Sewage Treatment Plant Calculator — What Size Do I Need?

The UK design method is the British Water Code of Practice 'Flows and Loads – 4'. For a single house, you use a minimum population (P) of 5 for up to and including 3 bedrooms, then add 1 person for each additional bedroom. So a 3-bedroom house = 5 PE, a 4-bedroom house = 6 PE, and a 5-bedroom house = 7 PE. The plant is then selected against this calculated population. Each person is taken at 150 litres/day. (Note: BS EN 12566-3 plants are rated by a tested PE figure, and BS 6297:2007 covers septic tanks and drainage fields — neither specifies a 'bedrooms + 1' rule.) Alternatively, use the actual or expected number of residents if higher.

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. Use our Septic Tank Calculator to compare sizing requirements.

Many installations operate under the General Binding Rules (England) without a bespoke permit, provided the conditions are met: discharge to ground (a drainage field) must be 2 cubic metres/day or less, and discharge to a surface water (stream, river, ditch) must be 5 cubic metres/day or less, with the plant meeting BS EN 12566. If you exceed these volumes or discharge in a sensitive area, you need a bespoke Environmental Permit from the Environment Agency. The reduced application charge for a small sewage discharge of 5 m³/day or less from a domestic household is currently £141.43 (Environment Agency charging scheme, Table 1.3), with no annual subsistence charge for domestic households; applications typically take 2–4 months. Always confirm the live figure against the current EA charge schedule, as it is uplifted by CPI each April.

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).

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.