A sewage treatment plant cost has three parts: the unit, the installation and the running costs. Replacing a non-compliant septic tank with a new plant and drainage field typically costs £5,000 to £15,000 or more. Once it is in, expect around £30 to £80 a year in electricity, an annual service and desludging once a year.
These figures are a ballpark, and an installer can give you an accurate price for your site. What sits around the unit, the ground, the discharge route and the electrics, moves the price most. This guide explains each part so you can budget and compare quotes like for like.
What makes up the cost of a sewage treatment plant?
Most quotes break down into the same six items.
- The plant itself, sized for the number of people it serves.
- Excavation, the base and backfill.
- Electrics, including a dedicated circuit to the control panel and compressor.
- The discharge route, either a drainage field or a pipe to a watercourse.
- Commissioning, the checks that set the plant up to treat properly from day one.
- Removing or decommissioning an old septic tank, if you are replacing one.
If you are upgrading an existing system, a specialist drainage inspection first typically costs £300 to £600, depending on the size of the system and access.
How does size affect sewage treatment plant cost?
Plants are sized by population equivalent (PE), which is the number of people the plant is designed to treat for. Domestic plants cover a single household, typically 4 to 20 people. The GRAF One2Clean, for example, runs from 5 PE (3,750 litres) through 7, 9, 12 and 15 PE up to 19 PE (13,000 litres).
A bigger plant means a bigger tank and a bigger hole, so the price climbs with each step. Undersizing is a false economy, though: the plant has to cope with your daily volume, peak flows and any likely growth.
Our sewage treatment plant sizer takes you through eight short steps and recommends a type and size, which is the best starting point for any quote.
What does sewage treatment plant installation cost depend on?
Installation is where quotes differ most, because every site is different.
The dig. The trench needs to be at least 500mm bigger than the tank at its base, at least 1,000mm from solid structures, and deep enough that pipework sits in the frost free zone, typically 600mm to 800mm down.
The base and backfill. The One2Clean sits on a compacted bed of round grain gravel 150mm to 200mm thick and is backfilled with gravel. That is quicker and more cost effective than traditional concrete installation methods.
Difficult ground. High groundwater or heavy clay needs extra drainage around the tank. A slope, mound or embankment within 5 metres needs a calculated retaining wall.
The outlet. If the outlet is lower than where the treated water needs to go, you need a pump. The One2Clean has an optional raised outlet that can remove the need for a pumped outlet on some sites.
Where it goes. A plant under a driveway needs a vehicle loading lid. A flush to ground lid keeps the garden looking as it did.
Electrics. The control panel and compressor need a dedicated circuit installed to current safety standards.
For the full process, read our step by step guide on how to install a sewage treatment plant.
Why does the discharge route change the price so much?
In England, the General Binding Rules came into full effect on 1 January 2020. Since then, septic tanks cannot discharge directly to a watercourse such as a stream, river, ditch or surface water drain. A non-compliant septic tank must either be replaced with a plant that produces effluent clean enough for a watercourse, or be connected to a drainage field built to BS 6297.
A drainage field should sit at least 10 metres from a watercourse and 50 metres from a water abstraction point, and away from areas that flood. On a small plot or in poor soil, a drainage field can be hard to fit and expensive to build. Discharging treated water from a plant to a suitable watercourse may then be the more practical route, and in many areas that needs a permit. Scotland, Wales and Northern Ireland have their own rules, so check with your regulator. Our guide to buying and selling a house with a septic tank explains more.
What are sewage treatment plant running costs?
Electricity. A plant’s compressor uses power. Most systems cost around £30 to £80 a year to run. The One2Clean uses about 0.63 kWh a day, around half of most systems, because its compressor aerates for around 10.5 hours a day rather than running non stop. That is well under £1 of electricity a week for most UK households. Our post on how much energy a sewage treatment plant uses explains how it works.
Servicing. GRAF UK recommends a professional service at least once a year. A service is cheap compared with replacing a compressor that has been left to fail.
Desludging. GRAF UK recommends desludging once a year. A registered waste carrier must do the work, and a visit typically costs £150 to £300. That is a ballpark figure: your local desludging company can give you an accurate price.
Our guide to sewage treatment plant servicing and maintenance explains what a service involves.
How do commissioning and warranty affect the total?
Commissioning checks, tests and adjusts the plant so it treats properly from the start and meets regulatory requirements. It takes 45 to 60 minutes. GRAF UK offers to get your new sewage treatment plant commissioned by GRAF UK at no extra cost when you buy it from us, carried out by a GRAFter trained in commissioning and servicing our plants. This is subject to the receipt of your completed pre-commissioning form at least 48 hours in advance.
Warranty is part of the long term cost too. The One2Clean comes with one year of warranty after commissioning and two additional years if it is serviced annually. Skipping services can cost you that cover, so build the annual service into your budget from the start.
Is a sewage treatment plant more expensive than a septic tank?
Usually, yes, to buy. But a septic tank can only discharge to a soakaway or drainage field in most cases, and its effluent is of a moderate standard. A plant treats to a higher standard, which gives you more discharge options and can suit stricter areas.
With proper maintenance, a packaged plant can last 20 to 30 years, so it is worth comparing the cost over that life, not just on the day you buy.
What should you check in a sewage treatment plant quote?
Check that it covers:
- The plant model and PE, and that it matches your household.
- Excavation, base, backfill and any extra drainage, retaining wall or pump.
- The discharge route, with the drainage field or outfall priced in full.
- The electrical connection to the control panel and compressor.
- Removal or decommissioning of the old septic tank.
- Commissioning, and the first year’s service.
Frequently asked questions
Do I need planning permission for a sewage treatment plant?
Requirements vary by area and by where the treated water discharges. Check local rules and any permits with your local authority or regulator before you start.
Can a sewage treatment plant be installed under a driveway?
Yes, with a vehicle loading lid. Keep a gap from surfaces used by vehicles heavier than cars that is at least equal to the trench depth.
Will a sewage treatment plant work while I am away?
Yes. Leave it switched on. Because the compressor runs intermittently, leaving it on while you are away should barely affect your electricity bill.
Get a size and a starting point
The quickest way to a realistic budget is to know what size plant you need. Use our free sewage treatment plant sizer for a recommendation in eight steps, or contact the GRAF UK team to talk through your site.
Posted by Callum Vallance-Poole, on November 2, 2026. Tags: packaged treatment plant, sewage treatment plant, wastewater treatment system