| Item | Typical cost | Source |
|---|---|---|
| 6–7 person treatment plant, unit only: Marsh Ensign, Tricel Novo UK6, Graf One2Clean 7PE. Equipment only: excludes installation. | about £2,190–£2,700 inc VAT | Cotterill Civils; Tanks For Everything; Tanks For Everything, Oct 2026 |
| Premium 6-person unit: Klargester BioDisc BA. Equipment only. | about £4,950 inc VAT | Tanks Direct, Oct 2026 |
| Manufacturer's guide for the unit only | £1,700–£3,000 (6-person), £1,900–£3,500 (8-person) | Tricel |
| Installation on top of the unit (delivery, excavation, groundwork) | £2,000–£8,000, depending on size and ground | Tricel |
| Supplied and installed, typical domestic job | commonly £7,500–£15,000; difficult sites cost more | Fastfix Drainage & Plumbing; Alton Facility Services; The Sewage Guy |
| Annual service | about £100–£300 a year, plus any parts | Alton Facility Services; Fastfix Drainage & Plumbing; Tricel |
What a supply-and-fit quote usually covers
- Primary settlement: solids settle out as sludge.
- Aeration: an electric blower pumps air into the middle chamber so bacteria can break down the waste.
- Final settlement: remaining solids settle and are returned to the first chamber.
- Treated effluent goes to a drainage field, or to a watercourse where your registration or permit allows.
For example, Fastfix's from-£7,500 package covers an EN 12566-3 certified plant, control panel, blower, drainage field and full commissioning. Alton Facility Services says a proper quotation should list plant supply, excavation, pipework, electrical works, commissioning, waste disposal, tanker attendance and reinstatement separately. Tricel puts installation at £2,000–£8,000 on top of the unit. Ask whether the discharge route and removal of an old septic tank are included, because these are often priced separately.
Common extras
- A drainage field or soakaway, if it discharges to ground
- Running an electricity supply to the plant
- A ground survey or percolation test
- Decommissioning an old septic tank or cesspit
- VAT
Running costs
Budget about £100–£300 a year for the annual service (Alton Facility Services; Fastfix £150–£250; Tricel around £120 plus parts), plus emptying and a small electricity cost for the air blower. Skipping the annual service can lead to smells, poor effluent quality and a shorter life for the unit.
Emptying
| Who | Price | Source |
|---|---|---|
| Scotland: Scottish Water household de-sludge, up to 9 m³ (2026–27) | £254.40 scheduled; £353.10 unscheduled; £509.70 urgent | Scottish Water |
| Northern Ireland: NI Water domestic desludge, up to 4.5 m³ (2026–27) | one free empty every 12 months, then £108 (£142 for difficult access) | NI Water; NI Water |
| England example: private tanker firm in Sussex (prices from September 2024) | £170 for up to 1,000 gallons, rising to £380 for up to 4,000 gallons | South East Tankers |
Converting an existing septic tank instead
| Option | Typical cost | Source |
|---|---|---|
| Add-on conversion unit fitted after the existing tank (Marsh Uni:Gem, certified to EN 12566-6). Equipment only. | about £2,510–£3,010 inc VAT | Drainage Superstore; BPPlastics, Oct 2026 |
| Conversion, supplied and installed | typically around £5,000 + VAT | The Sewage Guy |
| Replacing the septic tank with a new treatment plant | commonly £7,500–£15,000 | Fastfix Drainage & Plumbing; Alton Facility Services; The Sewage Guy |
| Keeping the tank but discharging to ground through a new drainage field instead | £3,000–£7,000 for the field | Clear Stream Drainage Solutions; Fastfix Drainage & Plumbing |
In-tank retrofit kits installed after 2007 need an Environment Agency permit. Add-on units need EN 12566-6 certification to discharge to surface water without one (GOV.UK; WTE).
Septic tank or treatment plant: which is cheaper overall?
A septic tank is usually cheaper to install and run, but only if your ground can take a drainage field and you're allowed to discharge to ground. Where a treatment plant is legally required, such as for a surface water discharge in England or Wales, comparing the two on cost alone doesn't apply.
Frequently asked questions
How long does a sewage treatment plant last?
With annual servicing, the tank shell can last for decades. Mechanical parts such as blowers and pumps wear out and need replacing from time to time.
What size treatment plant do I need?
It's sized on the number of people, using British Water's Flows and Loads guidance, so holiday lets and extra buildings must be included. Six-person and eight-person units are common for houses.