Ozonation

Ozone is an oxidizing biocide used for treatment of potable and industrial process waters. Environmental concerns regarding the use of chlorine have resulted in increased use of ozone as a biocide. Since ozone is an unstable gas that quickly decomposes to oxygen, it must be generated as needed by passing air or oxygen between a high-voltage discharge gap. Large capacity ozone generation units are not currently used for marine applications. Industrial units are relatively complex and bulky and require compressed air sources for the oxygen separator systems and cooling water for the ozone generator. Ozone generators have demonstrated good reliability in industrial applications, requiring about 10 hours of maintenance per month. However, the successful operation of these generators depends on the quality of the feed gas supply, adequate cooling, and a constant, relatively clean electrical power source. In addition, ozone systems have some serious materials problems, including increased corrosion rates of some alloys and deterioration of seals. Given the uncertainties associated with shipboard operation, the committee assigned negative ratings to ozonation for maintenance and crew impact. In salt water (brackish and sea water), ozonation produces the same residuals as chlorination.

Electric Pulse and Pulse Plasma Techniques

Electric pulse and pulse-plasma treatments have been grouped together since both inactivate organisms in a similar manner. Both methods have been successfully demonstrated in the laboratory, and prototype demonstrations are planned. Applied electrical voltages for these systems are in the 15 to 45 kV range, with pulse durations on the order of 1 µs. Although the power requirements for prototype systems are relatively modest (10 to 50 kW, large energy sources would probably be needed for systems capable of treating large volumes of ballast water. Neither the electric pulse nor the pulse plasma process produces toxic chemical residuals, but a pulse power system would generate gaseous decomposition products, notably carbon dioxide. Theoretically, pulses of electricity through seawater could generate chlorine, although none has been detected during laboratory tests of electric pulse systems.

Given the relatively immature status of both the electric pulse and pulse-plasma technologies for treating water, the costs of developing systems suitable for shipboard application are likely to be high, with resulting high equipment acquisition costs. Because both systems are automated and do not require attended operation, their technical complexity should not be a serious disadvantage if the mean time between equipment failures is long and repair can be achieved by replacement. Electric pulse and pulse-plasma systems are currently being designed for long lifetime and low maintenance. However, in the absence of supporting



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