The pressure gradient dosing of a fluid supply

The pressure gradient dosing system for a fluid supply system along comprising an additive supply system with a fluid inlet and a fluid outlet, a flow passage including a portion extending from the inlet to the outlet, a first pressure zone along the flow passage, which communicates with the inlet, a second pressure zone the flow passage which communicates with the outlet, wherein the second pressure zone at a lower pressure than the first pressure zone, so that a pressure gradient between the first and the second pressure zone present, and an additive reservoir which is responsive to the pressure gradient to the addition to the flow passage to deliver.