What Happens After You Flush?

The following is an article from Uncle John’s Bathroom Reader

toilet-bowl-flushREADY, SET, GO!

For you, the trip has ended. You’ve “done your business,” (hopefully you’ve also had a few minutes of quality reading time), you’ve flushed the toilet, and you’ve moved onto the next thing.

But for your “business,” a.k.a. organic solid waste, a.k.a. “Number Two,” the trip is just beginning. Here’s a general idea of what happens next.


If you live in a rural area, your house is probably hooked up to a septic tank. We’ll get to that later.

Before the 20th century, “sanitary systems” typically dumped raw sewage directly into rivers, streams, and oceans. Today, if you live in an urban area or a suburb, chances are your toilet and all of the water fixtures in your house—the sinks, showers, bathtubs, dishwasher, washing machine, etc.—are all hooked into a sewer system that feeds into a wastewater treatment plant. So the journey begins when Number Two mixes with all of the rest of the wastewater leaving your house. Then it enters the sewer main that runs down the center of your street (usually about six feet beneath the road surface), and mixes with the wastewater coming from the your neighbors’ homes.

From there the sewer main probably joins with other sewer mains to form an even bigger sewer main. Depending on how far you are from the wastewater treatment plant, the sewer mains may repeatedly join together to form ever larger pipes. By the time you start getting close to the plant, the pipe could be large enough in diameter to drive a truck through.


By now Number Two has a lot of company, especially if any storm drains feed into your community’s system. Anything that can be swept into the the storm drains—old shoes, tree branches, cardboard boxes, dead animals, rusty shopping carts—is now heading through the giant pipes toward the treatment plant.

This floating garbage would destroy the equipment in the plant, so the first step is to remove it from the wastewater. This is accomplished by letting the water flow through a series of screens and vertical bars that trap the really large objects but let everything else—including Number Two—float through. The big stuff is then removed and disposed of, often in landfills.


Now the trip starts to get a little rough:

  • The wastewater flows into a grinder called a communitor. The communitor is like a huge garbage disposal: It takes everything that’s still in the water, Number Two included, and grinds it down into a sort of liquefied mulch that’s easier to treat chemically and easier to remove. Number Two has now “become one,” so to speak, with all the other solid matter still in the wastewater.
  • Next this slurry flows into a grit chamber, where inorganic materials—stuff that can’t rot, like sand, gravel, and silt—settle to the bottom of the chamber. Later, they’re disposed of in a landfill.
  • The wastewater then flows from the grit chamber into a closed sedimentation tank, where it is allowed to sit for a while so that the organic matter still in the water has a chance to settle to the bottom of the tank, where it can be removed.
  • Have you ever dropped a raisin into a glass of 7-Up and watched the bubbles carry it to the top of the glass? So have the folks that design treatment plants. Some plants use a flotation tank instead of a sedimentation tank: They force pressurized air into the wastewater, then pump this mixture into an open tank, where the bubbles can rise to the surface. As they float up, the bubbles carry a lot of the organic matter to the surface with them (including what’s left of poor Number Two), making it easier to skim from the surface and remove.
    By the time the wastewater has been processed through the sedimentation tank or the flotation tank, as much as 75–80% of solid matter has been removed.


So what happens to all of the organic solid matter (i.e., Number Two and all his friends) that has just been removed from the sedimentation tank? It gets turned into fertilizer.

  • It goes into a thickener, where it’s—you guessed it—thickened.
  • Then it’s fed into a closed anaerobic tank called a digester, where it’s—right again—digested. Enzymes break down the solid matter into a soluble (dissolvable) form. Then acid-producing bacteria ferment it, breaking it down even further, into simple organic acids. Bacteria then turns these organic acids into methane and carbon dioxide gasses. The entire process of decomposition can take anywhere from 10 to 30 days, during which time it will reduce the mass of the organic matter by 45–60%.
  • What’s left of the digested sludge is pumped out onto sand beds, where it’s allowed to dry. Some of the liquid in the sludge percolates down into the sand; the rest evaporates into the air. The dried organic material that’s left can then be used as a soil conditioner or a fertilizer. (Moral of the story: wash your vegetables before you eat them.)


