Estimating fine bubble concentration in water with a time domain reflectometry

Abstract

In recent years, the use of fine bubbles (FBs) has been studied as a remediation method for groundwater contamination by volatile pollutants, and the behavior of FBs in soil has been elucidated. In this context, in-situ measurements of FB concentration in water and soil are required. In this study, we focused on the FB concentration in water. We investigated the estimation of FB concentration using the time domain reflectometry (TDR), a widely accepted method for measuring soil moisture content. FBs were generated in distilled water and surfactant solution, and the change in relative permittivity was measured with three different size TDR probes. The TDR probes were placed horizontally or vertically upward in water. The results showed that the FB concentration in distilled water was difficult to estimate due to its low concentration, while the FB concentration in surfactant solution may be captured. Relatively large bubbles originating from FB adhered to the TDR probes, and it caused an underestimation of the relative permittivity. It was found that the effect of air bubbles could be reduced by installing a relatively large probe vertically upward. The dielectric mixing model may be appropriate for converting the relative permittivity to FB concentration. These results indicate that the TDR has the potential to easily and effectively estimate the FB concentration in-situ. Comparison between TDR-based FB concentration and that measured with other methods is necessary to evaluate the accuracy of the TDR approach. It is expected that the TDR will be used to estimate the FB concentration in soil in the future.