15,16], as shown below:
e + O2 → e + O + O
O + O2 + O2/N2 → O3 + O2/N2
Reactive oxygen and nitrogen in a metastable state for ozone production can play a significant role in the three-body reaction. The quenching of the oxygen atoms is responsible for the absence of oxygen atom peaks during the generation of air plasma. Walsh et al. reported that the quenching of oxygen atoms is faster than the radiative processes of oxygen atoms 5S0-5P and 3S0-3P [24]. In this sense, in the compact air-cooled coaxial DBD, which produces a large amount of ozone, oxygen-related peaks were not detected because of the reaction of ozone formation.

3.4. Optimized Condition of Ozone Generation

Figure 4 shows the time-dependent concentration of ozone and the outlet gas temperature with respect to the applied flow rate. Ozone was generated from a compact air-cooled coaxial DBD according to the applied flow rate. The saturated concentrations of ozone were 235.1, 474.1, 500.5, 469.2, and 460.3 ppm at applied flow rates of 10, 15, 20, 25, and 30 L min–1, respectively, and the power consumption was 60 W. Figure 4b shows the outlet gas temperature according to the applied flow rates. Twenty minutes after the discharge of plasma, the gas temperatures were 125.4, 107.5, 98.4, 86.3, and 79.9 °C at the applied flow rates of 10, 15, 20, 25, and 30 L min–1, respectively, and the consumption power was 60 W. The ozone concentration was the highest at a flow rate of 20 L min–1. The gas velocity was low, and the gas temperature increased rapidly at a flow rate of 10 L min–1. As shown in Figure 4b, when the flow rate of the discharge gas was low, the gas temperature increased owing to the Joule heating effect of the electrodes. Ozone decomposes rapidly at high temperatures. The half-life of ozone at various temperatures of −50, −35, −25, 20, 120, and 250 °C was 3 months, 18 days, 8 days, 3 days, 1.5 h, and 1.5 s, respectively. As shown in reaction (3), ozone will always be formed when there is a process of oxygen dissociation. At high temperatures, when the concentration of atomic oxygen is high, the equilibrium of reaction (4) moves to the left as the ozone dissociation proceeds and the ozone concentration is low. However, the equilibrium shifts to the right at low temperatures [25]. In this system, the gas temperature at higher flow rates is lower because of the self-cooling effect. However, at high discharge gas flow rates, the concentration of ozone decreases because of the short residence time of the discharge gas in the electrode. In general, the key parameters for stable plasma generation and high ozone production include the applied frequency and voltage, kinds of gases, electrode configuration, gas flow rate, applied power, electrode cooling, etc. Also, long-term operation using only ambient air is very important for our application. So, we designed the coaxial DBD plasma with an air-cooled internal electrode for stable and long-term operation. According to our results, eventually, the compact air-cooled coaxial DBD was optimized to generate ozone at an applied power of 60 W and a flow rate of 20 L min–1 through repeated experiments. It was confirmed that a decrease in the gas temperature and residence time of the discharge gas in the electrode are important for high-concentration ozone production. According to the results, the developed compact air-cooled coaxial DBD was optimized to generate ozone at an applied power of 60 W and a flow rate of 20 L min–1. Consequently, the nonthermal plasma processes of the compact air-cooled coaxial DBD generated high concentrations of ozone at low temperatures.

