The District of Columbia, in the midst of a major building boom, could face the kind of sudden moratorium on development that struck Montgomery County early in this decade if more sewage-treatment capacity is not found, a new study shows.

Furthermore, although the metropolitan area is doing a better job of purifying its sewage before flushing it into the Potomac River, pollutants from a variety of other sources will continue to foul the river unless stricter controls are adopted.

These are among the central points in a $3 million, three-year study of Washington area sewer and water supply problems just completed by the Metropolitan Washington Council of Governments and its Water Resources Planning Board.

Although the situation for the District of Columbia is not dire today, it could become so quickly "if we do not find a solution" to the capacity shortage, D.C. planning director Ben W. Gilbert agreed in an interview. A pending lawsuit and continued cooperation among local area governments are the keys to keeping the District of Columbia out of trouble.

According to figures published in the report, new construction and sewage treatment requirements will exceed the District's allotted capacity in the regional Blue Plains sewage treatment plant in far Southwest Washington by 1980.

The District reached the same conclusion in an internal report one year ago. But since that time it has moved to buy temporary sewage capacity from other jurisdictions and find permanent solutions.

Central Montgomery County and the Reston-Herndon sections of Fairfax County race similar problems.

"We're not currently scraping the ceiling in Washington," Gilbert said, "and we have healthy growth for a change. We want to nourish that and don't want developers to worry." Gilbert said that an interim agreement the District reached with Montgomery and Prince George's County last year will permit Washington to use unneeded Maryland sewage capacity in Blue Plains for a few years.

In the meantime, the central regional sewer issue - the controversial proposed Dickerson plant on the Potomac in western Montgomery County - will have to be resolved. Either Dickerson will have to be built - and the U.S. Environmental Protection Agency has refused to approve plans to do so - or alternaties will have to be found.

Montgomery County is suing the EPA over the Dickerson decision. The District of Columbia, as part of the agreement to borrow sewage treatment capacity from Maryland, has joined the suit on the side of Montgomery County.

The politics of Dickerson permeate the "Water Quality Management Plan," as the new COG study is officially called. For the thing, the plan assumes that Dickerson will be completed, despite the EPA action.

That action came after work began on the plan. But Maryland officials, led by Montgomery County Executive James P. Gleason Jr., vigorously beat efforts to have the plan reflect the apparent reality: no Dickerson.

Despite the flaw, there are enough charts and graphs and tables and maps to tell the reader that Dickerson isn't real yet. Furthermore, the narrative portion of the plan clearly underlines the split between planners and politicians on one hand and environmentalists and no growth advocates on the other.

For example, the plan recommends that sewage treatment plants be permitted to purify less thoroughly in winter to purify less thoroughly in winter than in summer. Such as approache is cheaper and the added pollution load is offset by the higher dilution rate of heavier wintertime river flows, the theory goes.

The Citizens Advisory Committee urged top-level treatment year-round.

One of the central concerns about Dickerson is that it would discharge into the Potomac above the area's drinking water intakes. As a result, an extraordianarily high level of treatment was required in Dickerson plans. The plan suggests that if Dickerson is built, that level will be satisfactory.

The Citizen Advisory Committee voted to 'prohibit' new direct discharges above water supply intakes."

Rhea Cohen, a Prince George's County citizen activist and Sierra Club employe, who has been active in the environmental movement in this area and who was chairman of the citizens advisory committee, was critical of many aspects of the plan. The central point, she said, was that the plan "doesn't show how the Potomac River will be cleaned up."

The plan does make a number of specific recommendations, some of which area jurisdictions have already begun to adopt. It suggests that:

The use of phosphate detergents be banned in the area. Phosphates require extensive treatment to remove them from sewage; removing them from the source would save $4.6 million a year, the plan estimates.

Water-saving plumbing codes be adopted. Under such codes, now in force in Maryland, the District of Columbia and Fairfax County, water-saving plumbing fixtures are required in new construction. If less water is used, less sewage is created.

Impact on water quality should be assessed before new development occurs.

Better controls need to be applied to reduce the pollutants that flush into rivers and streams during a rain-storm.

The report suggests that land treatment of sewage - a favorite technique of many area environmentalists - should be considered, perhaps as one of the alternatives to Dickerson. But it also points out that land treatment includes "the need to acquire or lease approximately 330 acres" for every million gallons a day.

The plan supports the composting of sewage sludge - the semisolid residue left after raw sewage is purified. The disposal of sludge is a continuing controversial question in the region, and improved treatment techniques are producing increasing amounts of slude.

Pilot programs have found that sludge can be composted and used as a land reconditioner.

The pollutants that roll off rooftops, highways, parking lots and construction sites create one of the largest problems. According to the plan, more than 100,000 acres of rural land in the Washington area will be developed in the next 20 years.

That means that land used for residential, commercial or industrial purposes will increase by 34 percent, and pollution levels will increase by 25 percent, the plan said.

The plan projects that the area's population will grow from about 3 million to 4.2 million by 1995, although planners such as Gilbert acknowledge that recent trends show that figure to be too high.

There will be three public hearings on the plan:

April 10, 7:30 p.m., George Mason Junior/Senior High School Auditorium, Leesburg Pike and Haycock Road, Falls Church.

April 11, 7:30 p.m., Maryland-National Capital Park and Planning Commission, 8787 Georgia Ave., Silver Spring.

April 12, 7 p.m., D.C. City Council Chambers, Room 500, District Building 14th and E Streets NW, Washington