A study for the Canadian Coast Guard (1992) concluded that the physical removal of organisms by filtering may be an effective stand-alone treatment process or may be used in conjunction with other technologies, such as chemical oxidation or ultraviolet sterilization. Space constraints for media filtration units were identified as an issue, as were high flow rates during ballast uptake and discharge. A recirculation type treatment system was also considered in the Canadian study. The reader is referred to the original report for cost estimates and technical details of systems with different pumping rates and strainer slot sizes.

In another recent study of ways to prevent organism intake in ballast water by filtration (Carlton et al., 1995), a two-stage system using woven mesh screen filters is considered. The first filter would remove most of the larger zooplankton; the second filter would remove most of the smaller zooplankton and most of the large and medium-sized phytoplankton. The relative merits of woven mesh and wedgewire filters and discussed, as are space requirements, mesh size, and order of magnitude costs.

Biocides

Perhaps the most compelling reason to consider biocidal treatment is its ease of application. Biocides could be added to ballast water by metering concentrated solid chemicals or they could be generated electrolytically from sea water. These two methods of applying biocides are currently used on board ships, although not for treating ballast water. Alternatively, simple chemical injection pumps, feeding on line with the main ballast pumps, could routinely add a measured amount of premixed liquid biocide during any ballasting operation. The turbulence within the pumping system would ensure complete mixing of the biocide with the sediment and water column, resulting in efficient inactivation of target organisms. Chemical feed systems would require small amounts of power to pump concentrated bulk solutions to the ballast system. Electrolytic generation systems would require significant amounts of power, typically in excess of 200 kW. In addition, in-situ generation requires large and expensive equipment costing $400,000 to $800,000.

Biocides are among the most widely used industrial chemicals, and there is a large body of scientific data on their use in waste-water treatment. Effective biocide concentrations are typically in the range 1 to 5 mg/l (ppm). Dose levels and contact times need to be determined for effective treatment of aquatic nuisance species. However, if doses are similar to those used for waste-water treatment, only 5 m3 of biocide would be required for each million cubic meters of ballast water to give a 5 mg/l biocide concentration. Therefore, even large ships carrying thousands of cubic meters of ballast would be required to carry only a few cubic meters of biocide per voyage.

The effectiveness of many biocides is quite simple to monitor: others would require biological indicators to ascertain their level of effectiveness. There is no



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