Used plastic containers, long regarded as the unwanted and eternal dregs of a throwaway society, are being profitably resurrected as carpets, corrugated pipe, blankets and park benches.

An estimated 15 billion to 20 billion pounds of plastics land in the country's waste stream each year. Less than 1 percent is recycled.Manufacturers have routinely recycled plastic scraps, but only recently have companies and communities - faced with littered landscapes and bulging landfills - taken seriously the recycling of used plastic containers.

There's new hope for many formerly unsalvageable containers such as detergent and ketchup bottles, made of various plastics. New processes blend formerly incompatible plastics into everything from fence posts to automobile barriers. Like wood, the blended plastic can be nailed, screwed, cut and planed.

Even dirty garbage bags, once considered beyond salvaging, now can be blended with other plastics to create a lumber substitute. More and more clean bags that have been used are being recycled into new bags.

"We're doing research now to see if we can make these commingled plastics load-bearing," says Darrell Morrow, director of the Center for Plastics Recycling Research at Rutgers University. "Being impervious to weather and insects, the material would make wonderful railroad ties. Who knows, houses of the future might even be made of plastic."

Experiments at the center here indicate polystyrene foam, common in cups, trays and packaging, might add strength to commingled plastics. "At the very least, we expect the foam will be turned into housing insulation someday," Morrow told National Geographic.

The two plastics favored by recyclers, polyethylene terephthalate (PET) and high-density polyethylene (HDPE), come from bottles and other containers. Most soft-drink bottles are made primarily from PET; milk and juice jugs, from HDPE.

The largest U.S. PET recycler, Wellman Inc. of Clark, N.J., needed about 800 million bottles to produce 100 million pounds of recycled plastic last year. Much of it went into industrial resins used as fibers in carpets, blankets, furniture, wearing apparel and a thick fabric that helps stabilize railroad beds.

"We're expanding, and in the near future we could use three times as many bottles, but we have a collection problem," says Dennis M. Sabourin, vice president of Wellman's Trading Division.

Absence of an assured source of used plastics has been a major drawback for recyclers. Only nine U.S. states and three Canadian provinces now have deposit laws that encourage the return of empties for recycling. Only five states, New Jersey, Pennsylvania, Connecticut, Florida and Rhode Island, have statewide compulsory curbside collection of recyclables such as glass, aluminum, paper and plastics. Other states are expected to follow as they run out of landfill space.

"I'm convinced curbside collection, properly developed on a national scale, will be far superior to deposit laws in both cost effectiveness and level of participation," says Morrow.

He favors the development of regional recovery centers for recyclables that would serve 250,000 to 500,000 people each. "In many places," he says, "mom-and-pop-type operations might suffice. You wouldn't need multimillion-dollar complexes."

But most municipalities, concerned with lowering the weight of trash in their landfills, haven't mandated plastics in their curbside collections.

This is an oversight, says Luke B. Schmidt, president of the National Association for Plastic Container Recovery. "Plastics may account for only 4 to 7 percent of the total waste stream, but because of the size of containers, they account for more than 15 percent of the volume of trash at landfills."

Schmidt is convinced that if curbside collection of plastics becomes widespread, 600 million pounds of the material could be recycled by 1992. If export markets are found, the amount could be far greater.

Plastics are made almost entirely from petroleum and natural gas. Potential overseas buyers of recycled plastics are large petroleum importers such as Japan, which might dramatically increase purchases of recycled plastic in order to decrease the petroleum they now must use to manufacture plastics.