Abstract
It has been shown in previous studies that the use of ultrasound in the disinfection stage of water treatment using chlorine may help to alleviate some of the problems inherent in this process, e.g. the formation of harmful biproducts by the reaction of chlorine with chemicals in the water, and to counter the build up of chlorine resistance by bacteria. Ultrasound when used in the absence of a biocide is capable of either killing bacteria with high power at low frequencies or de-clumping bacteria with low power at high frequencies. This study explores the effects of ultrasonic treatment prior to chlorination (pre-treatment) and simultaneous treatment at varying frequencies (20kHz, 38kHz, 512kHz, 850kHz, and 1.2MHz) and ultrasonic times on the efficiency of sodium hypochlorite solution as a biocide towards Escherchia coli suspensions.It has also been shown that ultrasound alone is capable of inactivating bacteria at low frequencies and high powers with longer sonication times and also is able to de-agglomerate micro-organisms at short ultrasonic times with low powers. Ultrasound alone is not sufficient to act as a disinfectant but combined with other disinfection agents has been shown to be beneficial as demonstrated in previous investigations. Therefore the advantageous effects of ultrasound on sodium hypochlorite as a disinfectant is investigated in this study by using ultrasonic treatment prior to chlorination (pre-treatment) and simultaneous treatment at varying frequencies (20kHz, 38kHz, 512kHz, 850kHz, and 1.2MHz) and ultrasonic times towards Escherchia coli suspensions and monitoring the effects by viable plate counts.
For pre-treatment a remarkable frequency effect has been noted in that an improvement in biocide efficiency occurs with increasing frequency up to 850kHz. It was found that pre-treatment at 20kHz, 23.16W resulted in an adverse affect on the inactivation of the bacteria compared with chlorine alone by up to -1.7 logs. On the other hand pre-treatment at 850kHz, 1.34W was found to improve inactivation by up to 2.11 logs. These effects are thought to be due to the influence of ultrasonic agitation on the de-aggregation of bacteria before the addition of the biocide.
For simultaneous treatment a frequency effect was also observed but in this case the efficiency of the biocide decreased with increasing frequency. Thus simultaneous treatment at 20kHz, 23.16W, improved the inactivation of the biocide by up to 1.77 logs whereas at 850kHz, 1.34W simultaneous treatment has an adverse effect on the inactivation rate of the biocide by up to 1.7 logs. The positive effects at these frequencies is thought to occur due to the ultrasonic agitation producing a temporary permeability of the cell membrane thereby increasing the amount of biocide reaching target sites for inactivation of bacteria.
The presence of suspended solids in ultrasonic experiments was also investigated and was found to hinder the inactivation rate of the biocide both during pre and simultaneous treatment. Pre-treatment at 20kHz, 23.16W, with glass beads (212-300μm) resulted in a reduced effect on the inactivation of up to -2.11 logs compared with the control in the absence of glass beads of -1.7 logs. With simultaneous treatment under the same conditions the adverse effect was -0.4 logs compared with the control of 1.7 logs. In both cases it is thought to be due to the beads protecting the micro-organisms.
In summary ultrasound has been shown to improve the efficiency of the biocide tested, depending upon the conditions. This investigation demonstrates that, using the five different laboratory ultrasound sources available, the optimum conditions for ultrasonically assisted disinfection using 1ppm chlorine were 20kHz, 23.16W at 1 minute for simultaneous treatment and 850kHz, 1.34W at 1 minute for pre-treatment.
| Date of Award | 2006 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Timothy Mason (Supervisor) & Larysa Paniwnyk (Supervisor) |