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Ozone, used to sterilize fancy spas, has been bubbled through four miles of stainless steel pipes in the bowels of the Alaska oil tanker Tonsina in tests. It appeared to kill most bacteria and tiny animals in ballast water, according to a scientific report presented Friday to the Prince William Sound Regional Citizens' Advisory Council.

Tankers carry water in their tanks to maintain stability during empty runs from the West Coast to Alaska. All sorts of sea life, from bacteria to minnows and crabs, hitch rides. During about 550 voyages per year, tankers dump an estimated 17 million metric tons of ballast water into Valdez Arm.

"It gets into the rivers and burrows into the banks," Leland said afterward. "It's a huge mess in the Sacramento River."

Exchanging ballast water in the open sea, one method to avert alien transplants, does not always work, so shippers and regulators have been looking for other solutions, including filters, heat and ultraviolet light.

Last week federal lawmakers introduced bills into the House and Senate that would spend $160 million annually to fight, study and monitor invasive species. Among other things, the National Aquatic Invasive Species Act would eventually require all ships to eliminate at least 95 percent of the critters in the ballast water.

In anticipation of the law, British Petroleum and ozone specialist Nutech 03 installed the world's largest shipboard ozone generator aboard the Tonsina in 2000 and launched experiments since reviewed by a panel of 14 scientists.

Ozone is an unstable form of oxygen. It breaks down into deadly chemicals upon contact with sea water, but becomes harmless within seconds, according to the tests.

The Tonsina eliminated about 64 percent of bacteria and plankton and other creatures when replacing its ballast water in the open sea. But in three tests, ozone zapped 99.9 percent of the bacteria and up to 99 percent of the zooplankton.