A bunded heating oil tank simply consists of a tank within a tank. The fuel is stored in the inner tank and the outer tank acts as a failsafe so that in the event of a spillage, excess fuel will collect in the outer tank - thus averting a pollution incident.
Bunded heating oil tanks have been proven to significantly reduce the risk of pollution at oil storage installations. They are a requirement at commercial, industrial, institutional and commercial premises; all installations in the Channel Islands; and, at most domestic installations in the United Kingdom and Republic of Ireland.  Single skin tanks on the other hand, consist of only one shell or skin. This means that in the event of a tank split, valuable heating oil will be lost, and the surrounding environment will be polluted by the heating oil contained within it.