Septic system sizing

the system at a glance

What it was

A concept sizing for the septic system serving a two-bedroom dwelling on a rural property. Every number comes from Ontario Building Code Part 8 (the 2024 compendium) plus the soil test results, and each step feeds the next: flow, tank, chamber length, trench layout, then a check of how sensitive it all is to the soil. The property, owner and site details are left out here.

1. Daily design flow

The code sets a minimum daily design sewage flow by dwelling size (Table 8.2.1.3.-A): 750 L/day for one bedroom, 1,100 for two, 1,600 for three, 2,000 for four, 2,500 for five, and another 500 for each bedroom after that. I used the two-bedroom row directly, with no per-person reduction, so the system stays sized to the dwelling class:

Q = 1,100 L/day

2. Tank volume and compartments

Article 8.2.2.3 requires a working capacity equal to the greater of 3,600 L or 2Q:

2Q = 2 × 1,100 = 2,200 L, so the 3,600 L minimum governs.
That is 3,600 / 1,100 ≈ 3.27 days of design flow. It’s a useful comparison, not a pumping interval.

The tank also needs at least two compartments:

Check	Calculation	Minimum	Sized
First compartment	1.3Q = 1.3 × 1,100 = 1,430 L	2,400 L	2,400 L
Second compartment	50% of the first	1,200 L	1,200 L
Total	greater of 3,600 L or 2Q	3,600 L	3,600 L

3. Leaching chamber length

For Type II chambers taking septic-tank effluent, Article 8.7.3.2 gives the required length as L = QT / 300, with a 40 m minimum. The tested percolation time was T = 20 min/cm:

Required: L = (1,100 × 20) / 300 = 73.33 m
Provided: 6 × 12.7 m = 76.2 m

4. Six-trench layout

A level distribution box splits the flow evenly into six equal 12.7 m runs. Equal runs are simpler to level and balance:

Per trench	Calculation	Result
Minimum length	73.33 / 6	12.22 m
Flow share	1,100 / 6	183.33 L/day
Capacity of a 12.7 m run	12.7 × 300 / 20	190.5 L/day
Reserve	190.5 − 183.33	7.17 L/day

The trench geometry stays inside the Part 8 ranges:

Item	Code range	Used
Trench width	500–1,000 mm	600 mm
Trench depth	600–900 mm	750 mm, preliminary
Chamber size	555–575 mm wide, 300–320 mm high	approved Type II module
Run length	no more than 30 m	12.7 m
Centre spacing	2.4 m, or 1.6 m in fill	2.4 m

Final depth depends on finished grade and on keeping at least 900 mm of separation between the trench bottom and groundwater, rock or unsuitable soil.

5. Soil loading and the T-time sensitivity

If the trenches are built in leaching-bed fill, Table 8.7.4.1 allows a loading rate of 10 L/m²/day at T = 20, so the minimum fill area is A = 1,100 / 10 = 110 m².

The catch is that T = 20 sits right at the upper edge of that loading band. So I ran the numbers across a range of percolation times:

T (min/cm)	Chamber length	Loading rate	Minimum fill area
15	55.0 m	10 L/m²/day	110.0 m²
20	73.33 m	10	110.0 m²
21	77.0 m	8	137.5 m²
25	91.67 m	8	137.5 m²
30	110.0 m	8	137.5 m²
36	132.0 m	6	183.33 m²

If the accepted T comes back even slightly above 20, the minimum fill area jumps 25% (137.5 m² ÷ 110 m² = 1.25) and the 76.2 m of chamber no longer covers (at T = 21: 1,100 × 21 ÷ 300 = 77.0 m, more than the 76.2 m provided) the required length. That warning went in the report in bold.

6. Clearances

The site plan has to dimension the tank and the chamber centre lines from everything around them:

From	Tank	Chamber centre line
Structure	1.5 m	5 m
Drilled well, watertight casing ≥ 6 m deep	15 m	15 m
Any other well	15 m	30 m
Lake, pond, river or stream	15 m	15 m
Property line	3 m	3 m

What stood out

The arithmetic was short. The judgement was in what it depends on. Two numbers drive the whole design: the bedroom count and one soil test result, and that result happened to sit exactly on a code boundary. A sensitivity table turned “it passes” into “it passes if T holds”, which is the more honest thing to hand a client before anything gets dug.