Previous: Why Do We Burp and Fart (So Much)?!
Next: Science Superlatives of 2013



View count:1,195,503
Last sync:2023-01-09 19:15
You ever think about where your trash goes? How long it takes to decompose? And whether your garbage can become ... dangerous? You should! Hank explains the science of trash, how we've dealt with it (or not) over the ages, and both the risks and the potential it holds for the future.

President of Space subbable subscriber:

Like SciShow? Want to help support us, and also get things to put on your walls, cover your torso and hold your liquids? Check out our awesome products over at DFTBA Records:

Or help support us by subscribing to our page on Subbable:
Looking for SciShow elsewhere on the internet?

Thanks Tank Tumblr:

Sources for this episode:

Americans generate about 226 million metric tons of garbage each year, do the math, and that comes out about 2 kg of trash per person per day.  Thankfully, about a third of that refuse we generate ends up getting recycled or composted, a portion nearly six times greater than it was 30 years ago.  Still, the majority of my garbage ends up in a landfill.  It's easy to forget about your food wrappers, paper towels, and old light bulbs the second you throw them away, and when it comes to the science of trash, there is no shortage of questions like, 'how long will this stuff take to decompose?', 'what happens to it?', and 'can our trash become dangerous?'  But first, let us start with this: 'how did we deal with all this trash before landfills existed?'  The answer?  Not very well.

(SciShow Intro plays)

For centuries, most people disposed of their trash wherever they lived, whether that was the streets, their backyards, or even inside their own homes.  Historically, cities didn't consider garbage a problem until it affected their defenses, like in 500BC, the leaders of Athens decided they might want to dispose of their garbage a mile or so from the city, so invaders could no longer use the piles of trash to try and scale the city walls.  As recently as the late 19th century in many U.S. cities, barrels of trash were just put in public places for both people and animals to pick through.  In 1896, the Chicago city council reported that its unpaved roads were, quote, "Polluted to the last degree with trampled garbage, excreta, and other vegetable and animal refuse of the vilest description."  Eventually, people began to make the connection between disease and the squalor within their cities, and waste disposal began to change.  Which brings us to the dump.

A dump is little more than a hole in the ground, and for most of the 1900s, it's how many Americans got rid of their garbage. Sometimes sanitation workers covered the trash with a layer of dirt, but not always.  Dumps caused a number of environmental problems, many the result of liquids in the decomposing trash that made their way to the bottom of the hole, aided by gravity and rain. This foul concoction is called 'leachate', and since dumps contain no lining, any hazardous substances in the liquid, often heavy metals like mercury, chromium, nickel, and lead just ooze into the soil and in many cases, the ground water.  

In the 1970s, a U.S. government study found that 90 billion gallons of leachate was entering the groundwater supply each year thanks to shoddy dumps.  These masses of uncovered rotting garbage also released huge amounts of the greenhouse gas methane, which is 20 times more potent than carbon dioxide. 

Methane and CO2 make up between 90 and 98% of dump gases, the rest is a combination of nitrogen, oxygen, ammonia, and hydrogen.  These gases are produced as bacteria break down all of that organic waste, and how much of each gas there is depends a lot on the type of waste, though more gas gets produced as the dumps get wetter and hotter as well.  And when the dumps of yesteryear got full, cities often burned the garbage to create more space, which released a whole other array of toxic chemicals.  Even at dumps that were capped off with a layer of soil, the methane usually killed any vegetation that tried to grow on top.  Also, methane's super explosive, maybe I forgot to mention that.  

Unless it's properly vented, methane and other landfill gases move through the ground and try to escape to any open space, including nearby buildings.  It's called 'vapor intrusion,' and if the methane finds some poorly ventilated space, like your basement, it can build up and cause fires and even explosions.  In 2000, for instance, a house in Rochester Hills, Michigan was destroyed by an explosion caused by methane buildup from a landfill that had been closed for a long time. 

It wasn't until 1976 that Congress got serious about where we put our trash.  The Resource Conservation & Recovery Act not only required new standards for landfills, but it also, for the first time, divided waste into categories: hazardous and non-hazardous.  Important distinction.  That's right, until then, there was no distinction between an old newspaper and barrels of raw sewage, or like, just a million gallons of arsenic. There, just put it in the landfill with everything else.  So for the past 30+ years, landfills in this country have had to meet stricter guidelines, like being lined, usually with compacted clay and high density polyethylene.  They also have to have leachate collection pipes, so that all of that delicious trash juice can be safely hauled off to sewage treatment plants.  Modern landfills are also designed to collect and vent methane, lessening the chances of trash hole explosions.  

