Using bacteria to get sulphur out of oil.

That infamous bad-egg fetid scent is sulfur's hallmark. it is also a major problem for petroleum refiners, who by law must drive their fossil fuels through lengthy and costly catalytic steps to expunge sulfur from their best oil and gasoline blends.

Burning sulfurous fossil fuels causes air and water pollution, including damage to the environment from sulfur oxide-laden acid rain.

To rid petroleum products of noxious sulfur, most refineries use a method called hydrodesulfurization. At high temperatures and pressures, this technique forces petroleum compounds together with hydrogen gas and metal catalysts, yielding a disposable sulfur by-product.

Yet this method has a problem: It does a poor job of breaking down heavy sulfur compounds that can damage a fuel by lowering its octane level.

Attacking this removal problem from another angle, Steven W. Johnson, a chemical engineer at Energy Biosystems Corp. in The Woodlands, Texas, describes a new method using bacteria to remove sulfur from petroleum products. At the American Chemical Society meeting in Washington, D.C., this week, he explained that the bacterium Rhodococcous erythropolis has proved both effective and efficient at removing sulfur from hydrocarbon mixtures without harming the fuel itself.

"Microbes are great catalysts because their actions are very specific," says Johnson. "The beauty of this organism is that it directly attacks the bonds between sulfur atoms and hydrocarbons, snipping the sulfur out without damaging the fuel molecule. It replaces sulfur with a benign oxygen atom, which yields nonpolluting carbon dioxide and water when the fuel is burned.

"The organism uses three enzymes to take dibenzothiophene through a series of oxidation steps, converting sulfur to an inorganic form," Johnson adds. "It's an enzymatic, catalytic process, though the organism doesn't secrete any enzymes. Instead, the enzymes remain intact within each microbe, which acts as a carrier of the catalyst. To drive the reaction, the organism consumes a small portion of the fuel for energy."

Scientists at the Institute of Gas Technology in Des Plaines, Ill., originally identified the bacterium, called IGTS8, in 1989. Since then, research teams have studied its catalytic capacities - primarily its tendency to convert dibenzothiophene into a hydrocarbon without sulfur and a benign sulfur compound. The sulfate residue, says Johnson, can be readily made into ammonium sulfate, a widely used fertilizer.

A pilot plant demonstrating this new bio-desulfurization system will begin operating in early 1995 at Energy Biosystems, Johnson says. Initially the plant will produce only one to five barrels of desulfurized oil a day. Ultimately, the goal is to scale up from what Johnson calls "a shake-flask process to 40,000 barrels a day."