Each day, the Upper Occoquan Sewage Authority plant in Prince William County dumps high levels of nitrogen into Bull Run and, ultimately, into the Chesapeake Bay 150 miles away.

In the Chesapeake, nitrogen is deadly, creating what are known as "dead zones" -- places in the water where it helps spur the growth of algae, which depletes oxygen in the water and kills animals and plant life.

If state officials approve stiffer regulations this month, the sewage authority, which serves 80,000 households in Northern Virginia, will have to spend about $100 million to rebuild its facility to reduce nitrogen levels by more than 80 percent.

It is an expensive prospect, and one that about 120 plants statewide are facing. Total upgrades could cost $2.3 billion, and the bill will fall to Virginia residents whose sewer fees will rise by 50 percent over the next five years, officials estimate.

Already, virtually all the state's sewage treatment plants are trying to reduce nitrogen. Arlington's plant has embarked on a $350 million project to reduce nitrogen and make other improvements. County residents will see their bills rise from $500 this year to $570 next year.

But the Upper Occoquan Sewage Authority has decided to put up a fight.

Its officials say that saving the Chesapeake could hurt the Occoquan Reservoir downstream of the Bull Run plant. The reservoir, which is owned by the Fairfax County Water Authority and supplies water to more than a half-million Northern Virginians, is the largest reservoir in the United States that contains reclaimed wastewater. So far, it has had no problems with nitrogen levels or algae blooms, officials said.

If the plant cuts the nitrogen in its discharge by 80 percent, officials say, there could be an unforeseen negative effect on the Occoquan Reservoir. The matter needs further study, they say. The water authority agrees.

"You best know what you're doing before you make these large-scale decisions without adequate scientific information in hand," said Thomas J. Grizzard, director of the Occoquan Watershed Monitoring Laboratory, an independent agency.

In Virginia, there are no limits on how much nitrogen can be discharged by a sewage treatment plant, but the state's Water Control Board has given preliminary approval to capping nitrogen discharge at 3 to 8 milligrams per liter, depending on the age and technology of the plant. The Occoquan plant discharges between 16 and 21 milligrams per liter, according to James L. Bannwart, the authority's executive director.

The cap is scheduled for a final vote Sept. 27.

The proposed state regulations, which follow guidelines from the Environmental Protection Agency, are part of a massive effort, estimated to cost $10 billion, to save the Chesapeake.

Chuck Epes, a spokesman for the Chesapeake Bay Foundation in Virginia, said the Upper Occoquan sewage treatment plant should stop fighting the changes.

"They question the science. They question the timing. They question everything. And they are alone in that," Epes said. "There are several square miles of Gatorade green water [in the Chesapeake] that is so full of algae that it looks like green oatmeal floating in it."

An underlying problem, he said, may be the other green: money. "We are very much aware that localities and ratepayers will be saddled with the bill," Epes said.

The state water control board will decide whether there should be more research, said Thomas A. Faha, water permits manager for the Department of Environmental Quality's Northern Virginia regional office.

The foundation pushed for a "flush tax" on sewer users, similar to the one in Maryland, to be used to clean up the bay, but the tax was unpopular among state legislators.

Officials at the Upper Occoquan Sewage Authority say retrofitting its plant could lead to unintended negative consequences downstream. Virginia is considering limiting the amount of discharged nitrogen.