Since both electric pulse and pulse plasma treatment technologies are relatively immature, the above cost estimates are clearly very approximate.

Size, Cost, and Complexity

An electric pulse treatment system that meets System A requirements would be approximately 2.5 m wide, 1.5 m high, and 1.0 m deep, and weigh about 1.5 tons, mainly due to the transformers. Such a system would be designed for long lifetime (more than 105 hours), and no maintenance would be required over this period. Since the system would be a push-button device, no training in its use would be needed.

The gross volume of a pulse plasma treatment unit for System A would be between 4 and 6 m3, with a weight of about 4,500 kg, including safety screening and controls. Such a system would be self-monitoring with automated shutdown and out-of-service alarms, and would not require attended operation. Maintenance could be scheduled while a vessel was in port, but no information is available on the mean time between failures. One or 2 days of training would be needed to acquaint engine room personnel with the device and its safety provisions.

Monitoring

Both systems monitor energy flow, but additional tests (and equipment) would be needed to measure the mortality rates of specific organisms.

ACOUSTIC TECHNIQUES

Ultrasound in the appropriate frequency and power ranges destroys microorganisms in liquids by means of localized mechanical stresses resulting from cavitation. Ultrasonic treatment systems use transducers to generate alternating compressions and rarefactions in the liquid to be treated. The resulting cavitation is influenced by frequency, power density, time of exposure, and the physical and chemical properties of the liquid. Optimum frequencies for destroying microorganisms are reported to be in the lower range of ultrasonic frequencies, from 15 to 100 kHz. The application of ultrasound treatment to large volumes of liquid has given variable results. Treatment effectiveness decreases with increasing distance from the transducer as the energy density in the liquid decreases. The efficacy of ultrasonic treatment increases with exposure time and can also be influenced by resonance effects due to container geometry.

Safety

Any shipboard ultrasonic treatment system would be fully sound-insulated and shielded, even though the operating frequencies are not considered harmful.



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