Years ago, the story has it, pilots used to know that they were approaching Pittsburgh's airport from the heavy black plume above the smokestacks at Duquesne Light Co.'s generating plant on Brunot Island.

When they saw the smoke, they knew it was time to turn for the airport.

Today, the navigational aids - both in the cockpit and at the airport - are much improved. And that is for the good because pilots can no longer rely on Duquesne Light for guidance.

Brunot Island no longer throws much soot into the Pittsburgh air. Once a heavy coal-burning plant that supplied a large portion of the Pittsburgh area's electric needs, Brunot Island today burns expensive, but relatively clean, heating oil to supply this city's electric needs during periods of peak demands. Other times, the plant doesn't operate.

But here, in the middle of one of the biggest coal-mining regions in the United States, Duquesne Light could not transform all of its coal-fired generators to oil. While it did shut down some very old coal-burning plants as well as convert Brunot Island into a peak-load facility, the company could not afford to close all its old coal plants and build enough new ones to supply the electricity needed here.

When Congress passed its clean air laws in 1970, industries and local environmental control agencies across the country were put on notice that they would have to take serious, and often expensive, steps to clean up the dust and noxious gases that their industrial processes were putting into the atmosphere.

And in Pittsburgh, heavily industrialized as it is and trapped in a river basin by the Allegheny Mountains, the clean-up probably has been as difficult as for any city in the country.

The Carter administration is becoming concerned about how much environmental regulations contribute to inflation and is examining whether there may be more efficient ways to reach the same air and water quality goals without interfering with the industrial process or adding to costs as much as federal mandates have in the past.

Although Pittsburgh is no longer the nation's most important steel producer (the Chicago area has been for several years), steel mills - many of them old - are everywhere and still the worst contributor to Pittsburgh's air quality problem. But coal-burning utilities run a close second.

Pittsburgh's atmposphere is far from pristine, even today, notes Ronald Chleboski, deputy director of Allegheny County's Health Department, but the improvement in recent years has been marked. Both dust and sulpher dioxide emissions have been reduced by more than half since 1971. In 1974 Pittsburgh had 21 "episodes" in which pollution was so bad that industry was required to curtail output. "In the last 12 months we've had only two," Chleboski said.

Nonetheless, Allegheny County, remains a "non-attainment area" in the jargon of the federal Environmental Protection Agency. That means, among other things, that new industrial sites cannot open up in Pittsburgh unless a plant producing a like amount of pollution is shut down. At several of the county's monitoring stations there is more dust in the air than 75 micrograms per cubic meter that the federal regulations call acceptable. Two monitoring stations register more sulphur dioxide than is considered ambient.

Duquesne Light Co., which serves about 537,000 customers, was the first major company in the Pittsburgh area "to come forth with a total program for all their facilities," Chleboski said. At other companies we "often had to go after sources [of pollution] on a one-at-a-time basis."

The Duquesne plan included shutting down an old coal-burning facility, converting from coal to oil at its plant that supplies steam to downtown office buildings as well as shifting Brunot Island to a peak-load oil-burning facility.

But the company still needed the power it generated from two older coal-burning plants - Elrama and Phillips. At the same time, under the law (prescribed for new equipment by the federal EPA but adapted by Pennsylvania authorities to apply to already existing facilities) the utility was required to sharply reduce the amount of sulphur dioxide and particulates (dust) it was putting into the air at these two stations.

When the plants were built decades ago, they were equipped with so-called mechanical dust collectors and electric precipitators that in the case of Phillips collected between 85 and 90 percent of the dust that was created by burning coal to make steam to power turbines to make electricity.

At neither plant was there any equipment to remove the sulphur dioxide gas emitted by the burning coal (in fact, no technology existed that would remove sulphur dioxide when the facilities were built).

But in 1973, at a cost of more than $50 million, the Phillips station was retrofitted (jargon for putting new devices on an old facility) with a system of gigantic cone-shaped scrubbers to wash pollutants out of the emissions. Phillips also got a high smoke-stack to dissipate in the upper atmosphere what the company could not remove with its scrubbers, as well as an elaborate holding tank-disposal system to get rid of the wet sludge created by the scrubbers.

Today, the Phillips plant (like its Elrama cousin where another $50 million was spent) catches 99 to 99.5 percent of the particulates (most of the time) and 83 percent of the sulphur dioxide, according to Steve L. Pernick Jr., manager of environmental affairs for Duquesne Light.

But it has been a costly clean-up that goes far beyond the $105 million Duquesne estimates it spent on Elrama and Phillips or the $300 million in total it has shelled out for pollution abatement on all its old and new plants, notes Pennsylvania public utility commissioner Helen O'Bannon.

"The operating costs are enormous - whether it's the high price of low-sulphur coal, the disposal problems, or the maintenance of the facilities. We've found out this stuff is tricky. It doesn't always work the way you think it's going to."

The acid in the scrubbers often corrode the stell interior. Scrubbers clog up with sludge. The fans that move the dirty air have had their problems. Even the smokestack fell prey to acid attacks and had to be reworked.

O'Bannon, whose commission must pass on all rate increase requests from Duquesne Light, figures that 18 to 20 cents of every revenue dollar at Duquesne "has something to do with pollution abatement." That means Duquesne customers pay about 18 percent of their electric bills to clean up the environment.

