By day, James P. Knight is a federal bureaucrat, paid to help the Nuclear Regulatory Commission decide where nuclear power plants should be located. By night he is a citizen activist, fighting the proposals of Montgomery County bureaucrats who want to dump tons of sewage sludge near his home.

Like many of his neighbors in the Beau Monde subdivision -- a pocket of suburbia in the rural, hilly northern tier of the county -- Knight was outraged to learn last month that county officials had designated three large plots of nearby land as prime sludge dsposal sites.

Since then, Knight and his neighbors, several of them veterans of such federal agencies as the NRC and the Environmental Protection Agency, have mounted a sophisticated antisludge campaign. They are federal government workers fighting the workers of a county governement, and even as they accuse county bureaucrats of being short-sighted, arrogant and unresponsive, Jim Knight and his friends are acutely conscious of the irony of their situation.

"I remember looking out of the audience at one public hearing," said Larry Benner, a lawyer who worked with the NRC during the latter stages of the hearing on the controversial Seabrook, N.H., nuclear plant, "I saw a guy out there leafing through his papers like this" -- he makes frantic motions at a pile of papers before him -- "and I thought, 'Boy, is he obsessed.'

"Now, maybe I'm obsessed," said Brenner, who has temporarily abandoned his law practice to wage this new war.

One indication of the intensity with which the sludge proposal is being contested was the size of the crowd -- more than 400 persons, standing-room-only -- that turned out last night for the first of two public hearings before the Montgomery County Council.

Knight recalls that "one of the things that struck home with me was the psychological impact of bringing high technology, dangerous technology" into a populated area. "This has made me far more sensitive to people who look at plans for a nuclear plant and say, 'What if it blows up like a bomb?'"

"You can't just say, 'Well, it doesn't,'" he added. ". . . I think this (sludge battle) may make me a better regulator."

To keep the ditch-digging machines and the sludge-bearing trucks out of these acres of rural greensward 35 miles northwest of Washington, Knight and his wife Sue, along with Larry and Maxine Brenner, have founded a group that has spent between $3,000 and $4,000 in just three weeks, assembling the technical and legal expertise they need.

They have hired a hydrologist and a lawyer to look for flaws in the county's site selection study.They have tramped around the three sites near the intersection of Md. Rtes. 355 and 27, looking for telltale wet spots that mark high points in the water table -- unacceptable spots for sludge disposal.

Such sophisticated opposition is nothing new to the Montgomery County officials charged with finding a place to put the unwanted residue of the sewage treatment process. "I've yet to find any citizens group unsophisticated on the subject of landfills and sludge," said David Sobers, head of the county Office of Environment and Engery Planning.

"They're generally well-educated, sophisticated and well-heeled enough to hire experts," he added. ". . . But this group is as talented as any I've ever dealt with."

In part, that's because some people in this group know what it's like to be on the other side of such a fight. And they are perfectly willing to press that advantage, gathering advice from high-level acquaintances in the EPA and picking apart the experts' technical jargon with technical jargon of their own.

They say their main worry is their families' health. They fear that toxic elements in the buried sludge -- bacteria, viruses and heavy metals -- will seep into the ground water and thence into wells that supply the homes where they and 500 neighbors live.

They also worry that rats and ticks would flourish around a sludge disposal operation and infect their dogs and their children with the sludgeborne contaminants, or that a gas line on one site could be ruptured, leading to an explosion that could fling contaminants into their back yards.

Montgomery County officials counter that they have built-in safeguards to prevent any contamination of well water or surrounding homes.

The sludge trenching process, which involves filling 5-foot ditches with the muddy residues, treating it with lime designed to kill the bacteria and viruses, then covering them over, has been extensively tested, they say, and has never been shown to contaminate nearby areas.

County officials say they have identified areas where the water table is at least 25 feet below the surface of the soil and where chemicals and heavy metals from the sludge would be absorbed.

And they point to other safeguards: new wells that will be dug solely to monitor the groundwater, and berms and dikes that will be constructed to catch storm water runoff.

For the county, time is important for it is under pressure from a federal court order requiring it to dispose of about 25 percent of the 1,500 tons of sludge produced daily by the Blue Plains Regional Sewage Treatment Plant, and from lawsuits filed by other local citizens' groups opposing sludge disposal in their neighborhoods.

Legally blocked, for the moment, from incinerating the muck or reducing it to a nontoxic, loamy material, county officials say that sludge entrenchment is their only immediate option -- and the three sites in the Germantown area, near Beau Monde, are the last acceptable places to do this. g

That answer is not acceptable to the Beau Monde residents. Nor do they believe the safeguards are well thought out or adequate.

"We don't want to be the last casualties of this," said attorney David Kantaglia, who represents the Beau Monde group. "They say, 'we're almost out of land.' We say that they've run out already, and they'd better face the political consequences."

"There's something every engineer, after some experience learns to fear," said Jim Knight. "We've seen it in our own work at the NRC. Your options are restricted to such a degree that you begin to rationalize. First, you say, 'I think it will be all right.' Then, 'I'm sure it will be all right. It ought to be all right. It will be all right.'"

The County Council is scheduled to hold a second hearing on the three proposed sludge sites at 8 p.m. Tuesday. A decision could come later this month.