Cost guide
Sewage treatment plant cost
What a package sewage treatment plant costs to buy, install and run, including the ongoing costs a septic tank does not have.
A package sewage treatment plant treats waste water well enough that the treated effluent may legally discharge to a watercourse as well as to ground. For many rural properties, particularly on clay where soakaways struggle, it is now the practical route to a compliant system.
It is also the option with real running costs. A septic tank is a buried box that needs emptying; a treatment plant is a small machine with an air supply, a service schedule and an electricity bill. Comparing the two on installation price alone gets the decision wrong, so the ongoing figures are set out here alongside the installation ones.
What these figures are, and are not
These are typical market ranges for domestic systems in this part of the country, not quotes, and not prices this site can offer. They are gathered from generally published UK contractor price guidance and are intended to tell you whether a quote you receive is in the normal range. Real prices vary widely with access, ground conditions and how far a tanker or a digger has to travel. Anyone quoting you from a phone call alone, without seeing the site, is guessing.
Ranges last reviewed July 2026.
Typical ranges
| Work | Typical range |
|---|---|
| Plant supplied only, up to about 6 peopleA domestic unit certified to EN 12566-3. Larger units, and those built for deep invert or high water table installation, cost more. | £3,000 to £6,000 |
| Supplied and installed, straightforward sitePlant, excavation, concrete surround or backfill as the manufacturer requires, electrical connection, commissioning and handover, on a site with decent access. | £6,000 to £12,000 |
| Installed with a new drainage fieldWhere discharge is to ground rather than to a watercourse, the field is an additional excavation and an additional cost, sized on a percolation test. | £10,000 to £18,000 |
| Annual serviceMechanical checks, blower and diaphragm inspection, biological and sludge assessment, and a written report. Required by most manufacturers to keep a warranty valid. | £150 to £300 |
| De-sludgingTypically once a year for a domestic plant, and often combined with the service visit to save a second call-out. | £200 to £350 |
| ElectricityA small air blower running continuously. Modest, but it is a cost a septic tank does not have, and it belongs in any comparison. | roughly £30 to £90 a year |
What drives the variation
Sizing, which is set by the property not by preference
Plants are sized in population equivalent, normally derived from the number of bedrooms. Undersizing produces poor effluent and breaches the standard the plant was certified to; oversizing wastes money and can starve the bacteria. The size is a calculation, and a quote should show it.
Where the treated effluent goes
Discharge to a watercourse avoids a drainage field entirely and is much the cheaper arrangement where a suitable watercourse is genuinely available and the General Binding Rules conditions are met. Discharge to ground needs a field, sized on a percolation test, which adds materially to the cost.
Groundwater and burial depth
A high water table can float an empty tank during installation and needs a concrete surround or anchoring, which adds cost. Deep incoming drain inverts need a deeper unit. Both are decided by the site, not by the product.
The electrical connection
A plant needs a permanent, protected supply, often some distance from the house. Trenching an armoured cable across a garden or yard is a real cost and should be a named line in the quote rather than an assumption that a socket is nearby.
Servicing you will actually be able to get
Some makes are widely serviced and their parts are easy to obtain; others tie you to one supplier. Since you are buying twenty years of servicing along with the unit, it is worth asking who else in the county services that make before choosing it.
What a fair quote should itemise
Two quotes are only comparable when they cover the same work. Asking for these as separate lines is the quickest way to see which is genuinely cheaper, and it is a reasonable request that any decent contractor is used to.
- The plant make, model and its certified population equivalent, with the sizing shown
- Where the treated effluent is to discharge, and the permit position if relevant
- Excavation, and the concrete surround or backfill the manufacturer specifies
- The electrical connection, including cable run and any trenching
- Commissioning, handover and the manufacturer documentation
- Building regulations application and completion paperwork
- Decommissioning of the old tank, where one is being replaced
- The annual service cost, and what it includes
- The warranty, and what is required to keep it valid
In Shropshire specifically
Treatment plants have become the default answer across much of Shropshire for one reason: the ground. On the clays of the north Shropshire plain, percolation tests fail often enough that a conventional septic tank and soakaway is frequently not a legal option, and a plant discharging to a watercourse becomes the compliant route.
The county is well supplied with watercourses, which helps, but a discharge to one has conditions attached under the General Binding Rules and is not automatic. Whether your nearest ditch or brook is a lawful discharge point is worth establishing before committing to a plant on that basis.
The Clun catchment in the south-west is a special case. It holds some of England’s rarest river wildlife and is subject to particular scrutiny over water quality, so discharges in that catchment can face additional requirements. If you are in the Clun valley, treat the discharge question as the first thing to resolve rather than a detail at the end.
Power is worth checking early on remote Shropshire properties. Plants in outbuildings or a long way from the house need a proper cable run, and on holiday lets and second homes an intermittent supply is a genuine problem, since the bacteria in a plant die back when it is switched off for long periods.
Frequently asked questions
How much does a sewage treatment plant cost installed?
Typically £6,000 to £12,000 supplied and installed on a straightforward site discharging to a watercourse, and £10,000 to £18,000 where a new drainage field is needed as well. The unit alone is usually £3,000 to £6,000. These are general market ranges rather than quotes.
What does a treatment plant cost to run?
Budget roughly £400 to £650 a year all in: an annual service at £150 to £300, de-sludging at £200 to £350, and electricity of about £30 to £90. A septic tank costs less to run but has fewer legal discharge options, which is usually what decides the choice.
Is a treatment plant worth it compared with a septic tank?
It depends almost entirely on your ground. Where a percolation test passes and a drainage field is permitted, a septic tank is simpler and cheaper to own. Where the test fails, which is common on Shropshire clay, a treatment plant is often the only compliant option rather than a preference.
What happens if I skip the annual service?
The plant degrades quietly. Blowers fail, the bacteria die back, and the unit gradually becomes an undersized septic tank discharging poorly treated effluent, which breaches the standard it was installed to meet. It also normally voids the manufacturer warranty, and missing service records are routinely queried when a property is sold.
Not sure which of these applies to you?
Describe the problem and we will tell you what you are most likely dealing with, and put you in touch with a local specialist where one covers your area. No charge for that, and no obligation.