A bund consisting of a concrete base and a secure outer wall is designed to contain up to 110% of the protected tank’s total storage capacity. This is known as "secondary containment". (In the case of a bund constructed around multiple tanks, it must be able to contain 110% of the largest tank's capacity or 25% of the total capacity of the protected tanks, whichever is greater.)
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Some tanks these days are made with built-in bunds ("integrally bunded"); others need to have bunds installed at the tank site. Although some tanks are "single skinned" (single outer layer) and some are "double skinned" (double outer layer), in practical terms both types will need bunds. The space between the outer layers of a double skinned tank is not sufficient to hold 110% of its capacity.
When are you required to have your tank bunded?
For commercial facilities, if it holds 200 liters or more.
For private dwellings, if it holds 3,500 liters or more.
If it is sited within 10 meters of a watercourse, or within 50 meters of a well or spring that provides drinking water, or in any way that risks contaminating drains or groundwater. These are not the best places for your tank in any case, and neither are known flood zones.
If it is sited in such a way that the tank vent pipe outlet cannot be seen from the fill point.
A pollution assessment of your proposed tank site should uncover these issues and any others that are relevant.
The pertinent regulations for Scotland, Wales, Northern Ireland, and Ireland differ somewhat from those for England and should be consulted as geographically appropriate.
