How a Septic System Works

The path wastewater takes

Every drain in the house, from toilets to the washing machine, feeds one main pipe that leaves the house and runs to the septic tank. The U.S. EPA describes the rest of the trip in its septic pages (U.S. EPA, SepticSmart and septic systems): the tank holds the wastewater long enough for solids to settle, then the liquid flows on to the drain field, where the soil finishes the treatment.

The tank

Tanks are usually concrete, fiberglass or plastic. Inside, three layers form:

• Scum on top: fats, oils, grease and light solids.
• Effluent in the middle: the partly clarified liquid that moves on.
• Sludge at the bottom: heavy solids that bacteria slowly break down.

An inlet baffle or tee slows incoming water so it does not stir the tank. An outlet baffle keeps scum from leaving. Many newer tanks also have an effluent filter at the outlet that catches small solids. Two-compartment tanks add a wall that gives solids a second chance to settle.

The drain field

Effluent flows into perforated pipes or plastic chambers in shallow trenches or a bed. It soaks down through the soil, where bacteria and the soil remove germs and some nutrients. The field needs a layer of unsaturated soil under it to work. That is the single biggest design issue in low, sandy parts of the Panhandle.

Pumps and alarms

Where the drain field sits higher than the tank, as with raised and mound systems, a second chamber holds a pump. Float switches turn the pump on and off, and a high-water alarm, usually a light and buzzer on a post or wall, warns when water rises too far. If the alarm goes off, cut water use and have the pump checked.

Who oversees septic in Florida

Florida's onsite sewage program permits new systems, repairs and modifications and registers septic contractors. It moved from the Florida Department of Health to the Florida Department of Environmental Protection in 2021 (Florida Department of Environmental Protection, Onsite Sewage Program). Older paperwork for many Pensacola-area homes will still carry the Department of Health name.

What wears out

• Sludge and scum build up until the tank is pumped.
• Concrete baffles and lids can crack and crumble.
• Filters clog.
• Pumps and floats wear out.
• Drain fields slowly clog with a biological mat, faster when solids escape the tank.

See what shapes a septic estimate for how each part affects a written estimate.

Related pages: septic tank pumping, septic system repair, drain field repair & replacement, and what shapes an estimate.

Sources: U.S. EPA, SepticSmart and septic systems; Florida Department of Environmental Protection, Onsite Sewage Program.