What is a septic system?

A septic system is a private wastewater treatment system buried on your property. In its most common form it has two parts: a septic tank that separates solids from liquid, and a drain field where the liquid soaks into the soil and is cleaned. Homes in rural and many suburban areas use septic systems because they are not connected to a city sewer.

EPA calls septic systems “underground wastewater treatment structures” that use natural processes and proven technology to treat water from your bathrooms, kitchen drains and laundry. More than one in five US households rely on one, according to EPA.

What does septic mean?

“Septic” refers to decay. In a septic tank, bacteria that live without oxygen slowly break down part of the solid waste, and that decay is where the name comes from.

When people say a house “is on septic,” they mean its wastewater is treated on the property instead of being piped to a municipal treatment plant. EPA lists several other names you may see in listings, permits and inspection reports:

• Onsite wastewater treatment system (or OWTS)
• Decentralized wastewater treatment system
• On-lot system
• Individual sewage disposal system
• Private sewage system
• Cluster system (when one system serves several homes)

They all describe the same idea: wastewater is treated close to where it is produced.

What are the parts of a septic system?

Penn State Extension describes a conventional system as having three main parts, connected by a pipe from the house.

Part	What it is	What it does
Septic tank	A buried, watertight tank, usually concrete, plastic or fiberglass	Holds wastewater so solids sink and grease floats
Distribution box	A small concrete or plastic box between the tank and the field	Splits the liquid evenly among the drain field pipes
Drain field	Perforated pipes in gravel-filled trenches or beds, covered with soil	Lets the liquid seep into the soil for final treatment

Some systems add a pump tank, an alarm, a sand filter or an aerobic treatment unit. These are covered in our guide to types of septic systems.

How do septic systems work?

Here is the path wastewater takes, based on EPA and Penn State Extension descriptions.

1. Wastewater leaves the house

Every drain in the house, including toilets, showers, sinks, the dishwasher and the washing machine, flows into one main pipe that runs to the septic tank.

2. The tank separates and settles

Inside the tank, the water slows down and forms three layers:

• Sludge: heavy solids that sink to the bottom.
• Scum: fats, oils and grease that float to the top.
• Effluent: the partly clear liquid in the middle.

Baffles, which are walls or T-shaped fittings at the inlet and outlet, keep the layers in place. The outlet fitting draws only the middle liquid so sludge and scum stay behind. Many tanks also have an effluent filter on the outlet to catch small particles. Penn State Extension says wastewater usually stays in the tank for about two to three days. During that time, bacteria break down some of the solids. The rest build up until the tank is pumped.

3. Effluent flows to the drain field

The liquid leaves the tank and moves to the drain field, either by gravity or with a pump. A distribution box spreads it among several pipes so no one area gets overloaded.

4. The soil finishes the treatment

The drain field pipes sit in shallow trenches dug into unsaturated soil. Effluent trickles out through small holes and down through gravel and soil. EPA explains that as it passes through the soil, harmful bacteria, viruses and nutrients are removed. The cleaned water eventually reaches groundwater.

This is why soil matters so much. Pennsylvania, for example, requires at least 4 feet between the bottom of the drain field and the top of groundwater or bedrock. Where soil is too shallow, too wet or too tight, a different design is needed.

How much water does a septic system handle?

Ohio State University Extension says systems are designed for up to 50 gallons of wastewater per person per day. EPA estimates that average indoor use in a single-family home can reach 70 gallons per person per day. When a household uses more water than the system was built for, effluent moves through the tank before solids settle, and the drain field can flood. Spreading out laundry and fixing leaks keeps use within design limits.

For a closer look at the tank itself, including sizes, materials and lifespan, see our full guide to the septic tank.

Septic system vs. city sewer

The basic difference is who treats the wastewater and who pays for it.

	Septic system	City sewer
Where wastewater goes	A tank and drain field on your property	Pipes under the street to a municipal treatment plant
Who owns and maintains it	You, the homeowner	The city or utility maintains the public sewer lines and treatment plant
Monthly sewer bill	None; EPA notes septic homes usually show a $0.00 wastewater charge	Yes
Routine costs	Pumping every 3 to 5 years and inspections about every 3 years (EPA)	Covered by the utility bill
Where it is common	Rural and suburban areas without a sewer main (EPA)	Towns and cities
What you can flush	Only human waste and toilet paper (EPA)	Similar rules, but mistakes clog public lines rather than your own field

Pros of a septic system

• No monthly sewer charge. You pay only for pumping, inspections and repairs.
• Cost-effective where homes are spread out. EPA describes decentralized systems as a cost-effective, long-term option in less densely populated areas, because they avoid long runs of sewer pipe.
• Water returns to the local ground. EPA notes that properly working systems help recharge groundwater.

Cons of a septic system

• You are responsible for everything. The Washington State Department of Health points out that the homeowner owns and operates the system. A failed system can lower property value and create legal liability.
• Repairs can be expensive. Penn State Extension notes that the drain field is the most expensive and difficult part to repair or replace.
• Some habits have to change. Grease, wipes, chemicals and heavy garbage disposal use can shorten a system’s life.

If you are moving from a sewer home to a septic home, the habit changes are the biggest adjustment. Our list of septic safe products covers what can and cannot go down the drain.

How long do septic systems last?

EPA says a concrete septic tank can last 50 years or more, and plastic tanks can last about as long. Other parts wear out sooner. Pumps and controls often need replacing every 10 to 20 years, and after 25 to 30 years the drain field can start to clog as the biomat, a layer of slime where effluent meets soil, thickens. Ohio State University Extension puts the typical life of a properly maintained system at 20 to 30 years.

Good care is the biggest factor. EPA recommends:

• An inspection by a septic professional at least every three years, or once a year for systems with pumps, float switches or other mechanical parts.
• Pumping every three to five years for most household tanks.
• Using water efficiently and keeping cars, buildings and trees off the drain field.

To see how often your own tank should be pumped, try the pumping frequency calculator. For a full care routine, see our septic system maintenance guide.

Signs your septic system needs attention

Health departments list these warning signs:

• Slow drains throughout the house
• Sewage backing up into toilets, tubs or floor drains
• Soggy ground or bright green, spongy grass over the drain field
• Sewage odors outdoors
• Wastewater surfacing in the yard

The Snohomish County Health Department in Washington notes that not every sign means failure, but sewage on the ground calls for a septic professional right away. Keep children and pets away from wet areas, and never open or lean into a septic tank, because the gases inside can be deadly.

Who regulates septic systems?

EPA provides guidance, but septic systems are regulated by state and local governments, which is why EPA does not keep a national database of them. Your state or county health or environmental department sets the rules for permits, tank sizes, setback distances from wells and property lines, and whether an inspection is required when a home is sold. Before you buy, build, repair or replace a system, call your local health department to find out what applies to your property.