Guide
How a septic system works — tank, distribution box, drainfield and soil, and what each part actually does.
A septic system takes everything that leaves the house through one pipe and returns it to the ground as water clean enough that the soil can finish the work. It does this with no power, no chemicals and no moving parts in a conventional design. The whole thing is gravity and time.
A watertight box, usually concrete, sized so that a couple of days of wastewater sit in it before anything leaves. In that time three layers form. Grease and light solids float to the top as scum. Heavy solids sink as sludge. Between them sits a band of relatively clear liquid, and that middle band is the only part that should ever leave the tank. An outlet baffle or effluent filter enforces this.
Bacteria in the sludge break some of it down, but not all. The remainder accumulates, which is why the tank has to be pumped.
A small chamber that splits the outgoing effluent evenly between the drainfield trenches. Boxes settle out of level over the years and send everything to one trench, which is a common and very fixable cause of a field that appears to be failing.
A set of shallow trenches with perforated pipe laid in gravel or in plastic chambers. Effluent trickles out along the length of the pipe and soaks downward. The field is not a disposal pit — it is a distribution device, spreading the water thin enough that soil can treat it.
The actual treatment happens here. As effluent moves down through unsaturated soil, a biological layer at the trench bottom digests organic matter while the soil column filters pathogens. Two to four feet of unsaturated soil between the trench and groundwater is what makes the system safe, which is why the seasonal high water table decides so much about a design.
A code cannot size for a household, because households change. It sizes for the house. Bedrooms are a durable proxy for how many people could live there, and the number a future buyer inherits. Two people in a four-bedroom house still need a four-bedroom system.
Almost never the tank. The drainfield is the vulnerable part, and it fails in three ways: solids escape a neglected tank and seal the trench bottoms; the field is hydraulically overloaded by more water than it was sized for; or it is compacted by vehicles and building. All three are avoidable.