The year is 1858. The city is London. An oppressive heat wave is scorching the city’s two-and-a-half million residents, and the Thames River, where for centuries Londoners have disposed of their waste, begins to stink. BAD, like really bad.   Water levels drop as raw sewage bakes in the sun, casting a fetid smell so powerful that it’s said to strike men down.    It would go down in history as The Big Stink, all three of those words capitalized. And as unpleasant as it was, it’s remembered today as a tipping point of sorts that eventually ushered in the age of modern sewage treatment.   Humans have always peed and pooped, of course. But where it goes after we’ve done our business has changed a lot, especially in the last 160 years.    Point of fact: The water you just drank may well have been a part of someone’s urine just weeks ago! Tasty.   [Intro]   As a result of the Great Stink, London ended up building more than 20,000 kilometers of underground sewers to channel human waste downstream where it could be could be released but still untreated.   And it turns out that just diluting and dispersing sewage and hoping for the best isn’t great public health policy. I mean, the Greeks and Romans were doing the same thing 2,000 years earlier, not exactly progress.    Over time it became clear that human waste not only had to be disposed of, but it also had to be rid of the pollutants that are harmful to humans and the environment.   But what makes poop and pee so dangerous in the first place? I mean, the stuff comes from your body, so if it doesn’t make you sick when it’s in there, why is there a problem?    Well, we’re learning a lot these days about the human microbiome -- the wonderful habitat that your body provides for literally trillions of microorganisms -- and it includes your waste-making parts.    Like, there’s growing evidence that your urine isn’t actually sterile, though for a long time we thought it was. And your intestines are populated with HUGE bacterial colonies that you actually NEED in there to help digest your food and fend off some infections.   Unfortunately, we’re not all healthy 100 percent of the time, and the viruses and bacteria that make us sick are found in our poop. So, human waste can contain all kinds of pathogens, like salmonella and E. coli, and the microorganisms that cause cholera, typhoid, and hepatitis.   Then you got parasites like hookworms and and protozoa like giardia -- which campers may be familiar with after drinking improperly filtered water.    And it’s more than just a problem for humans. Wastewater also contains lots of nitrogen and phosphates that can harm wildlife and water quality if released into the environment.   So, while a lot of us take it for granted, dealing with these challenges is harder than you might think.    Lack of access to clean water and flushing toilets is is serious public health problem in many developing countries, so the treatment processes that most of us use aren’t available to well over a billion people.   The wastewater journey usually takes one of two directions. It either heads by way of a pipe to your town’s local sewer, or into a septic tank right there by your own home.   Even though many of us live in big cities, backyard septic systems are actually very common. As many as 25 percent of people in North America depend on them to process their waste and distribute it back into the ground.   Scientists learned long ago that the anaerobic digestion of sewage by microorganisms could be extremely effective in breaking down human waste and eliminating pathogens. The word “septic” actually comes from the Latin word for the bacterial action that you know as rotting.   All of the stuff that goes down your toilet -- which wastewater-management types call black water -- and all of the stuff that goes down the drain in your sink and shower, or gray water, flows into that tank, which usually holds about 4,000 liters of liquid.  Then, in the span of about two days, naturally-occurring anaerobic bacteria get to work, reducing solids in the wastewater into liquids and a kind of greasy, fatty residue. The insoluble particles will sink to the bottom and form sludge while the fats form a layer at the top of the tank. And yes I realize I’m probably ruining your day at this point. But within 48 hours, the bacteria will break down the wastewater in a septic tank by about 40 percent. And they stay in the water when it flows out of the tank and into an underground drainage field. Here, the bacteria continue to do their thing while the soil acts as a biological filter of sorts, keeping any harmful organic material from reaching the groundwater supply. The soil contains oxygen which allows aerobic bacteria to enter the mix, and it also takes up some of the phosphorus and forms of nitrogen like ammonium, which plant life really enjoys. So, yep, if you’re off the sewer grid, all of that is going on in your backyard at this very moment. Kinda makes you not to walk around in your bare feet any more, doesn’t it? But for those of you connected to a sewer system, what kind of magical journey awaits your waste?  