Toilet Reading Episode 3: Where Does My Poop Go?


Now that we’ve broken the “wrote poop in a blog post” barrier, we can begin.

Most people don’t think about poop very often. It’s a need that we can usually take care of when it arises, and, even as leaders in the plumbing industry, we can say it is pretty gross. After that handle on the toilet (we would hope) is pulled, we never have to think about it again. The poop simply disappears…

…except it doesn’t.

Before vanishing, your poop will embark on a long and adventurous journey. It will take probably miles and days of intense energy-producing digestion before your poop has been fully processed.

The specific fate of your poop depends on whether or not you are on the sewer grid.

Sewer under the rainbow

In urban environments and densely-populated residential areas, your number two gets to hop on Route 66 in the form of the Great American Sewer System.

The sewer system is a network of pipes which carries solid waste, liquid waste, household greywater (from sinks, showers and washing machines) and rainwater to a water treatment plant for processing.

Step 1: Garbage Day

Once the sewage train your leftovers were riding arrives at the water treatment plant, the water is pulled through a series of metal rods to filter out things like:

  • That cell phone you dropped because you understandably couldn’t wait to get caught up on the last few issues of our Toilet Reading series.
  • “What do you mean you can’t flush paper towel down the toilet?”
  • Socks, and other items that 5-year old admirals dispatched on their perilous voyage.
  • Important documents that secret agents promised to swallow upon reading but ended up chickening out (probably).
  • Condoms
  • Money

Basically, don’t flush any of these things down the toilet. Except paper money. My contacts in water treatment have told me it is 100% okay to flush paper money.

At this point, often the sewage will pass through another screen filter to remove stones and other smaller materials that may not have been caught by the large screen. These debris are returned to industrial processing and may even be used in building your next toilet!

Step 2: He sediment, she sediment

After the non-organic bits have been filtered out, the thrilling speeds of this mixture are reduced to allow separation in the sedimentation tanks.

The smell has been described as “otherworldly…in a bad way,” “biological warfare,” and “eh, it’s not that bad.” (Although the last guy was responsible for a professional baseball team’s laundry).

It is in this smelly sedimentation tank that the sludgy solid waste sinks to the bottom while a congealed layer of grease and oil forms on top. In the middle of this nightmare sandwich is the murky brown water containing (most of) your poop.

The grease and oil are removed via skimming and the sludgy waste at the bottom is scraped and transferred to digestion tanks. The remaining wastewater mix travels on.

Step 3: Bacteria the Future

The mixture (which is starting to look more like water and less like something that could ruin any number of foods if I were to make a comparison) now moves along pipes and into the aeration chamber where the remaining waste in the mixture is a smelly buffet for aerobic bacteria.

Oxygen may be added to give the aerobic bacteria (aerobic meaning “needs oxygen”) a boost in their numbers and level of consumption.

What comes next is the aerobic bacteria doing the one thing we’ve all yelled at drivers who have cut us off(Hint: rhymes with “seat fit”).

Remember that sludge we talked about earlier? That also goes into a digestion chamber to feed other bacteria who were born on the wrong side of karma. During the digestion process of the dry solid material, methane gas is produced as a byproduct which is often used to power the same treatment plants that are facilitating this process.

Indigestible “biosolids” are collected and used either as fertilizer or soil conditioners, or they’re dried out and sold to be used with other high energy fuels (think Coal).

The next time somebody asks you to chip in a little more, just let them know that you’re powering public utility facilities and growing food…with your poop.

Step 4: The Return of the-

At this point, the water is usually sent to the final clarifiers where it is again skimmed on the top and the bottom. Any sludge remaining on the bottom is sent to back to the aeration tank or digestion chambers for bacterial seconds.

The water now looks no dirtier than regular lake water. It flows from the large open air clarifiers through an outfall for the final treatment. Here, the wastewater is treated with chlorine to purify the remaining treated water (what we call “effluent” in the biz) and is returned to the environment through bodies of water or soil irrigation.


For those of us who do not live on the sewer grid, most of the above chemical processes take place in our own backyard. In a septic tank, the entire process from the wastewater treatment cycle happens inside of a large underground reservoir.

This tank usually holds around 1,000 gallons of water. That is enough water to fill up your bathtub around fifty times (if you’re a psychopath who would do something like that).

Instead of aerobic bacterial digestion (used by the wastewater treatment process), a septic tank uses anaerobic bacterial digestion, meaning it takes place without the use of oxygen.

Step 1: Your poop and other flushed “black water” is taken, via a network of pipes beneath your home’s foundation, and carried to the septic tank where it combines with “grey water” from your sink/washing machine/giant marble fountain Sylvester Stallone statue.

Step 2: The combined wastewater sits in the tank. After about two days, anaerobic bacteria arrive from whatever corner of hell they originate and begin their competitive speed-eating tournament on your poop.

During this process, a layer of sludge will sink to the bottom of the tank while a layer of grease and oil floats to the top as a byproduct of this bacterial digestion. After two days, the solid waste that remains in the water solution will have been reduced to about 40% of the original volume.

Step 3: Because the anaerobic bacteria stick with the wastewater, they are able to hang on tight as the water leaves the septic tank and flows into a drainage field.

A drainage field is a network of outflowing pipes beneath a plot of land that allows soil to filter out harmful organic material while the bacteria continue devouring your waste.

By the time the wastewater hits the groundwater supply, it is free from any harmful organic or inorganic material, and it is now able to nourish planted crops that may find their way back into your mouth for the process to begin anew.

There you have it. Whether the flush of the toilet sends your poop to far off lands (your city’s water treatment facility) to be converted to energy and useful water, or it simply travels to the underground science lab in your backyard, you now know the crazy processes your poop will endure before it finally “disappears.”

Next time you flush your toilet, think about that.
Or maybe, on second thought, don’t.

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