Dual-frequency ultrasound: Strengths and shortcomings to water treatment and disinfection

Since the early 2000s, dual-frequency ultrasound (DFUS) has received much attention for synergistically enhanced elimination of organic pollutants and pathogenic microorganisms from water. In the present review, we have surveyed recent developments in acoustic physics to elucidate the mechanism of synergistic effect under exposure of aqueous media to DFUS. Briefly, the nonlinear dynamics of microbubbles upon DFUS exposure produces additional frequencies, such as harmonics, subharmonics, ultraharmonics and combination frequencies. These increase the probability of bubbles collapse, thereby enhancing cavitation and generating more reactive oxygen species (ROS) for advanced oxidation processes (AOPs). Further, literature data on ROS generation, chemical degradation and microbial inactivation in aqueous media through DFUS alone and DFUS-based AOPs (involving oxidants or catalysts) have been discussed. In this regard, optimal frequency combination, sonoreactor type and transducer arrangement appear to be key parameters for achieving a high synergistic effect. Strengths and shortcomings of DFUS to water treatment and disinfection have been identified and future research directions have been proposed. Though most studies were conducted on pure (matrix-free) aqueous solutions, these AOPs could be applicable for treating real waters.