Pernick estimate that the operating costs add about $10 to $12 to the $22 to $23 the utility pays for each ton of coal. In 1977, Duquesne Light Estimated its operating costs at $249 million, of which pollution control accounted for $67 million. Curiously, the utility's net income was just about $67 million last year too.

O'Bannon said that the increasing costs of generating electricity and the increasing complexity of installing and maintaining anti-pollution equipment will make it hard for utility commissions to stay out of areas that used to one the prerogative of management - including what sorts of technologies a utility should adopt.

"There are extremely costly, sophisticated decisions to be made and utility management nationwide is not of the caliber to deal with these diverse systems. They are very one-product oriented. They don't deal well in cost-constrained environments.

"And increasingly, as the capital expednitures grow, utilities are coming in and asking for an up-front guarantee. If the project goes bust, they want to make sure they can still include it in their rate base."

Before there were pollution controls, of course, everyone in the Allegheny Valley paid the costs of pollution - with their lungs, their health and frequent paint jobs on their houses. Now the customers of the industry that caused the pollution pay more directly for the cleanup costs. In the case of a stell mill product, a Pittsburgh resident might have been subsidizing with his health cost of a car bought by a New Yorker.

Economists call this internalizing (putting into the price of the product) a cost that had been borne externally.

Because of the more local nature of a utility, those who benefit from the clean air or water are generally the same people who are paying the higher electricity rates.

At older plants like Phillips, where pollution-control devices were fitted on existing generating equipment (there are six boilers and four turbines), the problems are even greater than at new facilities, like Duquesne's Bruce Manfield station, two of which are open and one of which is under construction.

"Because the scrubbers are so unreliable, there's a limit on our generating capacity at Phillips," Pernick complained. "Of the four scrubbers at Phillips, usually one is out. That gives us a reliability factor of 75 percent, compared with 85 to 90 percent for our boilers."

Furthermore, because the system is old, some outside air leaks in and gets cleaned along with the noxious discharges from the boilers. Since the scrubbers can only handle a certain volume of air, the generating capacity is further limited.

"Phillips has a capability of generating 380 megawatts," Pernick said. But it can be counted on for only about 280 megawatts with the pollution equipment.

Under the rules, noted O'Bannon, companies are not permitted to install any form of scrubber bypass, so that it could continue to generate in an emergency if the scrubber broke down. When the scrubber goes down, so does generating capacity. She said she would like to see the rules changed so that utilities could temporarily bypass scrubbers in an emergency.

Pernick said that so far no brownouts or black outs have been caused by an environmentally caused shutdown of capacity, but said the threat is always there.

The four steam turbines and six boilers at the Phillips plant are housed in an eight-story, two-football field long building about six miles from Pittsburgh airport. To house the scrubbers Duquesne Light built a similarly sized edifice directly behind the generating plant.

The dirty gases poured off the boilers are sent first through a mechanical dust collector that shakes the big pieces of soot out of the air. Then the air shoots through electric precipitators that attract more dust from the air. Then the smoke is sucked through a 20-foot by 16-foot duct that crosses from the top of the generating plant across to the scrubber building.

The four, 50-wide scrubbers are connected in sequence. When one of the scrubbers is down for maintenance or repairs, as is usual Duquesne officials say and which was the case on a recent day in late June, the company has to reduce the rate at which it fires its boilers in order not to generate more smoke than the scrubbers can handle.

Gas is sucked into the cylindrical scrubber from the top and is bombarded with a high-speed mix of water and magnesium oxide lime. The limewater mist absorbs most of the remaining dust and about 83 percent of the carbon dioxide. The water droplets fall to the bottom of the scrubber (producing about a ton of sludge for every three tons of coal burned) while the clean gas continues on and emerges from the top of the 340-foot smoke-stack located at the back of the scrubber building.

The water-lime-dust-sulphur dioxide residue is washed into clarifying tanks. The water is separated out and put back into the scrubber system. The sludge is treated and disposed of in a landfill up the road.

Not only does this complicated system of anti-pollution devices often break down, the system consumes about 10 percent of the electricity generated at Phillips, electricity the company otherwise could sell.

The Cater administration is reviewing environmental regulation to see if the rules should be rewritten to give industries a clean air and water goal but not mandate how an industry must achieve those goals. Many industries such as steel, complain that they could have attained the same level of pollution control much more efficiently and cheaply than the ways they were ordered to do it by states and the federal Environmental Protection Agency.

Pernick, however, doubts that Duquesne would have done things much differently. Unlike a steel mill, where the processes are many and the sources of pollution multitudinous, there is only one important source of air pollution at a generating site: the smoke-stack. And, as unreliable as the wet scrubber may be, Duquesne officials say they think it is the best way to eliminate soot and sulphur from the air.

There are newer, more exotic systems such as fluidized bed boilers, but Pernick said the company will probably stick with scrubbers.

So far, the Pennsylvania Public Utility Commission has permitted companies to pass on in higher rates the full capital and operating costs of anti-pollution equipment, O'Bannon said. But, she said, "It is getting to be more of a nitty problem. EPA standards are getting more stringent, but no one is taking a look at the costs and the benefits."

At present O'Bannon said, 'We're saying environmental regulations are reasonable. I have no problems paying all the costs. We've gotten by for too long paying only the direct costs while people here have respiratory problems."

But, she said, "We've got to ask what our goals are. Is it 100 percent pure, 100 percent of the time, or should there be some recognition of technological fallibility and costs."