## 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 sterizing 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.  
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.  
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 (\(n=5\)).  
**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.