[SciShow Intro]

Hank Green:
    I don't know about you, but I take recycling for granted. You finish your drink, and you just toss the plastic bottle into a bin marked "Recycling." Then something happens to it and you're told it's good for the environment, and then you feel good because you're saving the planet in your own small way by not throwing that bottle into the garbage.
    But how does recycling really work? What happens to that plastic, or that can, or that piece of paper I toss into that little bin with the arrows on it?
    Recycling is basically the process of collecting waste materials and breaking them down into building blocks that can be turned into new products. Since each material is made of different things, it needs to be broken down in its own way; paper, for instance, becomes wood fibers, but glass is just crushed into tiny pieces. And since 1973, scientists and engineers have been working on the best ways to separate, clean, and process recyclables at material recovery facilities. They're also known as "MRFs" for short, which is a much more awesome and fun name that we will be using from now on. 
    There are typically two kinds of MRFs: single stream recycling plants and dual stream recycling plants. Dual stream recycling means that the curbside bins are split into two categories: Mixed Paper and Everything Else. These two categories are kept separate in the truck, dumped into two separate piles, and offloaded onto two separate conveyor belts. Single stream recycling, on the other hand, is exactly that: everything is thrown into the same recycling bin and sorted later by a combination of people and high-tech machines. Less than half of all material recovery facilities use this method, but the number is growing.
    So here's how it works. Almost anything can be recycled, but some materials, like computers, batteries, and light bulbs, are too complex, too large, or contain too many toxins to properly recycle at any given MRF. If they show up in the recycling pile, they're either thrown away or taken to different, specialized facilities. Single-stream recycling focuses on five different kinds of waste: paper, steel, glass, aluminum, and plastic.  As they make their way through the plant, each of these materials is separated from the mixture and is processed.
    The paper and the cardboard come first, thanks to a series of rubber star-shaped wheels called rotary screen separators. With the help of blown air pushing them along, the cardboard and paper ride the wheels at a 45-degree incline up to a higher conveyor belt, while the more three-dimensional, heavier objects, like containers and bottles, fall through the gaps in the wheels and land on the main conveyor belt. That cardboard and paper is sorted by workers who remove any remaining contaminants or stray plastic, then separate the materials into bins for newspaper, mixed paper, and corrugated cardboard.

Paper is made out of two basic ingredients: cellulose fibers from wood and water. So to recycle it, you need to break it down to those two component parts and let it reform.  Plus, there are contaminants like ink and dirt that need to be filtered out.  So first, the paper is compacted, baled, sent to a mill, and placed in a hot water bath.  This bath quickly breaks the paper down to tiny strands of cellulose fiber, creating a mushy substance called pulp, basically wet, lumpy paper.  That pulp is still dirty, though, so it's forced through screens that filter out any remaining residues like glue or bits of plastic.  It's then sent to the de-inker, which is another bath containing air bubbles and soap-like chemicals called surfactants that separate the paper from the ink.  Air bubbles carry the ink up to the surface while the pulp, which is heavier, sinks to the bottom.  That pulp, now clean, can be formed into new paper products.  You might have even eaten from a recycled cereal box this morning.

Meanwhile, back at the MRF, anything that didn't make it up the rotary separator enters another sorting line.  What's left on the conveyor belt is moved through a cross-belt magnet, which is a high-powered magnet that attracts iron-containing metals, mostly steel.  Aluminum gets left behind because aluminum isn't usually magnetic.  Steel isn't all that hard to recycle, it's just crushed and baled and shipped to foundries, where it's melted down.  After that, it's ready to mixed with new steel and refashioned into cans or cars or beams or anything else made of steel.  So now all we have left at the MRF are glass, aluminum, and plastic. 

