January 9, 2002
Dealing with aquatic nuisance organisms and corrosion
As an article in the current print Marine Log makes clear, ballast water management is going to be an increasing concern for shipowners in the coming months as the Coast Guard moves from a voluntary to a compulsory implementation of the National Invasive Species Act.
Now a new study of a method pioneered by Japan's Sumitomo Heavy Ind ustries indicates that a technique for combating ballast tank corrosion may also be a cost-effective way to prevent the introduction of foreign aquatic species in coastal waters. The method is based on using nitrogen gas to remove oxygen from the ballast water.Marine ecologist Mario Tamburri of the Monterey Bay Aquarium Research Institute (MBARI) led the study reported this month in the journal Biological Conservation.
"Biological invasions of non-native species are one of the most devastating threats to native communities," said study co-author Kerstin Wasson of the Elkhorn Slough National Estuarine Research Reserve, who became involved in this project because "we urgently need practical solutions to stem the tide of aquatic invasions."
Current solutionssuch as exchange of ballast water in mid ocean, intensive filtration, heat treatments, and biocidescan be costly, dangerous to crew members and can have negative effects on the surrounding environment where the treated waters are discharged. Costly treatments in particular are unlikely to be voluntarily employed by the shipping industry. On the other hand, says MBARI, deoxygenation presents the first solution that safely and effectively removes the majority of organisms found in ballast water while also providing an economic benefit for shipowners.
"Deoxygenation was seen as too expensive for controlling invasive species in ballast water but our study shows that the anticorrosion benefit of this technique is a strong economic incentive for the shipping industry," said Tamburri. "Its a win-win treatment for solving an environmental problem and reducing ship maintenance costs."
Tamburri and Wasson summarize research by Japanese scientists and engineers at Sumitomo Heavy Industries, Ltd. which developed a deoxygenation technique to combat the costly corrosion experienced in cargo vessels. The method involves bubbling nitrogen gas into ballast water to remove oxygen, thereby preventing oxidation or rust in the tanks.
The shipping industry currently uses expensive paints, which must be maintained over the lifetime of the vesseltypically around 25 years. It is estimated that nearly $100,000 per year can be saved for each new ship that uses this deoxygenation technique rather than paint to prevent corrosion.
Aquatic organisms are also sensitive to oxygen levels, leading Tamburri and Wasson to explore the nitrogen ballast water treatment as a deterrent to non-native species introductions. They constructed laboratory experiments mimicking the shipboard deoxygenation technique and examined the oxygen tolerance of larvae from three known nuisance invasive species now found in U.S. watersan Australian tubeworm, European green crab, and European zebra mussel. The resulting low oxygen environment was toxic to essentially all of the larvae after only two to three days while major ocean crossings by cargo vessels typically take weeks.
Tamburri also reviewed the scientific literature to assess how other aquatic organisms would respond to the low oxygen environments. The reviews show that significant mortality within only hours to days would be expected for animals commonly found in ballast water. However, there are some speciessuch as those with cyst stages or anaerobic bacteriathat will likely be able to survive the conditions found in a nitrogen treated ballast tank.
"While extremely effective, deoxygenation may not be as comprehensive in removing aquatic organisms as other proposed ballast water treatments but it has the huge advantages of being environmentally benign while also saving ship owners money through rust prevention," said Tamburri. "Until international law mandates total mortality of all ballast water organisms, deoxygenation at very least deserves further consideration as a high priority treatment."