3.5. Sterilization of Microorganism in Water-Soluble Cutting Fluids

The experiment was conducted using a water-based cutting fluid for 6 to 12 months. The bacterial concentration after 48 h of incubation was 1.14 × 106 CFU/mL, and six morphologically different bacteria were isolated from the water-soluble cutting fluid (Figure 5). Six morphologically different bacteria were identified; a list of them is presented in Table 1 [26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48]. Among these, Achromobacter, Citrobacter, Pseudomonas, and Staphylococcus were the major genera identified in the water-soluble cutting fluid. Among the isolated bacteria, Achromobacter xylosoxidans has been isolated from hospitals, homes, and water, has been reported to produce foul odors, and is infectious to humans [26,27,28,29,30], whereas Citrobacter farmeri has been reported to cause illness in humans and is also known to produce foul odors [31,32,33]. Strains of Pseudomonas chengduensis have been found in humans (disease), animals, waste cooking oil, and landfills and have been reported as odor-causing bacteria, similar to other bacteria. However, unlike other bacteria, P. chengduensis has not been reported in human disease cases [34,35,36,37,38,39,40,41,42,43,44]. The last isolate, Staphylococcus warneri, has been found in the environment and animals and has been reported as an odor-causing bacterium in disease reports [45,46,47,48]. Figure 6a shows the bacterial sterilization of the cutting fluid samples using the ozone treatment. It can be seen that the plasma-generated ozone treatment sharply reduced microbial growth by 40 min, and more than 99% were killed after 60 min of treatment. Figure 6b shows the morphology of the cultured bacteria as a function of the ozonation time, with small differences between 40 and 100 min. The bacteria tested in Figure 6 are a mixture of the six types.
Figure 7 shows a schematic of the process of bacterial death upon treatment with ozone. Ozone binds to bacterial cell membranes composed of proteins, and an oxidation process occurs. In this process, one molecule of oxygen weakens cell membrane protein binding and destroys the bacterial cell membrane so that organs related to growth and proliferation inside the cell are eluted and the bacteria die [49,50].

3.6. Analysis of Water-Soluble Cutting Fluid Properties

It is important to observe the concentration change of oil in the cutting fluid because it influences the machining performance. In this context, Figure 8 shows the oil concentration of the cutting fluid based on the ozone treatment time. The initial concentration of fresh cutting oil was approximately 8.4%. We treated it for 10 min with an interval of 1 h over 1 d, and the treatment was repeated for 5 d, showing that their concentrations were 8.36%, 8.36%, 8.40, 8.40, and 8.40%, respectively, with insignificant concentration changes. The pH of the cutting fluid is shown in Figure 9, where those concentrations have been compared with the initial pH value of 9.37. The concentration of cutting oil and the pH of the cutting fluids remained almost unchanged after the ozone treatment.
Figure 10 shows the changes in the chemical composition of fresh-water-based cutting fluids due to ozone treatment. The compositional changes in the fresh cutting fluid due to the ozone treatment were analyzed by a qualitative GC/MS analysis, and one intense peak and two additional peaks were observed at retention times of 11.24, 14.42, and 18.93 min, respectively. The molecular species identified were C12H23N, C6H12BNO3, and C12H23NO2. We confirmed that the shifts and intensities of the peaks remained almost unchanged. Consequently, the concentration of ozone generated from the compact air-cooled coaxial DBD was not sufficiently high to dissociate the molecules. The complex odor and discoloration of water-based cutting fluids occur owing to the growth of microorganisms and putrefying bacteria, which significantly reduce cutting performance, prevent rust, and deteriorate the working environment. Figure 11a shows the improvement in chromaticity of the waste cutting fluids after ozone treatment for 12 h. It can be confirmed with the naked eye that the color of the waste cutting fluids becomes brighter after plasma treatment. The chromaticity of the recovered waste cutting fluids was close to that of the fresh cutting fluids after the plasma treatment. The chromaticity was observed to improve with the removal of dissolved impurities from the waste cutting fluids after the ozone treatment. In addition, Figure 11b shows the complex odor measured from the waste cutting fluids according to the ozone treatment. The initial odor of the waste cutting fluids was strong owing to the growth of impurities and microorganisms. The complex odor concentration of the waste cutting fluid, measured using an odor meter, was 450 OU. A higher concentration of complex odors indicates a stronger odor. The complex odor decreased by approximately 80% after 24 h of exposure to the ozone generated in the plasma. The four genera contained in waste-cutting fluids produce a complex odor. In this system, it was proved that about 99.99% of the bacteria were sterilized after the ozone treatment. The complex odor of the fresh cutting fluid was measured to be approximately 100 OU because of the oil contained in the cutting fluid. However, the complex odor of the waste cutting fluids that were treated with plasma for 24 h was approximately 25 OU, and that for 10 h was 120 OU. Therefore, it is necessary to maintain the concentration of oil-containing cutting fluids and an appropriate ozone treatment time. Consequently, the complex odor reduction in waste cutting fluids was improved by sterilization.

