You think you're doing the right thing by recycling your plastic, glass, metal and paper. You dutifully wash out pet food cans and sauce jars before placing them in your recycling bin. But before you smugly pat yourself on the back for doing your part and creating less waste, you need to know that what happens at your house isn't the whole story — and it's what we don't know that can hurt the recycling process.

Single-stream recycling is convenient for consumers as it allows us to throw all our recyclables into one container without separating glass from plastic, and it has increased the rate of recycling participation within communities. A 2015 Canadian study that compared single-stream and multi-stream recycling systems in Ontario's 223 municipalities found that single-stream curbside recycling increased household participation by about 6 percent.

However, there are snags in the system — literally.

A contaminated process

Recycling facility When things that aren't recyclable go through a recycling facility, the system comes to a screeching halt. (Photo: Felix Lipov/Shutterstock)

Have you ever been unsure about whether something was recyclable and tossed it into the bin anyway? Maybe it was a wire hanger or a plastic grocery bag. Well, those particular items (and lots more) aren't recyclable through curbside recycling. When they get mixed in with the regular recycling and start moving through a waste management facility, they can tangle up the equipment, causing processing lines to stop and requiring someone to physically remove the object from the machine before it can be restarted.

A representative of one such facility recently told USA Today that "contamination" changes with the season:

Since it's spring, the facility is getting a lot of garden hoses. Around the holidays, they get broken strands of Christmas lights, another choking hazard for the sorting line. And all day every day there are plastic shopping bags (recyclable at a grocery store but not from a household), chunks of styrofoam, diapers, syringes, food-contaminated containers ... a nearly endless litany of things that residents throw into their curbside recycling carts figuring they are or ought to be recyclable.

It's a little-known secret that glass recyclables are a major contaminant. When they shatter and break, they make the entire load of recyclables unrecyclable. Susan Collins of the nonprofit Container Recycling Institute (CRI) explained to NPR that a quarter of single-stream recycling is hauled to the dump due to cross-contamination. So while consumers think that most, if not all, of what we put in our recycling bins stays out of landfills, that's not completely true; it may be heading there after all.

Paper can be a contaminant, too, according to Clean Water Action, an environmental advocacy group, which points out in a report published by CRI that contamination works both ways.

"Single stream materials are collected by trucks that compact the paper and containers. Paper comingled with containers becomes contaminated by broken glass; the metal, plastic, and glass are contaminated by paper," the group writes. And that's bad for multiple reasons. Aside from the recyclable material heading for the dump, "contamination poses expensive problems for the processors and end-users, including wear and tear on equipment and increased costs of disposal," the report continues. When paper gets mixed in with glass recyclables, the glass must go to "low-value uses," and the paper, which should be recycled, is sent to a disposal facility instead, according to Clean Water Action.

One such facility in Massachusetts — Strategic Materials, a processor of glass and plastic for bottle manufacturers — discards about 12 percent of single-stream material each year. As the NPR story says, there are machines designed to help with contaminants, such as a machine with air jets that blasts certain types of plastic containers off the conveyor belt, or another machine with a magnet belt that grabs steel items. But these (and other machines that can pick out broken glass) are extremely expensive.

Is multi-stream recycling better?

Multi-stream recycling Studies have shown it's the larger collection bins associated with single-stream recycling and not convenience that have increased participation rates. (Photo: Rawpixel.com/Shutterstock)

The answer to that question depends on how you define recycling success. If you're measuring by amount of material recycled, then yes, it might be. But it's not without its limitations.

The aforementioned Canadian study of single-stream vs. multi-stream recycling systems busted a common myth about multi-stream recycling: It's not monumentally more expensive to municipalities than single-stream recycling. The study, which looked through 10 years of data, found that costs increased by just over $5,000 for multi-stream recycling:

Municipalities who implement single-stream recycling face higher material management costs when compared to those who opt for multi-stream systems. This is contrary to our expectation that single-stream recycling is cheaper than multi-stream systems. While collection costs for single stream collection are lower when compared to multi-stream municipalities, this savings is offset by significantly higher processing costs (48.7 percent higher) and lower realized revenue from sale of recyclable material (9.6 percent lower).

Clean Water Action notes that studies have shown it's the larger collection bins of single-stream recycling and not the convenience of the concept that increased participation rates. So maybe we just need larger curbside multi-stream recycling bins?

Let's hope all the old small ones are recyclable...

Angela Nelson ( @bostonangela ) is an exhausted mom of two young daughters and two old cats, and a Pulitzer Prize-winning digital editor with more than 15 years of experience delivering news and information to audiences worldwide.