Example Species
Ballast Water Exchange
Most aquatic organisms can only withstand a narrow salinity range, and exposure to salinities outside the range can be lethal. Ballast water exchange is based on this physiological principle. Vessels pick up ballast water from coastal, river, and estuarine ports that is relatively low in salinity. When the ballast water is exchanged on the high seas, the organisms cannot survive the higher salinity environment of the open ocean.
Smithsonian Institution
Similarly, the organisms in the new open-ocean ballast water cannot survive the lower salinity environment of the destination port. Without mid-ocean BWE, organisms in low salinity water from the source port would survive when released into low salinity waters of the destination port.
Unfortunately, ballast water exchange has many weaknesses that prevent it from being an effective mechanism for preventing the introduction of nonindigenous species. Many ports have salinities that are similar to open-ocean water, so organisms do not die upon release at the destination port. If the salinities are sufficiently different, many organisms can form cysts if environmental conditions become inhospitable, which allows them to survive the change in salinities.

There are also weaknesses in the physical process of ballast water exchange. BWE cannot be performed under rough sea conditions because it decreases the ship’s stability and poses a safety threat. When BWE is performed, less than 100% of the water is exchanged as “nooks and crannies” trap pockets of water. Even if most of the water is removed, sediment and residual water remain at the bottom of the tanks. Large communities of benthic organisms may reside in the sediment. As ballast water is added and then removed, some of the sediment and organisms may be re-suspended and released.


Ballast Water Treatment (BWT)
Instead of mid-ocean ballast exchange, ship owners may treat their ballast water to kill the aquatic organisms. Researchers are exploring a variety of treatment methods. Mechanical treatments include filtration and separation (via centrifuge). Physical treatments include the application of ultraviolet (UV) light, electric currents, or heat. Other potential physical methods consist of ozone injection and deoxygenation. Chemical treatments use biocides to sterilize the water. These methods may be used in combination for greater effectiveness.

Hyde Marine
Ballast water treatments must be safe for both the crew and the environment and be cost-effective. They must be effective at removing the organisms and be able to work for large volumes of water (11) Scientists and engineers are still researching and developing these techniques. A combination of microfiltration and UV radiation appears promising. However, currently no BWT meets Coast Guard specifications for effective removal of organisms.