That takes care of the organic matter—the part of the process known as primary treatment. Number Two’s trip is now at an end. But what about the liquid in the sedimentation and flotation tanks? Taking care of that is known as secondary treatment:

  • Some treatment plants pump the water through a trickling filter, where the water flows over a bed of porous material that’s coated with a slimy film of microorganisms. The microorganisms break the organic matter down into carbon dioxide and water.
  • Another process utilizes activated sludge—living sludge that is made up millions upon millions of bacteria cells. The wastewater is pumped into a tank containing the sludge, and the bacteria absorb any remaining organic matter.
  • Finally, the wastewater is processed in something called a secondary clarifier, which removes the bacteria before they are discharged back into the environment.
  • Some water treatment facilities don’t use trickling filters or activated sludge, they just pump the water into a lagoon or a stabilization pond, where the water is allowed to sit while naturally occurring bacteria and other microorganisms do the same job on their own, only a little slower.


Most wastewater that has received both primary and secondary treatment is considered safe enough to go back into the environment. But some water does require further treatment, especially if it is going to be reused by humans.

  • Processes with such names as reverse osmosis and electrodialysis can remove “dissolved” solids—solids that can pass easily through other kinds of filters. Then the water is filtered and treated chemically to remove phosphorous, ammonia, nitrogen, and phosphates.
  • If the water is going to be made safe for drinking, it is also treated with chlorine or disinfected by ozone.

That’s it! The water is clean. (Uncle John wouldn’t want to drink it, but that doesn’t mean it isn’t clean.)


Not everyone is hooked up to a water treatment facility. If you live out in the country, you may be hooked up to a septic tank, which performs the same wastewater treatment functions, only more simply and naturally:

  • The water from your toilets, bathtubs, showers, and sinks feed into a simple tank, usually made of concrete, cinder blocks, or metal.
  • Solid matter settles to the bottom and the liquid remains on top.
  • The liquid overflows into a system of underground trenches, often filled with rocks or gravel, where it can safely dissipate into the surrounding soil and biodegrade naturally.
  • The solids settle at the bottom of the tank and break down organically. You can help the process along by adding special yeast and other treatments to the septic tank; if this isn’t enough, it will eventually have to be pumped out.

This article is reprinted with permission from The Best of the Best of Uncle John’s Bathroom Reader. They’ve stuffed the best stuff they’ve ever written into 576 glorious pages. Result: pure bathroom-reading bliss! You’re just a few clicks away from the most hilarious, head-scratching material that has made Uncle John’s Bathroom Reader an unparalleled publishing phenomenon.

Since 1987, the Bathroom Readers’ Institute has led the movement to stand up for those who sit down and read in the bathroom (and everywhere else for that matter). With more than 15 million books in print, the Uncle John’s Bathroom Reader series is the longest-running, most popular series of its kind in the world.

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  • This is honestly really cool. I don’t often think about what happens to my “business,” but it’s something I have wondered about on occasion. There are so many steps that the waste water goes through, but I’m glad it does; I want the water to be as clean as possible! I’ve seen plenty of sprinklers with signs warning that they are using “recycled water” that isn’t safe to drink; I’m assuming that that water didn’t go through the advanced treatment, but what about water that did? Is it actually safe to drink, or is it only used in certain appliances? Thanks.

  • I can’t even imagine a sewer pipe leading to a waste-water treatment plant that is as big as a truck in diameter! That is huge. My sister accidentally flushed some important piece of jewelry down the toilet the other day and after reading this I can see there is a low chance we are getting that back.Thanks for posting this it was really interesting to learn about what happens after you flush.


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