The $500 million biological nutrient removal equipment at the Back River plant - the state's largest sewage plant - pumps the sludge full of oxygen and microbes that consume about 30,000 pounds of its nitrogen on a typical day.

Despite the size, cost and sweep of the nitrogen removal project, the plant still falls short of voluntary limits set by the state - 8 milligrams per liter of nitrogen in discharged water. The Back River, which wends its way to the Chesapeake Bay, remains one of the state's most endangered waterways.

At a time when Gov. Parris N. Glendening is asking rural homeowners and businesses to pay thousands of dollars for septic tank upgrades to eliminate small amounts of nutrient pollution, the state's two largest sewage treatment plants pour nitrogen into the Chesapeake Bay and its tributaries because they can't meet voluntary state standards.

That doesn't sit right with Sen. J. Lowell Stoltzfus, R-Somerset, a vocal opponent of Glendening's septic legislation.

"It's absurd. We are letting them go free and putting regulations on others that are far more costly," he said. "It makes you wonder are we genuinely concerned about removing nitrogen or...about regulating everyone and getting our meat-hooks into everybody?"

Glendening spokeswoman Raquel Guillory said the governor is committed to nutrient management statewide.

"Everyone has got to do their part," she said.

Although the naturally occurring nitrogen and phosphorous sound innocuous, they're not. Excessive amounts feed algae blooms that choke aquatic life, and have been tied to the 1997 outbreak of a fish-killing microbe, Pfiesteria piscicida.

The effluent pumped into the Back River's imperiled water rarely contains less than 10 milligrams per liter of nitrogen, said Paul S. Ander, operating engineer for the plant. They're doing the best they can, he said. "Given what we have, (the technology) is doing as well as it can," he said. But to reach the goal, more needs to be done, he said.

An even bigger problem lies about 10 miles southeast of the Back River plant, along another endangered water body - the Patapsco River.

Here the state's second-largest sewage facility - the Patapsco Wastewater Treatment Plant - treats 65 million gallons of raw sewage and feeds the river about 6,600 pounds of nitrogen daily. The plant has no nitrogen limit and only a lenient one for phosphorous.

Last upgraded in the early 1980's, the Patapsco plant wasn't built to handle nitrogen-removal technology and doesn't have enough land for the dozens of tanks and high-powered oxygen pumps necessary for a new system.

Ander shakes his head when asked about the prospects for the Patapsco plant. "I am not sure where all of that is going to end up, to be honest," he said.

The septic tanks that Glendening wants to regulate produce about 6 percent of the bay's nitrogen. Sewage treatment plants and industrial sites contribute about 33 percent, according to Department of Natural Resources reports.

The presence of the elements was of such concern that the 1987 Chesapeake Bay Agreement between the federal government, Maryland, Virginia, the District of Columbia and Pennsylvania promised to reduce their levels in the bay by 40 percent of their 1985 levels by 2000.

The Baltimore plants handle waste from 1.4 million people in Baltimore, Baltimore County and parts of Howard and Anne Arundel counties. They sit at the bottom of the Patapsco-Back River watershed, one of the most urbanized and endangered of the 10 watersheds bound to the state's Tributary Strategies, which were drafted in 1993 to reduce nutrients in the bay's tributaries.

The two plants, along with industrial sites, supply 76 percent of their watershed's nitrogen.

While levels of nutrient pollution in the Patapsco and Back Rivers have dropped since 1985, they are still recovering from centuries of industrial waste and continue to exceed optimum standards.

Monitoring stations in the Back River and Patapsco River received "poor" ratings in 1998 for nutrient pollution by the Chesapeake Bay Program, which directs the interstate agreement.

The Patapsco monitoring station was one of only two upper Maryland tributary stations rated "poor" for each measured category: nitrogen, phosphorous, suspended solids and chlorophyll, a water clarity indicator. The Back River station rated poor in every category except suspended solids.

State officials say nitrogen-removal technology has been a success - eliminating 12 million pounds of nitrogen from wastewater plants and industrial sites yearly since 1985, or 73 percent of the goal. Phosphorous levels, too, have been cut, and are nearing 94 percent of the target.

But environmental groups say the state's two largest wastewater treatment plants need more attention if they are to meet their goals.

"It's a matter of scale," said Barbara Taylor-Suit, executive director of Save Our Streams. "When you have a large urban area with a heavy population, monies cut and the economic base declining, it gets harder. (The plants) need to be much more of a priority."

Theresa Pierno, executive director of the Chesapeake Bay Foundation, said she's been impressed by state efforts to install the technology in wastewater plants, but said she would "like to see them moving faster" at Baltimore's plants because "they are key."

The two plants are hampered in meeting the goals by tricky biological processes and a lack of space and funding.

Nitrification, the biological conversion of nitrates to inert nitrogen gas, is hypersensitive and often fails. Nitrogen-hungry microbes need warm temperatures and adequate amounts of oxygen, conditions fostered by lots of tank space.

These bacteria are "fussy little prima donnas" and when the environment changes, their digestion of nitrogen is the first thing to go, said John Martin, operating engineer for Baltimore's Wastewater Engineering Division.

The Back River plant has installed 36 different tanks and two buildings full of 1,500 horsepower oxygen pumps to circulate the effluent, but more are needed if the extra nitrogen is to be removed, Ander said.

And that means more money and more space, both of which the 634-acre plant has in short supply.

However, the plant's nitrogen emissions are nearly half of what they were before the technology was installed in 1997, said Virginia Kearney, manager of the water quality infrastructure program at the Maryland Department of the Environment.

"The state was aware of the fact that (the Back River plant) might not meet the 8-milligrams-per-liter goal, so this isn't a shock or a surprise," she said. "But 10 parts per million ain't bad."

As for Patapsco, she said, the plant "will be doing nitrogen removal, but when, I can't predict. Patapsco will be an investment for the state and Baltimore."

Eight other wastewater treatment plants have no nitrogen removal systems but are planning them. None of those plants come near the quantity of wastewater handled by the Patapsco or Back River plants.

Whether the Patapsco plant will be abandoned for a larger facility or expanded elsewhere is unclear, Kearney said.

However, the state is considering establishing mandatory limits for nitrogen and phosphorous, also known as total-maximum-daily-loads, which could lead to fines and lawsuits if the municipal plants fail to meet set goals.

With a sigh, Martin said that might mean a lot of work for Baltimore's wastewater plants.

"If that limit happens to be a number we can't achieve with our equipment, then we've got problems," Martin said. "But if laws are passed...then we'll have to bite the bullet."