Septic Tank vs Cesspit: Key Differences
Both septic tanks and cesspits handle sewage for properties not connected to mains drainage, but they work very differently. Understanding which system you have — and which is right for your property — can save you thousands of pounds per year in running costs.
| Feature | Septic Tank | Cesspit |
|---|---|---|
| How it works | Treats and discharges waste to a drainage field | Stores all waste — no treatment or discharge |
| Emptying frequency | Every 12–24 months | Every 4–8 weeks |
| Annual running cost | £150–£350 per year | £1,000–£3,000+ per year |
| Environmental impact | Treated effluent discharged to ground | No discharge (all waste removed) |
| Regulations | Must comply with General Binding Rules | Must be watertight with no leaks |
| Installation cost | £3,000–£6,000 | £2,500–£5,000 |
How Does a Septic Tank Work?
A septic tank is an underground chamber that partially treats wastewater through a natural biological process:
- Wastewater enters the tank from your property's drains
- Solids sink to the bottom, forming a sludge layer
- Oils and grease float to the top, forming a scum layer
- The liquid middle layer (effluent) flows out to a drainage field
- Bacteria naturally break down the solids over time, reducing volume
- The drainage field further treats the effluent as it percolates through soil
This continuous process means a septic tank only needs emptying when the sludge and scum layers build up enough to affect performance — typically every 12 to 24 months.
How Does a Cesspit Work?
A cesspit (also called a cesspool) is fundamentally simpler:
- Wastewater enters the tank from your property's drains
- Everything stays in the tank — there is no outlet or discharge
- When the tank is full, it must be emptied by a tanker vehicle
Because nothing leaves the tank, cesspits fill up much faster. A family of four typically fills a standard cesspit in 4 to 8 weeks.
Cost Comparison Over Time
The running cost difference is dramatic:
| Period | Septic Tank | Cesspit |
|---|---|---|
| Per emptying | £200 (average) | £200 (average) |
| Empties per year | 1–2 | 6–13 |
| Annual cost | £200–£400 | £1,200–£2,600 |
| 5-year cost | £1,000–£2,000 | £6,000–£13,000 |
| 10-year cost | £2,000–£4,000 | £12,000–£26,000 |
| 20-year cost | £4,000–£8,000 | £24,000–£52,000 |
Over 20 years, a cesspit can cost £20,000 to £45,000 more than a septic tank to maintain. This makes conversion from a cesspit to a septic tank system one of the best investments a rural homeowner can make.
Which Is Better?
For most homeowners, a septic tank is significantly cheaper to run and maintain. Cesspits are only the right choice where:
- Soil conditions don't allow a drainage field — heavy clay or high water table can make drainage fields impractical
- Insufficient land — drainage fields need a minimum area that may not be available
- Regulations prevent discharge — some environmentally sensitive areas have restrictions
- Temporary solution — if you're planning to connect to mains sewer in the near future
Can I Convert My Cesspit to a Septic Tank?
Yes, in most cases. The process involves:
- Percolation test — a specialist tests your soil's ability to absorb treated effluent. This determines whether a drainage field will work on your land
- Site assessment — checking distances to watercourses, boundaries, buildings, and water supplies
- Design — the new system is designed to meet British Standard BS 6297 and the General Binding Rules
- Planning permission — required in some cases, though many conversions fall under permitted development
- Installation — typically takes 3–5 working days
- Building Regulations sign-off — your local authority must inspect and approve the completed installation
Conversion Costs
| Component | Typical Cost |
|---|---|
| Percolation test and site survey | £250–£500 |
| Septic tank unit | £1,000–£2,500 |
| Drainage field installation | £1,500–£3,500 |
| Pipework and connections | £500–£1,000 |
| Total conversion cost | £4,000–£8,000 |
The payback period is typically 2–3 years. After that, you save £1,000–£2,000 per year compared to running a cesspit.
Sewage Treatment Plant: The Third Option
A sewage treatment plant (also called a package treatment plant) is a step up from a septic tank. It actively treats waste to a higher standard using aeration, which means treated effluent can be discharged directly to a watercourse — something a septic tank cannot legally do.
| Feature | Septic Tank | Treatment Plant |
|---|---|---|
| Purchase cost | £1,000–£2,500 | £3,000–£7,000 |
| Running cost | No electricity needed | £50–£100/year electricity |
| Can discharge to watercourse | No | Yes (with permit) |
| Drainage field needed | Yes | Not always |
| Servicing | Annual emptying | Annual service + emptying |
A treatment plant is worth considering if your septic tank currently discharges to a watercourse (which is now illegal), or if your site cannot accommodate a drainage field.
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How Do I Know Which System I Have?
Not sure whether you have a septic tank or cesspit? Here are some clues:
- How often is it emptied? Monthly or more = likely a cesspit. Annually = likely a septic tank
- Is there a drainage field? A grassy area through which no pipes run but the ground may be slightly raised = likely a drainage field for a septic tank
- Check the property's deeds — they often describe the drainage system
- Ask the previous owner or check with your local water authority
- Lift the lid and look — a septic tank will have an inlet and outlet pipe; a cesspit has only an inlet
If in doubt, a local operator can inspect your system and advise you. Get a free assessment.
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