4. Conclusions

We reported ozone treatment using a compact air-cooled coaxial DBD with a bubbler. A compact air-cooled coaxial DBD with a bubbler stably generated a high concentration of ozone, which was injected into water-based cutting fluids. The bubbler was used to efficiently dissolve high concentrations of ozone in cutting fluids containing microorganisms and impurities. The optimized conditions for the discharge gas flow rate were determined based on the discharge gas temperature and ozone concentration. Approximately 99.99% of the bacteria thriving in waste cutting fluids were sterilized after the ozone treatment. We confirmed that the composition, pH, and concentration of the water-based cutting fluids were not affected by the ozone treatment. The chromaticity and complex odor of the waste cutting fluids improved after bacterial sterilization. We conclude that ozone treatment using a compact air-cooled coaxial DBD with a bubbler is an effective approach for sterilizing water-based cutting fluids. Consequently, this study confirmed the concentration and diversity of bacteria in the cutting fluids used. Additionally, this method extends the service life of water-based cutting fluids by reducing turbidity, bacteria, and odors with less energy and time and contributes to human health by improving the working environment in the metalworking industry.

Author Contributions

Writing—original draft preparation, S.M.C. and G.R.A.; writing—review and editing, Y.C.H.; data curation, G.W.Y. and H.J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the R&D Program of ‘Plasma Convergence & Fundamental Research’ through the Korea Institute of Fusion Energy (KFE) funded by Government funds (MSIT) (grant number EN2321-11). This study was supported by the Korea Environmental Industry & Technology Institute (KEITI) through the Aquatic Ecosystem Conservation Research Program funded by the Korea Ministry of Environment (MOE) (No. 1485017954). This research was supported in part by an Innopolis Foundation of Korea grant funded by the Korean government (MSIT) (grant number 2021-DD-RD-0093-01). This work was supported by the Korea Environment Industry and Technology Institute (KEITI) through a prospective green technology innovation project funded by the Korea Ministry of Environment (MOE) (2021003160014).

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to restrictions.

