Single-stream recycling seems so simple. There is a bin. You put everything that's not trash—whether it's paper, plastic or metal—into that bin. The city comes and takes that bin away. Easy.

The first communities to use this system, in the 1990s, were in California. By 2005, about a fifth of all U.S. communities with recycling programs used it. By the beginning of this decade, that number was more like two-thirds.

Single-stream recycling has two main advantages: Since it's so much easier than sorting out recyclables for individuals, it increases household recycling rates, and since it's easier to dump one can of stuff into a collection truck with one compartment, it saves cities money. But someone, eventually, does have to sort out the paper from the containers and the glass and cans from the plastic and the different types of plastics from one another.

Material recovery facilities (called MRFs, which rhymes with "smurfs") have gotten pretty good at doing that sorting, with combination of machines and human workers:

But this process is, ultimately, more expensive than sorting things before they got to the dump, and MRFs can't separate recyclables quite as well as a system that never mixes them together to begin with. Glass is a particular problem, as the Container Recycling Institute explains:

Glass is the material most affected by the amount of breakage in each type of collection system. In single-stream programs, it is virtually impossible to prevent glass from breaking as it goes to the curb, is dumped in the truck, gets compacted, gets dumped on the tipping floor of the MRF, is repeatedly driven over by forklifts, and is dumped on conveyor belts to be processed by the MRF.

All of this broken glass means that not as much gets recycled—and that sometimes it contaminates other recyclables, like bales of papers. One of the main criticisms of single-stream recycling is that it's led to a decrease in quality of the materials recovered (which matters for the people who buy bales of recycled material and turn it into new products).

When single-stream recycling was first taking hold, this wasn't a problem: It was common for recycling plants to sell those bales to countries in Asia, where lower-quality material could be cleaned up cheaply. But more recently, recycling plants have had trouble, at times, off-loading their less pristine products.

The rates of "residuals"—would-be recyclables that end up in landfills—are also higher for single-stream recycling systems than for systems that require more sorting earlier on in the process. Those residuals eat away at the gains in initial recycling rates.

Critics of single-stream argue that there are other ways to pump up recycling volume. Like, for instance, limiting how much trash people can throw away—or charging them for bags of garbage headed to the landfill but not for bags of recycling. People might not like sorting out paper from plastic, but they'll do that before they'll pay to have it hauled away.