An elaborate and controversial fail-safe system at a new regional sewage treatment plant prevented millions of gallons of raw sewage from being dumped in Northern Virginia's Occoquan Reservoir during January.

The seven-month-old plant, whose sophisticated design led to a total cost of $84 million and provoked much controversy, handled twice its normal flows during the month -- one of the wettest Januarys on record. On one day alone more than 24.7 million gallons of sewage -- more than four times the normal amount and more than twice the plant's rated capacity -- was handled without incident.

The biggest portion of the extra sewage came from Prince William County, whose flows to the Upper Occoquan Sewage Authority plant near Manassas where as much as 6 1/2 times the normal amount.

Prince William has been one of the biggest critics of the plant, which was designed with a number of backup features to prevent contamination of the Occoquan Reservoir -- the drinking-water supply for more than 600,000 Northern Virginians. The plant is located in southern Fairfax County on a tributary of the Occoquan.

The heavy flows to the plant in January were caused by rain and groundwater seeping into sewer pipelines. Plant statistics indicate Prince William has the leakiest pipes of all the facility's users.

Ordinarily Prince William sends about 2 million gallons daily to the plant. But in January, its average daily flow was 5,336,000 gallons. On one day, the county's flow reached 13,650,000 gallons.

Prince William Supervisor Donald White (D-Gainesville), one of the main critics of the UOSA plant, said yesterday the facility is "a white elephant." As for the features -- which kept raw sewage out of the Occoquan Reservoir in January -- White said, "we are of the opinion they are not needed."

White said that if Prince William received the federal funds it requested for pipeline repairs, the leaky pipe problem would have been solved.

The sanitary district's pipeline system -- built by private developers in the early 1960s before Prince William had a pipe inspection system -- has been described as "one big failing septic tank" by Thomas M. Schwarbert Jr., regional director of the Virginia Water Control Board.

"They (Sanitary District officials) should have done repair work in years past," Schwarbert said. "Their inspection program stunk. They could have used their time a lot better and used excess funds for correcting their time a lot better and used excess funds for correcting their problems."