For more than a month now, a controversial sewage treatment plant near Rockville has been discharging 2-to-3 million gallons of treated sewage daily into Rock Creek, sewage effluent Montgomery County officials claim is cleaner than the water already in the polluted stream.

However, District officials are accusing the county of ruining Rock Creek, the only large stream in the nation's capital and the focal point of Rock Creek Park, the federal government's oldest urban park. The stream meanders through county parkland, also called Rock Creek Park, for about 12 miles before reaching the District line where the federal park begins.

John D. Brink, chief of the city's bureau of air and water quality, said this week, "We're monitoring it but don't have any test results so far. However, we don't think you ought to put sewage plants on every small stream, especially the only stream flowing into the nation's capital and a historic federal park."

Brink accused the Environmental Protection Agency of violating its own regulations in permitting the $9 million "interim" sewage plant to be built on the stream in the first place.

"EPA is not supposed to permit interim plants except to help reduce pollution, not add to it. This plant was put up solely for growth," Brink said.

The plant was built by a consortium of private developers to enable them to get around the sewer moratoriums that have prohibited new construction in that part of the county for several years. The plant, built at private expense, is soon to be given to the Washington Sububan Sanitary Commission as a public facility.

The District sued EPA, Montgomery County, Maryland and other governmental agencies that approved the plant, but the suit was dismissed two weeks ago by U.S. District Court Judge June L. Green. Green refused to block operation of the plant, ruling there was insufficient proof that the sewage effluent would permanently damage Rock Creek Park and its stream or cause "any adverse consequences" to the public. The city is appealing the decision.

While the District "lost the first round" in its battle against the plant, said Brink, "and may lose them all, it doesn't mean we've changed our thinking about the sewage plant. We just don't advocate swimming in effluent."

Although Rock Creek is too polluted for water contact by humans at the moment, it has been designated by EPA, the National Park Service and the city as a future recreational stream. The goal had been to make Rock Creek swimmable for both fish and humans.

The Park Service also has expressed concern about the sewage plant and its effect on Rock Creek Park. It has hired a consulting firm to study to pollution problem in the park and the effect of the new sewage effluent.A Park Service spokesman said this week that if there is any increased pollution because of the sewage treatment plant, Justice Department attorneys will go to court to try to stop it.

Both the developers and Montgomery County water officials claimed last year that effluent from the sewage treatment plant, the most advanced in the Washington area, would have more than 98 percent of all pollutants removed and actually be cleaner than the waters of Rock Creek.

"This kind of effluent is so pure that some public officials have drunk it to demonstrate how good it is," Charles Dalrymple, attorney for the developers, told a public hearing by the U.S. Army Corps of Engineers last October.

No public officials here have yet offered to drink it, but the effluent now coming out of the temporary 16-inch drain pipe beside the Southlawn Lane Bridge appears cleaner than the fetid, gray-green waters that is pours into. The Corps of Engineers must approve the permanent pipe structure being built a few hundred yards upstream, which it is expected to do shortly.

Upper Rock Creek has been badly polluted for years, primarily by run-off from nearby industrial areas, junk yards, quarries, subdivision streets and farms, according to Steve Poteat, a planner for the county office of environmental planning. Many county residents who live along Rock Creek forbid their children to play near it because of the pollution, said Carol Mullaney of 11800 Dewey Rd. "All those storm drains run into it and even my neighbors put gasoline and stuff in it," she said.

Water tests made since the plant began discharging into the creek Sept. 14 confirm that the effluent is indeed cleaner than the stream in every different kind of test used to defect pollutants, and well under the strict design levels for the plant, according to Kenneth Wood, WSSC suprvisor of the east Rockville plant.

The District's Brink objects, however, that a new plant may well produce a clean effluent, "but they're like new cars, they break down . . . and the county plans to monitor it only twice a week. What happens on the other five days?" asks Brink.

The 3-million-gallon-a-day Rock Creek Interim Advanced Sewage Treatment Plant is one of two county "interim" plants. The other is a 5-million-gallon-a-day plant now also going into operation on Seneca Creek. While both are relatively to process sewage until the county has larger "mid-term" or permanent plants, they are as well built as any permanent plant, says Poteat.

In fact, they probably will become permanent plants and may even be expanded, although the Rock Creek plant "doesn't really stand a chance of being enlarged since the District already has warned us they'll fight to the death over any expansion of Rock Creek," Poteat said.

But expansion of Seneca is one of three most likely sites for a 20-million-gallon-a-day "mid-term" sewage plant now being considered by the county-Seneca's 5 million gallons a day of effluent, unlike Rock Creek's effluent, constitute a small portion of the Seneca stream flow - less than 20 percent even in the rare once-every-10-year low flow periods, says Poteat. Effluent from the Rock Creek plant, by comparison, is expected to constitute more than 50 percent of the Rock Creek stream flow during dry summer months.

The other two possible sites are on Rock Run and Cabin John creeks, south of River Road near the Beltway. But if a plant were built at either side, it would discharge into the Potomac, below all drinking water intakes from the Potomac, and would not affect either creek, Poteat said.