Ultrasound as a sole or synergistic disinfectant in drinking water [electronic resource]

by Wong, Kar Yee

Chlorine as a disinfectant reacts with natural organic matter to produce undesired and possibly carcinogenic halogenated disinfection by-products (DBPs), which are regulated by the U.S. Environmental Protection Agency under the Disinfectant/Disinfection Byproducts Rule (DBPR). In order to comply with the increasingly stringent regulations, alternative disinfectants such as ozone, UV irradiation, and chloramines have been investigated. Unfortunately, these alternatives have their own limitations and disadvantages as well. Sonication is another alternative that has not yet received adequate research. The hydroxyl radicals, tensile stresses, and fluid shear generated during sonication may inactivate microorganisms. The goals of this research were to evaluate the effectiveness of sonication alone and combined sonication and chlorination for inactivation of E. coli. Four stages of disinfection experiments were conducted: chlorine alone, sonication alone, combined sonication and chlorination, and heating alone. Experiments were conducted in laboratory prepared phosphate buffered saline. The variables tested included the chlorine dose, chlorine contact time, sonication time, sonication system (probe or bath), sonication power-to-volume ratio, and sonication frequency. E. coli was enumerated by use of pour plates and/or membrane filtration before and after disinfection. Substantial temperature and turbidity increases were recorded after sonication, especially at 900 W/L. After 10 minutes of sonication at 900 W/L, the temperature and turbidity of ii the experimental solution rose up to 77oC and 23 NTU, respectively. At both 180 W/L and 900 W/L, sonication alone demonstrated little inactivation (less than 1 log10) of E. coli for temperatures below 60oC and greater than 7 log10 inactivation at temperatures over 60oC. The results from heating only experiments confirmed that temperature was responsible for the inactivation rather than other ultrasonic wave effects. Sequential application of sonication and chlorination was ineffective at inactivating E. coli. Chlorination alone achieved higher levels of E. coli inactivation than the combination of both disinfectants. When sonication and chlorination were applied simultaneously, the inactivation was greater than the additive effect of two disinfectants, indicating that there were synergistic effects between sonication and chlorination. For example, at 900 W/L, chlorination alone at 0.6 mg/L for 2 minutes provided 1.2 log10 inactivation and sonication for 2 minutes alone provided less than 1 log10 inactivation of E. coli. When the two disinfectants were applied simultaneously, 4.5 log10 was achieved. Sonication may have weakened the cell membranes, causing them to be more susceptible to chlorine disinfection. iii