Septic tank or sewage treatment plant?

Septic tank in short

• Separates solids and lets liquid pass out for further treatment in the ground.
• Must discharge to a drainage field or soakaway — never to a watercourse.
• No power, no moving parts, so nothing to break.
• Needs emptying, typically annually, and depends entirely on a healthy drainage field.

Sewage treatment plant in short

• Adds an aerated biological stage, producing much cleaner effluent.
• Can discharge to a watercourse where a permit or the binding rules allow it.
• Needs a power supply and a blower or motor, so it needs annual servicing.
• Still needs periodic de-sludging by tanker.

How to choose

A percolation test on the actual ground is the deciding evidence. Guessing from a neighbour's system is how people end up installing the wrong thing twice.

• No room for a drainage field, or ground that will not take effluent: treatment plant.
• Existing discharge goes to a ditch or stream: treatment plant (or a compliant upgrade).
• Good soakaway ground, plenty of space, want zero running costs: septic tank.
• Selling soon, or a purchase held up by a drainage report: get the system assessed before you commit either way.
• Nearby watercourse or a sensitive site: check the constraints early — this is what determines what you are permitted to install.

Ongoing maintenance either way

Whichever you have, the pattern is the same: scheduled emptying by a registered carrier, an annual look at the outlet and drainage field, and keeping wipes, fats and heavy chemicals out of the system. Where the site needs pumping to reach the drainage field, the pump station goes on the same annual service.