Ultrasound-induced inactivation of gram-negative and gram-positive bacteria in secondary treated municipal wastewater

The effect of 24 kHz, high energy ultrasound in the presence and absence of titanium dioxide particles on the destruction of different bacteria groups was studied. Applying a total of 1500 W/L for 60 min (this corresponds to 5400 kJ/L specific nominal energy), the mean destruction of gram-negative bacteria such as total coliforms, faecal coliforms and Pseudomonas spp. was 99.5%, 99.2% and 99.7%, respectively. More recalcitrant to sonolytic inactivation were the gram-positive bacteria Clostridium perfringens and faecal streptococci with a mean removal of 66% and 84%, respectively. The presence of 5 g/L TiO2 generally enhanced the destruction of gram-negative bacteria, yielding three to five logs reduction. On the other hand, the relatively weak sonochemical inactivation of gram-positive bacteria was only slightly affected by the presence of solid particles. Inactivation was found to follow first-order kinetics regarding bacteria population and was not affected significantly by the wastewater quality. Ultrasound irradiation at 4000 kJ/L specific nominal energy and in the presence of 5 g/L TiO2 achieved less than 103 CFU/100 mL total coliforms, thus meeting USEPA quality standards for wastewater reuse.