In 1978, when the proposed Rock Run sewage treatment plant was approved by Montgomery County and the Washington Suburban Sanitary Commission, some planners envisioned the plant's completion near Potomac by 1985. The facility was to be a technocrat's dream--Montgomery's first major waste treatment facility, a colossus next to the two tiny sewage plants in Damascus and Seneca.

Now, however, in little-noticed legislative hearings and in discussions between Montgomery County and the WSSC, those same experts are saying that Rock Run will probably not be needed until the year 2005--or even later. Officials at the WSSC, the utility that oversees the construction and operation of sewage treatment plants in Montgomery and Prince George's counties, say a slowdown in Montgomery's population growth after the boom years of the early 1970s, as well as recent dry winters, may make it possible to delay Rock Run for 20 years.

"Rock Run is a dead project--at least for the time being," said a senior official of the WSSC, which still holds a $20 million option to buy the plant's 1,000-acre site at Avenel Farm west of the Beltway off MacArthur Boulevard.

"It boils down to a question of need and the willingness to spend megabucks," said the official, who asked not to be named. "And the answer is: Put Rock Run on hold."

Opposed by environmentalists as a possible polluter of the Potomac River, Rock Run is also the root of a feud between Montgomery and Prince George's legislators over area sewer rights. Utility experts, meanwhile, say that after it is built, the annual sewer bill of the average suburban Maryland family would rise by $100 to $200.

As an advanced treatment facility, Rock Run would handle about 20 million gallons of sewage daily, emptying effluent into the Potomac downstream from the point where the county draws its drinking water. "It still represents the best technical solution that Montgomery County has for its future sewer needs," said David G. Sobers, director of the county's Environmental and Energy Planning office. "We can't say it's going away. But we have concluded that it might be 1995 or later before it's needed."

Rock Run's delay hinges on at least four variables, said Sobers: continued slowdown in Montgomery's population growth; an increase in the sewage treatment capacity at the Blue Plains plant in Washington, which now handles almost all of Montgomery's 62.7 million gallons of daily waste; a successful leak-plugging campaign in the WSSC's 7,000 miles of water and sewer pipes; and low rain and snowfalls over the next decade.

WSSC and county officials said they are optimistic about increasing Blue Plains' capacity from 309 million gallons daily to 370 million gallons--an expensive proposition but presumably cheaper than building Rock Run, the officials said. The WSSC's campaign to curb the amount of rain and groundwater leaking into sewer pipes, meanwhile, added 2 million gallons of extra capacity at the plant last year.

(In testimony last month before Montgomery County's state senate delegation, Craig S. Coker, the county's Rock Run project manager, said there was "a strong possibility" that Blue Plains could be upgraded. "Such a conclusion would obviate the need to proceed with Rock Run at least until the year 2005," Coker said.")

Rock Run's probable delay was welcome news to the civic groups that have attacked the proposed plant as unnecessary and dangerous to the Potomac River's drinking water supply.

"We've been maintaining all along that by controlling water infiltration, the WSSC could save on millions of gallons of capacity," said William S. Green of Bethesda, legal counsel to the Potomac Valley League, a coalition of 25 civic associations--one of several groups opposing the sewage plant. "We feel like our position has been vindicated," said Green.

But a spokeswoman for developers in the two suburban counties believes that delaying Rock Run could crimp the future growth of the area.

"There is enough demand--and there will be enough demand--for some kind of plant," said Susan J. Matlick, executive vice president of the Suburban Maryland Home Builders Association. Last year--the worst for home building since the Depression--there were 5,000 and 2,000 new homes built, respectively, in Montgomery and Prince George's, Matlick said.

With no Rock Run plant to serve the counties' new housing, "builders and developers won't feel comfortable about making future plans," she said.

As an alternative to building Rock Run, several Montgomery legislators want to force Prince George's to share its two underused sewer treatment plants with Montgomery County. Del. Idamae Garrott (D-Silver Spring), who chaired the county delegation's subcommittee on Rock Run, has introduced legislation that in effect orders Prince George's to make its plants at Western Branch on the Patuxent and Piscataway on the Potomac available to Montgomery. A WSSC spokesman said the plants now operate at half-capacity.

"Rock Run is no better for the people in my district than for the people of Oxon Hill, Upper Marlboro or any place else in Prince George's," Garrott said. "The plants in that county should be available to all, because that's the way they were paid for--by both counties."

Prince George's legislative delegation, however, hoping the county's abundance of sewage capacity will spur development through the end of the century, is fighting the Garrott bill, which has been endorsed by the Suburban Maryland Home Builders Association.

"I'm not at all happy with the prospect of financing Rock Run," said State Sen. Arthur Dorman of Beltsville, "but Montgomery County, by delaying a major plant of its own, has created a situation where the rate-payers in both counties are going to get clobbered."

Once hampered by the sewer moratoriums of the 1970s, Prince George's now has ample sewer capacity--a luxury that bodes well for future development, Dorman said. "You can bet we'll guard what we have very jealously. Montgomery was able to develop when we couldn't. But when it comes to capacity, we're the 'Haves' and they're the 'Have-nots.' "

Dorman, who chairs the general assembly's select bi-county committee, said Garrott's bill had no chance of committee approval.