Innovations

Empty Tanks

DEOXYGENATION purges ballast water of invasives—And it’s cheap

Startingthis year, the U.S. Coast Guard will require that ships entering U.S. watershave plans to rid their ballast water of invasive species or face finesof up to US$25,000 per day. The United Nations’ International MaritimeOrganization also recently adopted a policy that sets upper limits forthe number of organisms in discharged ballast water.

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

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

But that’s not why the technologywas designed. Tamburri stumbled on its value as a weapon against invasivespecies in February 2000 while attending a conference in Japan. Tamburrimet engineers who told him about a method they had developed to preventcorrosion in ballast tanks by purging tanks of oxygen. While the engineerselaborated on the fine points of steel and rust, Tamburri’s mind racedto zebra mussels (Dreissena polymorpha) and other ballast-water stowaways.

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

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

What has the group most excited isan added bonus: this method saves money. Deoxygenation dramatically reducescorrosion of steel ballast tanks, so hulls need to be painted less frequently.A cost-benefit analysis suggests the technique could reduce a cargo ship’slifetime maintenance costs by US$1 million. Of the many ballast water treatmentsbeing 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 stilltake days to expire. Tamburri hopes that adding a biocide will increasethe killing speed of deoxygenation, a theory he is now testing.

Later this year, Tamburri and hiscolleagues will place a prototype of the system, developed by Los Angeles-basedNEI Treatment Systems, on a tug barge in the Gulf of Mexico and a tankerdue to sail from Singapore to the U.S. This project is supported by a grantfrom the U.S. National Oceanic and Atmospheric Administration.