David Lewis is a lab rat with no lab, a researcher with no salary, a once-influential scientist whose only perk these days is a lonely cubbyhole.

In his heyday, he was a high-ranking Environmental Protection Agency scientist whose discovery that dental equipment could be a haven for the HIV virus in the 1990s earned him prestige and respect.

Now he's a pariah, working out of a spare office at the University of Georgia and waging a quixotic battle with his former patron over sewage sludge, the reason for his gradual fall from grace.

More than a decade ago, Lewis began to challenge the EPA's policy allowing farmers to spread the semi-solid byproduct of wastewater treatment plants over their fields as a free, nutrient-rich fertilizer.

He's investigated illnesses and deaths he claims are linked to the sludge; he said his work has helped prod government officials to issue guidelines for workers who handle the sludge. He's also filed a flurry of lawsuits, the latest in March 2006 claiming UGA was complicit in a scheme by EPA leaders to justify the agency's program that distributes sludge to farm fields.

"Science is getting trumped by politics and I want that fixed," said Lewis. "My case is the worst case scenario where politics is blocking good science."

There was more than a hint of a conspiratorial tone in his voice recently when he said he's up against "an effort organized by multiple federal agencies and powerful industry groups with support of tens of millions of dollars in congressional earmarks."

The UGA and EPA researchers have stood by their work and deny wrongdoing.

"There's no cover-up. There's no conspiracy," said Robert Brobst, an EPA environmental engineer and a defendant in the lawsuit. "We're a bunch of nerdy scientists. How the hell do we know how to cover up and do conspiracies? We're boring."

For Lewis, who said he's trying to reclaim his reputation, it's been a costly crusade. He's lost his job with the EPA and was spurned by UGA, where he once hoped to land a gig as a tenured professor.

But he has reason to be encouraged. A federal judge has refused to throw out Lewis' lawsuit against UGA, and his work is helping focus attention on sewage sludge beyond the small circle of scientists who now study it.

"There really has not been adequate research about what this material is, let alone the repercussions," said Rob Hale, an environmental chemistry professor at the Virginia Institute of Marine Sciences. "Folks were told that this stuff had been studied to death, and Lewis is concerned that they're overstating what they knew about the material."

Wastewater treatment plants across the nation produce about 7 million tons of the sludge each year as a byproduct, and slightly more than half of it is used as fertilizer. The EPA has long argued the sludge is safe as long as it's applied properly.

"If it's misused, if it's overapplied, if it doesn't meet quality criteria, of course it's going to be a problem," said Brobst, who has specialized in this area for 30 years.

Still, "Based on what we know today, yes, it's safe," he said. "Science takes little steps, but if you add up all the little pieces in 1,500 articles in the last five years, you have a safe argument."

Lewis, who was never shy to question EPA policies, turned his attention to sewage sludge in 1996 after the issue kept coming up during an informal poll of his colleagues.

He started collecting samples from sewage treatment plants, using some of the same methods he used while investigating dental products: Collecting gunk and analyzing it for harmful pathogens and toxic materials. He soon found some that certain pathogens in the sludge could survive disinfection by taking shelter in fatty greases and oils.

"It doesn't take but a high school education in science to understand this stuff: Bacteria hides in hunks of gunk," he said.

He presented his findings at a national conference in 1998, prompting a new round of media coverage — and more scrutiny from his employer. Other EPA researchers soon conducted a study that refuted some of his work, though Lewis has questioned their methodology.

Lewis' growing reputation didn't do him any favors with his bosses, who offered in 1998 to pay his salary at UGA for four years as long as he retired after the contract was up. At UGA, though, he said he wasn't granted the freedom he had hoped, and started conducting research on his own dime. UGA never did offer him a job, and when his EPA contract was up, Lewis refused to retire and was let go in 2003.

Since then, he's turned his full attention toward fighting his former employers in court, where he's had a few successes and some stinging defeats, such as a 2004 administrative judge ruling that said Lewis has not provided "scientific evidence to back up his belief" that the sludge could pose a significant danger to people.

On the walls of Lewis' modest home in Watkinsville are some of the proudest images of his career, and in his office is a silver cabinet full of sludge files.

To Lewis, they are a constant reminder of the score he still wants to settle. And to some of his colleagues, that's not such a far-fetched idea.

"He's got a lot of guts and fortitude, and he's been right in the past," said Hale. "You need people like that, whether you agree with him or not."