This is how it gets done in rural North America, with a relay of tankers which can often run to half a dozen or more, each carrying between 7500 and 13, 000 liters - depending on whether they are two or three axle rigs.
Usually each tanker carries a folding port-a-tank, so it can immediately dump its load and set off to refill at a hydrant or open water source.
Not often used in Europe, a few French SDIS are now carrying port-a-tanks on a powered roof gantry on their four axle 13, 000 liter tankers, although these are primarily focused on supporting brush fire fighting rather than structure fires.
Added by David L on 12 February 2010.