Acknowledgments

We are very grateful to Aryung Machinery Ind. Co., Ltd., Republic of Korea, for providing wastewater-soluble cutting fluids.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Applsci 13 12302 g001
Figure 1. (a) Diagram of the compact air-cooled coaxial DBD reactor with bubbler configuration to disinfect water-based cutting fluids. (b) Image of compact air-cooled coaxial DBD plasma.
Figure 1. (a) Diagram of the compact air-cooled coaxial DBD reactor with bubbler configuration to disinfect water-based cutting fluids. (b) Image of compact air-cooled coaxial DBD plasma.
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Figure 2. Voltage and current waveforms according to the applied power. Power consumption was calculated from the values of Vrms and Arms. (a) 40 W, (b) 50 W, (c) 60 W, (d) 70 W, (e) 80 W.
Figure 2. Voltage and current waveforms according to the applied power. Power consumption was calculated from the values of Vrms and Arms. (a) 40 W, (b) 50 W, (c) 60 W, (d) 70 W, (e) 80 W.
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Figure 3. The optical emission spectrum of the compact air-cooled coaxial DBD in the range of 200–900 nm.
Figure 3. The optical emission spectrum of the compact air-cooled coaxial DBD in the range of 200–900 nm.
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Figure 4. Time-dependent (a) concentration of ozone according to the applied flow rates measured by an ozone monitor and (b) outlet gas temperature measured by a thermometer. We carried out each measurement three times per given condition.
Figure 4. Time-dependent (a) concentration of ozone according to the applied flow rates measured by an ozone monitor and (b) outlet gas temperature measured by a thermometer. We carried out each measurement three times per given condition.
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Figure 5. Morphology of bacteria isolated from water-based cutting fluids. (a) Achromobacter xylosoxidans; (b) Citrobacter farmeri; (c) Pseudomonas aeruginosa; (d) Pseudomonas chengduensis; (e) Pseudomonas oleovorans; (f) Staphylococcus warneri.
Figure 5. Morphology of bacteria isolated from water-based cutting fluids. (a) Achromobacter xylosoxidans; (b) Citrobacter farmeri; (c) Pseudomonas aeruginosa; (d) Pseudomonas chengduensis; (e) Pseudomonas oleovorans; (f) Staphylococcus warneri.
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Figure 6. The sterilization of bacteria from the sample cutting fluid according to the ozone treatment. (a) Plot showing the death of bacteria at different treatment times (n = 3). (b) Bacteria image on Petri dish corresponding to (a).
Figure 6. The sterilization of bacteria from the sample cutting fluid according to the ozone treatment. (a) Plot showing the death of bacteria at different treatment times (n = 3). (b) Bacteria image on Petri dish corresponding to (a).
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Figure 7. Schematic diagram of bacterial death by ozone treatment.
Figure 7. Schematic diagram of bacterial death by ozone treatment.
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Figure 8. The change in concentration of cutting oil measured by the refractometer (n = 5).
Figure 8. The change in concentration of cutting oil measured by the refractometer (n = 5).
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Figure 9. The pH of the water-based cutting fluids depending on the ozone treatment time (n = 5).
Figure 9. The pH of the water-based cutting fluids depending on the ozone treatment time (n = 5).
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Figure 10. The compositional changes in water-based cutting fluids by ozone treatment were measured through qualitative analysis of GC/MS.
Figure 10. The compositional changes in water-based cutting fluids by ozone treatment were measured through qualitative analysis of GC/MS.
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Figure 11. (a) The chromaticity of waste cutting fluids was recovered close to that of fresh cutting fluids after plasma treatment. (b) The odor was measured from waste cutting fluids according to the ozone treatment (n = 5).
Figure 11. (a) The chromaticity of waste cutting fluids was recovered close to that of fresh cutting fluids after plasma treatment. (b) The odor was measured from waste cutting fluids according to the ozone treatment (n = 5).
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Table 1. List of bacteria isolated and identified from the water-based cutting fluids and their properties.
Table 1. List of bacteria isolated and identified from the water-based cutting fluids and their properties.
Species No.Species NamePropertiesReference
LocationOdorHuman Infection
AAchromobacter xylosoxidansHuman, environment○○[26,27,28,29,30]
BCitrobacter farmeriHuman, environment○○[31,32,33]
CPseudomonas aeruginosaHuman, animal, environment○○[34,35,36,37,38,39,40]
DP. chengduensisLandfill leachate○X[41]
EP. oleovoransHuman, environment○○[42,43,44]
FStaphylococcus warneriHuman, animal○○[45,46,47,48]
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Chun, S.M.; Ahn, G.R.; Yang, G.W.; Lee, H.J.; Hong, Y.C. Sterilization of Water-Based Cutting Fluids Using Compact Air-Cooled Coaxial Dielectric Barrier Discharge Reactor with Bubbler. Appl. Sci. 2023, 13, 12302. https://doi.org/10.3390/app132212302

AMA Style

Chun SM, Ahn GR, Yang GW, Lee HJ, Hong YC. Sterilization of Water-Based Cutting Fluids Using Compact Air-Cooled Coaxial Dielectric Barrier Discharge Reactor with Bubbler. Applied Sciences. 2023; 13(22):12302. https://doi.org/10.3390/app132212302

Chicago/Turabian Style

Chun, Se Min, Geum Ran Ahn, Geon Woo Yang, Hee Jae Lee, and Yong Cheol Hong. 2023. "Sterilization of Water-Based Cutting Fluids Using Compact Air-Cooled Coaxial Dielectric Barrier Discharge Reactor with Bubbler" Applied Sciences 13, no. 22: 12302. https://doi.org/10.3390/app132212302

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