Enhanced optical detection of microparticles and bacterial cells in water using stationary acoustic wave fields

The overall goal of the research was to determine the feasibility of combining ultrasonic concentration and optical sensing technologies in the development of a sensitive water quality monitoring system to detect and enumerate micron-size particles, especially bacterial cells. The presence of micron-size particles and solutes in water deteriorate its quality, but the presence of contaminating microorganisms poses an extremely serious risk to the public. Water utilities expend the latest technology in water treatment to remove or reduce the levels of these contaminants. Currently, there are no inexpensive, easy-to-use instruments that have the sensitivity to detect and quantify microbial populations below 104 cells/ml of drinking water in real time. In this research, it was hypothesized that the detection limit would be improved by concentrating the contaminants via acoustic forces at the pressure nodes of a standing acoustic wave field. The contaminants were to be concentrated to a level within the detection limit of an optical system. An ultrasonic concentration chamber, where a planar standing acoustic wave field was used to concentrate contaminants, was developed for this study. Results showed that 8 mum diameter polymethylacrylate particles responded to the acoustic field within seconds and were retained at the nodes within one minute of sonication. The initial concentration of particles had the largest impact on the degree of concentration achieved---the lower the initial concentration, the greater the degree of concentration. Furthermore, results showed that the viability of Lactococcus lactis and Escherichia coli cells were not affected by transducer driving voltage, exposure time, microbial type, and pre-sonication cell concentration. A technique for designing a cylindrical piezoceramic tube for ultrasonic concentration was also developed in this study. Finally, empirical models for predicting cell concentrations using light scatter intensities and ratios of light scatter intensities were developed. Results showed that a relatively simple scattering measurement system may be coupled with the ultrasonic concentration chamber in the future development of a sensitive water quality monitoring system.