Stemming the Tide Controlling Introductions of Nonindigenous Species by Ships' Ballast Water (1996) / Chapter Skim
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4 SHIPBOARD TREATMENT OPTIONS
4 SHIPBOARD TREATMENT OPTIONS
Pages 52-73
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From page 52... ...
The basis for many of the candidate technologies originates in the wastewater treatment area, although the uniqueness of shipboard ballast applications was a major factor in the committee's assessment. A discussion of treatment issues and options is followed by a listing of candidate shipboard treatment technologies.
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From page 53... ...
TECHNOLOGIES FOR TREATING BALLAST WATER Shipboard treatment methods are the most flexible for managing ballast water. For this reason, the committee determined that treating ballast water on board ships will be the ultimate solution to reducing introductions of nonindigenous aquatic species.
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From page 54... ...
Oxidizing and nonoxidizing biocides. Oxidizing biocides, notably chlorine and ozone, are widely used in waste-water treatment.
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From page 55... ...
COMMITTEE'S EVALUATION METHODOLOGY Once the treatment problem was defined, the committee developed a list of candidate technologies, drawing in part on wastewater treatment methods. A treatment options query developed by the committee combined a list of technology requirements and capabilities with shipboard application considerations.
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From page 56... ...
For any candidate technology, the effectiveness in destroying potential aquatic nuisance species for the volumes of water and flow rates required for shipboard ballast water treatment, and the associated operational safety for the crew and ship, are of primary importance. The committee's query about options for treating ballast water defines two ballasting scenarios: System A flow rate of 2,000 m3/h and tank volumes of up to 25,000 m3, with residence times as short as 24 hours; and System B flow rate of 20,000 m3/h and tank volumes of up to 25,000 m3, with residence times as short as 24 hours.
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From page 57... ...
Power on board ship is limited, and providing more power capacity to treat ballast water may not be practical. Chemical residuals may be created when treating fresh water and sea water.
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From page 58... ...
The committee grouped the candidate technologies for treating ballast water into three major categories: (1) promising options, (2)
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From page 60... ...
and thermal treatment, to be relatively mature technologies with the potential for adaptation to treating ballast water on board ships. Filtration Systems The physical separation and removal of organisms above a certain size from ballast water could be achieved during ballasting operations using a shipboard filtration system.
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From page 61... ...
New wedgewire filtration systems have high flow rates and are cleaned by scraping, rather than backwashing, which eliminates the need for storing and treating backwash water and thereby reduces the overall size of the filtration unit. Another way to circumvent the problems of simple screening systems described above is to use deep media filters.
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From page 62... ...
62 STEMMING THE TIDE ~ Vee-Shaped Strained Clean \ ~ \ ProfileWire renting ail Collector Bars RPvPre. -- ~ous Flou At_ Screen Element _ :~ ~ ...~....
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From page 63... ...
Thus, discharge of treated ballast water could occur at any time in any location without increasing the risk of introducing nonindigenous aquatic nuisance species. Further investigation of this option for treating ballast water is needed to determine the relationships between achievable flow rates and selected media for effective organism removal.
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From page 64... ...
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.
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From page 65... ...
Its specific application for ballast water will require data on the required concentrations and contact times for different categories of organismsranging from bacteria and viruses to adult crustaceans and fish and relative to water flow rates and/or tank volume and water residence time. Electrolytic chlorination creates active chlorine by passing an electrical current through water containing chloride ions.
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From page 66... ...
The use of chlorine gas would be inappropriate for enclosed spaces such as on board ship, but calcium hypochlorite or sodium hypochlorite could be used for treating ballast water. It is well known that the addition of a powerful oxidant such as chlorine to seawater can generate toxic compounds (byproducts)
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From page 67... ...
In addition, it will be necessary to incorporate appropriate sampling and monitoring procedures to determine the effects of treatment (see Chapter 5~. OPTIONS WITH POSSIBLE LIMITED APPLICATIONS The committee determined that ozonation and electric pulse and pulse plasma technologies are potentially safe and effective methods for treating ballast water, although their current status renders them unsuitable for immediate shipboard applications.
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From page 68... ...
Neither the electric pulse nor the pulse plasma process produces toxic chemical residuals, but a pulse power system would generate gaseous decomposition products, notably carbon dioxide. Theoretically, pulses of electricity through seawater could generate chlorine, although none has been detected during laboratory tests of electric pulse systems.
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From page 69... ...
Further discussion of electric pulse and pulse plasma systems is provided in Appendix F OTHER OPTIONS The committee judged four candidate treatments ultraviolet, acoustic, magnetic, and deoxygenation to be inappropriate for shipboard treatment of ballast water since these methods have not been proven effective for a broad range of freshwater and marine organisms.
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From page 70... ...
Anti-fouling Coatings The committee did not consider anti-fouling coatings to be suitable for controlling introductions of nonindigenous species because the surfaces of ballast tanks and the organisms that settle on these surfaces represent only a small fraction of the total problem. Ballast water is a suspension of organisms, and the bulk fluid must be treated to prevent introductions of nonindigenous species by discharged ballast.
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From page 71... ...
Furthermore, if ships heated their ballast water during the initial days of a voyage, the water would probably have cooled by the time it was discharged; therefore it would have even less environmental impact. Perhaps the most environmentally benign discharge from a system for treating ballast water consists of suspended solids washed from filters.
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From page 72... ...
These issues are discussed in the following chapter. SUMMARY Based on the comm~ttee's evaluation of a wide variety of technologies for treating ballast water, the committee determined that there are no off-the-shelf technologies currently available suitable for use on board ship without some redesign and modification.
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From page 73... ...
1993. Shipping Ballast Water Heating as a Means of Destroying Potentially Harmful Marine Organisms.
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