Application of Ultrasound Waves and Nano-Bubbles for Inactivation of Escherichia coli in Sludge

Abstract

Sludge treatment in wastewater treatment plants is one of the most difficult challenges for professionals and officials. Disinfection has become a challenging aspect of treatment because of the rapid elevation of health standards and the growing concern for pollution-free water resources. Chemical disinfection methods, like chlorination and ozonation, are often not environmental friendly (because of production of some undesirable by-products such as trihalomethanes) so alternative methods for disinfection are needed and the objective of this study is to determine the effect of ultrasonic waves with low wavelength and the high level of energy to improve removal rate of Escherichia coli in sludge and determine the optimum operating parameters of the ultrasonic method. In this research, variables include ultrasound density (0.375, 0.75, 1, 1.3 and 2.5) W per 1 mL and time (1, 5, 10, 15 and 30) min. Furthermore, the impact of these variables on the E. coli inactivation in sludge is measured. The results shows a significant increase in rate of E. coli bacteria reduction by increase of ultrasound density from 0.375 to 2.5 W/mL. By increasing of sonication time from 1 minute to 30 min, the similar increase in rate of E. coli bacteria reduction was observed. The experiments determined that the optimum operating parameters be sonification time of 30 min, ultrasound density of 2.5 W/mL in the frequency of 20 kHz. Inactivation rate of E. coli in these circumstances was 99.43% that is more than 2 logs. Based on the results of this research, high power and low-frequency ultrasound waves are capable of eliminating nearly all the E. coli bacteria population at relatively short irradiation time and they could remove E. coli and disinfect the sludge hence could increase the treatment rate.