How to size a soak away for septic tank or sewage treatment plant?

🧮 Step-by-Step Soakaway Sizing Process

This depends on the property size:

Domestic property: Use the British Water Flows and Loads guide:

1-bedroom = 2 PE

2-bedroom = 3 PE

3-bedroom = 4 PE

Add 1 PE per additional bedroom

For commercial sites, use actual usage data or design flows.

Determine the Daily Flow Rate (Q)

According to UK standards (Building Regulations / British Water):

Septic tank: 150 litres/person/day

Sewage treatment plant: 150–200 litres/person/day (depends on use and system efficiency)

Formula:

$Q = \text{PE} \times \text{Flow per person (litres/day)}$

$Conduct a Percolation Test (Vp)$

This tests the soil’s permeability and gives a Vp value in seconds/mm.

Dig a 300mm square hole to 300mm depth (below proposed pipe invert).

Fill with water and let it drain overnight.

Refill with water and measure the time it takes to drop from 75% to 25% full.

Repeat at least 3 times for each hole, use average.

Formula:

$Vp = \frac{\text{Time (seconds)}}{\text{Water level drop (mm)}}$

Acceptable Vp range: 15 – 100 sec/mm
Outside this range, drainage fields are unsuitable.

$A = Q \times Vp \times 0.25$

Where:

A = Soakaway area (m²) — length of trench x 0.6m (trench width)

Q = Daily flow (litres/day)

Vp = Percolation value (s/mm)E.g. for a 4-bedroom house (5 PE), Vp = 45:

$Q = 5 \times 150 = 750\text{ L/day}$ $A = 750 \times 45 \times 0.25 = 8437.5\text{ m²}$

This is trench area, not plot area — divide by trench width (0.6m) to get trench length.

Trench width: 0.3 to 0.9 m, typically 0.6 m

Minimum pipe invert: 200 mm below ground

Min 1.2 m between trench bottom and water table or bedrock

Trench spacing: At least 1 m apart

Use perforated pipe surrounded by washed gravel

🛑 Notes & Limitations

Do not use soakaways for surface water or in impermeable soil (clay).

Alternative if unsuitable soil: raised mound systems or reed beds.

Always consult local building regulations and possibly an engineer for exact design.