Still, the downside of creating what's essentially a dry tomb is that it takes forever for the buried garbage to decompose.  By compacting the soil and keeping the oxygen out, landfills have become anaerobic environments, where that diaper you threw out today will look pretty much the same decades from now.

Seriously, archaeologists have studied landfills and found totally recognizable 15-year-old hot dogs in 20 year-old buns, not to mention newspapers that remain legible after half a century.  And while estimates vary, there's a general agreement that stuff like glass and petroleum-based plastics will take a million years to decompose, if they do at all.  


Given all of the space we have in the US to build more landfills, if we fill up old ones, finding alternatives hasn't really been a priority, but there are ways to create more efficient landfills, and there are even ways to use trash to generate power.  It's not exactly Back to the Future, but waste-to-energy, or WTE facilities, are growing in popularity, especially in parts of the world where land is at a premium.  The process involves, like, just burning garbage and using the heat to produce steam or otherwise run turbines to generate electricity.  It's basically how coal-fired power plants work, although, if done properly, burning trash can be less polluting than you might expect.  This is especially true in the new generations of WTE facilities that use filters and scrubbers to capture the most dangerous pollutants like dioxins, sulfur dioxide, hydrochloric acid, nitrogen oxides and heavy metals.  

After incineration those heavy metals and extracted acids are often sold to manufacturers. Burning the garbage also limits the release of methane that would otherwise build up in a landfill.

But the technology has yet to really catch on in the U.S.: there are 87 trash-burning facilities in this country, although none have been built in the last fifteen years. Together they burn 25.4 million metric tons of waste to generate about 2700 mega-watts of power per year, or roughly 0.3% of our total usage.

But in Europe, there are more than 450 WTE facilities incinerating 73 million metric tons of trash per year, enough to provide electricity for more than 13 million people. Spurred by EU regulations that restrict the creation of new landfills, these numbers will continue to grow.


Back in the U.S. of A. it's apparent that landfills are here to stay. So, can we at least make them a little more useful? Arthur C. Clarke once said, "Solid wastes are only raw materials we're too stupid to use." But now, a new type of landfill called a bioreactor shows promise that perhaps Clarke would be proud of. Perhaps.

Bioreactors are new or modified old landfills that work to decompose as much of the trash that's put in them as possible by providing a more hospitable environment for the busy bacteria that do the dirty work of decomposition.

There are two main types of bioreactor landfills, anaerobic and aerobic, and some hybrids between the two.

Aerobic bioreactors cater to oxygen-breathing bacteria by injecting air into landfills and recirculating some of that nasty leachate. But some of the best garbage-eating bacteria work without oxygen, so anaerobic bioreactors work to create the low-oxygen, moisture-laden environments that they love. To make the most of anaerobic degradation, moisture levels need to be between 35 and 45%, so these reactors pump in sewage sludge and storm water to make a nice garbage soup. Delicious.

Only problem is, all of that decomposition creates more landfill gas. But landfill gas is burnable, so why aren't we just burning it and making power with it? One possible answer is just what Arthur C. Clarke said: people are stupid when it comes to their own garbage.

Like, you may have seen a dump that looks like it's glowing? That's because landfill gases are actually flared off as they escape because carbon dioxide, which is produced by the burning of methane, is a less potent greenhouse gas than methane is. But there is hope that future systems can capture methane and other handy byproducts and make good use of them. Right now, just a handful of bioreactors are in the early stages of testing this out but it's a technology that you will hopefully hear more about in the coming years.

So, while science is working hard to make your garbage decompose faster, or turn it into the energy that you're using to watch me right now, in my country, at least, we're stuck with recycling as much as we can and sticking the rest in our trash holes.


Thank you for watching this episode of SciShow and thanks especially to our first president of space, TheNilFacts. To find out how you could become the president of space, an honorary associate producer, or even pick the topic of one of our episodes then go to You can always find us on Facebook and Twitter too, and don't forget to go to and subscribe.