A compromise between the U.S. Environmental Protection Agency and Washington area governments has cleared the way for the $300 million expansion of the Blue Plains regional sewage treatment plant, according to local officials.

The plan to increase capacity at the Potomac River facility -- already one of the largest in the world -- had been hung up since the fall by an EPA requirement that plants upstream from Blue Plains meet certain conditions for treating waste before the expansion could proceed. The EPA is expected to provide 85 percent of the funds for the expansion under the federal Clean Water Act.

Area officials protested those conditions, particularly one that required a procedure at plants in Arlington and Alexandria called nitrification, in which nitrogen in the waste water is treated with oxygen. They said nitrification would be costly and might not be effective in keeping the river clean.

In the compromise, the EPA agreed to drop the nitrification requirement pending the results of a study being conducted by the Metropolitan Washington Council of Governments. In return, local officials agreed that phosphorus removal would be improved at area treatment plants.

Fairfax County Executive J. Hamilton Lambert, who led the area's negotiating team, called the agreement "a tremendous breakthrough."

"This essentially means that the logjam has been broken and that we can go forward with an appropriate sewage program for this area while further improving the Potomac," Lambert said.

Blue Plains is the District's only sewage treatment plant and is used heavily by Montgomery and Prince George's counties. Fairfax County sends a small portion of its waste to be treated there, while Arlington and Alexandria rely largely on their own plants.

Lambert said the localities had been eager to reach an agreement with the EPA quickly for fear that the agency's funding would soon be slashed under the Gramm-Rudman-Hollings balanced budget law.

Under the agreement, the EPA has issued a draft permit for the Blue Plains expansion to the District government, which administers the plant. After a series of public hearings, a final permit is expected to be issued in the fall. The expansion is expected to take six years.

Although the compromise separates the issue of nitrification from the Blue Plains expansion, the longstanding technical debate about nitrification's merits continues.

The EPA, as well as some District, Virginia and Maryland state officials, have favored implementing nitrification as a way to improve water quality in the Potomac. Environmental experts disagree, however, on whether the treatment would make the river cleaner.

If the Council of Governments study concludes that nitrification is necessary, Arlington and Alexandria may be forced to spend more than $100 million to introduce the procedure at their sewage treatment plants, officials say.

The expansion at Blue Plains will increase its capacity by about 20 percent, to 370 million gallons of waste a day. The plant now treats an average of 290 million gallons a day and has a capacity of 309 million gallons.