Seeking solutions to escalating costs of sewage treatment, regional planers have been taking a new look at the old but seldom used process called land treatment - and they have reached what they say are some surprising conclusions.

According to a study under way at the Metropolitan Washington Council of Governments, there are eight acres in suburban Maryland and Fairfax County that have a "high potential" for land treatment.

Furthermore, a consultant's study prepared for Montgomery County has concluded that land treatment at any of five sites there would be cheaper and just as effective as treatment at a sewage plant, which dumps its effluent into the water.

"For the first time, somebody has been able to put together all soil information, computerize it, measure it against some widely accepted criteria, and come up with some acceptable sites," COG planner Mark Alpert said yesterday.

The findings are likely to put pressure on officials in growing suburban jurisdications not to rely exclusively on advanced treatment plants - to which the area has already committed million of dollars.

The COG findings are part of a $4.5 million study, still incomplete, that will detail what the area proposes to do over the next 20 years to clean up the Potomac River. The final document has to be agreed to by local jurisdictions and then be submitted to the Environmental Protection Agency for final approval by March 26.

Under land treatment, sewage that has received basic treatment is spread on soil to remove remaining pollutants, which are actually converted to nutrients. Under one process - used in the only large-scale operation in the country, in Muskegon, Mich. - the effluent is sprayed on the soil through sprinklers. High-yield corn corps have been harvested from the sandy Muskegon soil.

However, EPA Administrator Douglas Costle has served notice that EPA intends to "press vigorously" for land treatment. To put some clout into that effort, Costle said localities will be "required to provide complete justification for the rejection of land treatment" before receiving funding for new or expanded plants.

The COG study gave a "high potential" to three sites in Montgomery County, three in Prince George's County and two in Fairfax County.

The sites in Montgomery are south of Poolesville near the Potomac River, between Olney and Montgomery County Airport and between Rte. 29 and the Prince George's border north of the Capital Beltway.

The Prince George's sites are southwest of Laurel along I-95, on the southern border of the county between Accokeek and Brandywine and east of Brandywine.

In Fairfax, one "high potential" site surrounds Burke Lake, south of Fairfax City, and the other is southwest of the city. Both areas are lightly settled areas, but development pressures - mainly for one- and five-acre estates - is increasing.

While COG's Alpert said the sites meet the criteria for soil - it must be porous enough to let the effluent filter through instead of running off - some Fairfax officials took issue with his statement.

Supervisor Marie B. Travesky (R-Springfield) said the Burke Lake site had adequate soil, but she said the other site - southwest of Fairfax City - has problems. "Most of the soil there doesn't percolate (let water flow through)," she said.

Other Fairfax officials disputed COG study estimates that the land in the two sites could be bought for an average of $5,000 an acre. They said development pressures would put a much higher price on the land - probably pricing it beyond reason.

The initial reaction in suburban Maryland has been less negative, perhaps because the Montgomery and Prince George's governments-unlike Fairfax's - commissioned what turned out to be largely favorable studies of land treatment.

"Land treatment is more feasible now," said John L. Menke, president of the Montgomery County Council. He said that earlier, largely negative studies - done when the country was considering whether to build a big regional plant at Dickerson - are of date today. For one reason, he said, the demand for new sewage capacity has been greatly scaled down, which in turn has reduced the amount of acreage that would be needed for land treatment. Secondly, he said, newer techniques require less land - which again reduces acreage requirements.