Septic tanks and sewage treatment plants on the Isle of Man
If a property isn't on the mains sewer, its waste water is treated on site. Here's how each system works, which suits which property, and what the Island's rules require. If you need one, we can design and install it.
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- Supply and installation
- Replacements and upgrades
- Help with planning, Building Control and DEFA
In short
- Septic tank
- Settles and separates the solids. The liquid that leaves is only partly treated, so it has to go to a drainage field, where the soil finishes the job.
- Treatment plant
- Settles the solids, then uses air and bacteria to clean the water much further. It needs electricity and regular servicing.
- On the Island
- Mains sewer first where practical. New systems need planning permission and Building Regulations approval. Only treatment plants may discharge to a stream or the sea, and only with a DEFA licence.
- What we do
- Site assessment, percolation tests, design, help with the applications, supply, installation and start-up, then maintenance.
- What it costs
- Your first consultation is free, and we give you a free estimate before any work starts.
How a septic tank works
A septic tank is a watertight tank, usually with two chambers, buried in the ground. It has no moving parts and doesn't need power.
- Waste water from the house enters through a dip pipe, below the surface, so the crust on top isn't disturbed.
- Fats, oils and light solids float and form a crust, called scum.
- Heavier solids sink and form sludge. Bacteria slowly break some of it down, and the rest is removed when the tank is emptied.
- Part-settled liquid passes through the dividing wall into the second chamber, where more solids settle.
- The liquid leaves for a drainage field. It isn't clean enough to go straight into a stream or ditch.
- Access covers for emptying and inspection.
How a sewage treatment plant works
A treatment plant does what a septic tank does, then goes further. By adding air, it lets bacteria break the waste down properly, so the water leaving it is much cleaner.
- Waste water enters the first chamber, where solids settle out, as in a septic tank.
- A small electric air pump (blower) sits in a kiosk above ground.
- Air is bubbled through the liquid in the middle chamber. Bacteria living on plastic packing inside the tank feed on the waste and clean the water.
- In the last chamber, the remaining fine particles settle out.
- Settled sludge collects in the tank and is removed when it's emptied. In many designs it's returned to the first chamber first.
- Treated water leaves, much cleaner than water from a septic tank.
Designs vary between makes. This shows the general idea.
Septic tank or treatment plant?
| Septic tank | Sewage treatment plant | |
|---|---|---|
| How it treats | Settling only | Settling, then air and bacteria, then settling again |
| Water leaving it | Partly treated | Much cleaner |
| Where the water can go on the Island | A drainage field where all the water soaks into the ground, subject to percolation tests | A drainage field, or a stream, river or the sea with a DEFA discharge licence |
| Electricity | Not needed | Needed for the blower |
| Moving parts | None | Blower, and in some designs pumps |
| Upkeep | Regular checks and emptying every few years | Regular servicing, plus emptying |
| Usually suits | Sites with ground that drains well and room for a drainage field | Sites with poorer ground, less space, or where the water needs to go to a watercourse |
Other types you might come across
Older brick or concrete septic tanks
Common on older rural properties. Many still work, but they can crack, lose their dividing walls or leak, and some were never given a proper drainage field. A survey will tell you what you've got.
Modern one-piece septic tanks
Moulded plastic tanks, often shaped like a bulb or onion, with the chambers built in. Quicker to install and watertight, but they must be set and backfilled properly to stay that way.
Cesspools (cesspits)
A sealed holding tank with no outlet. Nothing is treated; it simply fills up and must be emptied often. Under the Building Regulations it's the last resort, only used where nothing else is practical.
Reed beds
A planted bed that gives water from a treatment plant a final clean before it leaves the site. Sometimes used where extra treatment is needed before a discharge.
What the Island's rules say
The rules on the Isle of Man are different from England's, and a lot of what you'll read online doesn't apply here. In summary:
- Under the Building Regulations, foul water must go to the public sewer wherever that's reasonably practicable. A septic tank or treatment plant is for where it isn't.
