Septic tank vs sewage treatment plant: what's the difference?

If your home isn’t connected to the mains sewer, it treats its own wastewater — and there are two main ways it does that: a septic tank or a packaged sewage treatment plant. They’re often confused, but they work differently and the rules treat them differently.

How a septic tank works

A septic tank is a simple settlement tank. Wastewater flows in, solids sink to the bottom as sludge and lighter material floats as scum. The liquid in the middle flows out — but it is only partly treated. That liquid still needs to soak away through a drainage field (a network of perforated pipes buried in the ground), where the soil finishes treating it.

A septic tank has no power and no moving parts. It’s cheap to install and run, but it depends entirely on having ground suitable for a drainage field.

How a sewage treatment plant works

A treatment plant does more. It uses air and bacteria to actively break down the waste, producing a much cleaner effluent. Because that effluent is cleaner, it can — within limits — be discharged directly to a watercourse (a stream or ditch), as well as to the ground.

A treatment plant usually has an electric pump or air blower, so it costs a little more to run and needs regular servicing, but it works where a septic tank can’t.

Which am I allowed to install?

This is where the difference matters most:

• A septic tank may only discharge to the ground via a compliant drainage field. It must not discharge to a watercourse.
• A treatment plant may discharge to a watercourse (within volume limits) or to the ground.

Since 1 January 2020, a septic tank discharging directly to a surface water is not allowed and must be upgraded — very often by replacing it with a treatment plant. See our General Binding Rules guide for the detail.

Running costs compared

	Septic tank	Treatment plant
Install cost	Lower	Higher
Running cost	Very low (no power)	Small electricity + annual service
Emptying	~yearly	Per manufacturer, plus servicing
Can discharge to a watercourse?	No	Yes, within limits

Not sure which you have, or whether it’s compliant? Run it through our free compliance checker.

Sources: GOV.UK — General binding rules: discharge to the ground · GOV.UK — General binding rules: discharge to a surface water