Sewage Treatment Plant Size Calculator
Sewage Treatment Plant Recommendation
Population Equivalent (PE)-
Recommended Plant Size-
Daily Flow (litres)-
Plant Supply Cost-
Installation Cost-
Estimated Total Cost-
Sewage Treatment Plant Sizes & Costs 2025/26
| Population Equivalent | Suits | Supply Cost | Installed Cost |
|---|---|---|---|
| 6 PE | 1-3 bed house | £2,500-£4,000 | £5,000-£8,000 |
| 8 PE | 3-4 bed house | £3,500-£5,500 | £7,000-£10,000 |
| 12 PE | Large house / 2 dwellings | £5,000-£8,000 | £9,000-£15,000 |
| 20+ PE | Multiple dwellings / commercial | £8,000-£15,000 | £15,000-£25,000 |
Regulatory Requirements
Environment Agency Permit
Required (watercourse)
Building Regs
Part H required
BS EN 12566-3
CE marking needed
How to Use This Calculator
1
Enter property details
Input the number of bedrooms and residents. The calculator uses British Water guidelines to determine population equivalent.
2
Select property and discharge type
Choose your property type and whether the plant will discharge to a watercourse or drainage field.
3
Select existing system
If replacing a septic tank or cesspit, additional costs may apply for decommissioning.
4
Review plant recommendation
The calculator shows the recommended plant size in PE, estimated supply cost, installation cost, and total budget.
Frequently Asked Questions
What is a population equivalent (PE)?
Population equivalent is the standard measure for sizing sewage treatment plants. 1 PE represents the average daily sewage production of one person (approximately 200 litres/day). For domestic properties, British Water guidelines recommend PE = number of bedrooms + 1 (e.g., a 3-bed house = 4 PE minimum, but 6 PE plants are the smallest standard size).
Do I need an Environment Agency permit?
If your treatment plant discharges to a watercourse (stream, river, ditch), you need an Environment Agency permit (from £125). If it discharges to a drainage field (soakaway), you may be able to register an exemption instead (free). A percolation test is required for drainage fields. All plants must comply with the General Binding Rules.
What is the difference between a septic tank and a treatment plant?
A septic tank only provides primary settlement (separating solids). Effluent must pass through a drainage field for further treatment. A sewage treatment plant provides both primary and secondary treatment (biological aeration), producing cleaner effluent that can discharge directly to a watercourse. Since 2020, septic tanks cannot discharge directly to watercourses.
How much does a sewage treatment plant cost to run?
Typical annual running costs are £50-£100 for electricity (aeration pump), £150-£250 for annual service/desludging, and £0-£125 for the Environment Agency permit. Total: approximately £200-£475 per year for a standard domestic system. Gravity-fed systems are cheaper to run than pumped systems.
How often does a treatment plant need emptying?
Most domestic sewage treatment plants need desludging (emptying) once per year, typically costing £150-£250. Some modern plants need emptying less frequently. Annual servicing by a qualified engineer is also recommended (£100-£200). Keeping a service record helps when selling the property.
Can I install a sewage treatment plant myself?
While it is legally possible, professional installation is strongly recommended. You still need Building Regulations approval (Part H), must comply with the General Binding Rules, may need an Environment Agency permit, and must ensure the installation meets BS EN 12566-3. Most manufacturers require professional installation to maintain the warranty.
Official Sources & References
Data verified against official UK government sources. Last checked April 2026.