Turning Your Leftovers Into Fuel


It’s three in the afternoon on a Wednesday when you hit the afternoon slump. You’re tired, and probably not just because you were up late finishing season four of Buffy the Vampire Slayer. Your body’s low on energy, so you grab an apple for a quick snack. Food is energy, after all. That’s why we eat it. But instead of throwing away that core when you’re done, try putting it in the garbage disposal. There’s a lot of energy left in that core, and this way, it might be converted to something useful. Because when food waste ends up in landfills, it actually creates a lot of problems. It takes time for that uneaten apple core to decompose. And meanwhile, bacteria are turning it into methane. That’s an issue, because methane’s a greenhouse gas, and it’s actually significantly better at trapping heat than carbon dioxide. Which got scientists thinking: what if we could figure out a way to control how the food waste decomposes, and in the process, produce energy we could actually use? That had the potential to help the planet and make lots of money.

It was a win-win, really — but it wasn’t going to be simple. The first challenge was just… getting the food waste itself. Recycling programs are pretty successful, but it’s still hard to get people to agree to scrape the food out of the takeout container before they throw it in the trash, or to get restaurants to separate food scraps. Cities are slowly starting to get on board, mandating that commercial establishments like restaurants send no more than a certain percentage of their waste to landfills. But even once they’ve separated out the food waste, there’s still the matter of getting it somewhere to be processed. Not to mention figuring out how to process it. At first, cities decided to take advantage of existing sewage treatment systems. We’ve talked here on SciShow about where the stuff in your toilet or sink goes after you flush — and eventually, part of it ends up in an anaerobic digester. That’s basically just a giant tank full of sludge and some specially-selected microorganisms — but very little oxygen. Bacteria would normally digest the sludge using a reaction that has carbon dioxide as one of the products.

But making carbon dioxide requires oxygen, and when there isn’t enough around, they have to do the best they can with what they have. So they end up producing methane instead, a product that’s much more useful to us, because the treatment plant can harness that methane and turn it into a usable fuel. When your apple core goes down your drain, it lands in the sewage system and eventually ends up in the sludge, where bacteria can use it to produce even more methane. So at first, cities simply added their food waste to the mix. Once they’d figured out that this was an effective way to turn food waste into methane, companies started building treatment plants specifically designed to handle food waste. Many of these plants have deals with local utility companies, and are able to sell the extra fuel in the form of heat or electricity, or just as compressed natural gas. Sell enough, and they turn a profit.

But methane still isn’t an ideal fuel. To move the gas around around or burn it, you have to compress it first, which takes extra energy and makes the fuel less efficient. And while methane is easy to use in power plants once they get it, it’s more difficult to use compressed natural gas to, say, power a car. That’s why these days, many companies are looking to turn food waste into an even more useful form of energy… ethanol. Yeah. The stuff in your tequila — ethyl alcohol. There are plenty of industrial plants that produce ethanol from things like corn, and they’ve been doing it for decades. One of the more common techniques uses enzymes to convert cellulose into sugar, then allows yeast to turn that sugar into ethanol and carbon dioxide. Since those plants use very specific raw materials, or feedstock, the chemical reactions are easier to control. Producing ethanol from an unpredictable mixture like food waste is more complicated. But there are already biorefineries that use other kinds of waste to produce ethanol, like the one in Edmonton, Alberta that started operations last year.

That plant takes what’s known as municipal solid waste — anything that can’t be recycled or composted. Then, it sorts that waste, separating out what can be used as feedstock for that plant, like non-recyclable plastics and soiled cardboard. The feedstock is processed using gasification, where high temperatures, hot sand, steam, and oxygen are used to convert it into a mixture of carbon monoxide and hydrogen called syngas. Using catalysts to help spur the reactions along, the syngas is turned into chemicals like methanol and ethanol. Companies haven’t yet applied the technology to food waste on an industrial scale, but they’re working on it. As we develop more ways to use renewable fuels like methane and ethanol, scientists and engineers all over the world are working to take advantage of untapped energy resources like food waste. Among them are engineers at the Emerson Electric Company, which sponsored this episode of SciShow.

Thanks to them for their support, and thank you for watching this SciShow Dose. If you want to keep getting smarter with us, just go to youtube.com/scishow and subscribe..