Secondary containment is required where a leak could reasonably reach the environment or a building. We assess whether your installation needs it, and where it does, either an integrally bunded tank or a purpose-built bund brings it into compliance.
What a bund is, and what it is for
A bund is containment around or beneath an oil tank, able to hold more than the tank’s full contents. If the tank splits, the fuel is caught rather than going into the ground.
Two ways of achieving it:
- An integrally bunded tank – a tank within an outer shell, manufactured as one unit. The usual domestic answer: simpler, takes less room, and the containment capacity is guaranteed by manufacture.
- A single-skin tank inside a constructed bund – a masonry or concrete structure built around it, impermeable, of adequate capacity. Common on larger and commercial installations, and the route for retrofitting containment to a tank that is otherwise sound.
This is drained agricultural land. Field ditches, dykes and drainage channels run alongside a very large proportion of rural property, and they count as watercourses even when dry for most of the year. Oil that reaches a ditch reaches a watercourse, and that is a reportable pollution incident with a clean-up liability that sits with the property owner.
When it is required
For a domestic installation the answer comes from a risk assessment carried out as part of the installation. Secondary containment is required where any of these apply:
- Within 10 metres of a watercourse – a stream, river, pond, ditch, dyke or drainage channel. The condition that catches most Norfolk sites.
- Where a spill could reach a drain, gully, manhole or soakaway, including surface water drainage on a driveway.
- Within 50 metres of a borehole, spring or well used as a water supply.
- Where the tank stands on hard ground that would channel a spill rather than contain it.
- Where the vent outlet is not visible from the fill point, so an overfill would not be noticed by the delivery driver.
- Above the domestic capacity threshold, regardless of position.
- Where the tank is inside a building or an enclosed structure.
It is a measured assessment rather than a judgement call, and it takes minutes on site. More detail in bunded or single-skin.
Non-domestic storage is different again
Above the threshold volume at a non-domestic premises, oil storage falls under the Control of Pollution (Oil Storage) (England) Regulations, where containment stops being the result of an assessment and becomes mandatory with specific conditions:
- Secondary containment of at least a specified proportion of the tank’s capacity.
- All fill points, draw-off points, valves, sight gauges and vent pipes within the containment – the requirement most commonly not met on older installations, where the fill point sits outside the bund.
- Sight gauges supported and fitted with a valve that closes automatically when not in use.
- Fixed pipework properly supported, protected where buried, with a valve at the tank.
- Fill points outside the containment requiring their own drip tray arrangement.
This applies to businesses and to farms above the relevant thresholds – see agriculture.
Adding containment to an existing tank
Where a tank is sound but its installation is not compliant, there are two routes.
Build a bund around it
A masonry or concrete containment structure, impermeable, of adequate capacity, with the fittings inside it where the regulations require. It is real construction rather than a low wall, and it needs designing – capacity calculated, materials suitable, and a means of removing rainwater without a permanently open drain.
Replace with an integrally bunded tank
Frequently the better value answer where the tank is more than a few years old, because you get a new tank with a new life as well as compliance. If the existing tank is fifteen years or more, this is usually what we would recommend – see oil tank replacement.
Which makes sense depends on the tank’s age and condition, the site, and whether the base needs work anyway. We will give you both with the reasoning rather than defaulting to the larger job.
Looking after a bund
A bund is only containment while it is empty and intact, and this is where existing installations most often fail on inspection.
| Problem | Why it matters |
|---|---|
| Rainwater sitting in it | Reduces containment capacity, and on a steel tank the standing water accelerates corrosion |
| Drain plug left out | Some bunds have a plug for emptying. Left out, the bund is a decorative outer shell |
| Cracked or split outer skin | On a plastic bunded tank the outer skin degrades in UV exactly as the inner one does. A split outer skin means no containment at all |
| Debris and silt | Reduces capacity and holds moisture |
| Fill point outside the bund | Non-compliant on regulated non-domestic storage, and a spill route on any installation |
| Never inspected | The gap between the skins is where a slow inner-tank leak shows up first – it is the most useful thing to look at |
Not sure whether your site needs containment? The distances to the nearest ditch and drain answer most of it, and they take minutes to measure.
Questions we get asked about this
Does my oil tank have to be bunded?
It depends on a risk assessment of your specific site. In Norfolk the proximity-to-watercourse condition catches a great many properties, because field ditches count – including ones that are dry for most of the year.
My existing tank is single-skin. Do I have to change it?
A tank installed compliantly under earlier requirements is generally not required to be replaced. When it is replaced, the new installation is assessed against current requirements – and in the meantime the pollution liability is yours.
Can I build a bund around my existing tank?
Yes, and it needs designing properly – adequate capacity, impermeable construction, the fittings inside it where required, and a way of removing rainwater. It is real construction rather than a low wall.
Is it cheaper to bund or to replace?
It depends on the tank’s age and condition. On a tank over about fifteen years, replacing with an integrally bunded tank frequently makes more sense, because you get a new tank as well as compliance.
How much does a bund have to hold?
More than the tank’s full contents, with a margin. Specific proportions apply to regulated non-domestic storage, and an integrally bunded tank is manufactured to meet the requirement.
Does a ditch really count as a watercourse?
Yes, including ones that are dry for much of the year. In drained Norfolk it is the condition that most often triggers the containment requirement.
Regulations and standards referenced on this page
- The Control of Pollution (Oil Storage) (England) Regulations 2001 (SI 2001/2954) — UK Statute Law Database
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