How Septic Systems Work

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Peter Aschwanden
In a septic tank, waste separates into (A) the scum layer, (B) the effluent and (C) the sludge layer.

If you live on a farm, you become accustomed to certain smells. I remember the first time I realized there was a particular … odor, to our barnyard. Returning from vacation, we had just crossed the cattle guard that kept our purebred Simmental cattle from wandering in the neighborhood, when my dad said, “Ahh, the smell of money.” (I’m pretty sure hog and sheep farmers say this to their children, too.) But even with a certain level of odor acceptance, the day we opened the septic tank and I learned how septic systems work, the stench was shocking.

Festus (a character truly worthy of the name) drove the honey wagon in our area. He opened the lid to our septic tank and slid a scary-looking accordion tube inside. I snuck a peek to see what the interior looked like – and the smell, well, it’s stayed with me as well. What was going on in there? Was something broken that we had to call Festus? How does the system work?

Well, it all rests on the backs (cell walls?) of some bacteria and some soil mechanics. Your septic system has three parts: the septic tank, the drain field, and the soil surrounding the drain field. Each has an important function in treating your waste water.

After waste goes down the drain, it runs into the septic tank. Lighter fats and greases float to the top and form the scum layer. Heavier solids sink to the bottom and form the sludge layer. The tank is designed so that the liquid portion, known as effluent, eventually flows through the tank and into the drain field. The tank also stores the waste between cleanings, allowing some of it to be digested by bacterial action. The process here is similar to what happens in your compost pile, except your septic tank is an anaerobic (without oxygen) environment. (Anaerobic conditions are called “septic,” which is where the tank gets its name.)

When the effluent leaves the tank, it enters the drain field. Most of the treatment of your wastewater happens here. Different types of soil absorb water at different speeds, called percolation rates. The drain field’s purpose is to diffuse the liquid into the soil at the proper rate using different types and configurations of drainage pipe and gravel.

When the effluent arrives at the drain field it contains both disease-causing germs (pathogens) and chemical pollutants. As the water percolates through the soil, the pathogens are killed or filtered out by soil contact. Well-adapted to their surroundings, bacteria already in the soil easily outcompete bacterial pathogens for available nutrients. Viruses and bacteria are held in the soil until they die over time. Some chemicals can become attached (adsorbed) to the surface of soil particles. Adsorption works better in soil that has the right amount of clay and humus (decayed plant matter) because they bind more easily to chemicals (including water).

If your septic system is built properly, it can do a good job of decontamination, but it still must be maintained (see “Septic System Design and Maintenance,”).  All septic tanks need to be emptied periodically. Digestion happens more rapidly in warm climates than in colder climates, so you may not have to have your septic tank pumped as often in warm climates.

Bringing us back around to thinking about Festus. Do you suppose fathers who drive honey trucks tell their children that sewage smells like money? 

Web Editor Jenn Nemec rates the septic tank stench as slightly more pleasant than that of branding and dehorning day at the ranch.