CLEARMASTERS
The short version
What’s the difference between a septic tank, a cesspit and a treatment plant?
If your home isn’t connected to the mains sewer, your waste goes to one of these three. The difference is what happens to it next.
Cesspit
Stores itA sealed tank with no outlet. It holds everything until a tanker empties it, so it fills up fast.
Septic tank
Settles itSolids sink, fats float, and the liquid runs out to a drainage field where the soil finishes treating it.
Treatment plant
Treats itAn electric blower feeds air to bacteria that clean the waste, so the water leaving it is much cleaner.
Five quick questions
Which system do I have?
Answer what you can, and choose “I don’t know” for the rest. We’ll tell you what you most likely have and what it needs.
Under the lid
How a cesspit, septic tank and treatment plant work
Pick a system to see what happens underground.
Cesspit
A cesspit (or cesspool) is a sealed holding tank. Nothing is treated and nothing leaves: it simply stores everything that goes down your drains until a tanker takes it away.
- Treats sewage?
- No. It only stores it.
- Outlet
- None. It must not leak or overflow.
- Power
- None needed.
- Emptying
- Often: every few weeks to a couple of months, depending on size and use.
- Good for
- Sites where nothing can be discharged at all.
Septic tank
A septic tank separates sewage. Solids sink as sludge, fats float as scum, and the partly treated liquid flows out to a drainage field, where the soil finishes the job.
- Treats sewage?
- Partly. The soil in the drainage field does the rest.
- Outlet
- To the ground, through a drainage field. Not to a ditch or stream.
- Power
- None needed.
- Emptying
- At least once a year, or as the manufacturer says.
- Good for
- Homes with ground that drains well and room for a drainage field.
Treatment plant
A sewage treatment plant is a small sewage works. An electric blower feeds air to bacteria that break down the waste, so the water that leaves is clean enough for a drainage field or, in many cases, a ditch or stream.
- Treats sewage?
- Yes, to a much higher standard than a septic tank.
- Outlet
- To the ground, or to a ditch or stream.
- Power
- Yes. It must stay switched on.
- Emptying
- Desludged at least once a year, or as the manufacturer says, plus regular servicing.
- Good for
- Poorly draining ground, or where the only outlet is a watercourse.
In your garden
Clues to look for
No single clue is certain, especially on older properties, but two or three together usually give it away.
How often it’s emptied
The biggest giveaway of all.
- CesspitEvery few weeks or months
- SepticOnce a year or less
- PlantOnce a year, plus servicing
Power and noise
Treatment plants need electricity for an air blower or pump.
- PlantCable, kiosk or a gentle hum
- SepticNothing electrical
- CesspitNothing electrical
Where the water goes
Follow the ground away from the tank.
- CesspitNowhere: no outlet
- SepticDrainage field or soakaway
- PlantDrainage field, ditch or stream
What’s on top
Covers vary a lot, so use this with the other clues.
- PlantLarge plastic lid, often with a kiosk
- SepticOne or two covers, or brick chambers
- CesspitA cover, often over a large tank
Alarms
A beeping box usually means a treatment plant, or a pump station.
- PlantUsually has an alarm
- SepticNo alarm
- CesspitSometimes a high-level alarm
Paperwork
Check your deeds, survey, building control records or old invoices.
- SepticSeptic tank or soakaway mentioned
- PlantMake and model, e.g. Klargester
- CesspitCesspool or holding tank
At a glance
Septic tank vs cesspit vs treatment plant
| Cesspit | Septic tank | Treatment plant | |
|---|---|---|---|
| What it does | Stores sewage | Separates solids and partly treats | Fully treats with air and bacteria |
| Where it goes | Nowhere: tanker takes it away | Drainage field in the ground | Drainage field, ditch or stream |
| Electricity | No | No | Yes, always on |
| Emptying | Every few weeks to months | At least once a year | At least once a year, or as the maker says |
| Servicing | Check it’s sealed and not leaking | Check tank, baffles and drainage field | Blower, pumps, alarm and media, usually yearly |
| Can discharge to a stream? | No outlet at all | No, not under the current rules | Yes, within the rules |
| Common problems | Overflowing if not emptied often enough | Blocked or failed drainage field | Blower failure, alarms, poor effluent |
| Running costs | Frequent emptying | Annual emptying | Electricity, servicing and emptying |
A typical year
How often does each one need emptying?
The biggest practical difference between the three is how often a tanker needs to call.
Cesspits fill fast
Building Regulations use 150 litres per person per day to size cesspits, and a new one must hold at least 18,000 litres for two people, plus 6,800 litres for each extra person. At that rate, a family of four with an older 18,000 litre cesspit fills it in about a month. A new tank sized for four (31,600 litres) lasts about seven weeks. Rainwater getting in fills either one faster.
Work out a rough figure for yours with the cesspit fill-up calculator.
Septic tanks and plants: once a year
The rules say the sludge must be removed at least once a year, or as the manufacturer says. Leaving it longer lets solids escape into the drainage field, which is the most expensive part to put right. Our guide to drainage field problems explains why.
