Efforts to find a regional home for a major sewage sludge compositing plant suffered another serious blow Wednesday night when the Prince William County Planning Commission recommended rejection of a proposal to locate such a facility in the country.

"This is a monstrous proposal," said the Commissioner Christopher Thomaidis, asserting that the enviromental safety of the plant has not been demonstrated and expressing fears about increased traffic. The plant would be located at Cockpit Point on the Potomac River between Woodbridge and Quantico Marine Base in the Dumfries area.

The plan, which would provide a solution to the mounting output of sludge produced by the Blue Plains regional sewage treatment plant in the District of Columbia, had encountered stiff opposition from neighboring residents. At Wednesday night's meeting, resident Barbara Kirby said Prince William would become the "dung capital of the country and perhaps the world" if the composting facility were built.

The planning commission vote turned aside a heavily qualified recommendation by county staff supporting construction of the facility. The Prince William Board of Supervisors will take up the matters Spet. 5. While the supervisors make the final decisions, they are likely to be strongly influenced by the unanimous vote of the planning commissioners.

An official of Dano Resources Recovery Inc., the Washington firm that wants to build the facility, expressed little surprise or dismay at last night's vote.

"We knew for a month this was coming," vice president Henry Valentino said.

Valentine said Dano is already exploring other sites in the metropolitan area, and that some of them are likely to encounter less opposition because they are more remote from residential neighborhoods. He would not say where the sites are.

The Dano plant would have mixed sludge with trash from the District to make compost that if claimed it would be able to sell commercially. Opponents of the plant said at last night's meeting that previous scheme elsewhere in the nation have failed to find a market for compost because of fears of harmful disease organisms and toxic heavy metals in the material, which is used to condition and fertilize soil.

Dano has a contract from the District government to dispose of Blue Plains Sludge. All Blue Plains users except the District have agreed to dispose of their share of sludge produced at the regional plant. Earlier this month a federal judge ordered the District to take care of its share