- A new septic tank, treatment plant or soakaway needs planning permission and Building Regulations approval.
- Replacing an existing system in the garden of a house can be permitted development (meaning you don't need to apply for planning permission), but conditions apply, including distances from trees and rules in conservation areas.
- A septic tank can only be installed where its water can go to a full soakaway, subject to percolation tests.
- DEFA (the Department of Environment, Food and Agriculture) policy is that septic tanks should no longer discharge to streams, rivers or the sea. Only treatment plants can, and only with a discharge licence.
Read our full guide to the Isle of Man drainage rules, with links to the official sources.
A plain-English summary, not legal advice. Check with the relevant authority before relying on it for a decision.
Got an old septic tank that runs into a stream?
This can be the case on some older properties. DEFA's policy is that septic tank water can no longer go into streams, rivers or the sea. The usual answers are a proper drainage field or an upgrade to a treatment plant. Talk to us and we'll explain the options.
What decides the right system for your property
Can you reach the mains?
If a public sewer is close enough to connect to, that normally comes first. We'll check.
How your ground drains
A percolation test shows whether the ground can take the water, which decides a lot of what follows.
Space and distances
Drainage fields need room and have to be kept well away from buildings, wells, boreholes and watercourses.
How many people
The system is sized for the number of people it serves, so plan for the house, not just who lives there now.
Power and access
A treatment plant needs a power supply, and every system needs access for a tanker to empty it.
Upkeep over the years
A septic tank is simpler to live with. A treatment plant cleans the water better but needs servicing. We'll talk you through both.
From first visit to handover
Site visit and advice
We look at the property, the ground, the space and where the water could go, and tell you the realistic options.
Tests
Trial holes and percolation tests show what the ground will take.
Design and drawings
We size the system and prepare the drainage drawings for the applications.
Permissions
Planning permission, Building Regulations approval and, where needed, a DEFA discharge licence. We'll help you with the applications.
Installation
We excavate, set the tank level on a proper base, connect the pipework, build the drainage field and backfill, following the manufacturer's requirements. Building Control inspects the drainage before it's covered.
Start-up and handover
We start the system up, test it, and show you how to look after it. A maintenance notice is left in the building, as the regulations require.
Septic tank and treatment plant FAQs
What's the difference between a septic tank and a treatment plant?
A septic tank only settles the solids, so the water leaving it is partly treated and must go to a drainage field. A treatment plant adds air so bacteria can clean the water much further. It needs electricity and servicing, but on the Island it's the only type that can discharge to a watercourse, with a DEFA licence.
Can my septic tank drain into a stream or ditch?
Not for a new system. DEFA's policy is that septic tank water should no longer go into streams, rivers or the sea, and a new septic tank can only be installed where the water can soak fully into the ground. If an existing tank does run into a stream, you can check DEFA's public discharge licence register or ask DEFA's Environmental Protection Unit (01624 685885), and we can explain the options for an upgrade.
Do I need planning permission for a septic tank?
A new septic tank, treatment plant or soakaway needs planning permission. Replacing an existing system within the garden of a house can be permitted development, as long as the conditions are met. We'll help you work out which applies.
Do I need Building Regulations approval?
Yes. Septic tanks, treatment plants and soakaways all need Building Regulations approval, and Building Control inspects the drainage before it's covered up.
What size septic tank do I need?
It depends on how many people the property can house. The guidance used on the Island sets a minimum of 2,700 litres for up to four people, plus 180 litres for each extra person. We size each system for the property.
Does a treatment plant need electricity?
Yes, to run the blower that pumps air into the tank. A good installation will also warn you if the blower stops.
Can I replace my septic tank with a treatment plant?
Yes, and it's a common upgrade, particularly where a drainage field has failed or the old tank discharges somewhere it shouldn't. We'll check what permissions are needed for your property.
Related pages
Planning a new system, or worried about an old one?
Tell us about the property and we'll talk you through the options before anyone spends any money.