It isn't often that Robert Scott sallies into the tangled jungle of federal regulations, grants and political intrigue. But when you start talking sewers, you're talking Scott's kind of game.

A plumber by trade, Scott has spent most of the last seven years telling anyone in earshot that a $12 million federally financed sewage-treatment plant on the drawing boards for his tiny township in Franklin County, Pa., was an over-engineered white elephant that would drain more cash than sewage.

"The situation is that nobody would listen to me," he said. "I tried to propose a less expensive system in 1978. It isn't that I haven't tried."

Today, with ground-breaking just around the corner, the 3,700 residents of St. Thomas, a rural township nestled in south-central Pennsylvania's dairy country, are looking at hookup costs as high as $2,500 per house, monthly sewer bills of $32 and up and the potential loss of one-fifth of the township's half-mile-long business district.

Having belatedly looked the gift horse in the mouth, however, residents of St. Thomas Township have discovered one of Washington's lesser-known truisms: The only thing tougher than getting a slice of the federal pie is trying to give a piece of it back.

So, when Robert Scott talks now, people listen.

Swept into office as township supervisor with more than 75 percent of the vote last month, the plumber-turned-politician is campaigning to have the government scrap the $12 million plant in favor of a less ambitious system that he says would handle the township's sewage adequately for about $2 million.

Scott's campaign has solid backing in St. Thomas, where 76 percent of voters signed a petition opposing the plant.

But the effort comes to the dismay of the engineering firm that designed the plant and stands to gain an estimated $3 million fee when it is constructed.

It also comes to the befuddlement of state and federal environmental officials who approved the project and to the consternation of the congressman who thought he had done the township a good turn when he helped secure federal funds for the plant nearly a decade ago.

"It's a unique situation," said an aide to Rep. Bud Shuster, a Republican who has represented the area for 13 years. "Obviously, the congressman doesn't want people to be unhappy. But you have to have sewage treatment to attract jobs and increase property values.

"Look at where it's located," the aide said. "It's one of our better areas. It's not right to have property this close to the Washington metropolitan area and not be able to participate in the expansion."

St. Thomas residents say they would rather keep their town alive than attract new industry, and they are not particularly interested in increasing property values for developers who they believe will move in to take advantage of the sewage system when residents are forced out.

According to Scott, 12 of the township's 55 businesses, including three antique shops, a motel, a greenhouse and the Odd Fellows Lodge, will close rather than pay hookup costs for the new sewage plant.

The township school will have to abandon its state-approved sewer system, not yet paid for, and hook up to the new plant at a cost of nearly $10,000 plus monthly fees of $1,155.

Scott also expressed concern about the impact on the township's residents, nearly half of whom are retired and on fixed incomes, many of them residents of three mobile home parks.

"Of the township's 1,165 housing units, 356 are mobile homes," he said. "A grave concern of mine is that they can put wheels on those homes and move out. If we have to absorb their share of the cost on the other 800 [permanent houses], we're in serious trouble."

The sewage plant for Franklin County was proposed in 1976, when regional systems were the rage and the federal money to build them looked endless.

As designed by the Nassaux-Hemsley engineering firm in nearby Chambersburg, Pa., more than 24 miles of sewage lines would carry waste water from all points in the township to a central treatment plant.

The Environmental Protection Agency, however, is now promoting smaller projects and cheaper "alternative" technologies in rural areas, exactly what Scott had in mind when he proposed a decentralized system that would collect effluent in smaller, separate treatment areas near the township's residential sections.

Scott said his proposal is based on a system under construction in neighboring Peters Township, an area about one-third the size of St. Thomas that expects its system to cost less than $300,000. "We wouldn't have all those pipes running around," he said.

Scott's proposal was endorsed last month by the state director of the Farmers Home Administration, which funds many rural sewage treatment plants but is not involved in the St. Thomas project.

In a letter to Scott's Washington lawyer, FmHA official D. Elmer Hawbaker said he is "firmly convinced" that the township's pollution problems could be solved "at a fraction of the cost of the proposed project."

"I have witnessed many minor insurrections when people in rural communities become aware of the cost of the sewage disposal projects designed by the same principle as a system serving a large metropolitan community," he said.

The letter did not impress promoters of the $12 million project, who quickly suggested that Hawbaker had his nose where it did not belong.

"Everything was fine and proper until this Farmers Home director thought it should be looked at again," Shuster's aide said.

By that time, Scott also had the attention of Sen. Arlen Specter (R-Pa.) and, not coincidentally, of Specter's 1986 reelection challenger, Rep. Bob Edgar (D-Pa.). Both asked the EPA to take another look at the project.

After a meeting with Scott and 50 other St. Thomas residents, Shuster also agreed to another review.

The residents, however, are skeptical of EPA's new review, conducted in a whirlwind visit to St. Thomas last week. The agency is expected to make its decision before the year's end, which Scott says is not enough time to evaluate his alternative fairly.

"We're just asking for a eight-week reprieve," he said. "The biggest delay in this project so far has been engineering delays, not caused by the citizens. It's ironic that we went all these years and now we can't get eight weeks."