What Is a Septic Tank and How Does It Work?

A septic tank is a buried, watertight container that holds the wastewater from a house long enough for the solids to separate from the liquid. It is the first half of a septic system. The second half is the drainfield, where the liquid soaks into the soil. Homes that are not on a public sewer line usually have one.

What a septic tank is

The tank is a rectangular or round box, buried a foot or two below the surface, with a lid or riser at the top for access. Most hold 750 to 1,500 gallons. Codes size the tank by bedroom count, not by how many people live in the home, and many current codes call for about 1,000 gallons for a typical three-bedroom house. Older homes may have smaller tanks.

Tanks are made of:

• Concrete: the most common and the heaviest.
• Fiberglass: light and rust-proof.
• Polyethylene (plastic): also light and rust-proof.

Some old tanks are steel. They rust out, and a steel tank in an older home is a reason to ask questions.

How a septic tank works, step by step

• Wastewater leaves the house. Toilets, sinks, showers, and the washing machine all drain into one pipe that runs to the tank.
• It enters through the inlet. An inlet tee or baffle slows the flow and directs it down into the tank so it does not stir up what has already settled.
• The tank separates it into three layers. Heavy solids sink to the bottom as sludge. Grease, soap residue, and other light material float to the top as scum. Between them sits a band of clearer liquid, sometimes called effluent.
• Bacteria digest part of the solids. Bacteria that live in the tank without air break down some of the sludge and scum. They never digest all of it, which is why the tank fills over time and has to be pumped.
• Clear liquid leaves through the outlet. An outlet tee or baffle draws liquid from the middle layer, so scum and sludge stay behind. Many tanks add an effluent filter on the outlet that catches small solids. It needs cleaning, usually at each pump-out.
• The liquid goes to the drainfield. A pipe carries it to a network of perforated pipes in gravel trenches or beds, also called a leach field. The liquid seeps out of the pipes and down into the soil.
• The soil finishes the job. As the liquid moves down through the soil, a thin biological layer and the soil particles filter out germs and many pollutants. By the time the water reaches groundwater it is meant to be clean.

The drainfield is the part that fails

The tank is a simple box. The drainfield is a living filter, and when solids, grease, or too much water reach it, the soil clogs and stops accepting liquid. Wastewater then backs up into the house or comes to the surface of the yard.

It is also the expensive part. A conventional system installed new runs about $3,600 to $12,500, and an alternative system such as an aerobic or mound system runs about $10,000 to $20,000. Our septic installation cost guide explains what moves the price, and the signs of drain field failure are worth learning before you need them. A routine pump-out, by comparison, costs $250 to $600.

How long a tank and a drainfield last

These are typical figures, not promises. A concrete tank often lasts 40 years or more, and a plastic or fiberglass tank can last as long when installed correctly. Steel tanks give out much sooner. Baffles and filters wear out first and are repaired.

A drainfield commonly lasts 20 to 30 years, and some last far longer. Skipped pumping, heavy water use, flushed trash, and vehicles shorten that life. Pumping on schedule and moderate water use extend it.

What goes wrong

• Solids reaching the field. When a tank is not pumped, the sludge and scum layers grow until solids escape with the liquid. They clog the soil, and the damage is expensive to fix.
• Hydraulic overload. Too much water at once, such as many loads of laundry in a day, a leaking toilet, or a houseful of guests, leaves no time for solids to settle and floods the field.
• Roots. Roots seek water and can crack pipes and block drainfield lines. Keep large trees away.
• Driving or building on it. Vehicles crush pipes and compact the soil, and a shed or patio over the field can ruin it.
• Additives that do nothing. Treatments sold to "restore" a tank are not needed, and no product replaces pumping. A healthy tank already gets its bacteria from normal use.

What a conventional tank does not need

A standard gravity system needs no electricity and no chemicals. Gravity moves the wastewater, and settling, bacteria, and soil do the treatment.

Some sites cannot use a conventional design. The soil may be too shallow, too tight, or too close to groundwater. Those sites use alternative systems, such as an aerobic septic system, which blows air into the tank to speed up treatment, or a mound system that raises the drainfield above poor soil. These systems use pumps, blowers, or alarms, so they need electricity and regular service, and many places require a maintenance contract for them. The tradeoff is cost: about $10,000 to $20,000 installed, compared with $3,600 to $12,500 for a conventional system.

What the owner has to do

Pump on schedule

Plan on pumping every 3 to 5 years for a typical home, with an inspection about every 3 years. The right interval depends on tank size and household size, and our guide on how often to pump a septic tank shows how to work it out. Routine pumping costs $250 to $600, and most homeowners pay around $400. See the pumping cost guide for what changes the price.

Mind what goes down the drain

Only wastewater and toilet paper belong in the system. Wipes, grease, paint, and medications do not belong. The what not to flush list covers the common mistakes.

Watch your water use

• Fix leaking toilets and dripping faucets. A running toilet can send hundreds of gallons a day to the field.
• Spread laundry over the week instead of doing it all in one day.
• Keep roof drains, sump pumps, and surface runoff away from the drainfield.

Keep records

Keep the permit, the as-built drawing, and every pumping receipt in one folder. A buyer will ask for them.

Septic versus sewer

A sewer sends wastewater through public pipes to a treatment plant, and you pay a monthly bill. A septic system treats it on your own lot, and you are responsible for the pumping and repairs. The septic versus sewer comparison sets out the costs and tradeoffs.

What to check when buying a house on septic

A septic system should be checked before you buy, not after. Do these things:

• Ask for the records. Get the permit, the as-built drawing, and the pumping receipts. A seller with no records is a flag.
• Match the system to the house. The tank size should fit the bedroom count. An added bedroom on an old permit can mean an undersized system.
• Hire an inspector who is not the seller’s. A proper inspection looks at more than the tank. See what happens in a septic inspection, and plan for $150 to $900 depending on the type of inspection; our septic inspection cost guide explains the range. You can request septic inspection quotes from licensed providers.
• Walk the drainfield. Look for wet spots, odors, and lush grass, and check that nothing is built on it.
• Ask about a soil test. If the property has no system yet, a perc test runs about $250 to $1,000.
• Check local rules. Permit and sale-inspection rules vary by state and county, so find your state and ask the local health department what applies.

Our guide to buying a house with a well and septic covers the rest of the purchase.

Common questions