On Wednesday, Philadelphia officials will announce that cartons for juice, milk, soup and other liquids can now be recycled curbside.

"This is one of the last pieces of the easily recyclable waste stream that's left," said the city's recycling coordinator, David Biddle. "We're really winding down."

The new "carton" category includes the waxy-looking (actually plastic over paper) refrigerated containers for milk and juice, and the unrefrigerated cartons that contain soup, broth, soy milk, wine, and other liquids.

Since the cartons are lightweight, they are not expected to greatly budge the current residential recycling rate of nearly 20 percent.

But adding them to the stream is seen as a way to make recycling easier.

"The message is much simpler," Biddle said. "If it's a consumer products package, it's recyclable. That's what we've done with our system."

Officials are so pleased with the growth of recycling - the rate was 7 percent in 2008 - that they have begun to reconsider what to do with the rest of the trash.

The city recently sought proposals for evaluating the environmental and fiscal implications of building a waste-to-energy plant. In the past, this meant incineration.

Now, said Rina Cutler, deputy mayor for transportation and utilities, "we are pretty convinced that the technology has so significantly changed . . . that we think it is something we should at least seriously take a look at."

She said she was not yet sure whether she would recommend it. No potential site has been identified. Nor is it clear if the city would build and own a facility, or have someone else do it.

Joseph O. Minott, executive director of the Clean Air Council, said he was skeptical. "Certainly there are different models out there, but I have yet to see one that does not emit air pollution and create a hazardous waste," he said.

Others say the city should instead take the next step in recycling, albeit not an easy one - adding a collection system for organic material that can be composted, 34.8 percent of the city's waste stream.

"If we added composting to our collection at the curb, we'd get to 40 percent in a hurry," said Maurice Sampson II, president of Niche Recycling Inc., a longtime Philadelphia recycling advocate.

The U.S. Environmental Protection Agency's hierarchy of solid-waste management puts source reduction at the top, recycling and composting in the middle, disposal in landfills or combustion facilities at the bottom.

Nationally, "if we were at a 60, 70 percent recycling rate, then we could talk about burning waste," said Allen Hershkowitz, senior scientist with the Natural Resources Defense Council. "But we're very, very far away."

The city sends 55 percent of its non-recycled trash to traditional waste-to-energy facilities in Chester and Falls Township, Bucks County. The other 45 percent is landfilled.

The city has to pay to dispose of trash - about $68 a ton. But because recyclables are a commodity, they earn the city money - about $67 a ton currently, although that fluctuates.

Carton recycling is seen as a glowing example of what can happen when industry decides to solve a problem.

On the front end, cartons have ranked high on the sustainability scale. Because they are lightweight, it takes less energy to transport them. Because they are square, more can be packed into a smaller space. The foil-lined cartons also are sterile, so the contents don't have to be refrigerated.

But because they are a sandwich of materials, cartons had not been recyclable.

"They were considered a perfect package," said Sampson. "But also perfect trash."

Derric Brown, director of sustainability for the Carton Council, said the industry fostered new technologies.

In Philadelphia, the council invested in new equipment at two sorting facilities - Waste Management's in the Northeast and Blue Mountain in Grays Ferry.

The industry also worked to build "end markets" for the materials: recycled paper products, plastic lumber, and other uses.

The refrigerated "gable-top" cartons look waxy but are actually paper and polyethylene. The "shelf-stable" cartons are paper, polyethylene, and aluminum.

In another example of manufacturer's responsibility, Philadelphia also has begun a pilot project for recycling polystyrene foam in partnership with Dart Container Corp., a manufacturer of foam food-service products.

The foam, identified by the "6" inside the recycling symbol, is used in cups, other food containers, egg cartons, and electronics packaging.

Residents and businesses can bring foam to the city's Northeast Philadelphia Drop-Off Center at State Road and Ashburner Street. Dart collects the material and recycles it at its facility in Leola, Lancaster County.

Contact staff writer Sandy Bauers at 215-854-5147, sbauers@phillynews.com, or @sbauers on Twitter. Visit her blog at philly.com/greenspace