When Montgomery County Executive James P. Gleason, a Republican, and the all-Democratic County Council agreed earlier this summer to put aside their longstanding differences and proceed with plans for new sewage treatment facilities they ended one battle - and started a new one.

The first round of the latest skirmish was fired last week with Gleason recommended six sites for one or more new sewage plants to accommodate anticipated development in the county until the 1990s.

Two of the locations are in the posh Potomac area, where Gleason recently proposed a new landfill to the outrage of nearby residents.

In presenting his six favored locations to the County Council in advance of a public hearing Oct. 5, Gleason drew from an inventory of 11 sites, which will be discussed at the hearing. The council is expected to make its final decision Oct. 17.

Gleason described two locations in the Potomac and Cabin John areas and another in the Seneca basin at Riffleford Road as "the most pragmatic in terms of cost, water quality and time of implementation.

These locations and another in the Seneca area at Schaefer Road would be suitable for conventional waste water treatment plants, he said. Three others - on Muncaster Road, River Road in the Upper Potomac region and at Dawsonville - could be used for land treatment in which partially treated effluent would be sprayed onto thousands of acres of land for filtration through the soil.

The country is seeking treatment capacity of 20 million gallons per day, partially for the District of Columbia.

After eight years of countywide sewer moratoriums that crippled development, and a nationwide recession that further slowed growth, county officials now see the opportunity to resume a "normal" growth pattern. Gleason's initial proposal for a plant at Dickerson has been struck down by federal court.

Anticipated new sewage facilities could permit the construction of about 5,500 new homes annually, the officials said.

Gleason said he is determined to locate a site before he leaves office at the end of the year.

After they thread their way through the neighborhood opposition to the proposed sites, Gleason and the council will face even more hurdles.

Among them will be obtaining Prince George's County support fro the sale of bonds to finance the plant construction through the Washington Suburban Sanitary Commission, a joint operation of the two counties.

These bonds would raise sewer rates in both counties. To minimize the potential financial impact, Gleason has asked Prince George's County Executive Winfield Kelly Jr. to consider leading Montgomery some of the unused capacity of existing Prince George's sewers.

A spokesman for Gleason said that Montgomery officials already have heard "rumblings" that Prince George's officials are not enthusiastic about either of these arrangements.