People often use "septic tank" as a catch-all for any off-mains drainage system, but there are three distinct types — cesspits, septic tanks, and sewage treatment plants. They look similar from the outside but behave very differently. Knowing what you've got matters: it affects what you can do with it legally, how often it needs servicing, and how much you'll spend over the next 20 years.
Quick comparison
| Cesspit | Septic Tank | Treatment Plant | |
|---|---|---|---|
| How it works | Sealed holding tank | Anaerobic settling + soakaway | Aerobic treatment + discharge |
| Discharge? | None — sealed | Effluent to drainage field | Clean effluent to soil or watercourse |
| Install cost | £3,000–£6,000 | £3,000–£6,000 | £5,000–£15,000+ |
| Emptying frequency | Every 4–6 weeks | Every 6–12 months | Annual sludge removal |
| Annual running cost | £1,500–£3,500 | £150–£400 | £300–£600 |
| Lifespan | 30+ years | 20–30 years | 15–25 years |
| Best for | No-soakaway sites | Most rural properties | Sensitive locations / watercourse |
Cesspit — the sealed holding tank
A cesspit (sometimes called a cesspool) is a fully sealed tank. Everything that goes down the drain stays in the tank until it's pumped out. No treatment happens, no discharge — just storage.
Pros:
- Works anywhere — no soakaway needed
- Can be installed in tight or low-percolation sites where septic tanks aren't viable
- Long-lasting if properly maintained
Cons:
- Massively expensive to run — you're paying to empty everything that comes out of your house
- Frequent emptying needed (every 4–6 weeks for a typical household)
- Larger volumes mean bigger trucks and tipping fees
- No legal "discharge" path, so any leak is a serious environmental issue
Cesspits are usually a last resort. If you have one, it's almost certainly because a septic tank or treatment plant wasn't viable on that site. See our cesspit emptying service for ongoing pump-outs.
Septic tank — the workhorse of rural Cornwall
A septic tank is what most Cornwall rural properties have. Wastewater enters the tank, solids settle to the bottom, oil/grease floats to the top, and the cleared middle layer (effluent) discharges to a drainage field where soil bacteria finish breaking it down.
Pros:
- Far cheaper to run than a cesspit (annual emptying vs. monthly)
- Mostly passive — no electricity, no aerator, nothing to break
- Long lifespan (20–30+ years, longer for well-maintained concrete tanks)
- Established technology — every Cornwall waste carrier handles them
Cons:
- Needs a drainage field — won't work on tight sites or where soil percolation is poor
- Cannot legally discharge directly to a watercourse (since the 2020 General Binding Rules)
- Drainage field eventually fails (typically 20–30 years) — expensive to rebuild
- Less effective treatment than a treatment plant — discharged effluent is still mildly contaminated
For most rural Cornwall homeowners with reasonable land, a septic tank is the right answer. See our cost guide and maintenance schedule.
Sewage treatment plant — the upgrade
A sewage treatment plant (sometimes "package treatment plant" or "STP") is a more sophisticated system that actively treats wastewater using aeration. It produces effluent clean enough to legally discharge to a watercourse — making it the standard solution for properties where a drainage field isn't possible.
Pros:
- Can legally discharge directly to a watercourse (with appropriate permit)
- Produces much cleaner effluent than a septic tank
- Works on smaller sites where soakaway isn't viable
- Better for sensitive environments (e.g., near rivers, protected zones)
Cons:
- Higher install cost (£5,000–£15,000+ for typical residential)
- Needs electricity to run the aerator — won't work in a power cut for long
- More moving parts means more things to maintain/replace
- Annual servicing by a competent engineer is standard
- Sludge build-up still requires regular pump-out (similar to septic tank frequency)
See our installation service for new builds or replacements.
Which one do you have?
Three quick checks:
- Is there a soakaway / drainage field downstream of the tank? Yes = septic tank. No = either cesspit or treatment plant.
- Is there an electric supply to the tank area (humming sound, vent)? Yes = treatment plant. No = septic tank or cesspit.
- How often does it get emptied? Monthly = cesspit. Annually = septic tank. Annually with quarterly service = treatment plant.
If you've recently bought a property and the previous owner didn't leave documentation, a CCTV survey can confirm the type and condition.
Which one should you have?
The decision tree, in rough order:
- If you can install a septic tank with a drainage field, do. Cheapest to run, longest-lasting, simplest maintenance. Most Cornwall properties suit this.
- If your soil won't percolate or you're near sensitive watercourses, install a treatment plant. Higher cost but cleaner discharge and more flexibility.
- If neither will fit, install a cesspit and budget for high running costs. Last resort. Some Cornwall coastal cottages have no other option.
Cost over 20 years
Rough total cost of ownership across a 20-year period (installation + running):
- Septic tank: ~£8,000–£15,000 (£3,000–£6,000 install + £3,000–£8,000 running) — assuming one drainage field rebuild
- Treatment plant: ~£10,000–£25,000 (£5,000–£15,000 install + £5,000–£10,000 running)
- Cesspit: ~£35,000–£75,000 (£3,000–£6,000 install + £30,000–£70,000 running)
Yes — over 20 years, a cesspit costs roughly 3–5x more than a septic tank. The frequent emptying adds up fast. Unless you have no choice, avoid.
The bottom line
For most rural Cornwall properties, a properly-sized septic tank with a drainage field is the sweet spot — cheap to run, simple to maintain, long-lasting. Treatment plants are the right answer for sensitive locations or tight sites, and worth the extra install cost when needed. Cesspits should be the last resort, only when nothing else will work.
Not sure what you've got, or what you should have? Submit your postcode for a free assessment quote — we'll match you with a Cornwall specialist who can advise honestly.