Ultrasonic Destruction of Biological Contaminants in Water

In order to achieve complete destruction of biological contaminants in water through sonication very high ultrasonic intensities are necessary. Unfortunately this makes the technique expensive to use for general microbiological decontamination. However over the last two decades some conventional disinfection techniques involving chemicals, ultraviolet light and heat treatment have become less effective as some bacteria become more resistant. Such processes have become a focus for the use of sonication as an adjunct to other techniques. 

Low power ultrasound offers the possibility of enhancing the effects of chlorine. The results of a study of the combined effect of low power ultrasound and chlorination on the bacterial population of raw stream water are shown (Tab. 4.2). Neither chlorination alone nor sonication alone was able to completely destroy the bacteria present. When sonication is combined with chlorination however the biocidal action is significantly improved [10]. The effect can be ascribed partly to the break-up and dispersion of bacterial clumps and floes which render the individual bacteria more susceptible to chemical attack. In addition cavitation induced damage to bacterial cell walls will allow easier penetration of the biocide. 

A continuing problem in water treatment is the occurrence of algal blooms. Algae may be killed relatively easily on exposure to ultrasound and a lightly polluted system 

Conditions 1 10 dilution of raw stream water, using ultrasonic bath (power input to system = 0.6 W cm T = 20 C. 

Similar results were obtained in a study of the combined effect of ultrasound (20 kHz) and heat treatment on the survival of two strains of Bacillus subtilis in distilled water, glycerol and milk [17]. When spores, suspended in water or milk, were subjected to ultrasonic waves before heat treatment little or no decrease of the heat resistance was observed. However when heat and ultrasound were applied simultaneously the heat treatment times in milk were reduced by 74% for B. subtilis var, niger-40 and by 63 % for B. subtilis var, ATCC 6051 and similar results were obtained in glycerol. Thermosonication in water was more marked reducing the heat resistance of the spores by up to 99.9 % in the 70 - 95 °C range. The effect of thermosonication was slightly diminished to 75 % as the temperature reached the boiling point of water. 

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