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Ballast water management can involve mechanical, physical, chemical, and biological processes, either on their own or in combination, to remove, render harmless, or minimize the uptake or discharge of harmful aquatic organisms and pathogens within ballast water.
Ballast water exchange, treatment or retention on board are considered to be best management practices for ballast water and accumulated sediment. Shoreside disposal is an option that is also being studied.
Mid-ocean ballast exchange that occurs 200 nautical miles from shore in waters that are at least 2,000 metres deep currently provides the best available option to reduce the risk of alien species introduction and transfer; however, it is subject to serious ship safety limits.
Under the current Canadian regulations, all ships entering Canadian waters must exchange ballast water outside of the Exclusive Economic Zone. If offshore exchange is not feasible for safety reasons such as heavy seas or storms, current Canadian regulations allow for exchange in alternate exchange zones located off the East and West Coasts, and seasonally in the Laurentian Channel of the Gulf of St. Lawrence.
The principles behind ballast water exchange are that water taken from the foreign port is flushed out, thereby reducing the concentration of alien organisms in the ballast tanks, and the high salinity level of the water (30 parts per thousand or greater) results in mortality to any remaining freshwater organisms. However, there is growing concern among scientists that freshwater organisms may be able to survive the saline environment created by mid-ocean exchange, and there are examples of species that are salinity tolerant, such as alewives and the sea lamprey that normally spend parts of their lives in salt and freshwater.
When a ship is ballasting and deballasting, the tanks are flooded or emptied by gravity and by pumping. The two most common approaches to ballast water exchange are sequential exchange and flow-through exchange:
- Sequential exchange requires ballast water to be discharged until suction is lost and stripping pumps or eductors to be used if possible. There are concerns associated with sequential exchange because the tank has to be completely emptied before refilling. During the emptying process the ship's safety and maneuverability may be compromised if the volume of ballast water is temporarily less than optimal for safe operation of the ship. Also, in a partly emptied tank, the water sloshing back and forth creates heavy impacts on the hull and bulkheads in addition to affecting stability.
- Flow-through exchange requires an entry and exit for the flow, and involves pumping through at least three times the volume of the ballast tank. Calculations indicating the amount of water utilized and pumping rates need to be recorded. Since water is simultaneously added to and discharged from the tank, the flow-through method does not greatly affect ships' stability.
Even when ballast exchange can be fully implemented, it is not totally effective in removing all alien organisms from residual ballast water and sediment that remains in ballast tanks. It has also been suggested that deballasting at sea may even contribute to the wider dispersal of alien species, and that island states located 'down-stream' of mid-ocean deballasting areas may be at particular risk from this practice. Because of this, it is extremely important that alternative, effective ballast water management and/or treatment methods be developed.
Significant research and development efforts are well underway by a number of scientific and engineering research establishments around the world, all aimed at developing a more complete solution to the problem. The only international standards at present are those of the International Maritime Organization and the technologies being developed are to those standards.
Additional examples of ballast water research and development are available in the IMO 's Ballast Water Treatment R&D . The National Oceanic and Atmospheric Administration ( NOAA ) in the United States has also funded a variety of ballast water research initiatives.
Select this link for information on those technologies that may be applied on ships in Canadian waters.
It is estimated that 90 per cent of ships entering the Great Lakes through the St. Lawrence Seaway are fully loaded with cargo and therefore do not require ballast water for stability or safe operation. These vessels declare "no ballast on board" ( NOBOB ) and therefore are not subject to comply with the standard ballast water management procedures for ballasted vessels.
Nevertheless, when these NOBOB vessels off-load their cargo they take on water to stabilize the ship. NOBOB ships cannot completely empty their ballast tanks due to structural and pumping limitations and, as a result, they can still contain residual amounts of ballast water and sediment. This water and sediment provide a potentially suitable environment for organisms in various life stages to survive and be introduced into the Great Lakes if this added water is subsequently discharged at another port.
Some organisms have a life cycle stage during which they are considered to be "resting eggs". These embryonic forms of organisms have been found to be resistant to adverse environmental conditions, making them primary candidates to survive in ballast tanks. During the time period between the initial uptake of ballast water and its discharge, organisms may have reproduced and resting eggs may have hatched. Once discharged into freshwater they can become viable.
Residual sediment and water in a ballast tank. (Photos courtesy of Philip T. Jenkins and Associates Ltd.)
Best Management Practices
A three-year survey of NOBOB vessels showed that a high percentage entering the Great Lakes system had fresh or low-salinity ballast water residuals. These vessels posed the greatest risk for new introductions due to the live freshwater-tolerant organisms found therein.
In August 2005, the United States Coast Guard introduced best management practices for NOBOB vessels, which encourages them to conduct mid-ocean ballast water exchange on ballast-laden voyages. If the vessel cannot conduct mid-ocean exchange, they are encouraged to conduct a saltwater flushing of their empty ballast tanks.
Regular and consistent flushing of ballast tanks results in a reduction of residual sediment accumulation, while providing the added benefit of raising the salinity level of the residual ballast water. However, the effectiveness of "salinity shock" in eliminating freshwater-tolerant organisms varies widely and therefore must be viewed with some caution while this approach receives additional evaluation.
The United States Coast Guard and Transport Canada have an ongoing sampling program of residual water and sediment from the ballast tanks of NOBOB vessels in order to determine the success of this initiative in reducing or preventing the introduction of alien invasive species to the Great Lakes.
More information about NOBOBs and the Great Lakes is available in the report Assessment of Transoceanic NOBOB .