Empty Tanks

Deoxygenation purges ballast water of invasives—and it’s cheap

By Melissa Hendricks

Starting this year, the U.S. Coast Guard will require that ships entering U.S. waters have plans to rid their ballast water of invasive species or face fines of up to US$25,000 per day. The United Nations’ International Maritime Organization also recently adopted a policy that sets upper limits for the number of organisms in discharged ballast water.

But this will be no easy feat. An estimated 10,000 different species are transported in ballast water everyday. Studies have shown that millions of bacteria and virus particles may reside in just one gallon of ballast water. Scientists have been searching for new treatments for many years, yet none is without drawbacks. The conventional method of exchanging ballast water on the high seas destabilizes some ships and is impractical for short voyages and coastal journeys. Chlorination yields chlorine waste, and ultraviolet light may not work in turbid water.

Now, Mario Tamburri and a team of engineers at the University of Maryland’s Center for Environmental Science may have a better solution. They are testing a way to remove almost all of the water’s dissolved oxygen by bubbling nitrogen into ballast tanks. In dockside trials and laboratory tests, they’ve shown that deoxygenation kills most organisms found in ballast water.

But that’s not why the technology was designed. Tamburri stumbled on its value as a weapon against invasive species in February 2000 while attending a conference in Japan. Tamburri met engineers who told him about a method they had developed to prevent corrosion in ballast tanks by purging tanks of oxygen. While the engineers elaborated on the fine points of steel and rust, Tamburri’s mind raced to zebra mussels (Dreissena polymorpha) and other ballast-water stowaways.

He wasn’t the first to conceive of the idea, however. In 1996, the U.S. National Research Council weighed the merits of deoxygenation against other candidate ballast-water treatments but rejected it. “They concluded that deoxygenation doesn’t kill everything,” says Tamburri. Most worrisome were facultative anaerobes, such as Escherichia coli and Vibrio cholera.

Tamburri’s prototype solves this problem by adding a small amount of carbon dioxide to the water to lower the pH. Pilot tests indicate that E. coli, V. cholera, and Enterococcus succumb to this rapid pH change. Tamburri reported the results in May at the Second International Conference on Ballast Water Management in Singapore.

What has the group most excited is an added bonus: this method saves money. Deoxygenation dramatically reduces corrosion of steel ballast tanks, so hulls need to be painted less frequently. A cost-benefit analysis suggests the technique could reduce a cargo ship’s lifetime maintenance costs by US$1 million. Of the many ballast water treatments being explored, deoxygenation is the only one with this economic benefit, says Tamburri.

However, the system is not perfect. Although pH and oxygen levels drop within seconds, some organisms still take days to expire. Tamburri hopes that adding a biocide will increase the killing speed of deoxygenation, a theory he is now testing.

Later this year, Tamburri and his colleagues will place a prototype of the system, developed by Los Angeles-based NEI Treatment Systems, on a tug barge in the Gulf of Mexico and a tanker due to sail from Singapore to the U.S. This project is supported by a grant from the U.S. National Oceanic and Atmospheric Administration.

Photo courtesy of Australian Quarantine and Inspection Service