Conclusion 8. The most promising technologies for successfully treating ballast water on board ships appear to be physical separation techniques. Filtration could remove all materials, down to a predetermined size, during uptake, thus removing many unwanted organisms from ballast water. Using filtration is applicable to most ships, regardless of the characteristics of the water being taken up for ballast. Continuous backwash filtration is the most favorable candidate technology for use on board ships based on environmental, health, safety, effectiveness, and cost considerations. The size of current media filtration systems precludes their use on board ship, but their performance and design could be optimized for compatibility with ships' operations, for example, by reducing their size and providing the capability to handle high flow rates.

Conclusion 9.  Chemical and thermal technologies are potenially acceptable alternatives to filtration for treating ballast water on board ships. Further investigation is needed to establish, with more confidence, the relationship between biocidal dose or temperature and the time needed to kill or inactivate unwanted organisms. Development work is also required to optimize system designs for shipboard implementation.

Conclusion 10. Some emerging electro-treatment technologies have shown encouraging results in laboratory tests. However, all technologies investigated—aside from filtration, chemical and thermal treatments—appear to have major limitations and, therefore, require extensive development for possible onboard application. The transition from laboratory scale to shipboard systems carries high technical risk, and development may be extremely costly.

Conclusion 11. Ultraviolet, acoustic, magnetic, and oxygen-deprivation technologies for treating water have been successful in selected land-based applications, but these technologies have not demonstrated their effectiveness in treating a wide range of aquatic species, especially on board ships. Therefore, they are not considered to be prime candidates for treating ballast on board ships when they are used alone.

Conclusion 12. Certain treatment approaches would cause byproducts or residuals to be discharged overboard. As a general practice, discharges to the environment (e.g., chemicals filtrates, heated water) resulting from treating ballast water should be minimized. The trade-offs between the risk of introducing nonindigenous aquatic nuisance species and the risk of releasing small quantities of residuals produced from treating ballast water require further evaluation.

Conclusion 13. Questions remain regarding appropriate levels of risk of introducing unwanted organisms: What level of reduction of biological activity is necessary? What standard can be used to declare that a particular treatment has

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