The users of the big regional sewage treatment plant at Blue Plains have devised a novel way of expanding the facility without actually making it any bigger.

The beauty of the solution is that it would give the hard-pressed District and Montgomery County the extra sewage capacity they need to keep revenue-producing development moving along -- without encroaching on the share jealously guarded by Prince George's County.

Under the proposal, which must be approved by the Environmental Protection Agency, the calculation of sewage flow from Montgomery and the city would no longer include all of the rain and ground water that penetrate leaky pipes on the way to Blue Plains. This seepage, which has been estimated to account for 20 to 30 percent of all flows to Blue Plains, is called "infiltration and inflow."

The solution was worked out in a closed meeting of the chief administrative officers of the District, the Maryland suburbs and Fairfax County. The meeting was held Wednesday at the Metropolitan Washington Council of Governments, and its proposals were revealed yesterday, with the release of a letter from Fairfax Acting County Executive J. Hamilton Lambert to Montgomery County Administrator Robert W. Wilson.

Lambert said that a "modest" credit of 5 percent "would go a long way toward relieving the immediate crisis." For example, a 5 percent credit to the District would give it, for immediate use, an additional 6 3/4 million gallons of sewage capacity daily -- or probably enough to handle every bit of the $1.3 billion worth of development underway or planned in the downtown area, as well as commercial and residential construction elsewhere in the city.

Montgomery's credit about 3 1/2 million gallons daily, would handle shortterm needs, but, according to county estimates, would not handle all the requests made by developers for sewage capacity.

Montgomery has developer requests amounting to about 5 million gallons -- enough to serve an additional 50,000 people -- beyond what it can honor with its present capacity.

However, Montgomery's situation may not be as severe as the numbers indicate, because some of the development would not occur for years to come -- until after 1985, when the county is expected to have a new plant on line with a capacity of 20 million gallons.

According to various sources close to the negotiations, any jurisdiction receiving credit for seepage would have to pay back what amounts to a short-term loan. This could be done, the sources said, by correcting leaky pipes (all the local jurisdictions are already seeking 75 percent federal funding under an EPA program) or by instituting water-conservation programs.

In either case, some flows to Blue Plains would be reduced to make room for other flows (the so-called "credits").

Environmental activists are likely to view the entire proposal as a subtle scheme to increase Blue Plains' capacity and thus open the Potomac River to more pollution. But according to sources close to the negotiations, if the jurisdictions take steps to reduce seepage by the same 5 percent they are to receive as a credit, Blue Plains will not exceed its present EPA-imposed limitation of 309 million gallons a day.

Yet there is some fuzziness on this point. For example, Lambert's letter to Wilson says that even if credits are given, "the actual sanitary flows at Blue Plains should still be well below the 309 million-gallon-a-day limit and the strategy for environmental protection of the Potomac will remain intact."

One source interpreted "sanitary flows" as sewage and infiltration-in-flow, but another said they included only sewage. The answer is critical in determining whether Blue Plains is within its permit limitation.

The search for a solution to the District's and Montgomery's problems was precipitated when the city announced it would unilaterally take a bigger share of Blue Plains. Suburban Maryland sued the District, then both sides agreed to try an out-of-court settlement.