Our crews empty septic tanks and desludge treatment plants with our own 2,000 to 4,000 gallon tankers, and you get a waste transfer note every time.
The rules in England
The rules for each system
Septic tanks and treatment plants come under the Environment Agency’s general binding rules. Meet them and you don’t need a permit.
Cesspits
Must be kept in good condition and must not leak or overflow. They must be emptied regularly by a registered waste carrier. The Environment Agency or your council can make you repair or replace a cesspit in poor condition.
Septic tanks
Must discharge to the ground through a drainage field, not to a ditch, stream or river. Rainwater soakaways installed after 2007 don’t count as drainage fields. A discharge to ground over 2 cubic metres a day needs a permit or a sewer connection.
Treatment plants
Can discharge to the ground or to a watercourse. Discharges to a watercourse are limited to 5 cubic metres a day without a permit. The plant must be desludged and maintained as the manufacturer says, and kept in good working order.
Selling your home
You must give the buyer written details of the system: what it is, where it is, any changes made and its maintenance records. A septic tank that discharges to a watercourse must be dealt with, and the buyer and seller should agree who will do it.
Summarised from GOV.UK guidance for England. Wales, Scotland and Northern Ireland have their own rules. This is general information, not legal advice.
Easy to confuse
Common mix-ups we come across
“It’s a cesspit” (it’s a septic tank)
The names are often used for each other, but if liquid leaves the tank, it isn’t a cesspit. Under the rules, anything that discharges to the ground or a watercourse is a septic tank or a treatment plant.
A “cesspit” emptied only once a year
A sealed cesspit couldn’t last a year without emptying. If yours does, it’s either a septic tank, or a cesspit that’s leaking into the ground, which is pollution and needs fixing.
A treatment plant that’s been switched off
Without its blower, a treatment plant is just a tank. The bacteria die off, treatment stops and the outlet gets dirty. It must stay on, even when you’re away.
Soakaway or drainage field?
A soakaway is a pit; a drainage field is a network of perforated pipes in gravel trenches. Septic tank effluent needs a proper drainage field. If your drainage field has failed, it usually needs replacing.
Upgrades
Thinking of changing your system?
The most common reasons people change are a septic tank that discharges to a stream, a drainage field that’s failed, or a cesspit that costs too much to keep emptying.
Septic tank to a stream: replace it with a treatment plant, add a drainage field, or connect to the sewer. A conversion unit is sometimes possible, but needs a permit.
Expensive cesspit: a treatment plant can cut emptying to about once a year, if there’s somewhere suitable for the treated water to go. That may need planning and building regulations approval, and sometimes a permit.
Mains sewer nearby: connecting may be the simplest long-term answer, if it’s practical.
Questions answered
Septic tank, cesspit and treatment plant FAQs
What is the difference between a septic tank and a cesspit?
A cesspit is a sealed tank that simply stores sewage until it is emptied, so it fills up quickly and needs emptying often. A septic tank separates the solids and lets the partly treated liquid flow out to a drainage field, so it only needs emptying about once a year.
What is the difference between a septic tank and a sewage treatment plant?
Both settle out the solids, but a treatment plant also pumps air into the waste so bacteria can break it down. That makes the water leaving a treatment plant much cleaner, which is why it can often go to a ditch or stream, while septic tank effluent must go into the ground through a drainage field. Treatment plants need electricity; septic tanks do not.
How do I know which system I have?
The biggest clues are how often it is emptied, whether it uses electricity, and where the liquid goes. A tank emptied every few weeks with no outlet is a cesspit. Something with a blower, a kiosk or an alarm is usually a treatment plant. A tank with no power that drains to a soakaway or drainage field is usually a septic tank. If you are not sure, we can identify it on a visit.
How often should a septic tank be emptied?
At least once a year, or as often as the manufacturer recommends. The general binding rules require the sludge to be removed before it builds up too far, and a tank emptied too rarely can push solids out into the drainage field and block it.
How often does a cesspit need emptying?
It depends on the size of the tank and how many people use it. Building Regulations use 150 litres per person per day for planning, so a family of four could fill an 18,000 litre cesspit in about a month. Our cesspit page has a calculator to work out a rough figure for yours.
Can a septic tank discharge into a stream or ditch?
No. Under the general binding rules in England, septic tanks must not discharge directly to a watercourse. If yours does, it must be replaced with a treatment plant, connected to a drainage field or connected to the mains sewer, and the buyer and seller should agree who will do this if the property is sold.
Can I convert my septic tank or cesspit into a treatment plant?
Often, yes. A septic tank discharging to a stream can be replaced with a treatment plant, and some can be upgraded with a conversion unit, which needs a permit. A cesspit can sometimes be replaced with a treatment plant if there is somewhere suitable for the treated water to go. We can look at your site and explain the options.
Do I have to tell a buyer about my septic tank or treatment plant?
Yes. When you sell, you must give the new owner written details of the system, including what it is, where it is, any changes made to it, and its maintenance records if you have them.
Got a different question? Call 03333 233566 or send us a message.