Septic Tank and Soak Pit Sizing for Residential Projects
Where there is no sewer connection, on-site treatment is a septic tank followed by soil absorption. Each is sized separately, and the soil test governs the second.
Design population and flow
Sewage flow = population × wastewater generation (120–150 L/person/day)
Septic tank capacity
The tank provides settling, flotation of scum, and anaerobic digestion. Capacity is the sum of a liquid retention volume and a sludge/scum storage volume:
where P = population, q = flow per person per day, t = hydraulic retention time (1–3 days; use 2), s = sludge accumulation per person per year (~40 L), N = years between desludging (2–4).
Example: 8 persons, 130 L/day, t = 2 days, s = 40 L, N = 3:
Provide two compartments (2/3 + 1/3), minimum liquid depth 1.2 m, and 300 mm freeboard.
Soak pit / drain field from a percolation test
Dig a test hole, presoak, then time the drop of 25 mm of water. The percolation rate (minutes per 25 mm) sets the allowable absorption:
| Perc rate (min/25 mm) | Absorption (L/m²/day) |
|---|---|
| ≤ 5 (sandy) | ~50 |
| 15 | ~30 |
| 30 | ~20 |
| > 60 (clay) | unsuitable — use a mound or package plant |
For 1040 L/day at 20 L/m²/day → 52 m² of trench bottom, or an equivalent soak-pit wall area.
EngEst Pro sizes the septic tank from design population and retention time and the soakaway from your percolation rate — the Plumbing Calculator outputs tank dimensions, concrete or pre-cast units, and drain-field materials in the BOM.
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