May 13, 2008

Microwaves Remove Alien Species from Ballast Water

Researchers from Louisiana State University have developed a way to destroy unwanted animals and plants that reside in the ballast waters of cargo vessels. The scientists found that a continuous microwave system removed all marine life within the water tanks.

According to the U.N., "invasive species" dispersed by ballast water discharges are one of the four main threats to the world's marine ecosystems. The agency's data shows that five billion tons of ballast water is transferred internationally each year as shipping moves more than 80% of the world's commodities. These vessels, especially large container ships, use ballast tanks for stability and to correct shifts in the ships' mass. Once a ship's cargo is unloaded, it fills with ballast water. Once it is reloaded, many times in a distant location thousands of miles away, the water is discharged.

Dorin Boldor from Louisiana State University's Agricultural Center was the study's co-author. He said the research team envisioned the microwave device being fitted to the ballast tank's exit valve.

"The basic idea is that you take the ballast water and pump it through a microwave cavity," he told BBC News, adding that the system would use the same principle as a household microwave oven.

"The power level is much higher and a different frequency, but it creates a very high intensity electric field in the centre of the cavity that oscillates rapidly.

"The water molecules are going to start spinning around very fast and they are going to create a lot of friction that generates heat," he explained.

"But it generates heat in the whole volume at the same time, unlike if you try to use another heating mechanism where you have to take the heat from somewhere else and conduct it through the liquid."

"It is extremely fast and very efficient at transferring the energy from the microwaves into heat," he told BBC News.

Historically, marine species have been transferred throughout the oceans by natural means for thousands of years, often with currents and drifting on debris. However, natural barriers such as landmasses and temperature variations limited the dispersal of some species and allowed different marine ecosystems to evolve. But these natural barriers have been broken down following the emergence of the modern shipping fleet and growing international trade, allowing the introduction of alien species that disrupt the balance of ecosystems.

The Global Ballast Water Management Program (GloBallast), led by the U.N., estimates that at least 7,000 species are carried around the world in ships' ballast tanks. While many of these plants and creatures do not survive their trip, some find the new environment habitable enough to establish a reproductive population and that can have a deleterious effect on native species.

For example, European zebra mussels (Dreissena polymorpha) have infested more than 40% of the US's inland waterways, according to GloBallast. Up to $1 billion was spent controlling the spread between 1989 and 2000.

In the Black Sea, an invasive jellyfish-like organism called Mnemiopsis leidyi led to a major ecological "regime change" that played a central role in the collapse of the region's commercial fisheries. At one point, 90% of the sea's entire biomass consisted of the species, and other organisms were unable to compete for the native plankton stocks needed to sustain or re-establish viable populations.

To address the issue, the international shipping community established a consensus in February 2004 on tougher measures to prevent discharges of ballast water releasing potentially invasive species. The International Convention for the Control and Management of Ships' Ballast Water and Sediment mandates that all vessels over 400 tons must eventually equip systems to treat ballast water.

Dr Boldor said his team's development is ideally suited to help commercial operators meet this obligation.

"It will probably work very well for it to be installed on very large ships themselves, but when you are talking about smaller vessels it may be more cost effective to have some sort of barge system based in the ports.

"It can just pull up to the ship, take and treat the ballast water while the ships are waiting to berth at the dock," he said.

The findings appear in the journal Environmental Science and Technology, and can be viewed here.