A King George County judge yesterday extended his order prohibiting an Alexandria firm from building a trash compositing plant in the county to dispose of District of Columbia sewage sludge.

Circuit Court Judge John A Jamison barred Dano Resource Recovery INC. from constructing the composting plant until it has obtained permits from Virginia's Board of Health, Air Pollution Control Board and State Water Control Board.

Virginia Attorney General J. Marshall Coleman requested the court order to block the plant's construction. Jamison issued a similar order March 5, but it expired this month. Dano attorneys did not object to the new injunction at yesterday's hearing.

Dano already has $27.5 million in contracts with the District of Columbia government for disposal of the city's sewage sludge and trash. It plans to bild an $11 million composting plant on a 195-acre site in King George County, about 50 miles south of Washington.

King George County residents and officials have opposed the firm's plans.

Yesterday, Jamison also ordered Dano to disclose information about its financial backing by September 1. Coleman, who was at yesterday's court hearing, contended that such financial disclosure was needed to make sure the company had sufficient funds "to clean up any mess that may be created" at its proposed plant.