The proposed Rock Run sewage treatment plant is as controversial as it was in 1978 when Montgomery County and the Washington Suburban Sanitary Commission initially approved the project, to be located in Potomac. In the meantime, the cost to build it has tripled.

The original $55 million cost estimate has risen to $144 million, with some officials now predicting it could cost $200 million.

Civic groups fighting the project say sewer bills for most homeowners in Montgomery and Prince George's counties would go up almost $100 a year for 20 years to pay for the project, estimates that the commission has not contradicted. Opponents also question whether the plant is even needed.

The Rock Run debate has heightened the antagonism between Montgomery and Prince George's officials over sewage and sludge.

The two counties are supposed to cooperate on sewage plans as members of the bicounty sanitary commission. But Montgomery contends that Rock Run would not be necessary for at least another 20 to 25 years if Prince George's would agree to share its two large and underused sewage treatment plants. The plants belong to the commission and were paid for by both Montgomery and Prince George's residents.

Prince George's officials insist the two plants -- Western Branch on the Patuxent and Piscataway on the Potomac -- are for the exclusive use of Prince George's residents; Montgomery contends there is no legal basis for that claim. Prince George's officials have blocked all efforts to share the plants, saying they want to preserve the capacity for future development in the county. They support construction of Rock Run for Montgomery County, even though Prince George's homeowners would pay a large share of its cost.

Montgomery County could take its sister county to court to try to force Prince George's to share the jointly owned sewer plants. But County Attorney Paul McGuckian says a decision on suing will not be made for another year or two, when Montgomery must make its final decision on Rock Run.

Although a final decision is not expected before 1984, at least four public meetings were held this week on the project. The site proposed for the plant is the 1,019-acre Avenel Farm west of the Beltway off MacArthur Boulevard. A public hearing is scheduled Jan. 25 at the new county office building in Rockville.

The latest meetings were held by the county to consider a consultant's study of the site, and by state legislators and civic groups to discuss the escalating cost of Rock Run and whether the plant is actually needed.

"It was sold to the public as a $55 million project in 1978 and now it's at least $144 million and maybe 30 percent higher ($187 million), not counting the costs of operating the plant," says William Green, legal counsel for the Potomac Valley League. The league represents 25 Montgomery County civic associations and is one of several groups opposing the sewage plant.

Dean Gibson, president of the Montgomery County Civic Federation, a 57-year-old organization representing 65 civic groups, has prepared detailed cost estimates for the construction and operation of Rock Run.

"We don't think it will be needed for at least 25 years," says Gibson, "if we share (sewage) capacity with Prince George's residents."

Nikki Roy of the Clean Water Action Project, the 10-year-old environmental group founded by Ralph Nader, predicts Rock Run will cost "at least $450 million, when you include debt service. And the crazy thing is that the plant is not necessary. Rock Run is a political artifact. Prince George's is going to make its own citizens pay for an unneeded sewage plant when there's already enough sewage capacity in both counties for the next 25 years."

The environmental group also claims there are no standards for discharge of nitrogen, viruses or heavy metals into the Potomac from Rock Run. And it foresees potential problems because Rock Run will discharge effluent just above the District's new emergency drinking water intake pipe near Chain Bridge -- although plans call for Rock Run to reroute effluent to Blue Plains if the District needs to use the new water intake pipe during a drought.

Potomac civic groups are concerned about odors that could be given off by the plant affecting nearby residents. And the National Park Service is concerned about the large effluent pipes that under one plan would be built beneath the George Washington Memorial Parkway for most of the distance between the Beltway and Chain Bridge.

A bill to kill the project has even been introduced by Del. Robin Ficker (R-Potomac) in the General Assembly, which yesterday opened its 1982 session in Annapolis. It has virtually no chance of passing, according to other delegates from the two counties.

Prince George's officials admit that the county has large amounts of unused sewage capacity at its two 30-million-gallon-a-day treatment plants. Prince George's and Montgomery also have large shares of the 309-million-gallon-a-day capacity of the District's regional Blue Plains Sewage Treatment Plant, where virtually all Montgomery sewage is now routed.

Montgomery Delegate Idamae Garrott (D-Silver Spring), who is heading a county delegation subcomittee on Rock Run, said this week, "I have studied all the documents and nowhere is there (anything that says) Montgomery cannot use Piscataway or Western Branch. This is a political problem. . . . There is now 49 million gallons a day of unused sewer capacity for both counties. It does seem insane to raise sewer rates . . . $100 a year at least. Is it really worth it for Prince George's residents?"

Garrott says Rock Run may be necessary if all else fails. But she hopes to get officials of the two counties together soon in another attempt to reach a political compromise, although she admits that efforts over the past three years have been unsuccessful.

Prince George's County Executive Lawrence J. Hogan has written repeatedly to Montgomery officials insisting the sewage plants in his county were "designed solely to meet the wastewater treatment requirements within the county" and reaffirming his county's support for construction of Rock Run.

The sanitary commission, meanwhile, is in the middle, with three members each from Prince George's and Montgomery.

Prince George's officials have said they also are opposed to construction of giant pipes across the county to carry Montgomery sewage to Western Branch or Piscataway. But Garrott notes that Montgomery is constructing a giant water pipe across Montgomery to supply Prince George's with Potomac River water.

Rock Run is not to be confused with the Rock Creek sewage treatment plant, the small and also controversial $9 million plant near Rockville that opened in 1978 and was closed indefinitely in 1979. It was built by private developers as a "temporary" plant. It is similar to the 5-million-gallon-a-day plant at Seneca, which was built at the same time.

Rock Creek was mothballed, commission officials say, because the recession and high interest rates stopped the housing construction boom and existing homes have been using less water, causing a temporary drop in sewage flows in the county.