Water-on-water filtration system with precision metering device

A method of delivering filtered water with a water filtration system, the water filtration system including at least one filtering member, first and second water-on-water storage vessels, at least one concentrate source, and a control system, the first and second storage vessels each being configured to alternate between a fill state wherein the storage vessel is filled with filtered water and a service state wherein filtered water is expelled from the storage vessel, the method comprising:

• generating a supply of filtered water with the filtering member;
• adding concentrate from at least one concentrate source to the first storage vessel using a dosing device;
• and
• adding concentrate from at least one concentrate source to the second storage vessel using a precision metering device;
• wherein the precision metering device includes:
• a first chamber having a fixed volume and a second chamber having a fixed volume, each chamber having at least one outer wall;
• a first piston, disposed in the first chamber so that edges of the first piston slideably contact the outer wall of the first chamber forming a seal that divides the first chamber into a mixing portion and a driving portion;
• and
• a second piston, disposed in the second chamber so that edges of the second piston slideably contact the outer wall of the second chamber forming a seal that defines a concentrate portion concentrate portion in the second chamber,
• wherein the first piston and the second piston are in mechanical communication with each other so that when the first piston is displaced in the first chamber, the second piston is displaced in the second chamber, and
• wherein the concentrate portion of the second chamber is in fluid communication with the mixing portion of the first chamber.