between ports; more time is available for treatment; and flow rates through the treatment system may be lower. In addition, water can be recirculated until the number of organisms inactivated is sufficient to provide the appropriate level of protection. As noted earlier, ships can discharge ballast water at an extremely high rate (upwards of 20,000 m3/hr at flow velocities up to 3 m/s), with resulting drawbacks for inline treatment in terms of system capacity and associated space and power requirements. However, inline treatment during ballasting/deballasting is potentially more effective in inactivating large organisms throughout the entire sediment/water mixture than onboard treatment. In the latter case, the sediment fraction settles to the bottom, and at least a two-phase system (water column and sediment fraction) is present. Therefore, even treating ballast water at a lower flow rate for a longer time period may not easily achieve inactivation in the sediment fraction.
The committee identified 10 candidate technologies for shipboard treatment:
Filtration Systems. Filtration systems are widely used in municipal and industrial applications. Systems designs are determined by the size and type of particles to be removed. Filter systems require periodic cleaning, either manually or using automatic backflush systems.
Oxidizing and nonoxidizing biocides. Oxidizing biocides, notably chlorine and ozone, are widely used in waste-water treatment. Organic structures, such as cell membranes, are destroyed by the addition of strong oxidizers. Nonoxidizing biocides include a large inventory of compounds commonly used in industries for treating the growth of organisms in cooling tower water and other areas where large amounts of biological growth or sediment accumulation occur. Nonoxidizing biocides work in a manner somewhat analogous to pesticides by interfering with reproductive, neural, or metabolic functions of organisms, such as by inhibiting respiration.
Thermal techniques. High temperatures are commonly used to sterilize water in a wide variety of applications.
Electric pulse and pulse plasma techniques. The application of a pulsed electric field or an energy pulse to water can kill organisms. Electric pulse systems generate an electric field; pulse-plasma systems deliver a high energy pulse to an inwater arc mechanism and generate a plasma arc in water.
Ultraviolet treatment. Treating water with ultraviolet energy to inactivate bacteria is a well-established technology. Ultraviolet irradiation in the fluid at wavelengths of approximately 200 nm can destroy cellular components.
Acoustic systems. Acoustic systems use transducers to apply sound energy of specified amplitude and frequency to water to be treated. The sound energy causes cavitation, and the resulting mechanical stresses disrupt cells.