Sewage Treatment Plant Calculator — What Size Do I Need?

This calculator sizes a sewage treatment plant by the population equivalent (P) of your property, following the British Water Code of Practice Flows and Loads – 4, the recognised UK design method.

Population (P)

For a single house, use a minimum P of 5 for up to and including 3 bedrooms, then add 1 person per additional bedroom:

• 3-bedroom house = 5 PE
• 4-bedroom house = 6 PE
• 5-bedroom house = 7 PE

The calculated population is then rounded up to the nearest standard manufacturer unit (5, 6, 8, 10, 12 PE…). For multiple dwellings, Flows and Loads 4 also applies balancing reduction factors (×0.9 for 13–25 people, ×0.8 for 26–50 people).

Daily design flow

Daily flow = P × 150 litres/person/day. 150 l/p/d is the domestic hydraulic loading in Flows and Loads 4 (1 PE = 150 L/day flow, 60 g/day BOD). Higher figures may be used for properties with unusually high water use.

Plant capacity

A packaged treatment plant is notsized by a volume formula — its capacity is the manufacturer-tested PE rating under BS EN 12566-3. Select the unit whose rated PE meets or exceeds the calculated population. (The reference “primary-settlement volume” shown is the Approved Document H 2015 §1.18septic-tank volume rule — at least 2,700 litres for up to 4 users, plus 180 litres for each additional user — included only for comparison; it is not the treatment-plant rating.)

Standards

Sewage treatment plants must meet BS EN 12566-3 (packaged/site-assembled domestic wastewater treatment plants) and be UKCA/CE marked. The installation must comply with the General Binding Rules (England) — discharge to ground 2 m³/day or less, to surface water 5 m³/day or less — or hold an Environmental Permit. Building Regulations Approved Document H also applies.