Mad About Science: Septic tanks and sewer systems

By Brenden Bobby
Reader Staff

Have you ever wondered what happens after you flush your toilet? Where does it all go, and how are we not waist-deep in sewage right now?

You’re about to find out, because I had the misfortune of finding out what happens when the waste has nowhere to go. Before I continue, I’d like to thank Ace Septic and Case Plumbing, respectively, here in Bonner County for helping me out of a really crappy situation.

The average’s home wastewater system can be operated completely without electrical power, with one exception: If you have an electric pump for your well water, you won’t be able to replenish your system when the power goes out. So long as you are able to fill the tank of your toilet, you should be able to flush away waste without any problems. How does this happen?


A diagram of a residential septic system. Courtesy photo.

Water will always move downward until it can’t anymore because of obstructions. Imagine a puddle, or our beautiful lake. All of the wastewater pipes in your house are designed to travel downhill, generally at a sloped grade to maintain a predictable speed at which your water will transport your waste. If the slope is too steep, the water could travel faster than your waste, risking buildups of waste that will eventually block the water from passing through the pipes. At that point, the water has nowhere to go, and adding more water will only force that excess water and waste back up your pipes, into your house and all over your floor.

The final destination of your bodily waste can vary based on where you live and the age of your house. Living in rural North Idaho, you are likely familiar with what a septic tank is, though you probably have not spent a tremendous amount of time researching that particular technology unless you are trying to learn more about yours to avoid costly repairs. A septic tank is a large container made of plastic, fiberglass or concrete buried underground where most of your household waste will end up. Your waste enters the tank via an inlet baffle, a T-shaped pipe that is meant to help slow and direct solid wastes downward, and to keep scum from flowing back up the pipe and into your house.

The contents of your septic tank naturally separate into three layers, like a disgusting tiramisu: scum, liquid and sludge. The scum layer at the top is made primarily of oils and fats — things that are lighter than water and naturally float to the top. The liquid layer is a mixture of urine and wastewater that’s heavier than the scum layer above it, but lighter than the sludge beneath it. 

Sludge is a mass of solids and bacteria that work tirelessly to decompose your solid waste — I would assume this translates to lots of gas, but you would have to consult a septic specialist or a biologist about how all of that gas escapes, and whether or not it will coalesce back into water after it has been broken down. Liquid is allowed to escape your septic tank, and does so by exiting through a drain field, where it is dispersed into the soil, which acts as a natural filter to clean the water before it can return to the underground reservoirs.

This process is similar in many ways if you are hooked up to a sewer system rather than a septic tank. Sewers are built in towns where a larger volume of waste is produced, and though they take considerably more maintenance and upkeep than a septic tank, they’re easier to maintain when everyone’s money is pooled together to help out — just like the public library, fire department or police.

Waste from the sewer is transported to a sewer treatment plant where it undergoes a number of steps to be converted into drinkable water. Upon entry into the facility, it is chemically treated and transported to filtration, where the waste is pushed through filters that separate larger items from the effluent, or liquid waste. All sorts of weird things end up in the sewer, including teeth, watches and (in at least one instance) half a car, according to some internet sleuthing. Good luck flushing your dad’s Camaro at home. 

These larger items, including half of that car, are transported to a separate facility where they can be processed to be buried and fermented to be used as fertilizer, or taken to a landfill or recycling center for inorganics. The wastewater left behind at the treatment facility is pumped into bioreactors filled with bacteria that consume other bacteria that could harm us. At the end of its journey, it’s mixed with chlorine to kill any residual bacteria before it flows back out into a nearby body of water.

While that might gross you out, literally all of the water on Earth has been recycled. Earth is built to recycle water by evaporating surface water, allowing it to coalesce into clouds, rain and snow that will inevitably be consumed by you and me and released back onto the surface to evaporate and start all over again.

That’s right, that $8.99 bottle of designer water your favorite Instagram influencer duped you into buying was caveman pee at one time. You’re welcome.

Sorry for ruining your lunch. Stay curious (and hydrated), 7B.

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