Most of a septic system diagram is a straight line: house, tank, distribution box, trenches, soil. The details worth understanding are the ones the line hides, why the baffles sit where they do, why the trenches run along the contour, and why the outlet is a few inches below the inlet. The tools on this site draw your own system to scale from your bedroom count and soil, which is more useful than a generic picture.
The components, in order
| Component | Job | Typical dimension |
|---|---|---|
| House sewer line | Carries everything to the tank | 4 in, 1/4 in per ft fall |
| Inlet baffle or tee | Directs flow down, protects the scum layer | Extends 6 to 12 in below liquid |
| Tank | Settles solids, stores them | 1,000 to 1,500 gal typical |
| Outlet baffle or tee | Draws only the clear middle layer | Extends 12 to 16 in below liquid |
| Effluent filter | Catches fibers heading for the field | Clean annually |
| Distribution box | Splits flow between trenches | Must sit level |
| Drainfield trenches | Release water to the soil | 18 to 36 in deep, 6 to 10 ft apart |
| Soil | Does the treatment | 2 to 4 ft separation required |
Reading a tank cross section
A tank drawn in section shows three bands: scum on top, effluent in the middle, sludge on the bottom. The inlet enters near the top and its baffle points the incoming rush downward so it does not push the floating layer straight across to the outlet. The outlet sits two to four inches lower than the inlet, and its baffle reaches well below the liquid surface so that what leaves is drawn from the middle band only. That vertical offset and those two baffle depths are most of the engineering in a septic tank.
The three chamber tank diagram
Multi-compartment tanks split the volume so that settling happens in stages. The first compartment usually takes two thirds of the tank and does most of the work. The second polishes, and in a three chamber design the third is a small final settling or dosing chamber. The advantage is that a surge of water through the house cannot stir the whole tank at once. Many codes require two compartments as a minimum on new installations.
The drain field diagram
- Solid pipe from tank to distribution box, perforated pipe only within the trenches.
- Trenches parallel and along the contour, level along their length.
- Six to ten feet between trench centers, each with its own soil to work into.
- Perforations facing down at roughly four and eight o'clock.
- Gravel below and above the pipe, then a permeable barrier, then soil cover.
- The reserve area drawn beside the field, the same size, left clear.
The aerobic system diagram
An aerobic diagram adds three things to the conventional picture: a blower on the surface feeding air to a diffuser in the aeration chamber, a clarifier with sloped walls where floc settles and returns, and often a chlorine tube and a pump tank before dispersal. The dispersal end may be trenches, a small bed or spray heads, depending on what the state permits. If a diagram you are looking at has a blower but no clarifier, it is a retrofit aerator rather than a full ATU.
Permit files usually include an as-built sketch with distances measured from the house corners. That is far more useful than any generic diagram, because it tells you where to dig. Call the health department with your parcel number before you rent a probe.
Setbacks, the part diagrams usually leave out
| From | Tank | Drainfield |
|---|---|---|
| Private well | 50 ft | 100 ft |
| Stream, pond or lake | 50 ft | 50 to 100 ft |
| House foundation | 5 to 10 ft | 10 to 20 ft |
| Property line | 5 to 10 ft | 5 to 10 ft |
| Driveway or parking | 5 ft | Not permitted over |
Draw your own
The homepage tool draws a scale section of the tank sized for your household, and the drain field size calculator draws the plan view with the trench count, length and spacing your soil requires. Between the two you get a diagram of the system your house needs rather than a picture of somebody else's.