valve 41 when open and exits the cartridge 30 at the water outlet port 42 from here the purified water is carried by a tube 44 through a bicarbonate cartridge 46 in a known manner to generate a purified water and bicarbonate solution the purified water and bicarbonate solution is carried by a tube 48 and re admitted into the cartridge 30 via the bicarbonate inlet port 50 the temperature of the bicarbonate solution is measured at sensing port 52 and the bicarbonate solution pressure is measured at sensing port 54 the bicarbonate solution passes a bicarbonate control valve 56 when open before entering a bicarbonate solution reservoir 58 having an inlet and an outlet the bicarbonate control valve 56 is closed when flow therethrough is not required a bicarbonate dosing pump chamber 60 having an inlet and an outlet receives the bicarbonate solution from the bicarbonate solution reservoir 58 through a bicarbonate dosing pump inlet valve 62 the bicarbonate dosing pump chamber 60 is closed by a diaphragm to define a bicarbonate dosing pump which upon actuation of the diaphragm pumps the bicarbonate solution from the bicarbonate dosing pump 60 to a first mixing pump chamber 66 bicarbonate pump chamber the bicarbonate dosing pump 60 has a bicarbonate dosing pump outlet valve 64 which is closed when the bicarbonate dosing pump inlet valve 62 is open the bicarbonate dosing pump outlet valve 64 is opened to permit bicarbonate solution to be pumped.