This episode of SciShow was brought to you by Bill Gates.

Most of the time, you don’t have to think too much about our toilets. I mean, you go, you flush, and unless you’ve caused a catastrophic clog or something, it just kind of works.

But 4.5 billion people worldwide don’t have access to safely managed sanitation. It’s a serious global health problem, with sanitation-related diseases causing an estimated 500,000+ deaths every year in kids under 5. The simplest solution seems obvious: get more people access to safe toilets.

But you can’t just drop off a bunch of porta-potties. Those only work if you have the infrastructure to treat the waste once you collect it. That’s why research teams around the world are working on more sustainable types of toilets: ones that are cheap, don’t require access to water, sewer, or power lines, and can last for a long time.

And some of the most promising options are surprisingly cool. Take the toilet being developed at Caltech, for example. It’s one of the more advanced systems out there, using solar power to turn waste into treated water, hydrogen, and fertilizer.

It even flushes. Here’s how it works:. You do your business and flush, sending the water, along with whatever you deposited in the toilet, into a holding tank.

There, the larger particles of waste settle to the bottom and microbes start breaking them down, eventually turning the poop into fertilizer. Meanwhile, the liquidy leftovers go into a separate tank, which uses electricity to drive chemical reactions that disinfect the water, producing hydrogen gas in the process. That’s what the solar panels are for, they charge the batteries.

In theory, you could also use the hydrogen gas to generate some electricity, but that hasn’t been incorporated into the toilet’s design yet. After it’s treated, the water travels through a filter into another tank, and is clean enough to use the next time you want to flush the toilet or for things like irrigating crops. This all sounds pretty awesome.

But once the team started testing toilet prototypes in the real world, they ran into a problem: maintenance. The Caltech toilet is complicated, and when parts broke down, there was no one around to diagnose and fix the problem. The group is working on a set of sensors to help figure out issues automatically so people can fix them without much training.

But clearly there are some drawbacks to a system where so many things can break. So other researchers are working on solutions that would be more viable for developing countries, like terra preta sanitation, a method that actually goes back thousands of years. Back in the day, some indigenous groups in the Amazon used to mix excrement, other types of organic waste, and charcoal in airtight containers, which produced super-fertile soil now known as terra preta.

This stuff is so effective as a fertilizer that hundreds of years after they stopped doing this, there are still areas in the Amazon where the soil is three times more fertile than the surrounding dirt. Over the last decade or so, researchers have been using the same idea to develop a safe sanitation system. The main difference is that you need to do something with the pee.

So terra preta toilets divert liquid and solid waste into separate tanks, using a special mix of microbes to treat the urine and turn it into liquid fertilizer. The poop tank also gets a special blend of microbes, along with a mix of charcoal and wood that helps make sure there’s enough carbon for the composting process later on.

Bonus: the charcoal gets rid of the smell. After about a month, the processed poop is ready for the next step, called vermicomposting, which is what will actually turn it into fertilizer. The “vermi-” prefix comes from the Latin word for “worm,” and the whole thing is basically driven by earthworms. You put the processed poop into either a pile or a container, then add some worms.

Together, the worms and microbes take nutrients like potassium and phosphorus in the processed poop and turn it into compounds like humic acids, which help plant roots absorb more nutrients. And, voilà: you’ve got some nice, fertile terra preta. There are some challenges to this system: for one thing, you need to be able to either make or get more of those microbe and charcoal mixes.

Otherwise the process won’t work. There also needs to be some demand for the terra preta, or else you’ll just end up with a bigger and bigger pile of fertilizer. So, too complicated, like the Caltech toilet, and you risk the toilet breaking.

Too basic and you have to figure out what to do with the leftovers. But maybe there’s a nice middle ground for Goldilocks to poop in. Like a toilet being developed at the University of Toronto that disinfects liquid waste and incinerates the solid stuff.

Like the terra preta toilet, the Toronto toilet is designed to do the separating for you. Solids end up on a conveyor belt, which we’ll get to in a sec, and liquid goes into a tank. First, the liquid is filtered through sand, which removes contaminants that might be in there, like parasite eggs, which you really don’t want in your water.

Then it’s sent to a chamber with a UV light, which kills off any microbes. Meanwhile, you turn a crank that moves the solid waste along that belt I mentioned. As other people use the toilet and also turn the crank, the poop gets flattened by the belt and is dried by fans.

The belt leads to a smoldering chamber, which works kind of like a charcoal grill, in that, it will just keep going until there’s nothing left to burn. The fan and UV light rely on a battery connected to a solar panel, but there are no complicated electrochemical processes involved like with the Caltech toilet. And there’s nothing like fertilizer that could accumulate if you don’t use it.

Poop and pee go in, and water and ash come out. Perfect for Goldilocks. There’s still a lot of work to be done to make safe sanitation available to everyone, no matter how the toilets are designed.

But, since 2000, efforts like these have helped 1.4 billion people get access to at least basic sanitation, the type where they have access to some type of facility, even if the waste disposal process isn’t totally safe. So hopefully, we’ll be able to extend that to the others who still need it. This episode of SciShow was brought to you and sponsored by Bill Gates.

Visit GatesNotes.com or check out the link in the description below to learn more about the safe sanitation problem and strategies that can help, including other toilets being developed by teams around the world and the progress that’s been made so far.