As America's culture of convenience continues to flourish, single-cup coffee makers have become increasingly popular among coffee drinkers. But beneath those expedient one-cup coffee pods lies a growing environmental problem.

"These things aren't readily recyclable, if recyclable at all," said Darby Hoover, senior resources specialist for the Natural Resources Defense Council. "Anytime you've got this kind of small, single-use packaging option, especially when there are clearly alternatives, it just leads me to question why you would promote that system over another that works just as well for many applications."

Approximately 13 percent of the U.S. adult population drinks a coffee made in a single-cup brewer every day, according to a 2013 study from the National Coffee Association. That's up from only 4 percent in 2010, said Joe DeRupo, director of communications for the NCA.

But as consumers become more and more aware of the recyclability issue, single-serve coffee manufacturers are looking for more sustainable solutions.

Green Mountain Coffee Roasters Inc., which acquired Keurig in 2006 and has since become the largest manufacturer of single-use coffee brewers in the country, has been struggling with the recyclability of Keurig's K-Cups from the beginning.

"The system has a lot of pretty demanding technical requirements in terms of being able to withstand certain amount of temperature and to have a certain kind of rigidity, and provide the right kinds of moisture barriers and oxygen barriers and the like," said Mike Dupee, vice president of sustainability for Green Mountain. "So it isn't the simplest challenge."

Like most single-cup coffee pods, K-Cups consist of a combination of plastic, aluminum, organic material (coffee grounds) and a paper filter. While all said items can be easily recycled separately, the K-Cup as a whole cannot be recycled on a municipal level.

"It's that hybrid packaging that makes it very hard for traditional recycling companies," said Albe Zakes, global vice president of public relations at the recycling company TerraCycle. "If you think of something as simple as a chip bag for example, a chip bag is fully recyclable. The challenge is that it's both plastic and aluminum, a hybrid packaging, which is very common, especially in food and beverage."

Since 2009, TerraCycle has partnered with various single-serve coffee manufacturers to provide recycling solutions for spent coffee pods. By teaming up with Tassimo, Mars Drinks, Nespresso and more recently Illy, TerraCycle has developed a customized take back program for each company that has helped divert millions of coffee pods from landfills across the country.

"Over the last couple years, TerraCycle has already collected I believe 25 million coffee capsules of the various applications through all these programs," said Zakes. "As we expand with more companies, bringing Illy into the fold, we've really ended up in a place where we work with basically everyone except for the K-Cup."

Despite reaching out to the company multiple times, TerraCycle has not been able to develop a relationship with Green Mountain, Zakes said.

However, that doesn't mean the company isn't working toward a solution on its own. In recent years Green Mountain has been looking for ways to use design to combat the issue of recyclability, Dupee said.

"With the VUE [brewing system] we're now in a polypropylene cup that you're able to use the tab and peel out the filter, coffee and lid and recycle the cup. So that's a great example I think of using design to try and improve; it's certainly not the end game by any means, but it is a demonstrable step forward," he said.

All materials recovery facilities have different size limits for items they can accept, and whether or not the VUE cups are recyclable, they may not be recycled down the line, said Hilary Gans, facility operations contract manager with the South Bayside (Calif.) Waste Management Authority.

"Because the items are so small, they typically wind up either as residue mixed in with broken glass in the glass stream, or they might wind up in mixed paper," he said.

Green Mountain is experimenting with various materials for a new, recyclable K-Cup, though nothing is in production yet.

"We tried a paper-based tea portion pack probably two years ago, and it didn't perform so we took it down," Dupee said.

In addition, the company has a pilot take-back program for its workplace customers where the coffee grounds are sent to a compost facility and the rest of the materials are sent to a Covanta Energy Corp. waste-to-energy facility. Green Mountain recovered more than 7.5 million portion packs since the program began in July 2011, Dupee said.

Incineration, of course, doesn't sit well with many environmentalists who often fight against new and old WTE facilities.

"I know and I appreciate that Green Mountain has been trying to wrestle with ways to make its pods more environmentally sustainable pretty much the whole time that they've been offering them," said Hoover, the senior resources specialist for the NRDC. "But incinerating them instead of recycling them doesn't reduce the need to rely on virgin source materials to make oodles more of these disposable pods."

Dupee said the end-of-life portion of K-Cups' lifecycle has the least overall impact

The question remains whether the convenience of single-serve coffee systems is worth the overall environmental impact.

"The trade-off that we need to consider is, for shaving a few seconds off the amount of time it takes me to make my coffee in the morning, what is it costing in terms of what's going to the landfill, what kinds of non-renewable source materials like fossil fuel-derived plastics are being used, and what other environmental costs are associated with that."