A new permit will slash the amount of nitrogen that can be discharged from the watershed's largest wastewater treatment plant by 45 percent, or 3.8 million pounds a year.

The EPA in September issued a five-year permit that would limit the amount of nitrogen that can be discharged from the Blue Plains Advanced Wastewater Treatment Facility to 4.7 million pounds a year, effective in 2015.

The previous permit limited discharges to 8.5 million pounds of nitrogen a year, although the plant typically discharged less than that amount.

The levels prescribed in the new permit will be achieved when an expected $950 million upgrade begins shortly that is scheduled for completion by July 14, 2014.

"These reductions are critical to protecting the health of the Chesapeake Bay as well as the Potomac River" said Shawn Garvin, administrator for EPA Region III, which includes most of the Bay watershed. "By significantly reducing nitrogen pollution from the Blue Plains plan, we're taking a major step on the road to restoring the Bay for future generations."

Although the plant, operated by DC Water (the District of Columbia Water and Sewer Authority), is the single largest discharger in the watershed, it has already greatly reduced nitrogen discharges since 1985 through previous upgrades.

That has resulted in noticeable improvements in water quality.

A recent paper credited reduced nitrogen discharges from the facility for a resurgence of underwater grasses in the Potomac River over the last 15 years.

"DC Water was the first to meet the Chesapeake Bay Program goals to reduce nitrogen levels by 40 percent of the 1985 levels," said DC Water General Manager George Hawkins. "And we have continued to meet the program goal every year since."

The plant already achieves the phosphorus removal goals in the permit.

Blue Plains serves the District of Columbia, Montgomery and Prince George's counties in Maryland, and Fairfax and Loudoun counties in Virginia. Overall, it treats wastewater from about 1.6 million people.

The plant, which covers 150 acres, is the largest advanced wastewater treatment facility in the world. It has the capacity to handle 370 million gallons of wastewater a day.