And because I know some of you are curious, what happens to the stuff that you flush down your toilet that isn’t your own excreta? Well, before it flows into your local treatment plant, wastewater goes through a screen of metal rods that catches all the big stuff  -- sticks and clothing and small animals and other weird non-crap that finds its way into the sewers -- so that none of that gets into the moving parts of the facility. From there it’s on to the settling tank, aka the grit chamber, which is where solids like sand and gravel that have been picked up along the way, settle to the bottom.  It’s also at one of these two early treatment stations where other flushables are removed.  And there are literally tons of this stuff. According to one estimate, as much as 50% of so-called non-dispersable material in wastewater is paper towels from public restrooms. Another 25 % is nothing but baby wipes! And the remaining 25% is a mixture of condom and cosmetic wipes and tampon applicators and random things that toddlers probably got their hands on. The process for removing them varies depending on the system, but usually involves another mechanical screen or a skimmer.  The real treatment begins at the next stage, called the primary clarifier. This circular basin slows down the sewage so that the heavier organic solids fall to the bottom. Those solids at the bottom will be removed and transferred to a digester, sludge press or drying bed, which we’ll get to, I promise. But back to the primary clarifier, most facilities also use a skimmer here to remove fats and oils and grease that float to the top. These substances, most of which come from cooking materials or road runoff, will also often be diverted to the digester. The largest change in the science of wastewater treatment over the last few decades is that the process used to stop right here. People were like “hey, okay, we got the sludge out, the water is good to go back into the rivers.” Except it’s not. As we discussed earlier, the wastewater is still full of potentially dangerous pathogens, which is why we now have what’s called secondary treatment.  Wastewater from the clarifier flows to an aeration basin, where air is added to foster an efficient environment for microorganisms that, unlike those used in most septic systems, consume both organic matter and oxygen. At this point, the water is known as activated sludge, and it looks like dark mud. It’s rich with active bacteria and protozoans that go to town on the oxygen-rich water. From there, the water heads to a secondary clarifier and yet another sedimentation basin. It’s one last time for any leftover heavy stuff to fall to the bottom, where it’s pumped to the digester. And while its name is kind of gross in this context, the digester does pretty much what it sounds like. It eats your poop. Inside, it has an anaerobic environment similar to a septic tank, where bacteria can break down sludge, reducing the volume, the odors, and most importantly disease-causing organisms. Some facilities use drying beds for this step, but either way, the material left over from this process is called biosolids, or treated sewage sludge. It wasn’t so long ago that we took these biosolids and tossed them into a landfill once they went through treatment.  But today, this leftover, dry sludge is often used as fertilizer for agriculture or used to make compost mixtures that are sold commercially.  So, again, yes! You may have used the very diluted byproduct of human poop to grow the veggies in your garden.  We here at SciShow love recycling! But after those solids are taken care of, we still have a bunch of wastewater left over. Now it needs to be disinfected before being released into the wild, also known as our drinking supply. There are lots of ways to do this. Some facilities use chlorine to kill any harmful bacteria still left in the water. Chlorine can be introduced as a gas or a salt. Either way, it reacts with the water to form hypochlorous acid, which ultimately breaks down bacteria’s cell membranes and kills them. And chlorine is really effective, but if you live in a town that uses it, you know that it does not taste good, at all.   Chlorine also can be dangerous for organisms in the rivers and streams that receive the wastewater, which why some places now dechlorinate water AFTER chlorinating it. This is done using sulfur dioxide gas, which quickly turns chlorine compounds into chloride ions, which makes the water less toxic. A more expensive but less chlorine-y method involves using ultraviolet light, which effectively sterilizes microorganisms, changing their genetic composition enough that they can no longer reproduce. Some newer wastewater treatment plants now even go an extra step with what’s called tertiary treatment. It can be a physical process, where water is run through sand filters to reduce any remaining solids; or it can done biologically -- some places divert the water to natural wetlands, where plants remove excess nutrients. But this is the end of the line. No matter what the disinfectant process, the effluent will be released, usually into a nearby river or creek -- ready for you to possibly drink again someday.    Your pee and poop has come full circle.   Thanks for watching this SciShow Infusion -- especially to our Subbable subscribers. To learn how you can support us in exploring the world, just go to Subbable.com/SciShow.   And as always, don’t forget to go to YouTube.com/scishow and subscribe!