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Sewage treatment plants: how they work, cost & installation
A sewage treatment plant (also called a package treatment plant) treats wastewater to a much cleaner standard than a septic tank, using aeration and bacteria to actively break the waste down rather than just letting solids settle. Because the treated effluent is cleaner, it’s allowed to discharge to a watercourse — a stream, ditch or river — which a septic tank is not permitted to do. Installing one typically costs £9,000–£13,000, plus a modest ongoing electricity and servicing cost.
How a treatment plant works
Most package plants use three stages:
- Primary settlement — solids sink and scum floats, similar to a septic tank’s first stage.
- Aerated biological treatment — air is pumped in (or the chamber is otherwise aerated) so bacteria can break down the waste far more thoroughly than settlement alone.
- Final settlement — the treated liquid clarifies before it discharges.
The result is a much cleaner effluent than a septic tank produces, which is why the rules allow it to go to a watercourse.
Why that matters for compliance
Since 1 January 2020, a septic tank discharging directly to a stream, river, ditch or canal has not been allowed under the Environment Agency’s General Binding Rules. A treatment plant is the usual fix, because — unlike a septic tank — it’s permitted to discharge to a surface water, up to 5 m³/day, provided it meets the BS EN 12566 standard. See our General Binding Rules guide for the full rules, or discharge to a watercourse vs to the ground for how the two discharge routes compare.
A treatment plant can also discharge to the ground via a drainage field, the same as a septic tank — see soakaways and drainage fields.
Electricity and servicing
Unlike a septic tank, a treatment plant has moving parts — typically a pump or air blower — so it needs a mains power connection and costs a little to run. It also needs an annual service (or per the manufacturer’s schedule) to keep the biology working and stay compliant. Skipping servicing is the most common way a previously-compliant plant drifts out of standard.
When to choose a treatment plant over a septic tank
- Your only practical discharge point is a watercourse and there’s no room or suitable ground for a drainage field.
- You’re replacing a septic tank that currently discharges to a stream or ditch (non-compliant since 2020).
- You’re building new and there’s no mains sewer within reach.
- Ground conditions — heavy clay, high water table, small plot — won’t support a septic tank’s drainage field.
See our full septic tank vs sewage treatment plant comparison for running costs and the compliance differences side by side.
Cost
| Item | Typical cost |
|---|---|
| Sewage treatment plant installation | £9,000–£13,000 |
| Annual servicing | A few hundred pounds |
| Electricity | Modest — a small pump or blower running continuously |
Run our cost calculator for a figure closer to your property, or see the full septic tank cost guide.
Not sure which system is right for you?
Our free compliance checker asks a few simple questions about your property and tells you, in plain English, whether a septic tank would be compliant or whether you need a treatment plant.
Sources: GOV.UK — General binding rules: small sewage discharge to a surface water · GOV.UK — General binding rules: small sewage discharge to the ground
Common questions
How much does a sewage treatment plant cost?
Installation typically runs £9,000–£13,000 for an average domestic property, more for larger homes or difficult ground. On top of that there's a small electricity cost to run the unit and an annual service, usually a few hundred pounds a year.
Can a sewage treatment plant discharge to a stream?
Yes, within limits. Because it treats effluent to a much higher standard than a septic tank, a sewage treatment plant is allowed to discharge to a watercourse under the General Binding Rules — up to 5 cubic metres a day — provided it meets the BS EN 12566 standard and is properly maintained.
Does a sewage treatment plant need electricity and servicing?
Yes. Most plants use an electric pump or air blower to aerate the wastewater, so they need a power supply and cost a little to run. They also need servicing to the manufacturer's schedule — lapsed servicing is the most common reason a compliant plant drifts out of compliance.
Sewage treatment plant or septic tank — which do I need?
It depends on where you can discharge to. If you have suitable ground for a drainage field, a septic tank is cheaper to install and run. If there's no room or suitable soil for a drainage field, or your only practical discharge point is a stream or ditch, you need a treatment plant. See our septic tank vs treatment plant guide for the full comparison.
Need this sorting in Cornwall?
Kernow Septic covers Cornwall's towns and rural parishes. Call or run the free tools.