Do you wonder how septic systems work? I never gave it much thought in all the years I lived in Cincinnati. Every house I lived in was connected to a municipal sewer line. Just about every house I built I connected to a sewer line. Few of the houses I built needed their own septic systems.

One such house, however, stands out in my mind. The lot wasn’t big enough to support a traditional leach field. (I’ll get into what a leach field does in a moment.) It required a septic system that was in essence a miniature sewage treatment plant!

There was a propeller on a shaft that extend down into the septic tank. It was attached to a motor that was protected from the weather. For 10 minutes an hour this motor would spin, working much like a kitchen blender. When the propeller spun around, fresh air from outdoors was also injected into the swirling mass of water and waste inside the septic tank.

Visit a medium- or large-sized sewage treatment plant and that’s what you’ll discover. Before it sends the sewage back into the closest river, the plant aerates the wastewater. Introducing oxygen to sewage is a fantastic way to get rid of all the harmful things that one might find in wastewater.

If you’re unfamiliar with how septic systems work, here’s a primer. When you flush your toilet or when water drains from a tub, shower, vanity or kitchen sink, the wastewater flows through a 4-inch pipe that connects to a large precast concrete tank. The capacity of the tank can range from 500 to 1,000 gallons or more. They’re sized by septic designers based on the amount of projected waste that might be created within the house each day. Typically the designer goes by the number of bedrooms in the house.

Some tanks have different walls and baffles within the tank. The ones I see most often in New Hampshire have a small wall that’s about a foot from where the house drain pipe enters the tank. The purpose of this suspended concrete wall is to help break down any solids and toilet paper that rush into the tank. These things are supposed to crash against this wall as they enter the tank.

The trouble is that there are inlets into most tanks that allow the plumber to install the drain pipe parallel to this wall. Be sure your tank is installed correctly so the drain pipe enters the tank aimed directly at this small wall.

The waste from your body, foodstuffs and oils from your skin all contain bacteria. This bacteria starts to work in the tank to break down the waste. At the other end of the tank, opposite the inlet pipe, is an outlet pipe. For each gallon of water that enters the septic tank, a gallon of water flows out of it. This partially treated water that leaves the tank has lots of microscopic bacteria and pathogens in it.

It flows from the tank, or is pumped up a hill, to the leach field. The wastewater enters a maze of pipes that have perforations in them. The pipes typically are set upon a thick layer of washed sand. The wastewater is distributed into multiple pipes where it then slowly enters the sand.

There’s lots of oxygen in the sand, as well as countless small organisms. These work in tandem to purify the wastewater that drips out of the leach field pipes. It’s a simplistic system that’s time-tested. Best of all, it works very well if you watch what you put in your septic tank.

Years ago when I lived in Cincinnati, I would put anything I could down my drainpipes. Foolishly, I felt that as long as it made it out to the sewer line, it wasn’t my problem. That was a bad attitude, and municipal sewer plant operators wish more people would care. For example, I’d clean my paint brushes in a sink thinking nothing of it. I’d emulsify grease from kitchen pots and pans, and it no doubt congealed farther down in the sewer.

You never want to put any of these things, or chlorine bleach or any chemicals, into a septic tank. The only thing that should go into the tank is waste from your body and toilet paper. The cheaper the toilet paper the better. Never put flushable wipes in a septic tank or a city sewer system. (Why? Go to AsktheBuilder.com website and watch my flushable wipes video!)

If you plan to build in a rural area where a septic tank is in your future, install a utility sink in the laundry room or garage that drains directly outdoors. In other words, don’t connect the sink to the septic tank. Many inspectors allow this gray water to flow onto the ground away from your home because they don’t want you to put paint, grease, or who-knows-what into your septic tank. Wash all the bad things in this sink, not the other sinks in your home.

It’s vital that you pump your septic tank at least every three years. It’s affordable and it ensures that you won’t ruin your leach field. It can cost thousands of dollars to replace a leach field. I only pay, in 2021 dollars, $285 to pump out my 1,000-gallon septic tank. You can see why it really pays to do this. The average cost per year is less than $100.

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