How Does a Septic System Work? Everything You Never Wanted to Know—but Should

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Sure, you probably know a septic system is something located underground that deals with waste. But what else do you know about it? And do you really need to know anything else?

If you’re buying a house that has one, the answer is a resounding yes.

“The purpose of having a septic tank is to enjoy good, clean, affordable, eco-friendly drainage for the full span of your time on a given property,” explains Glenn Gallas, vice president of operations for Mr. Rooter Plumbing. “And when you do decide to sell, a perfectly operating septic system could also add value to your property.”

Here’s everything you probably never wanted to know about septic systems—but should.

What is a septic system?

Let’s start with the absolute basics. A septic system is an underground waste treatment structure. Not everyone has one or needs one. If you live in a city or town that has its own sewer system (like so many do), you’re good! But if you have well water, don’t get a sewer bill each month, or don’t have a water meter on your property, you very likely have your very own septic system.

The main part of a septic system is a steel or concrete tank that’s buried under the soil. And they aren’t small.

“Most are capable of holding 1,000 gallons of material,” Gallas says.

One main drainage pipe connects this tank to all of the other pipes in your house—the ones that carry wastewater from your toilets, bathtubs, sinks, washing machine, and so on.

How does a septic system work?

Wastewater goes into the tank, where over time it settles naturally into three layers. (Thanks, gravity!) Grease and oil rise to the top, aka the scum layer. Anything heavier than water sinks to the bottom and is known as the sludge layer.

“The tank also produces rancid gases, which are filtered through a vent pipe,” Gallas says. (It generally leads out the roof of your house so you’re less likely to smell gross odors.)

Floating between these two layers is wastewater, which contains chemicals like phosphorus and nitrogen. These substances appear courtesy of human waste, food, and some detergents and soaps, and they actually work to help decompose the literal crap in your water.

Called effluent, this section of wastewater eventually leaves your tank. A system of lateral perforated pipes allow effluent to slowly trickle out into a designated underground area in your yard known as a drain field.

What happens in a drain field?

Contrary to popular belief, not all the septic magic happens inside the tank. Much of the actual water treatment takes place in your drain field.

Once effluent drips out of these buried pipes and makes its way into the soil, microbes in the soil quite literally gobble up potentially harmful gunk, like bacteria and viruses.

Where your drain field is in your yard will depend on where you live and the type of property you have. Your town will likely have specific requirements on where a drain field should go, especially if your land backs up to, say, a stream or wetlands (contaminated groundwater is always a risk with septic systems).

Generally speaking, a drain field for a three-bedroom house could be sized anywhere from 450 to 900 square feet. (To put that in perspective, that’s about the size of the average American apartment.)

Unless you’re mucking around the area with an excavator, you shouldn’t really see or smell the natural water treatment happening. Any kind of pooling or a nasty, sewage smell isn’t normal—and if you experience this, call in a pro, pronto.

Still, it’s not safe to plant a garden on top of your drain field. (Do you really want to eat fruits and veggies that have been fertilized by human waste?) Or drive on top of it. And that bocce court you’ve been wanting to put in? Find another spot for it. Please.

Do I have to do anything to maintain a septic system?

“A septic tank needs periodic maintenance to remain working and healthy,” Gallas says.

That means pumping solid waste from your tank on a regular basis. While systems vary, experts advise that a 1,000-gallon septic tank (what a typical household of four has) should be serviced every five years or less.

And don’t cheap out—such maintenance work should never be a DIY undertaking, “because the germs and gases from the septic tank can be dangerous,” Gallas notes.

(Fun fact: Untreated wastewater can give you diseases like dysentery and typhoid fever.)

You may balk at shelling out money to maintain a septic system, which can run a few hundred dollars. But lack of maintenance is a big contributor to septic system failure, as Audrey Monell, president of Forrest Anderson Plumbing and AC, in Glendale, AZ, points out—and that can mean thousands of dollars in cleanup, plus an unthinkable mess.

Ignore your septic system for years and you might have to replace it altogether—which can ding you between $5,000 and $10,000.

Plus, you’re not just making a mess for yourself; problems with septic systems “can be detrimental to lake water purity and hazardous to the surrounding environment,” Gallas says.

To responsibly run your septic system, you need to treat the pipes that run to your system. Prevent yours from clogging by keeping grease, hair, cigarette butts, or hard items like plastic from slipping down your drains. Never flush baby wipes, pads, or tampons. And don’t even think about planting trees or other heavily rooted plants on, or even near, your system.

Sure, they’ll pretty up the area, but “roots and bulbs can grow stronger as time passes and ultimately damage your drainage pipes,” Gallas says.

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