Every day 4 1/2 million gallons of water - enough to supply about 45,000 people - leaks, unused, from the District's McMillan Reservoir in Northeast into the city's sewer pipeline.

The leak deprives the city not only of potential drinking water but of an even more critical resource - treatment capacity at the Blue Plains regional sewage plant.

"The plant doesn't know the difference," one Washington official said. "It treats water just as it treats sewage." Because it has to treat the McMillan leak - and other clean water that infiltrates the District's sewer pipe network - the plant is running out of its ability to treat sewage generated in Washington, now in the middle of a building boom.

Based on current building projections, the city could run out of capacity at the plant - the only one it has - as early as 1980. If the McMillan leak could be plugged, the city could buy another three to four years' time before having to build a new plant, expand Blue Plains or declare a moratorium on new development.

The leak underscores what city and suburban officials are beginning to identify as a significant problem - infiltration of water into sewer pipelines.

When sewage treatment costs were low and plants were large enough to handle all the area-s wastes, pipeline maintenance had a low priority and standards for the construction of new lines were low. As a result, millions of gallons of water began to seep daily into area pipelines and had to be treated as sewage.

The Washington Suburban Sanitary Commission estimates that of the 147 million gallons of sewage it produces daily, 52 million gallons are actually water that has infiltrated the 3,300-milelong pipeline network in Montgomery and Prince George's counties.

The District estimates that of the 131 million gallons daily it sends to Blue Plains for treatment, 30 million to 40 million are from infiltration. During wet periods, officials say, daily infiltration is as much as 70 million gallons.

The WSSC estimates that 20 million gallons' worth of its daily leakage could be plugged under what the federal Environmental Protection Agency calls "cost-effective" standards. The cost would probably be about $12 million, according to Michael Bonk, who heads the research office of the WSSC's "Find It and Fix It" program.

A pipeline repair meets EPA standards - and is eligible for 75 percent funding - if the cost of the repair is less than the cost of continuing to treat the water as sewage.

Although the District has, so far, surveyed only two of its eight study areas, it has discovered 26 1/2 million gallons of daily infiltration. While the biggest single source appears to be McMillan Reservoir, another 500,000 to a million gallons leak daily into the sewerage from the swimming pool at Banneker Recreation Center, officials say.

A consultant's study of the ROck Creek basin shows that of the 18.8-million-gallon daily pipeline flow in that area of Northwest, 11 million gallons are water from infiltration. According to Edward M. Hailey, who heads the city's water and sweer engineering and construction division, it would probably be practical to repair 9 1/2 million gallons' worth of leaks.

The cost, Halley says, could approach $10 million. The cost fo= repairing leaks throughout the city's 3,500-mile network of pipes, he said, would be about $59 million.

While the federal government could pay up to 75 percent of the costs, the District would still have to pay aminimumo f $14.8 million as its share.

"We're trying to sell something," Halley said, "and it's a hard thing to sell because there are so many constraints on the city's budget. The numbers are frightening to the budget people."

To win support from the budget office, the Department of Environmental Services, which includes Halley's office, has put together a slide show called "All You Wanted to Know About Your Sewers."

After tracing the history of sewer building in the city - an ambitious program that began in the 1870s and converted hundreds of acres of marshy and perennially flooded land into prime areas for development - the slide show gets to the message:

"Today's very modest budget allocation does not permit proper maintenance . . . Washington's sewer system is suffering from neglect, and there will come a time tomorrow when someone must pay."

Major sections of Washington's sewer network were laid when Rutherford B. Hayes was in the White House. According tr Harold M. Stearn, chief of the bureau of water and sewer services, funds for maintenance and improvement for the past 10 years have been only one-teeth of what they should have been.

"There is a heck of a lot catching up to do," Stearn said. "I don't think they (the budget office) have any idea. There is a big communications gap."

But that gap, Stearn and his colleagues hope, will be closed by the prospect of the District running out of capacity at Blue Plains before facilities elsewhere are available. A more certain spur to the education of District Budget officials in the costs of leaks will be the rapid increase in the cost of threating sewage officials predict.

Each million gallons of sewage treated at Blue Plains today costs about $250.When the plant is improved, sometime in 1980, according to the schedule, that cost willn rise to at least $375 for each million gallons. Sewage processed through "Blue Plains, after 1980, could cost $115,875 a day.

While the estimated cost of fixinb 30 million gallons' worth of leaks in the District has been put at $59 million, the cost for building a sewage treatment facility that could handle that amount would be at least double that sum, according to current construction prices.

While fixing leaks offers a way to reduce the costs of treating sewage, such repair work can be a formidable task - especially if streets in densely populated areas have to be torn up. In addition, no one knows for sure if leaks plugged in one area will not simply divert groundwater - the major source of infiltration - to another area of the pipeline susceptible to seepage.