Recently, destruction of the ecological environment caused by plankton in ballast water on oceangoing ships is one of the most crucial topics for ocean environmental protection. The author has proposed a technique for water treatment using ultrasonic cavitation induced by the squeeze-film effect and has made a prototype of an ultrasonic sterilizer. This sterilizer can efficiently inactivate oosysts of
Cryptospridium purvum which is pathogenic protozoan. In this paper, the effect of this ultrasonic water treatment on
Heterocapsa circularisquama, which is phytoplankton and has sometimes caused red tide pollution, is experimentally investigated with the aim of applying to ballast water treatment. As a result, the number of killing
H. circularisquama cells per unit input energy is almost constant independently of the thickness of the squeeze-film. The efficiency of inactivation of
C. purvum oosysts are also rediscussed from the viewpoint of the number of inactivated oosysts per unit input energy.
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