They're separating using an air classifier, which is a fan that pushes lighter goods, like aluminum and plastic, toward a higher conveyor belt, while allowing glass, which is heavier, to fall down to a lower conveyor belt.  Then the glass takes a ride through a rolling drum, it's shattered into pieces and filtered though sieve-like screens that ensure that no glass piece is larger than 5cm wide.  Those glass pieces are then sorted by color: clear, brown, and green.  The different colors are important.  They make recycling tricky because they're permanent.  Glass is made of silica, plus a bunch of other ingredients, depending on the color, that's been melted by high heat and then rapidly cooled.  Around 60% of all the glass in the US is clear.  It's the easiest glass to make because it's just silica, limestone, and soda ash.  But clear glass can sometimes cause the substance inside to degrade, due to light exposure from the sun.  Beer, for example, has a light sensitive chemical called isohumulone inside of it that breaks down into free radicals when struck by UV light.  The side effect of this reaction is a skunky smell and some carbon monoxide, generally not what you're looking for in a beer.  That's why we have brown and green glass, which act like sunglasses and protect the contents from UV rays.  But they also have ingredients in them to make them that color, like nickel, iron, and sulfur.  This means that the brown and green colors can't be removed from glass, so they need to be recycled separately.  Once the glass is all separated, it's crushed into tiny pieces called cullet.  These cullets melt at a lower temperature than new glass, because the ingredients have already been mixed and fused together when that glass was initially made. 

Back at the MRF, we've only got aluminum and plastic left.  We are almost done.  Aluminum can be separated from the plastic using a machine called an eddy current separator.  It's a big drum with a spinning rotor that contains magnetic poles, which creates a really strong magnetic field, called an induction field.  Remember how I said that aluminum isn't magnetic?  Well, that is true, most of the time.  This induction field is so powerful that it causes electrons in a conductor, like aluminum, to create their own magnetic field.  The two fields repel each other, and the aluminum is pushed away to a different conveyor belt while the plastic continues on.  Like steel, once it's separated, there isn't much to do when it comes to making the aluminum reusable.  It's shredded, washed, and turned into chips, which are melted in a large furnace and poured into molds.  These molds are shipped to manufacturers, where they're melted again and rolled out into thin sheets that can be cut and bent and shaped into new products like cans and license plates and aluminum foil. 

So now all we have left is the plastic.  Kinda.  It turns out that plastics are made of one of six different types of chemicals which correspond to the numbers one through six that you'll see on the bottom of that soda bottle or yogurt container.  Sometimes you'll also see a 7, but that just means it's made of any one of the less popular types.  Each kind of plastic has a different molecular structure, which determines the physical properties of the plastic.  It also means that some plastics are a lot easier to recycle than others.  Plastic is made out of long carbon chains.  Usually, the hydrocarbon molecules are extracted from fuels like crude oil or natural gas, then linked up forming big repeating structures called polymers.  Take PET for example, which corresponds with code #1.  It's made of a polymer called polyethylene terephthalate, which has rings of carbon separated by chains of carbons, some of which have single and double bonds to oxygen atoms.  Those rings and double bonds don't give PET much flexibility, so the polymers tend to pack closely together and are harder to melt.  Since PET will soften at temperatures more than 100 degrees below that high melting point, it's easy to reshape without damaging its molecular structure.  That's pretty helpful when you're trying to recycle it.

Then there's code #3, PVC, which is made of a polymer called polyvinyl chloride, it's a strong and durable plastic often used in piping or bottles of shampoo.  It's also known as the poison plastic, because when it's melted, it can release a corrosive and toxic gas called, yeah, hydrochloric acid.  Most of the time, recycling PVC involves grinding it into a powder at a specialized plant and then mixing it with additives so that it can be reused.  But you definitely do not want it contaminating the rest of your plastic, which is just one reason why all the different kinds of plastic are separated at the MRF. 

More importantly, separating plastics by types preserves their special properties.  For example, PET is harder to shatter, while PVC is more resistant to harsh chemicals.  In most plants, the separation is done using human sorters, but in newer plants, there are infrared sensors that identify the plastics based on the spectrum of light they reflect.  Since each type of polymer reflects light differently, the sensors can identify which plastic is which.  Once identified, little puffs of air blast those plastics on to different conveyor belts.  Depending on the MRF's capabilities, plus other factors like market demands, some of these plastics are recycled, while others are thrown away.  Take expanded polystyrene, for example, which you know as Styrofoam.  While it can technically be recycled, it's not particularly practical, because a truck full of lightweight air filled Styrofoam doesn't melt down into a lot of polystyrene.  But soda bottles, which are made of PET plastic, are recycled at almost all MRFs.  They can be melted down, mixed with new plastic, and used to create things like clothing and carpet or even to fill pillows.  So that soda bottle you just recycled?  Someday, you might end up wearing it. 

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