The short answer
A septic tank separates solids and lets settled liquid soak away through a drainage field — it does not treat effluent enough to discharge to a watercourse. A sewage treatment plant adds a biological treatment stage, producing cleaner effluent that can discharge to a watercourse where the General Binding Rules are met. A cesspool (cesspit) is a sealed holding tank with no discharge at all — everything is stored and tankered away, so it needs frequent, costly emptying. Which you need depends on whether you have suitable ground for a drainage field and whether you must discharge to surface water.
These three are often confused but work very differently, and choosing wrong is expensive. Here is what each does and how to decide.
At a glance
- Septic tanksettles solids, soaks to ground
- Treatment planttreats effluent, can reach watercourse
- Cesspoolsealed store, no discharge
- Drainage field neededseptic tank: yes
- Running cost drivercesspool: frequent emptying
How each system works
- Septic tank: wastewater settles; solids sink and break down; the liquid flows out to a drainage field and soaks into the ground. Simple and lower cost, but needs suitable, percolation-tested ground and cannot discharge to a watercourse.
- Sewage treatment plant: the same settling, plus an aerated biological stage that produces effluent clean enough to discharge to a watercourse (where the rules allow) or to a smaller drainage field. Higher cost and needs power.
- Cesspool: no treatment and no outlet — a sealed tank that simply stores sewage until a tanker empties it. Useful where the ground will not take a drainage field, but the emptying bills are the highest of the three.
How to choose
Start with your discharge point. If you have suitable ground for a drainage field and no watercourse involved, a septic tank is often the lower-cost choice. If you must discharge to a watercourse, or your ground will not pass a percolation test for a full drainage field, a sewage treatment plant is usually the route to compliance. A cesspool is normally a last resort where neither soakaway nor watercourse discharge is possible, because of the ongoing emptying cost.
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Frequently asked questions
What's the difference between a septic tank and a treatment plant?
A septic tank only separates solids and lets the liquid soak away to ground; it cannot discharge to a watercourse. A sewage treatment plant adds a biological stage, producing cleaner effluent that can discharge to a watercourse where the rules are met.
What is a cesspool?
A cesspool (cesspit) is a sealed holding tank with no outlet. It stores all the sewage until a tanker empties it, so it needs frequent emptying and has the highest running cost of the three systems.
Which system do I need?
It depends on your ground and discharge point. Suitable ground with no watercourse usually suits a septic tank; a required watercourse discharge or poor ground usually points to a treatment plant; a cesspool is a last resort. A percolation test confirms it.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.