Continuous liquid treatment by high-density microwave plasma in flowing liquid

In recent years, much attention has been paid to the generation of plasma in liquids and the interaction between plasma and liquids. Application of such plasma in liquids to organic decomposition and sterilization, and the chemical reaction species in the plasma are being actively studied. We have developed microwave-excited plasma sources using slot antenna, which minimizes damage to the slot electrodes, increases the plasma volume, and have improved the stability of plasma production. We also reported improvement of liquid treatment efficiency under reduced pressure conditions during discharge using a pump [1]. However, since these processes were performed in batch processing, the liquid processing speed of these plasma sources were not sufficient for practical use, and drastic improvement of the liquid processing speed was an issue. To solve this, we have developed an in-line microwave plasma system utilizing the Venturi effect, and an improvement in the efficiency of decomposition treatment of organic solutions compared to the batch processing [2].

In this study, the decomposition efficiency of organic matter in liquid is experimentally measured with respect to flow speed, and the effect of liquid flow speed on the interaction between high-density plasma and liquid is analytically evaluated. These results suggest that increasing the flow speed not only facilitates plasma generation due to reduced pressure by Venturi effect, but also allows reactive species to be efficiently utilized in the process. Based on these results, a novel plasma source is developed that can process large flow rate of 60 L/min while maintaining high flowing speed for large volume liquid process.

[1] R. Saito et al.: Curr. Appl. Phys. 11 (2011) S195-A198.

[2] M. Ito et al., Jpn. J. Appl. Phys., 56 (2017) 026201.