introductions of nonindigenous aquatic species through ballast water, a vessel's master may consider not loading ballast water if the vessel is capable of operating safely and efficiently without it. Locations and times when ballast is likely to contain such organisms include:

• global "hot spots" where targeted organisms are known to be present in the water column

• ports with high sediment loads

• areas of sewage discharge or known incidence of disease

• certain sites at certain times of year, depending on seasonal variations in populations of organisms

• at night, when some planktonic organisms migrate higher up the water column

Despite concerns about nonindigenous species, ships may need to take on ballast water in port to preserve safety. The need for ballast to ensure ship safety during cargo loading and unloading imposes practical restrictions on available control options (see Chapter 2, Ballast Conditions in Port). For example, certain classes of vessels, such as bulk carriers, need to load ballast water simultaneously with the discharge of cargo. Thus, the option of treating ballast water as it is loaded may merit consideration under some circumstances.

Most large ships routinely load or discharge ballast at rates of thousands of cubic meters (metric tons) per hour. Pumping systems associated with ballast movements meters abroad ship are large, and flow velocities may be as high as 2.6 to 3 m/s. These high flows are similar to those encountered in large drinking or waste water treatment plants common in the United States. However, typical units required for these treatment plants are much larger than could be fitted on board a ship.

In addition to handling high flows and volumes, a treatment system used in conjunction with ballast loading operations would be required to remain on standby when not in use. During loading operations in port, for example, the system could be cycling on and off as the ship discharged ballast to accommodate cargo being loaded. This is not an optimum scenario for existing complex water treatment systems, which are designed to operate continuously at a predetermined load factor.

Despite these technical challenges, treating ballast water as it is loaded remains a potentially attractive option because it would obviate the need for controls en route. Thus, the operational hazards accompanying ballast change would be avoided, as would the need for onboard treatment of a sediment/water mixture (see Chapter 4).

Most ballast water is taken on board in coastal and estuarine areas that tend to contain high levels of suspended solids. In some cases, ships taking on ballast are floating very close to the bottom and can scour bottom sediment into the ballast system. As a result, ballast water often contains high concentrations of