Destruction of Escherichia coli and Enterococcus faecalis Using Low Frequency Ultrasound Technology: A Response Surface Methodology

Background: Ultrasonic irradiation has been used for a variety of purposes. Ultrasound is able to inactivate bacteria and deagglomerate bacterial clusters through a number of physical, mechanical and chemical effects. Objectives: The current study aimed to investigate the effect of ultrasound technology on Escherichia coli (E. coli) (ATCC 25922) and Enterococcus faecalis (E. faecalis) (ATCC 11700) reduction in drinking water. Materials and Methods: Fifty mL inoculated samples of drinking water were sonicated by ultrasonic homogenizer with the dissipated power (Pdiss) of 70 watt and 20 KHz frequency at 2, 6 and 10 pulse/s ultrasound cycles, with the retention time of 5 and 10 minutes and also the microbial suspension concentration of 3, 6 and 9 CFU/mL. Microbial colonies were counted by McFarland and plate count methods. Response surface methodology (RSM) was applied to optimize the operating conditions. Design-Experts 8 (trial version) was employed in order to perform an ANOVA to analyze the ultrasound efficiency for the selected bacterial inactivation. Results: The results showed that E. coli and E. faecalis were effectively treated at 10 pulse/s in 9 minutes and 6 log CFU/mL bacterial suspension (P < 0.0001 for E. coli with 99.99% (4 log) and P = 0.0002 for E. faecalis with 97.5% removal efficiency). High coefficient of correlation (R-2 = 99.85 for E. coli and R-2 = 99.49 for E. faecalis) indicated that the model was reproducible. Conclusions: ANOVA results showed that the effect of cycle and time on the selected bacterial removal efficiency were more important than that of the microbial concentration.