Understanding Septic Systems | Ask This Old House

In this video, This Old House plumbing and heating expert Richard Trethewey teaches host Kevin O’Connor everything he needs to know about septic systems, how they work, and how to maintain them.

Host Kevin O'Connor and plumbing and heating expert Richard Trethewey meet at the shop to talk about septic systems. Richard explains how septic systems work, what they look like inside, and what homeowners can do if their septic begins to fail.

Septic Systems are Contained Units
A septic system consists of piping running from the home’s drain system to a sealed tank, and then out to a series of pipes called leach pipes. As waste is flushed down the drain, it travels down the waste pipe to the tank where it separates, breaks down, and eventually ends up back into the soil.

There are Two Chambers
Septic tanks are typically very large, with over 1000 gallons of volume, and they’re made from concrete or fiberglass. Inside those tanks are two chambers divided by a half-wall. As waste comes down the drain pipes and lands in the tank, solid waste falls into the first chamber, while liquid waste continues over the wall and into the liquid chamber. From there, the liquid flows out into the leach fields through the leach pipes.

The First Chamber is Where Solids Break Down
Solids that travel down the drain pipe land in the first chamber and break down. The septic tank is an anaerobic chamber filled with beneficial bacteria and enzymes, and these bacteria and enzymes get to work breaking down the solid waste and turning it into liquid. Once it turns to liquid, other solids will displace it and allow it to flow over the half wall into the liquid chamber.

The Second Chamber is for Liquids
Liquids land in the first chamber and then overflow into the second chamber. Here, bacteria and enzymes break down any additional waste that may exist, but once the liquid reaches the height of the drain inside the tank, it overflows out into the leach pipes, dispersing into a leach field.

Earth is the Last Filter
After the wastes are broken down and flow out to the leach field, it then drains into the sand and soil where it will continue to be filtered until it heads back into the water table. This is the reason why properties require perk tests, as the wrong type of soil will not allow the waste to drain properly.

Certain Items Don’t Belong in Septic Systems
Septic systems are great at breaking down waste, but some items don’t belong. For instance, sanitary products, baby wipes, grease, dental floss, and other items that are flushed down drains can disrupt the harmony in a septic system. For the most part, these items float over the top of the other waste in a layer known as a “scum layer.” However, if these items make their way to the leach pipes, they can quickly clog the system and prevent it from draining properly.

Also, certain solutions like paint and other chemicals can actually kill the beneficial bacteria and enzymes inside the tank. This will prevent the breakdown of waste, and result in a backup, requiring the homeowner to call and have the system pumped out and restored.

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From the makers of This Old House, America’s first and most trusted home improvement show, Ask This Old House answers the steady stream of home improvement questions asked by viewers across the United States. Covering topics from landscaping to electrical to HVAC and plumbing to painting and more. Ask This Old House features the experts from This Old House, including general contractor Tom Silva, plumbing and heating expert Richard Trethewey, landscape contractor Jenn Nawada, master carpenter Norm Abram, and host Kevin O’Connor. ASK This Old House helps you protect and preserve your greatest investment—your home.

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Understanding Septic Systems | Ask This Old House

[matthockman]

This is way better than some generic 3D animation. A human who is passionate and a little scale model he built to help teach. Like the good ol days.

[toddoneto]

Great video w/ clear, concise description of how a Septic System works, what can damage it & how to ensure functionality.
All in 6 mins!

[nylonguitar]

I've had mine since new for 43 years, standard 2 story house. I've had it pumped out and cleaned every 3-5 years throughout its life. Pump and clean around $250.00. Maintaining properly will last a lifetime maybe more.

[jamesboyer]

Make sure to get a high end lint filter for the washer, polyester lint is a huge killer of drain fields.

[woohunter]

I was told by a guy that installs septic systems that one of the worst things for the leach field is laundry lint. I installed a fine screen with a frame that fits snuggly in my laundry sink about 3” from the bottom, clean it out every couple of weeks.

[QuadroNVS]

This was a wonderful presentation and explanation of the workings of a septic system. Richard is a professor of master plumbing and always learning with each TOH episode.

[tjroicub]

I had 3 houses with septic systems over 38 years and never (thank God) had any trouble. As far as flushing things down the toilet, I had a rule: if you haven’t eaten it don’t put it in there.

[timothykeith]

An effluent filter can be added before the lateral lines - ongoing filter cleaning is needed, can greatly increase the life of the leach field. Also, a grease trap can help - also needs cleaning. I think it would help if the grey water were plumbed to the 2nd chamber of the septic tank - taking long showers tends to push floating material into the laterals

[scotsmanofnewengland]

My house was built in 1870 before they had septic tanks and city sewers. The only bathroom was on the second floor and used gravity to flow into a dry well in the back yard. It now has city sewers but when digging in the back yard I uncovered the dry well and made sure it stayed closed up !!

[anabellamainetti]

We all just want someone who would describe us as that man describes the septic system 😂 wow. I love him and I loved his explanation and his passion for septic tanks.

[heinzotto]

Best explanation of a septic system I have seen yet. Thank you Rich 🙏

[robinw]

First time off grid, septic, this video really taught me so much, thanks!

[zakman]

I'm very glad you did say that the septic system does need to be cleaned out or pumped out every few years !! I can't tell you how many people don't ever have their septic system cleaned out or pumped out . Most people don't even know what a baffle in the septic system is for . Only until it's backed up, then they go nuts . Good video !!

[ricksmith]

That was by far the best demonstration of a septic system. Being a country boy I know all about them having grown up with them and actually installed a few myself. We never had our tank pumped, were diligent about what went down the pipe, had some of the greenest grass over the leach field. Every spring we would pull effluent from the tank and fertilize the garden with it and had what i call Chernobyl carrots, they were huge. It was a messy job but my parents kept a septic system for 30 years with no issues other than really thick green grass.

[Elbowendj]

Really interesting and informative. Love how Richard is so enthusiatice about the subject.

[Sjrick]

i hope to own my own home one day. Been watching these guys for years

[TheDailyConversation]

Excellent model, Richard! As a proud owner of a septic system, I didn’t have a clue how they worked when we bought our house a couple years ago. This video and the others like it you’ve made in the past explaining septic (and heat pumps!) have really helped me understand the systems in our house. Great work guys, keep it up!

[dalesworld]

I had a new tank and a cesspit put in (it was allowed by the inspector as I didn't have the square footage for a big field). Installer never put an effluent filter on the outbound side and only two years later the cesspit stopped leaching liquid into the soil and I had a massive wet spot in the lawn. I had the system pumped, then I put risers on the tank and cesspit for ease of access, installed an effluent filter myself (you do not want to be hanging upside down in your septic tank but you do what you have to) and put a koi pond air pump in my basement and ran the hose to a diffuser in the cesspit to start aerobic breakdown. I also ran the grey water elsewhere. Just taking your washing machine off the septic will go a long way to helping keep your septic system alive.

[jeffclark]

I bought a house last year with a hybrid septic system, sump pumps, extra drain fields, and an alarm system, been trying to learn how it operates, great video, lots of information

[jamesstuart]

Well done. Also should add that if enough sludge/scum gets into your distribution field, it can permanently clog it up and you'll have to replace the pipes