The board of the Metropolitan Washington Council of Governments yesterday approved a far-reaching plan to clean up the Potomac River and keep it clean for the next 20 years.

The keystone of the plan is construction of a big regional sewage treatment plant on the Potomac at Dickerson in upper Montgomery County.

The Dickerson proposal has provoked considerable opposition because effluent would be charged above pipes collecting most of the region's drinking water. Furthermore, the federal Environmental protection Agency has vetoed the project, although that decision is being appealed by Maryland.

With Dickerson in doubt, a last-minute contingency was added to the 20-year clean-up plan. The alternative says that if the proposed plant is torpedoed, excess sewage flows will be treated equally in Montgomery, although no site is the specified, and at the Piscataway plant in Prince George's County.

Prince George's officials said they are unhappy with the Piscataway contingency. They are afraid that if it is adopted, their county could become "the sewer capital" of the region.

The COG-approved plan must still be certified by regional localities, as well as Maryland and Virginia, and the possibility of a vote in Prince George's is likely, officials there said.

The plan must also be approved by EPA, which is on record as opposing the main component - dickerson.