An EPA sponsored study has indicated that sonic energy can enhance ozone effectiveness for disinfection. ozone is already widely used for drinking water treatment applications including disinfection. Currently ozone is produced using the corona discharge method and dissolved into the water either using venturi injectors or fine bubble diffusers. The ozonated water is than sent to a contactor where the ozone can act on the micro organisms.
The sonic technology utilizes a mechanically driven resonator to radiate fluids using high-intensity, low-frequency acoustic energy, was evaluated for its ability to enhance ozone disinfection of drinking water. The device consists of an oscillator coupled to an elastic rod or bar. The oscillator drives the elastic member at a resonant bending mode. The vibrations are then transmitted to the fluid where they can be used to promote mixing and mass transport.
The acoustic radiation was evaluated for its ability to enhance ozone disinfection of drinking water, specifically in the treatment of resistant spore and cyst forming protozoan forms such as Cryptosporidium and Giardia. A surrogate microbe had to be used in place of the protozoans. B. subtilis var niger ATCC 9372 endospores were chosen as a reasonable approximation to the protozoan cysts.
There are two ways in which the acoustic energy could enhance the disinfecting power of ozone:
1. The acoustic waves themselves could damage or otherwise compromise the spores to make them more susceptible to ozone;
2. The mixing and mass transfer effects could give better contact between bubbles and spores, as well as enhance the transfer of ozone from the gas into the liquid.
Experiments were conducted to evaluate the effect of acoustic energy alone, ozone alone, and ozone in combination with acoustic energy on the ability to deactivate the spores for short periods of radiation. (5 minutes).
A 5 log colony forming unit (CFU/mL) was used as a starting population for each test. The contactor was 35 liters in capacity. The tests were conducted in sodium bicarbonate buffered deionized water. Samples were taken as a function of time over the challenge period and quenched with sodium thiosulfate immediately. They were serially diluted and analyzed by standard plate counting techniques, with checking by fluorescence microscopy. The reduction in population was expressed in terms of exposure (ozone concentration times length of exposure).
In tests of one hour duration, the acoustic energy, at both high and low intensity, showed no effect on the viability of spores. The ozone challenges, with and without sonics, were conducted at 0.5, 1,5, and 2 mg/L target dissolved ozone concentrations. The decrease in exposure necessary to reach log 2 population and the 5 minute residual population indicated an enhancement of inactivation, related to increased mass transport.
The sonic contactor discussed here is a mechanically simple technology that enhances the utilization of ozone during liquid contacting which could be incorporated into existing facilities.