## 3 Results
### 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 C\({}_{12}\)H\({}_{23}\)N, C\({}_{6}\)H\({}_{12}\)BNO\({}_{3}\), and C\({}_{12}\)H\({}_{23}\)NO\({}_{2}\)  
Figure 8: The change in concentration of cutting oil measured by the refractometer (\(n=5\)).  
Figure 9: The pH of the water-based cutting fluids depending on the ozone treatment time (\(n=5\)).  
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 puttefying 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.  
Figure 10: The compositional changes in water-based cutting fluids by ozone treatment were measured through qualitative analysis of GC/MS.