suspended material, which presents a problem for both the ship operator and the treatment system engineer. Sediment accumulates in ballast tanks in large quantities (hundreds of cubic meters) and is costly to remove and to transport. Suspended sediment also interferes with most treatment systems, whether they are physical separation processes or advanced oxidation processes. In the first case, suspended sediment tends to plug screening systems rapidly, necessitating the use of substantial bypass systems. In the second case, suspended material may shield target species from any treatment process. Thus, treatment efficacy—defined in terms of inactivation of target organisms—may be severely limited in the presence of suspended solids.

En Route

Ships that traverse large areas of open-ocean water have different options for controlling ballast water than ships that traverse inshore and coastal waters. For instance, many ships crossing a large body of open water may have the option of changing ballast water at sea. If ballast can be changed, treating ballast water on board may not be necessary. However, in some cases only a partial or incomplete change of ballast is possible, particularly in bad weather. Thus, changing ballast and treating ballast water on board are not always mutually exclusive.

Shipboard Treatment

Once ballast water has been loaded on board, the ideal mechanism for preventing subsequent introductions of nonindigenous aquatic species is to kill or remove the organisms prior to discharging ballast water overboard. This can be done by chemical, physical, mechanical, or biological treatments, or by any treatment combination. In all cases, the offending organisms or treatment residues must be dealt with in an environmentally safe way before deballasting or subsequent disposal. A detailed discussion and evaluation of possible onboard technologies for treating ballast water is provided in Chapter 4.

Ballast Change

The change of coastal and port ballast water with ocean water is an approach to the reduction of nonindigenous species inoculations.2 There are two major biological and ecological principles that provide the scientific basis for this control option. First, the probability of reciprocal introductions is virtually nonexistent. The oceanic environment is inhospitable for freshwater, estuarine, and most inshore

2  

It is accepted that ballast change serves little or no purpose for lake carriers that do not travel outside the Great Lakes waterway system, with the possible exception of controlling the spread of localized infestations.



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