If you fill with concrete, this has to be dug out before you can move the tank.

With foam filled tanks, the whole tank can be removed from site.

Concrete does not absorb hydrocarbons and if it leaks it will cause pollution.

Foam absorbs hydrocarbons and liquid products therefore no pollution risk.

Filling with concrete is often not complete, leaving "live" voids.

Foam injection systems ensures a complete filling of tank without "live" voids.