Specially picked sewer workers, dressed in futuristic life-support costumes, drive miniature bull-dozers through dark and dank 11-foot concrete tubes.

In surroundings that would befit a science-fiction thriller, they battle a foul-smelling, gummy enemy that eats rubber boots and tires and attacks in the form of a deadly blue cloud.

The enemy in this unlikely scenario is a mixture of two very toxic industrial chemicals discovered in Louisville's sewer system March 29. It has proved itself a more formidable opponent than officials anticipated.

Its discovery forced the closing of the city's main sewege treatment plant seven weeks ago. Since then, about 100 million gallons of raw sewage has been dumped into the Ohio River each day.

The untreated sewage, more than 5 billion gallons of it so far, has had a profound effect on levels of dissolved oxygen in the Ohio downstream from Louisville, and could lead to a major fish kill in the already polluted water-way.

However, water-quality officials say the oxygen levels in the river are still above the minimums necessary to support aquatic life, although some life forms may be feeling "some trauma."

Initially, officials hoped the treatment plant would be reopened by the first week in May: but a series of thorny problems forced them to miss a number of target dates.

The latest one - yesterday - was missed because toxic fumes penetrated the respirators used by the workers, forcing them from the lines. Work was suspended for several days earlier this week while new equipment, with self-contained oxygen supplies, was shipped in from Pittsburgh. The new target date is June 1.

The culprit chemicals are hexachlorocyclopentadiene (HCP) and a related hydrocarbon, octachlorocyclopentine ("Octa"). It has not been determined how the chemicals found their way into the sewer system. Local and federal investigations into the mater are continuing.

While HCP and Octa have not specifically been named as potent carcinogens - cancer-causing agents - they are chemically related to such famous "lethal chemicals as endrin, chlordane, Kepone, heptachlor and Tris.

The clean-up began with a thorough scouring of the contaminated treatment plant. According to Tierney, this part of the process went fairly smoothly, and was finished on schedule weeks ago.

To clean equipment in the plant's "screen and grid" building, which Blocks particulate mater, MSD officials had to contract with a sandblasting firm for hoses capable of spewing water at a pressure of 6,400 pounds per square inch. THe HCP-Octa compound is a gummy, glue-like substance that is impervious to a normal firehose, which has a pressure of only about 450 pounds per square inch.

Tierney said MSD officials do not know whether the use of the high-pressure hoses may have damaged equipment in the facility.

According to Tierney, the estimated cost of disposing of the material taken from the plant is $4 million. He said the material will be shipped to a specially conditioned landfill in Wilsonville, III. More than 250 tons of contaminated waste has been shipped to the landfill, and about 6 million gallons remain in holding tanks at the plant.

The clean-up's next stage involves the decontamination of more than three miles of sewer line.

Most important is a 2,744-foot section leading into the treatment plant. Once this section is decontaminated, sewage can be routed through the plant without passing through other contaminated lines. Workers are now past the 2,000-foot mark: it is this phase of the project that officials hope will be completed by Wednesday.

At first, workers in the sewer lines were using scrapers that had to be pushed along like lawnmowers. But in the thick sludge this technique proved unworkable because the sludge "almost sucked the boots off their feet," according to Tierney. So they were provided miniature bulldozers, "so at least they could ride instead of walk."

Workers were disheartened to learn that the contaminated sludge has an appetite for rubber boots and tires. The rubber workboots wore away after 19 or 12 hours of contact with the material. Since the clean-up began, 36 rubber tires have blown out. MSD engineers have begun spraying silicone into the tires, a procedure that seems effective.

Yet another problem was the creation of blue clouds of toxic gas, which forced workers out of the sewer line time and again.

The chemicals, heavier than water, settled into the foot-thick layer of sludge at the bottom of the sewer line. When the sludge was disturbed, the chemicals tended to income liquid and rise to the surface. Upon reaching the surface, they became gases, which filled the sewer line with the blue haze.

When the haze was present, workmen could smell the chemical (its abrasive smell has been likened to rose food's), and could feel it irritating their eyes - even when they were wearing respirators.

The safety limit for workers with full-face respirators is 5,000 parts per billion parts of air. Levels of the substances in Louisville's sewers have reached 12,000 parts per billion at times.

Moreover, the fumes that disrupted work this week were hydrogen sulfide, which, unlike the "blue cloud" wasn't stopped by the activeated charcoal filters of the respirators. That is why the self-contained oxygen supplies were necessary.

Each of the workers has been subjected to rigorous medical screening and each received training in the use of specialized equipment required.

"Nobody to my knowledge has ever had a problem of this magnitude." Tierney said. "Consequently, there is no technology to deal with it ... We're trying to take it task by task."