A plan to build a mammoth plant to convert solid waste into energy at Fairfax County's Lorton landfill has upset nearby residents, who believe smoke from the facility's incinerator could cause harmful air pollution in the southwestern part of the county.

The plant, which would be the largest such facility in the metropolitan area, would burn trash to boil water, producing steam that would generate electricity. The power would be sold to Virginia Power Co.

The facility, which would cost an estimated $300 million to $350 million and would be financed through industrial revenue and development bonds, would handle about 3,000 tons of trash a day, reducing it to an ash residue that would be dumped in the 23-acre landfill. County officials said the new facility would extend the life of the county's only solid waste landfill, which could reach its capacity by the early 1990s.

The proposed incinerator also would be used by the District of Columbia, which manages the property for the federal government.

Members of the Federation of Lorton Communities and some residents of the town of Occoquan, which overlooks a portion of the landfill along the Occoquan River, said they fear the proposed plant's 320-foot smokestack would emit toxic fumes and gases containing carcinogens. Residents also said they did not want thick clouds of smoke to pour from the stack and pollute their community.

Chuck Pugh, who serves on a citizens task force appointed by the Fairfax Board of Supervisors to study the proposal and who is also a member of the Occoquan Town Council, said he was disgusted with the idea of building an incinerator opposite the small riverside community.

"I know there's a problem with storing solid waste," he said, "but this is not the way to get rid of it. I don't want this thing because the stench of the landfill is already unbearable. It will make you sick sometimes."

Officials at the county's Division of Solid Waste, which is responsible for studying and developing plans for the facility, said the plant would be built with highly efficient pollution control devices designed to control noxious odors. Officials said the devices also would eliminate nearly all traces of smoke or particles from the building's giant stack.

The disposal plant would turn the solid waste to energy through a system of water-filled pipes that would absorb heat from furnaces in which trash would be burned at 2,000 degrees Fahrenheit. The water in the pipes would change to steam, which would then turn turbines to produce the electricity.

Robert Snow, a spokesman for the county's solid waste division, said the facility would use about 2.3 million gallons of water a day from the nearby Occoquan River. Snow said the water ultimately would be channeled into a sewer line and carried to a sewage treatment plant. "But you won't be getting any exposure of water to waste or anything. It just goes through a pipe, cools and goes out," Snow said.

Local residents remain skeptical. "The river water used around the landfill has got to seep somewhere," said one Lorton resident.

Officials said the plant would accept only household and commercial solid waste. Hazardous waste would not be burned.

If the plant is built, it would have to pass a series of rigorous safety tests conducted by the state's Air Pollution Control Board before being allowed to operate.

John C. Doherty, the board's regional director, said state and federal environmental standards allow a weight of 0.08 grains of dust or other tiny particles per cubic foot of air to escape from such facilities. Doherty said county officials plan to allow only 0.025 grains per cubic foot to be released from the Lorton smokestack, well below the maximum allowed under the state standard.

"That's about the weight of a gnat's eyelash," Doherty said.

Baltimore last May opened a smaller version of the same "energy resource recovery plant," as county officials call the facility. It burns 2,250 tons of trash each day and operates at a weight standard of 0.019 grains. Arlington County and Alexandria plan to open a smaller waste-to-energy center on Eisenhower Avenue, which will operate at a 0.03 grain standard.

Supervisor T. Farrell Egge (R-Mount Vernon) said that while he was not thrilled at having the huge incinerator in his district, he favored the project because it would extend the use capacity of the landfill.

"The new plant will certainly beat the trash blowing all over creation and the stench that comes up from there," Egge said in a telephone interview. "But I'm not going to support going forward with an unsafe facility. If the facility is made safe, then you won't have any smell, no more paper blowing around, it will be self-contained and will extend the life of the landfill."

Marcia E. Hanson, Lorton community association chairwoman, said she is pessimistic about the facility's potential environmental impacts, despite state and county officials' promises to run the plant with strict pollution control standards.

The county "will have a hard sell to convince us it's safe," Hanson said. "We're not saying the county is not doing a good job, but what's down the road? Who's going to manage it?"

Snow said that if the Board of Supervisors approves the facility sometime in the fall, county officials would then hire a private firm to run the waste disposal operation.

Doherty, the state pollution control official, said the county would carefully examine prospective operating companies to ensure proper management of the trash facility.

"The companies that bid on something like this have an incredible amount of experience. If the company is not running the disposal to the standards the county wants, the county will turn them off," said Doherty. "And then the investment the operating outfit has in it goes down the tubes."

Snow said Camp, Dresser and McKee, an Annandale-based consulting firm, is studying the environmental effects of the proposed incinerator. He said the company's findings would be made public by next week.