They don't know what to call it yet, possibly "Capital Compost," "Metro-Earth" or maybe "Washington's Best," but flowers and vegetables love it, it will be free and available by this fall in almost unlimited quantities for farmers, nurseries, parks and homeowners.

By whatever catchy name it's called, it will be Washington's 600-ton-a-day sewage sludge problem transformed - through composting with wood chips - into the world's largest mother lode of odor-free, almost sterile organic mulch.

It will be available to Northern Virginia residents at as yet undetermined sites, even though area residents contribute little sewage to the Blue Plains sewage treatment plant on the Potomac, opposite Alexandria, where the huge composting program will be run. Composting is also soon to be the sludge disposal method at the Upper Occoquan sewage treatment plant that will serve parts of Fairfax and Prince William counties.

U.S. Department of Agriculture scientists, who have been testing 60-ton-a-day sludge composting for more than three years in Beltsville, Md., have given a clean bill of health to Blue Plains' product - even for use in home gardens.

And a marketing study now being completed for the Metropolitan Washington Council of Governments has found that the peat-like compost will be so popular, whether given away free or sold at low cost, that even in 1980 when Blue Plains alone will be producing 1,800 tons of sludge a day there will be no problem in getting rid of it.

"This is good quality sludge compost compared to other cities, and we've done hundreds of anaylses," says Dr. James F. Parr, chief of USDA's biological waste management laboratory in Beltsville. "It's very exceptional compost for use in gardens. I use it in my flower pots and my colleagues use it in flower and vegetable gardens. I would, too, but I'm not a gardener."

The only major reservations about the use of composted sewage sludge in gardens concern the presence of heavy metals (zinc, copper, cadmium and lead) or microorganisims like salmonella, which can cause food poisoning.

The seven-week composting process, which mixes sludge with wood chips and in which natural decay raises temperatures above 150 degrees, kills virtually all salmonella and most other bacteria, leaving the compost odor-free and almost sterile.

However, USDA soil microbiologist Dr. Wylie Burge says, "It's still possible to locate salmonella in low numbers from time to time in the compost, but I don't think it constitutes a hazard, and I'm not afraid to use this compost in the vegetable garden." He said there is much more danger from salmonella from letting foods sit around at room temperature or walking in a park where dogs are given free rein than from the compost.

Maryland health officials are not concerned about the rare presence of salmonella in the compost but are worried about the presence of cadmium, residues of which can build up in the body and cause liver or kidney damage and hypertension. However, USDA scientists, while praising Maryland's caution, say the presence of heavy metals in Washington sewage is extremely low - since there are few industrial plants that pollute the area's water - and that it constitutes no threat.

A spokesman for Maryland's health department said, "We're very much in favor of this, with only one reservation, that with indiscriminate use in home vegetable gardens, especially with root crops that are eaten raw, there could be cadmium buildup."

Neither Virginia nor District officials have questioned its quality, however, and free distribution should begin as soon as the Blue Plains composting gets under way and distribution points are settled on.

Jean Levesque, director of the District's water resources management administration and an enthusiastic composter, said, "We never had any problems getting rid of sludge from 1938 until 1969" when the suburbs had become so built up tha the odoriferous raw sewage could no longer be spread on open farm fields. Blue Plains' raw sewage sludge is now buried - currently in upper Montgomery County - at great expense, although "digested" sludge, which is heat treated to reduce gas and liquid, is still used on area farms as well as for composting at Beltsville. About 50 per cent of Blue Plains' daily output of 600 tons of sludge is "raw" (not heat-treated). The advanced waste-treatment equipment now going in at Blue Plains will remove twice again as much sludge, or solid matter, from area sewage water, thus greatly improving quality of the remaining liquid dumped into the Potomac River. Blue Plains has long been the river's major polluter.

Levesque said sewage sludge has gotten such a bad name in the past few years that "we're going to have to advertise the compost in order to get rid of it. I imagine we'll come up with some catchy name but initially at least we'll have to give it away free."

The principal users of Blue Plains (the District and Montgomery and Prince George's counties) are expected this spring to approve construction of sludge incinerators at Blue Plains, in case composting isn't so wildly popular as most officials expect it to be and as backup for the composting program.

The U.S. environmental Protection Agency had opposed early designs for the incinerators as wasteful of fuel - large quantities of fuel are needed to burn wet sludge - but the new designs could burn sludge that is 60 per cent water with almost no fuel at all, says Levesque, "because at 60 per cent sludge will theoretically burn by itself."

But composting, recycling human waste, would still be cheaper and better environmentally, said Levesque. "We feel composting is the best way."

The marketing study done for COG by an environmental consulting firm here found in its final draft report that much of the compost would probably be used by farmers, especially since the use of chemical fertilizers in recent years has left "the organic content of farmland . . . somewhat depleted. Farmers have traditionally used organic materials such as manure, straw and husks to increase organic content of soils" but such use has dwindled.

Sewage compost, while generally a soil conditioner - improving both light sandy soils and heavy, compacted soil - it is also a low-grade fertilizer, containing about 1 per cent nitrogen, 2 per cent phosphoric acid and a trace of potash, according to USDA. Thus to provide the same nutrients as a 5-10-5 chemical fertilizer it would take 5 to 10 times as much composted sludge as chemical fertilizer.

Washington's composted sludge, from the Beltsville pilot study, has already been used extensively here. The National Park Service is spreading it as mulch and soil conditioner throughout its gardens and flower beds - although in 1975 the White House balked at spreading composted sewage on the lawns just in case it was more odoriferous than advertised - and more than 9,000 tons of it was used in creating Constitutions Gardens, which opened last spring beside the Reflecting Pool. One hurdle to the composting plan is that Blue Plains needs 30 acres on which to put 600 tons a day of sludge in long piles for mixing and aerating and wants 17 acres of National Park Service land at Oxon Cove, where the Park Service planned to build a public golf course in 1978.

A lease is being worked out now, subject to public hearings and an environmental impact assessment, to lend the 17 acres for five years and condense the golf course's 18 holes, with Blue Plains enlarging the course at the end of five years. The Park Service expects some opposition to the move from Oxon Hill residents who, though they live at least a mile from the site, might fear it would be odoriferous.

A reporter's visit to the Beltsville site found little odor, and that not unpleasant, because the "aeration" method of composting uses pipes underneath the piles to suck air into the compost and the air is then filtered.

The marketing study found many U.S. cities already selling or giving away sewage sludge of one kind or another - some of it heat-dried and others blended - but all with "catchy" names from the Famous Milorganite, now a "household word," according to the report, to names like Chicago's "Nu-Earth," Houston's "Hou-actinite," and Los Angeles's "Nitrohumus." Milorganite is Milwaukee organic nitrogin, sludge heat-treated to kill organisms. None of the cities mentioned above has as large a sludge composting operation as Washington is proposing.