Bunded or single-skin? How to tell which tank you are allowed

What a bund actually is

A bunded tank is a tank within an outer shell, with the space between them able to hold the full contents of the inner tank plus a margin. If the inner tank splits, the fuel is caught by the outer one rather than going into the ground.

The alternative is a single-skin tank inside a separately constructed bund – a brick or concrete containment structure built around it. That is common on larger and commercial installations and unusual domestically, where an integrally bunded tank is simpler and takes up less room.

The key numbers: the bund must hold more than the tank’s full capacity, and it must be impermeable – a bund with a crack in it, or a drain plug fitted for convenience, is not a bund.

What makes it mandatory

For a domestic installation, the requirement comes from a risk assessment carried out as part of the installation, following the guidance installers work to. Secondary containment is required where any of these apply:

• Within 10 metres of a watercourse. A stream, river, pond, ditch, dyke or drainage channel. This is the one that catches most Norfolk properties, because this is drained land and field ditches are everywhere.
• 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 or a surface 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, where bunding becomes required regardless of position.
• Where the tank is within a building or an enclosed structure.

Non-domestic storage is different

Above the threshold volume at a non-domestic premises, oil storage falls under the Control of Pollution (Oil Storage) (England) Regulations, and containment stops being the result of a risk assessment and becomes a mandatory requirement with specific conditions:

1. Secondary containment of at least a specified proportion of the tank’s capacity.
2. 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 is outside the bund.
3. Sight gauges supported and fitted with a valve that closes automatically when not in use.
4. Fixed pipework properly supported, with protection where buried, and a valve at the tank.
5. Fill points outside the containment requiring their own drip tray arrangement.

This applies to businesses, and to farms above the relevant thresholds – see agriculture.

Why it is worth having even where it is not required

The regulatory answer is one thing. The practical case is separate and, in this county, fairly compelling.

• The clean-up liability sits with you. A single-skin tank that splits contaminates the ground it stands on, and remediation of oil-contaminated soil is expensive – frequently a multiple of the cost of the tank.
• Oil travels. In drained land it reaches a ditch, and a ditch reaches a watercourse. What starts as a garden problem becomes a reportable pollution incident.
• Insurance. Some policies exclude or limit pollution cover, and some require containment. Worth reading yours.
• Resale. A non-compliant single-skin tank near a ditch is something a buyer’s solicitor may raise, and it is a poor moment to discover it.
• Failure is not dramatic. Tanks rarely burst. They weep slowly, and a slow weep from a single-skin tank can run for months undetected. A bund catches it and makes it visible.

The cost difference between a bunded and a single-skin tank of the same capacity is real but modest relative to the tank’s twenty-year life, and negligible relative to the cost of a spill.

Looking after a bund

A bund is only containment while it is empty and intact.

• Rainwater. An open-top bund fills with rain, which reduces the containment capacity and, on a steel tank, sits against the tank accelerating corrosion. Bunds need checking and emptying.
• Drain plugs. Some bunds are fitted with a plug for emptying. It must be closed. A bund with the plug left out is a decorative outer shell.
• Cracks and splits. The outer skin of a plastic bunded tank degrades in UV exactly as the inner one does, and a split outer skin means no containment at all.
• Debris. Leaves and silt reduce capacity and hold moisture.
• Inspection. The gap between the skins is exactly where a slow inner-tank leak shows up first, and it is the most useful thing to look at on any bunded tank.

It is part of what an inspection covers – see oil tank inspection.