Northern Virginia officials are trying to reduce the cost of operating a controversial and highly sophisticated sewage treatment plant near Manassas, raising charges that the changes could endanger a major source of drinking water for suburban Washington.

Officials from Fairfax and Prince William counties and Manassas and Manassas Park, which use the facility, denied the change would harm the public health, arguing that technology advancements enable them to do things more efficiently now than when the plant was designed in the 1970s.

Those officials have invited five engineering firms to submit proposals by tomorrow for a $100,000 study on several changes the local governments say will save money.

Critics claim the changes also will mean that more viruses and other harmful pollutants will enter the Occoquan Reservoir, the source of drinking water for more than 600,000 Northern Virginians.

Local officials are not asking the consultants to determine "if there is a more efficient way to meet existing water quality standards. The bulk of the thrust is to lower the standards. Anybody can save money by lowering standards," said Noman Cole, former chairman of the Virginia State Water Control Board and a key architect of the Occoquan policies.

"Just about any virus you can get is in sewage. Drinking water is no place to go taking chances," said Cole, a major advocate of the expensive, high-tech facility. "There is a good and prudent reason why we have a safety margin on the system and we have no business taking it out."

"We're not interested in lowering standards," says Fairfax County Executive J. Hamilton Lambert, chairman of the subcommittee examining the issue. He said the regional sewer authority wants to save money on construction "as long as no damage is done to the stream."

The $82.3 million facility of the Upper Occoquan Sewer Authority (UOSA), located about 25 miles from Washington near the Fairfax-Prince William line, dumps its discharge into the Occoquan Reservoir. During dry spells the discharge can make up as much as 80 percent of the reservoir water.

The debate on the expansion is only the latest controversy to erupt over the Occoquan Basin. It is also the focus of a court fight over Fairfax County's efforts to limit growth and of a political fight between Fairfax officials and Fauquier County farmers, who plan to put sludge on their farmland. Fairfax officials argue that both development and sludge pose a serious threat to the water quality of the Occoquan.

The UOSA facility has itself been controversial. Since it began operation in the late 1970s, officials from Manassas, Manassas Park and Prince William have questioned the need for the elaborate layers of treatment and backup facilities. There have been protests in Manassas Park over costly sewer bills from the plant.

Virginia state officials, who must approve any major changes in the facility, usually have opted for the maxim often stated by Fairfax Supervisor Audrey Moore of Annandale: If you "put your outhouse over your well," you've got to be extra cautious.

The current move to reduce costs was sparked by a 1981 decision to expand the facility. The "driving force" behind the expansion was a request by International Business Machines Corp., a major Northern Virginia industry, for more waste disposal capacity, although the firm will use only a small amount of the total new capacity, said UOSA executive director Millard H. Robbins Jr.

About two years ago IBM, which operates a microchip plant in Manassas, informed local officials it would be expanding, meaning it would generate more waste. But Manassas already was sending about as much as it was allowed to the UOSA facility. Because Manassas Park and Prince William also were nearing their limit, plans were drawn up in 1981 to expand the treatment facility and IBM was given the go ahead for its expansion. IBM already has started building the new manufacturing plant and it will be ready for operation by late 1985, when the company has been promised more sewage disposal capacity, said IBM spokesman James Dameron.

In 1981, the cost of the sewage plant expansion was estimated at $18.2 million. But because of inflation and some modifications in the design, those costs have risen to $25 million in 1984 dollars, Robbins said. IBM is negotiating to pay part of the costs, but the price tag is still making the local governments, already strapped by what their officials call tight budgets, unhappy.

"The expansion costs are extremely high. We might be able to expand the plant for lesser dollars," said Prince William UOSA representative John Sloper, "There must be a way."

In the request by the four governments for a proposal to study the issue, the consultants were told to focus on a handful of potential changes, including:

* Relaxing standards for certain things in particular seasons. Supporters of this change argue, for instance, that algae does not grow in the winter, so standards on ammonia or nitrates should be relaxed during that period. Critics argue that such a relaxation would allow the concentration to build up in the water, and when warm weather comes, the nuisance algae would flourish.

* Eliminating a process designed to eliminate pollutants that are not biodegradable, such as Kepone and DDT. The local governments questioned whether the process is "economically justified since it marginally improves the quality of the effluent discharged."

* Replacing a sophisticated and expensive lime treatment for removing phosphorus with a chemical additive in an earlier phase of the treatment. Critics contend that lime treatment does a better job at removing phosphorus and also reduces viruses and heavy metals, something the chemical would not do.

* Calculating the waste flowing through the facility on an annual rather than seasonal basis. Critics say it would make it appear that the facility was handling much less than it actually was. "What that would mean is the sewage wouldn't get treated in the spring," Moore said.

* Using emergency backup facilities for regular operations to gain additional capacity, and relaxing standards during periods of heavy flow, when the untreated or less treated waste would be diluted.

* Eliminating the third part of the treatment--which includes the chlorination process that disinfects the water to reduce viruses--during heavy flows.

"It's not very good for the people who are drinking the water," Moore says. "The standards were already lowered as much as they could safely be lowered."

"Fairfax says it doesn't want sludge dumped anywhere near the Occoquan Basin and then it turns around and considers this kind of thing. It's the height of hypocrisy," Cole said.

Sloper, Prince William's representative to UOSA, said, however, that there is a substantial amount of excess capacity at the plant. Sloper estimated that the plant could handle at least 10 percent more sewage without any new construction.

The Occoquan Watershed Monitoring Laboratory, an independent, government-funded agency, earlier this year conducted a study similar to--though smaller than--the one being commissioned Monday. The study, issued in May, concluded "that the criteria set forth in the policy are, with a few exceptions, necessary to protect public health and water quality."

One person familiar with the issue said the decision by the local governments to get a new study means: "That what they're saying is: we don't like that report so we'll get another one."