Journal of Hazardous Materials 146 (2007) 492–495 Sonochemical disinfection of municipal wastewater Apostolos Antoniadis a , Ioannis Poulios a,∗ , Eleni Nikolakaki b , Dionissios Mantzavinos c a Laboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece b Laboratory of Biochemistry, Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece c Department of Environmental Engineering, Technical University of Crete, Polytechneioupolis, 73100 Chania, Greece Available online 20 April 2007 Abstract The application of high intensity, low frequency ultrasound for the disinfection of simulated and septic tank wastewaters is evaluated in this work. Laboratory scale experiments were conducted at 24 and 80 kHz ultrasound frequency with horn-type sonicators capable of operating in continuous and pulsed irradiation modes at nominal ultrasound intensities up to 450W. For the experiments with simulated wastewaters, Escherichia coli were used as biological indicator of disinfection efficiency, while for the experiments with septic tank wastewaters, the total microbiological load was used. Complete elimination of E. coli could be achieved within 20–30 min of irradiation at 24 kHz and 450 W with the efficiency decreasing with decreasing intensity and frequency. Moreover, continuous irradiation was more effective than intermittent treatment based on a common energy input. Irradiation of the septic tank effluent prior to biological treatment at 24 kHz and 450 W for 30 min resulted in a three-log total microbiological load reduction, and this was nearly equal to the reduction that could be achieved during biological treatment. Bacterial cell elimination upon irradiation was irreversible as no reappearance of the microorganisms occurred after 24 h. © 2007 Published by Elsevier B.V. Keywords: Disinfection; E. coli; Ultrasound; Wastewaters 1. Introduction Disinfection has become a challenging aspect of water treat- ment because of the rapid elevation of health standards and the growing concern for pollution-free water resources. The most commonly used disinfection methods utilize cheap and effective chemicals such as chlorine and its products, which unfortunately, have a number of serious drawbacks. They produce chlorinated organic products, which are dangerous for mankind as well as the environment, since they are toxic, carcinogenic, and mutagenic [1]. Additionally, they cannot totally inactivate all the pathogenic microorganisms that can be present in wastewaters or the drink- ing water due to their low oxidant action [2]. Due to these problems alternative methods for water disinfection are being investigated, and amongst these is the application of ultrasound. Sonochemical oxidation techniques involve the use of ultra- sound waves to produce an oxidative environment via cavitation that yields localized microbubbles and supercritical regions ∗ Corresponding author. Tel.: +30 2310 997785; fax: +30 2310 997784. E-mail address: poulios@chem.auth.gr (I. Poulios). in the aqueous phase. The collapse of these bubbles leads to extremely high local temperatures and pressures. These con- ditions are very short-lived, but have shown to result in the generation of highly reactive radicals [3], which are capable of initiating or promoting many fast reduction–oxidation reactions. Sonochemistry is an example of advanced oxidation process (AOP). AOP owe their enhanced reactivity, at least in part, to the generation of reactive free radicals, the most important of which is the excited hydroxyl radical ( • OH) [4]. Ultrasound is able to inactivate bacteria and deagglomerate bacterial clusters through a number of physical, mechanical, and chemical effects arising from acoustic cavitation. On collapse, cavitation bubbles produce enough energy to mechanically weaken or disrupt bacteria or biological cells via a number of processes [5]: • Forces due to surface resonance of the bacterial cell are induced by cavitation. Pressures and pressure gradients result- ing from the collapse of gas bubbles which enter the bacterial solution on or near the bacterial cell wall. Bacterial cell dam- age results from mechanical fatigue, over a period of time, which depends on frequency. 0304-3894/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.jhazmat.2007.04.065