How do I calculate population equivalent for a sewage treatment plant?

Population equivalent (PE) is calculated as the number of bedrooms plus 1 when using the bedroom-based method. For example, a 3-bedroom house has a PE of 4. This is the standard method used by the Environment Agency and specified in BS 6297. Alternatively, you can use the actual number of residents if known, but the bedroom method provides future-proofing.

What is the difference between a septic tank and a sewage treatment plant?

A septic tank only provides primary settlement (solid/liquid separation) and requires a drainage field for further treatment before effluent can be dispersed. A sewage treatment plant (also called a package treatment plant) provides both primary and secondary treatment, producing a much cleaner effluent that can be discharged directly to a watercourse (with an Environment Agency permit) or to a drainage field. Since 2020, septic tanks can no longer discharge directly to watercourses in England.

Do I need an Environment Agency permit for a sewage treatment plant?

If your sewage treatment plant discharges to a drainage field (soakaway), you can usually operate under the General Binding Rules without a permit, provided certain conditions are met (under 2m³/day discharge, no discharge to sensitive areas). If discharging to a watercourse, you will need an Environmental Permit from the Environment Agency, which typically costs £125 and takes 2-4 months to obtain.

How much does a sewage treatment plant cost?

A domestic sewage treatment plant for 4-6 PE typically costs £2,000-£5,000 for the unit itself. Installation costs (excavation, concrete surround, electrical connection, drainage runs) can add £2,000-£5,000 depending on ground conditions and distance from the property. Total installed cost is typically £4,000-£10,000 for a domestic property. Annual running costs include electricity (£30-£80/year) and desludging (£150-£300/year).

How often does a sewage treatment plant need servicing?

Sewage treatment plants require annual servicing by a qualified engineer to check the mechanical and electrical components, air blower, settlement chambers, and effluent quality. They also need desludging approximately annually, though this depends on usage. Most manufacturers require proof of annual servicing to maintain the warranty. The servicing typically costs £150-£250 per visit.

Sewage Treatment Plant Calculator — What Size Do I Need for My Property?

How We Calculate This

This calculator sizes a sewage treatment plant based on the population equivalent (PE) of your property, using the standard UK methodology.

The formula

Population Equivalent (P) = Number of bedrooms + 1

Daily capacity = P × Daily water usage per person

The standard daily water usage assumption is 150 litres per person per day (l/p/d), which is the figure used by most manufacturers and the Environment Agency. This can be adjusted for properties with higher water usage (e.g. en-suite bathrooms, power showers, or multiple bathrooms).

Tank capacity

The minimum tank capacity is calculated as approximately 3 times the daily flow for domestic installations (up to 10 PE), allowing sufficient retention time for settlement and biological treatment. For larger installations, a factor of 2.5 is used.

Standards

Sewage treatment plants must comply with BS EN 12566-3 (Packaged and/or site assembled domestic wastewater treatment plants) and be CE marked. The installation must comply with the General Binding Rules (England) 2020 or hold an Environmental Permit from the Environment Agency. Building Regulations Approved Document H also applies.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Sewage Treatment Plant Calculator — What Size Do I Need?