Two jobs, two parts: a tank that settles solids, and soil that finishes the treatment. Here's what actually happens underground.
Short answer: A conventional septic system has two main parts. Wastewater first flows into a buried, watertight tank, where solids settle to the bottom as sludge and fats/oils float to the top as scum, leaving a layer of relatively clear liquid in the middle. That liquid then flows out to a drainfield (a network of perforated pipes in gravel-filled trenches), where it seeps into the soil and is filtered and broken down by naturally occurring microbes before it ever reaches groundwater. The soil itself is doing real treatment work, not just drainage -- which is why a system has to be designed around the specific soil on that lot.
Every drain in the house -- toilets, sinks, showers, washing machine -- feeds into a single buried tank, usually concrete, fiberglass, or heavy plastic. Inside, wastewater sits long enough (typically a day or two of retention time) for gravity to do the separating:
A baffle or effluent filter at the outlet is designed to hold back sludge and scum so only that middle liquid layer moves on to the drainfield. Over time sludge and scum both build up faster than bacteria in the tank can break them down, which is why periodic pumping is necessary -- see our pumping frequency page for how often.
The effluent leaving the tank flows into the drainfield -- a series of perforated pipes laid in gravel-filled trenches (or, on tighter or wetter lots, engineered alternatives like mound or sand-filter systems). From there it seeps slowly into the soil, where the real final treatment happens: soil microbes consume remaining organic matter and pathogens, and the soil itself filters out much of what's left, before the water eventually rejoins the water table.
This is the part of the system that's easiest to damage without realizing it. Driving or parking on a drainfield, planting trees with invasive roots over it, or covering it with a deck or pavement can compact the soil or physically block the pipes, and a saturated or damaged drainfield is one of the more expensive things to fix on a home.
A percolation ("perc") test -- checking how quickly water drains through the specific soil on a lot -- is a required step before a septic system can be permitted in nearly every jurisdiction, because the soil isn't just there to drain water away. It has to be permeable enough to accept the flow, but fine and biologically active enough to filter and treat it before it reaches groundwater. Soil that's too sandy may drain too fast to treat effluent properly; soil that's too dense (heavy clay, or a high water table) may not drain fast enough at all, which is why some lots need alternative system designs instead of a conventional gravity drainfield. Requirements and allowed system types vary by state and county -- see our state regulations guide for how a given state handles this.
Some older homes, particularly those built before the 1970s, may have a cesspool instead of a conventional septic system -- essentially a single perforated pit that combines the settling and soil-treatment functions a modern system splits into two separate stages. Because a cesspool provides far less treatment before wastewater reaches the soil, most states stopped permitting new cesspool installations decades ago, and some now require them to be upgraded to a conventional system at the point of failure or property transfer. If you're not sure what type of system a home has, that's one of the first things a pre-purchase inspection should establish.
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