The ballast operation is essential to maintain stability

The water is taken on board vessels to balance them when empty. When the ship loads its cargo, the ballast is gradually released so that weight stays constant and the vessel does not list.

However, this water can contain thousands of tiny marine creatures and plants, and while harmless in their port of origin they may be potentially disruptive invaders at their destination.

Now, the Queensland researchers are developing a filter system to screen out the aliens.

"At present, the generally approved method of ballasting is to pump water into a ship from the port where it unloads cargo," Steve Hillman, of James Cook University, explained to BBC World Service's Science In Action programme.

"It then travels to take cargo on board, and then discharges that ballast - in that water can be all kinds of organisms that are foreign to the destination port.

"These can turn into pests - and indeed have done in many ports throughout the world."

The invasive species taken into ballast tanks can include all manner of life - from fish, crabs and snails to seaweeds, algae and bacteria.

Truck size

Dr Hillman said the problem of controlling the pests that had established themselves in foreign ports was "in the vicinity of $10-20bn a year".

The solution, his team believes, is to filter the ballast before it goes on to the ships.

"We would anticipate that this kind of equipment would be on the ship and would filter the material so that what was in that port remains in that port," he explained.

After extensive work on small-scale lab models, a prototype - which occupies the back of a large truck - has now been built.

"What we're trying to do is simulate as closely as possible the actual conditions that would be encountered at a port, so we can fine-tune each step in our process," explained co-researcher Phil Schneider.

"Ultimately [we will] go to a variety of different ports across Australia and test our equipment and our processes under real world conditions."

Monkey barrier

Dr Schneider explained why the team felt its work was important.

"A lot of the organisms that we would really like to target are toxic either to humans or to other fish," he said.

"We pick things like Artemia - also known as sea monkeys - that display certain life phases that we can test for in the operation of our ballast water treatment system."

The sea monkeys - tiny shrimps - are put into giant tanks at around 100 organisms per litre to test the effectiveness of the filters.

A self-cleaning filter element, protected by wire meshing, collects the pests as the water passes through it.

The detritus is then deposited back in the water at the point of origin. The idea is ultimately to have such a system on ships themselves.

"With the sea monkeys we've had some quite dramatic results - effectively we're killing everything that passes through," Dr Schneider said.