Mix sewage sludge with ground up paper cups and old reports from the Occupational Safety and Health Administration, and what do you have?

What you have, according to a Swiss-based firm caled Dano, Inc., is an excellent compost that can help tomatoes grow and keep weeds out of the flower garden.

Danowants to build a $10 million composting plant in Prince William County that, the company says, could solve one of the region's messiest problems - what to do with the smelly and gooey sludge that is turned out, at the rate of 800 tons a day, by the Blue Plains sewage treatment plant in the District.

Prince William, which is generally hostile to regional solutions, has expressed interest in the proposal, and today a group of county people are flying to a Dana-built factory outside London to see how well the process works.

The plant, which would mix and pasteurize Blue Plains sludge and the District's paper trash to produce a salable compost that could be sold at $20 a ton, would be located at Cockpit Point on the Potomac River between Woodbridge and Quantico.

Although the site is already zoned industrial, the county attorney has decided that Dano will need a special use permit before it can build the facility, according to Supervisor Andrew J. Donnelly, who represents the area.

But Prince Wiiliam officials are showing interest because the plant could generate $400,000 annually in county taxes.

The impressions brought back by the group visiting London will be an important consideration in what county officials decide to do about the special use permit, Donnelly said. Both the Planning Commission and Board of Supervisors must vote approval - after a public hearing.

The Prince William possibility took on more significance yesterday when the District told U.S. District Court Judge John Lewis Smith Jr. "that it is just not feasible at this time to locate a composting site in the District of Columbia."

The District and other jurisdictions using Blue Plains - Montgomery, Prince George's and Fairfax counties - were called to court to present plans for how they would each handle their share of sludge - a byproduct of wastewater treatment.

water treatment.

Montgomery representatives said that for nine months, the county would bury its share in trenches on a 60-acre, county-owned site at Clarksburg,in the upper part of the county west of I-270. After the nine months, the county said, it would compost sludge at an 80-acre site in the Montgomery Industrial Park on Rte. 29.

The Montgomery County Council approved the site last year, but plans were shelved when the Prince George's County Council objected on grounds that the location was too close to Prince George's residents. Objections could kill the site this time, too.

In the plan it gave Smith yesterday, Prince George's said it would continue trenching at its Cheltenham site and spray some sludge - digested to reduce ador and harmful, disease-causing micro-organisms called pathogens - on land at Andrews Air Force Base.

Fairfax County said it would incinerate its share at the county's Lower Potomac treatment plant.

Because the District had no solution for its share of sludge, another court session was scheduled for June 22.

District assistant corporation counsel Frederick F. Stiehl said Dano made the lowest bid when the city asked for commercial proposals earlier this year, and that if the Prince William site is approved by the county, Blue Plains sludge - mixed with paper trash - could be converted to compost at the Dano facility.

"It sounds too good to be true," Stiehl said.