Ambient water temperature. The heat input needed for thermal treatment can be reduced significantly where water temperatures are at tropical or summer levels (30°C or higher). The heat loss due to ambient water temperatures outside the hull must be considered. Due to low ambient water temperature, ballast tanks located along the hull shell may have a higher total heat input than those adjacent to cargo.
Specificity to target organism(s). Higher organisms, such as fish, are more easily killed by thermal treatment than are microbes. Studies will be required to determine the effectiveness of heat treatment to specific target organisms.
Further discussion of thermal treatment is provided in Appendix F.
The promising technology options described above—filtration, the addition of biocides (excluding ozone), and thermal treatment—are currently the most attractive for shipboard demonstration. However, the selection of a technology for demonstration will require an analysis of voyage features such as trading patterns and ports of call. For example, thermal treatment is unlikely to be chosen for short voyages because of the time needed to heat and treat ballast water. The results of the committee's assessment indicate that filtration with state-of-the-art strainers is likely to be the most widely applicable option for treating ballast water on board ships. Shipboard demonstration would provide an opportunity to resolve engineering issues associated with the installation and operation of a strainer system on an existing vessel. Regardless of which technology is chosen for demonstration, a control tank will be needed to establish a baseline against which to compare the results of ballast water treatment. Ideally the treatment and control tanks should be of similar configuration. In addition, it will be necessary to incorporate appropriate sampling and monitoring procedures to determine the effects of treatment (see Chapter 5).
The committee determined that ozonation and electric pulse and pulse plasma technologies are potentially safe and effective methods for treating ballast water, although their current status renders them unsuitable for immediate shipboard applications. Nonetheless, limited applications may be possible in the future. The transition of ozone treatment systems from industrial to shipboard applications is a major challenge. Further development of electric pulse and pulse plasma systems for land-based applications should provide more information regarding the suitability of these technologies for shipboard use.