How a Septic System Works
Short answer: A septic system treats wastewater on your own property in two stages: a septic tank settles out solids and scum, and a drainfield lets the remaining liquid soak into the soil, where naturally occurring organisms finish breaking it down.
The two main parts
Wastewater from your home flows first into the septic tank, a buried, watertight container. Inside, solids settle to the bottom as sludge, and fats, oils and grease float to the top as scum. The relatively clear liquid in the middle, called effluent, flows out to the next stage. Homes with no municipal sewer line nearby rely on a system like this instead; see our septic vs sewer guide for how the two compare.
In a conventional system, that effluent moves into a drainfield: a set of trenches or a bed of perforated pipe laid in gravel or sand, where it slowly filters into the soil. Naturally occurring soil organisms treat the wastewater as it moves through, removing most remaining contaminants before it reaches groundwater.
Why the soil matters
A conventional system depends on healthy, well-drained soil to do the final treatment. Where soil conditions are poor, such as shallow bedrock, high groundwater, or clay-heavy soil, an alternative system type is used instead. See our guide to septic system types for what those look like.
Frequently asked questions
Does a septic system need electricity?
A conventional gravity system doesn't. Some alternative systems, including aerobic treatment units and drip distribution systems, use a pump or aerator and do need power.
Sources
Checked July 2026.
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