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encountered, the movement of ballast on and off a ship can be highly variable (see Chapter 2). Substantial power and space limitations on ships, together with safety concerns for the vessel and its crew, make onboard treatment options complex. Space on board any ship is at a premium, and some ships are more seriously constrained with respect to space than others.
Defining effective treatment scenarios is complicated by the variability of ships, shipping routes, and ports. Lakes and waterways typically contain large quantities of minerals but have low chloride-ion concentrations. Oceans are perhaps less complicated because of their chemical consistency worldwide when they are away from the continental shelf. The pH and salinity of seawater are kept relatively constant by a complex balance of chemicals. The unpredictable mix of ocean and fresh water in estuaries, shallow tidal bays, and inlets, where major port facilities are located, tends to create a zone rich in nutrients and resulting marine life. A short list of water parameters that must be addressed by systems for treating ballast water comprises temperature, salinity, pH, dissolved oxygen, suspended solids, and biological constituents.
Ships' ballast tanks and cargo/ballast holds represent a unique challenge for treating water due to the high flow rates, large volumes, organism diversity, and ballast water residence time. It appears that with currently available technology one overall onboard treatment scenario will not be effective for all vessels carrying ballast water in and between various regions of the world. As discussed in Chapter 3, a range of preventive measures, including onboard and shore-or port-based methods for treating ballast water, will be needed to address the problem of controlling introductions of nonindigenous aquatic nuisance species. Earlier reviews of options for treating ballast water, such as Canadian Coast Guard (1992), AQIS (1993), and Carlton et al. (1995), provide more detailed discussions of these methods.
TECHNOLOGIES FOR TREATING BALLAST WATER
Shipboard treatment methods are the most flexible for managing ballast water. For this reason, the committee determined that treating ballast water on board ships will be the ultimate solution to reducing introductions of nonindigenous aquatic species. The following sections describe candidate technologies and methods for evaluating them.
Treatment technology options can be incorporated during three different phases of ballast operations: (1) during ballasting at the cargo discharge port; (2) during the voyage, between ports; and (3) during deballasting at the cargo loading port (see Figure 3-1). Each of these scenarios has significantly different constraints with respect to treatment options. In the first and third scenarios, large quantities of water must be treated in a short period of time, while water is taken on board or discharged and flow rates through the treatment system are high. In the second scenario, water resides in the ballast tanks or cargo/ballast holds