ENGINEERING CHALLENGES FOR CONTROL OF AQUATIC NUISANCE SPECIES IN SHIPPING BALLAST WATER

摘要

Aquatic nuisance species (ANS) transported by ballast water or sheltered in ballast tank sediments, also referred to as nonindigenous aquatic species, are organisms that enter a body of water or aquatic ecosystem outside of their historic or native range. The transport of ballast water in ships is now recognized as the primary vector for the movement of aquatic organisms within oceans, between oceans and between coastal and inland waters. These species include microorganisms (protozoa, dinoflagellates and cholera), planktonic species, plants, insects, other arthropods, worms, mollusks and vertebrates. Size ranges of these organisms range from approximately 1-μm to a thousand-μm or larger, although the greatest number of organisms are found in the 1 to 100-μm range in size. These organisms can be found suspended in the water column of the ballast tank or in the sediments at the bottom of these tanks. Sediments deposited in ballast tanks are also of this size range (1 to 1000-μm) and represent a significant habitat for many ballast tank organisms and biofilms. There are three objectives of this paper. The first objective is to provide an introduction as to the challenges faced when considering the concept of engineering controls for shipping ballast water treatment. For example, the United States receives 21 billion gallons of ballast water per year from 35,000 vessels, with a significant number of these vessels handled by ports in the Northern Gulf of Mexico. The second objective is to summarize the primary forms of treatment that are, or may offer potential, for vessel or shore-based treatment. A review of ballast water exchange (BWE) indicates that while BWE may be considered as the standard of comparison, more development and examination of the methodology is required. It is argued that some form of separation, followed by an inactivation or disinfection process will provide a viable and flexible unit operation/process design for control of ANS. The third objective is to examine a separation-based unit operation, including dissolved air flotation (DAF) for the size-based separation of particles/flocs in a 1 to 250-μm range. DAF results were demonstrated at a laboratory test facility utilizing reproducible ballast water. Both freshwater (< 1-ppt salinity) and saltwater (> 20-ppt salinity) ballast surrogates were synthesized from aquaculture systems, standardized, and utilized in the test facility. Results demonstrated the potential of DAF as a competitive and effective size-based separation technology. Results from size-based laser diffraction analyses indicate DAF has a weighted effectiveness across the entire gradation of 97% for saltwater while DAF is 81% for freshwater.