In recent years, there has been an increase in the application of ultraviolet (UV) light as an alternative to chemical disinfection technologies. However, in the case of poor quality effluents, the practical limit of UV disinfection of wastewater is dictated by disinfection-resistant, particle-associated bacteria. Although these particles may be removed by filtration, an alternative method to reduce the impact of suspended particles on disinfection efficiency is to decrease particle size using ultrasound technology. Mechanical forces exerted on particles due to the collapse of cavitation bubbles created by sonication break suspended particles into small fragments. In this paper, a critical review of ultrasound application for wastewater treatment is presented with emphasis on disinfection. Much of the work in this area remains at the laboratory scale. As a result, there is a need for fundamental information regarding the effect of sonication on the kinetics of disinfection and interaction of ultrasound with suspended particles. Such information is necessary for process engineering, design, and scale-up of ultrasound systems.
It is generally known that sonication improves ultraviolet (UV) disinfection kinetics of municipal effluents by breaking large suspended particles. However, the feasibility of sonication as a pretreatment technology largely depends on wastewater quality and discharge requirements. The purpose of this study was to investigate the potential benefits of ultrasound for improving the UV disinfectability of various effluent types, including primary, activated sludge, and trickling filter effluents. It was found that the tailing level of the dose-response curve at high UV doses (>40 mJ/cm2) decreased with the increased sonication time. The reduction in the tailing level had a strong correlation with the decrease in the number concentration of large particles (<60 µm) such that 1 log reduction in the number concentration of large particles resulted in 1.4, 1.1, and 1.7 log reductions in the tailing level for primary, activated sludge, and trickling filter effluents, respectively. However, the improvement in the UV disinfectability due to sonication was partly offset by the reduction in the UV transmittance of the effluent.
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