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After the FLUSH: Meet the Septic System

Updated: Mar 20, 2023

When we attend events, one of the most common questions from the public is, "What happens when I flush my toilet??" The beauty of this question is that it depends on where you are and how your system works! When we think about how toilets generally work, we often point to the sanitation value chain to explain it simply (see image below). But it may not help you understand how exactly your toilet works.

After the flush, toilets should generally go through this step-by-step process. (Credit: Unknown Author under CC BY-SA)

So, FLUSH wants to help explain what could happen after your toilet…flushes. In our new blog series, "After the FLUSH", we will explain how your poo and pee are managed by looking at specific places to make it easier to understand.


This Edition: Sewers & Treatment Plants

We talk a lot about sewers at FLUSH, but not all homes are connected to those systems for their toilets. In our second in this series, let's see what happens when your toilet connects to a septic tank!


About a decade ago, census data showed that about 23 million homes in the US had septic tanks – that's a good amount of people! But how do they work, and where do they come from? We're here to give a brief overview of this system.


How Does a Septic System Work?

According to the US EPA, several types of septic systems exist- we'll focus on the conventional version for this blog post.


You have pipes from your home that carry your flush from the toilet to an underground tank outside your house – a septic tank. Septic tanks can typically hold around 1,250 gallons.

Much like in wastewater treatment plants, gravity becomes a leading character in managing our toilet waste in the septic tank. Septic tanks often have two separate areas connected by a hole in the dividing wall. In the septic tank, heavy solids settle to the bottom while scummy grease floats to the top. The dirty water from the flushing sits in the middle, then travels through the hole in the wall to the second chamber to go through the same process again – using gravity to separate solids from the water. Remember – you likely have sinks and shower drains that also go into the same pipes as your toilet, so your system has a lot of water.

A septic tank (Credit: US EPA)

The term "septic tank" may sound gross to you because the word septic comes from the ancient Greek word for "make rotten." We use the term "septic" for this sanitation system because tanks have an anaerobic (aka, no oxygen) digestion process at the bottom of the tank. There, anaerobic bacteria help decompose the heavier organic matter. As a result, the heavier solids in the tank start to break down over time until the tank fills up and needs to get emptied.


Anatomy of a Septic System (Credit: Minnesota Pollution Control Agency)

What about all that water traveling through the two rooms in the septic tank? The water continues through a pipe that goes to a drainage field – often called a leach field – of holed-up pipes. There, the somewhat cleaner water percolates into the ground over time, getting filtered as it makes its way to groundwater systems and going back into the water cycle.


When the Septic System Fills or Breaks – What Then?

How often do these tanks fill up? It depends on its size and the number of people using the system, but families with well-design septic tanks can fill them up after 3-5 years (poorly-designed or too small tanks take much less time). Sometimes it's easy to tell that the septic tank is filling up – maybe water drains more slowly than usual, it takes a few more flushes to get things down, or a lush plot of green grass may show up on a lawn. That's when it's time to call a septic vacuum service to guzzle the solids out of the septic tank. They sometimes will bring the vacuumed solids to a wastewater treatment plant nearby for further treatment, if not brought straight to landfill for disposal.


On average, septic systems have a 40-year lifespan, but septic tanks can still break. In fact, lots of septic tanks used in the US are broken! This could happen for many reasons – encroaching tree roots, an aging system, or when people flush things they shouldn't (like wet wipes!).


Also, sometimes septic tanks break simply because the geography isn't right. For example, in some places, the installed septic tanks break when the soil can't handle absorbing the leaching water and the tank floods too quickly, like in clay-based soils.


Where do Septic Tanks Even Come From?

Fosse Mouras Details from 1860

The septic tank we use today has a history about as old as the modern-day sewers we use in many high-income countries. In the 19th century, cities were piling up with feces in the streets, and so were more remote towns. In France, engineer Jean-Louis Mouras built a cement chamber underground to stockpile the poo in town. The purpose of this underground chamber was to reduce bad odors, which people used to think made them sick. He left this chamber alone for 12 years; when he opened it back up, he was surprised that the waste had liquified – through anaerobic digestion! Mouras shared his technology with people to get some interest, but his invention didn't go far.


Sixteen years later, a priest, Abbott Moigno, read about Moras's tank and decided to install a Fosse Mouras at his rectory, adding a window for daily observations. According to his journals, Abbott Moigno found it amazing to watch organics breaking down and published a couple of articles about his findings. These articles finally brought Mouras into the limelight, and he became a bit famous for his original tank.


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