Montgomery County Executive James P. Gleason yesterday selected a farm in the middle of Potomac as the site for the county's new 20-million-gallon sewage treatment plant.

The $62 million plant would be built in the same place - the 455-acre Avenel farm at Brickyard and Persimmon Tree roads - as Gleason has been trying to build a garbage dump.

Gleason said that if the County Council would approve the Potomac site for a sewage plant, he would give up his fight to make Potomac the site for the county's new landfill. At present, Montgomery is appealing the state veto of Avenel farm as the garbage dump site.

In an appeal to the council, Gleason said construction of a treatment plant in Potomac would "bring to an end the county's eight-year litany of sewage problems.

"I have finished my work . . . there's no reason to hold back," Gleason said in urging the council to act before the election Tuesday. But council members, most of whom are running for reelection, decided to keep the item on their agenda for Nov. 10 - three days after the election.

The Potomac facility would be designed to handle Montgomery's sewage treatment needs for 15 years. The 20-million-gallon capacity - enough to handle about 200,000 people - would have a spare 5 million gallons that would be offered to the District of Columbia.

Gleason's decision to build a 20-million-gallon per day plant represents a scaling down of his original plan - for a facility at rural Dickerson that would be three times as big (60 million gallons) and cost perhaps eight times as much (close to $500 million).

The federal Environmental Protection Agency vetoed Dickerson as too big. EPA suggested a smaller plant, but earlier this month, the agency said it would not fund a 20-million-gallon plant, either, because it was designed to handle only growth, not "existing needs."

That means that the plant's cost would have to be borne by Washington Surburban Sanitary Commission's customers in both Montgomery and Prince George's counties. Prince George's officials, edgy about possibly having to raise sewer rates, have asked Montgomery to exhaust all efforts to get U.S. funding - a prospect that Gleason has rejected as futile.

Gleason said the Potomac plant would be the "most cost-effective" of a range of alternatives he and his staff considered.

He also noted that the plant's effluent, which would be given a high level of treatment, would be discharged into the Potomac River below the pipes that take out water to supply suburban Maryland and the District of Columbia.

Gleason also said the plant would be "heavily buffered" to reduce its impact on its affluent neighbors, which include the River Falls development and, about half a mile away, Congressional Country Club.

Gleason's proposal ran into immediate opposition in the Potomac area. William S. Green, attorney for the Potomac Valley League of Montgomery County, which helped lead the fight against the landfill, said: "It's incredible . . . We are opposed to the sewage treatment project - absolutely. We will do everything we can to oppose it."

The league is the umbrella group for all civic associations between River Road and the Potomac River from the District of Columbia line to Frederick County.