At the elaborate new regional treatment plant near the banks of Bull Run in Fairfax County, sewage will be routed through pipes painted in shades of aquamarine. At different treatment points, the pipes become lighter blue - a symbol that what they carry is pure enough to drink.

The effluent will not be drunk, but it will be emptied into water that forms the drinking supply for 600,000 Northern Virginians. The goal is to lessen the pollution of that supply.

To its supporters, the plant, which will open around June 1 after four years of construction, is "the state of the art" in treatment technology, the best hope for saving the Occoquan Reservoir from being destroyed by pollution.

To its critics it is a "gold-lined chamberpot," and overdesigned facility so expensive that some of its users "absolutely and postively cannot afford it."

For short, both supporters and critics call the plant "UOSA," an acornym for the Upper OCcoquan Sewage Authority. The authority was created seven years ago the agency to build and operate one big regional plant that would replace 11 smaller, outmoded plants in Fairfax and Prince William counties.

The smaller plants were discharging such dirty effluents that the down-stream Occoquan Reservoir was dying an early death.

The new plant was expected to reverse the death sentence. It was designed with the latest technology.Effluent standards were the strictest in the country. As a fail-safe measure everything was built in duplicate, except the power system, which is in triplicate (connected to two separate power substations as wall as a generator).

But all the technology and safe-guards cost dearly. The original estimate was about $42 million. Then the federal Environmental Protection Agency insisted that whatever was built during a funding cycle had to be a "separate operable unit." This, and inflation, helped escalate the total cost to $82 million and delayed construction by two years, according to UOSA executive director Millard H. Robbins Jr.

EPA picked up 75 per cent of "grant eligible" costs, but the state and localities had to pay the rest, including all land-acquisition costs. The localities burden became heavier when the Virginia General Assembly, after one year, stopped appropriating the 10 per cent share of costs that had been committed by the state.

For residents in Western Prince William, according to county officials UOSA would mean an almost 400 per cent increase in sewage-treatment costs - from average of $36 per quarter to $140.

"The people in this sanitary district absolutely and postively cannot afford it," said Donald L. White, chairman of the Prince William board of Supervisors, who would like to see the county back out of hooking up to the new plant and pay only its share of construction costs.

Former Board Chairman Alice Humphries, who coined the phrase "gold-lined chamberpot" said the federal government should take over the plant.

On Tuesday, the supervisors will consider whether the county will continue to operate its own sewage facilities - a move that would be strongly opposed by the State Water COntrol Board.

Regional official of the board are already annoyed with the Greater (Manassas Sanitary District for Failing to report sewer backups that have occurred during heavy rains. The District director John Sloper acknowledged in an interview that backup sewage was pumped into nearby creeks, and said all such instances were reported to the State Water Control Board.

However, the Manassas district's report an January operations said there were no overflow, even though Sloper, in an interview, said there were some overflows during heavy rains that month. The sewers back up when groundwater seeps through leaky pipes.

Engineer Noman G. Cole Jr., who while chairman of th Water Control Board was largely responsible for the UOSA plant being built, said the Prince William criticisms are a "smokescreen" so the county can continue bypassing sewer overflows. He said the county's rates at UOSA would be high because of all the stormwater that would have to be processed.

When the UOSA plant was conceived sewage was identified as the chief cause of polution in the Occoquan. But a six-year monitoring program has shown that stormwater runoff from both agricultural and urban areas contributes enormous quantities of all kinds of pollution - not just debris like paper cups and scraps of wood but algae-producing nitrogen and phosphorus and even heavy metals like zinc and cadmium.

The UOSA can do nothing about the pollution from runoff, and those who are studing the problem concede, like Dr. Clifford W. Randall, director of the monitoring program that the 'technology of stormwater control is not known." Futhermore, the new plant, with its extra capacity, will hasten development, which in turn will create more urban runoff.

Cole points out that the latest findings of monitoring back up the old theory that sewage was the chief culprit - at least in time of drought.

According to monitoring during last fall's drought most of the pollution-caused algae formed when effluent from existing plants enters the reservoir.

Cole also said it appears that most pollution from runoff does not come from myraid sources but just one - quick-release fertilizer, used on farms and in suburbia. If farmers and suburbanites shifted to a fertilizer that didn't release its phosphorus so quickly, he said, the chemical wouldn't become a big problem when runoff carried it into streams.

When critics complain about the high costs of treatment at the UOSA plant, Cole, a practiced defender, says "The wholesale cost is 90 cents a thousand gallons. That's 9 cent a day per person - not a whole lot to protect our water supply."