Most of us go to great lengths to recycle our products, and feel pretty good when we dump our old envelopes and coffee cups into the designated paper bin instead of the regular trash. But now an expert in waste management has filled Vicky Gan from City Lab in on what actually ends up being recycled, and what holds up the process, and it turns out we've been doing it wrong all along.

To encourage people to recycle, most cities around the world give homes and workplaces bins broken down by plastic, paper or glass recycling bins. In some suburbs, these have been replaced by 'catch-all' recycling bins that take all these materials. 

This has worked in one sense, because it gets people to recycle more and throw away less, but it's also led to much more contamination in the system. Because in reality, not all papers, plastic and glassware is made equal, and there's a lot more that dictates whether something is recyclable or not.

In fact, in Washington DC in the US, introducing larger residential recycling bins last year led to a 50 percent drop in the amount of profit that the city made from selling off its recyclables, according to a report by Aaron C. Davis over at The Washington Post. And they actually had to pay more to filter through the bins.

"By pushing to increase recycling rates with bigger and bigger bins - while demanding almost no sorting by consumers - the recycling stream has become increasingly polluted and less valuable, imperilling the economics of the whole system," Davis wrote.

So what are the contaminants that are so detrimental to the system? We've listed below the ones we've been most guilty of.

Paper, cardboard and polystyrene food containers

Any of these containers that have been contaminated with food are a big no-no for recycling. Which means no pizza boxes, even if you've gone to the effort of scraping off all the food scraps.

This all comes down to oil soaking into the paper fibres. "Since the paper is mixed with water in a large churner, the oil eventually separates from the paper fibres. The oil does not dissolve in the water, instead it mixes in with the paper. The eventually result is new paper will [have] oil splotches," writes Stanford University's Buildings & Grounds Maintenance team.

Plastic and glass containers, on the other hand, are usually good to go in the recycling bin if they've been rinsed.

Plastic bags

These are a pain because they tend to blow off the conveyer belt during processing and clog up all the other machinery in a recycling facility.

"At the material recovery facility in San Francisco, they have to shut down all the machines every night for at least an hour to go in and manually pick out all the pieces of plastic bag that have gone in there and jammed up the various machines," Darby Hoover, a senior resource specialist at the Natural Resources Defence Council in the US, told Gan over at City Lab. Bottom line? Put them in the regular bin.

Paper cups

Turns out, these aren't actually paper. Most of the paper cups we buy our coffee in are actually coated with plastic, as Gan explains, which means they can't be processed properly with paper or plastic and can't be recycled. Same goes for Tyvek envelopes used for express post in the US.

Condiment packets

These are either made up of plastic or an aluminium-plastic blend. Either way, no one really wants them. "There's nobody recycling those right now," Hoover told Gan.

Paper scraps 

Sure, these are recyclable. But when they're put into catch-all recycling bins, they're pretty much impossible to sift out from all the other plastic and glass waste. This means that they - and everything around them - end up being thrown into the trash at the recycling plant anyway. A handy way to get them recycled is to staple them shut into a paper envelope marked 'shredded paper', says Hoover.

Find out more in Gan's report over at City Lab. And for an explanation of how recycling actually works, check out the episode of SciShow below: