Operational and kinetic parameters of ultrasonic disinfection

Abstract

Ultrasonic pressure waves are powerful means of destroying organic matterand pathogenic organisms in polluted water bodies. The potential is due to "cavitation phenomenon", which consists of the formation, growth and implosive collapse of acoustic cavities that are made of microbubbles filled with gases andvapors of the surrounding liquid. When these bubbles grow to sufficiently largesizes, they implode violently in a "catastrophic collapse", releasing extremely hightemperatures and pressures and creating "local hot spots" in the surrounding. Atsuch, molecules of gases entrapped in the microbubbles are thermally fragmentedinto atomic or radical species. When the surrounding liquid is water, gaseousbubbles are filled with molecules of water vapor, which undergo pyrolysis duringcavity collapse to fragment into hydrogen and hydroxyl radicals. The destruction oforganic matter and bacterial cells in sonicated water is due both to the chemicalreactivity of the radicals and to very unique mechanical effects of cavity collapse.The purpose of this study was to investigate the operational parameters ofultrasonic disinfection and to develop a kinetic model to describe the rate ofbacterial kill under ultrasonic irradiation. The method involved the use of threedifferent frequencies, 20 kHz, 300 kHz, 520 kHz to inactivate pathogenicorganisms in water represented by Escherichia Coli. Bacterial count tests wereperiodically performed throughout the sonication period to determine the quantityof surviving cells in the reactor effluents. Effects of the operation mode (continuousvs pulse), the buffer concentration, initial E.coli density, and solid particle additionwere investigated to select optimum conditions.It was found that maximum rate of kill was accomplished at 20 kHz within 20min. The degree of cell inactivation could be enhanced by the addition of sand ortalc with effective diameters of 53 um. There was no significant difference betweencontinuous and pulse mode of operations, to be attributed to the relatively longpause period applied. The rate of inactivation was described by a modified form ofChick̕s law, showing that ultrasound mimicked the role of chemical biocides inwater disinfection systems.