Waste Management's recycling center on Chicago's far South Side is meant to sort cans, bottles, paper and cardboard. It is not meant to sort plastic bags, but you wouldn't know it by the sheer quantity of them mingling with newspaper and aluminum on the receiving floor, wrapped around equipment in the sorting screens, tucked inside cardboard baled into neat cubes for sale and dangling from metal structures throughout the plant like dirt-crusted laundry on clotheslines.

"Plastic film migrates to every orifice of the plant," said Mike Tunney, area recycling director for Waste Management, which receives residential recycling from Chicago and several suburbs. "They make their way into every commodity stream."

Plastic bags, set to be exiled from Chicago starting Saturday, when the city's plastic bag ban goes into effect, are among the greatest headaches for recyclers grappling with growing contamination of their recycling streams, which slows their systems, drives up their costs and hurts the quality of the materials they sell to be reborn into new products.

Tunney said that he hopes the city law prohibiting large retail chains from distributing the ubiquitous thin plastic bags will mean that fewer arrive in his recycling center. But with many retailers planning to instead give away thicker reusable plastic bags that comply with the ordinance, he exhorts people to remember that plastic bags are not to be thrown into curbside recycling bins.

"There's a gap in the education," he said. Clean plastic bags should be returned to retailers like Jewel and Walmart that collect them for proper, bag-specific recycling. People who use plastic bags to carry their recyclables should empty them into the recycling bin and then toss the bag into the regular trash, he said.

Jewel, Target, Walmart, Walgreens and CVS all plan to offer customers free reusable plastic bags allowed under the city's ordinance: at least 2.25 mils thick and able to carry at least 22 pounds for at least 125 uses. The thicker bag "may not tear as easily," Tunney said, "but it still can wrap around the equipment."

Five times a day, the whir of the conveyor belts at the recycling center comes to a halt and crews wearing kneepads clamber onto the sorting screens. Using hook knives and reciprocating saws, they cut away the plastic bags, garden hoses, videotape and occasional sweatshirt that have wrapped around the rotating shafts that help separate paper from cans and bottles.

"If we didn't do this, four hours from now, none of the containers would fall through because the plastic would plug the gaps," Tunney said one recent morning as the workers, clipped to safety ropes, untangled the plastic bags from the equipment. Each day, more than five hours are spent cutting away materials from the shafts, mostly plastic bags. Doing a back-of-the-envelope calculation, Tunney estimates that it costs an additional $9,500 a month in labor. Sometimes, the bags can break the equipment.

The problem has grown with the increasing use of plastic bags and the increasing confusion among consumers about what is recyclable. As well-intentioned companies with sustainability goals put the recycling logo on their packaging, well-intentioned consumers toss them into their curbside recycling bins and assume the recycler will figure it out.

About 18 to 20 percent of the materials that come through the South Side plant are not recyclable, either because they have become contaminated or are not meant to be recycled, Tunney said. Across the country, contamination levels are around 16 percent, up from 5 or 6 percent 15 years ago, and in some large urban areas, they exceed 20 percent, he said.

"If that trend continues and suddenly you're at a place where 40 to 50 percent of your inbound material is contaminating, then that would have a threat to the long-term sustainability of the recycling program," he said.

Contamination challenges come as recyclers also are seeing falling prices for some commodities, including plastic, which loses its recycling value when oil prices are low and manufacturers can get the virgin stuff for cheap. Houston-based Waste Management, which last week reported $3.32 billion in second-quarter revenues, said that average recycling commodity prices were approximately 13 percent lower last quarter compared with the prior year, and recycling volumes declined 5.7 percent.

In addition to jamming equipment, plastic bags are a pain for recyclers because they are used to carry things and therefore are often sullied with food or other products that then contaminate the rest of the stream. On the "tip floor," where Waste Management receives 500 to 600 pounds of residential recyclables a day, workers ripped open a plastic bag to reveal a pair of sunglasses and earbuds.

Workers manning conveyor belts try to pluck out the plastic bags as they roll by, but "the sheer quantity is overwhelming," Tunney said, and some make it into the sorted bales bound for manufacturers. There, more quality-control workers are tasked with pulling intruders — mostly plastic bags — out of the compacted cubes. The piles of accumulated plastic bags go to the landfill.

Meantime, there are plastic bag recyclers pining for those bags, if only more people would take them back to the retailers for clean recycling.

"It has high value in the marketplace, and we can't get our hands on it," said Steve Alexander, executive director of the Association of Postconsumer Plastic Recyclers. "We are trying to work on the collection, which has been one of the most difficult processes we have tried to undertake."

About 62 million pounds of grocery store-type plastic bags were recycled in the U.S. in 2013, and if you include other overwraps and bag types, like newspaper bags, it is 141 million. Add in commercially generated agricultural plastic film, and the number rises to 1.1 billion pounds, according to the trade association.

Almost half of it is used to make composite plastic lumber for products like decking, and most of the rest goes into making new plastic film and sheet products, including plastic bags, the group said.

About 100 billion plastic bags are consumed in the U.S. annually, and 12 percent are recycled, said Phil Rozenski, senior director of sustainability for plastic and paper packaging manufacturer Novolex, a member of the American Progressive Bag Alliance. However, he said, up to 87 percent are reused in some way, be it to pick up dog poop or line a trash can.

Chicago's bag ban, which starting Saturday applies to franchise stores with more than 10,000 square feet, and a year from now will apply to smaller chains, has raised questions about whether it will do anything to cut down on plastic waste if shoppers can just get a sturdier plastic bag for free, albeit one that lasts longer. The new bags are five times thicker than the typical flimsy ones and consume five times more energy, mostly in gas to transport them because fewer fit in a truck, Rozenski said.

But not all retailers plan to stick to plastic. Mariano's plans to offer shoppers paper bags at no charge, and the grocery chain has added a new line of reusable bags for a $1 charge. Sears, Home Depot, Best Buy and other retailers are going with paper. And that you can throw into the recycling bin.