The Stafford County Board of Supervisors yesterday unanimously rejected a Swiss company's plan to construct a sewage treatment facility that would turn 800 tons of District of Columbia waste a day into marketable compost.

More than 100 area residents attended the board meeting to oppose he Dano Resource Recovery, Inc., proposal to use a tract of land along Aquia Creek as a waste treatment center.

The board rejected the plan because of "too many unanswered questions" about the plant's environmental and health impact, according to Peggy Marshall, assistant county administrator.

Prince George's County Council rejected a similar proposal by Dano in January, saying it was undesirable to have a waste project close to residential areas.

If Dano does not locate a treatment site by October 31 it will lose a $4-million-a-year contract with the District of Columbia, a company spokesman said yesterday.

The spokesman said he was confident the company will find an alternative sire before the October 31 deadline, and that the 5-year contract with the District of Columbia will be saved. But he refused to discolse any sites under consideration by the company.

Dano's District of Columbia contract is the only one the Swiss-based firm has in the United States, although it says it has similar operations throughout Europe, Africa and South America.

The company specializes in a process it says combines sludge and household refuse to produce compost.