Method and system for monitoring the operation of a liquid treatment apparatus including a replaceable liquid treatment cartridge

A liquid treatment apparatus (1) includes a replaceable liquid treatment cartridge (3) including at least one liquid treatment part (12) including at least an amount of ion exchange material. The liquid treatment apparatus (1) includes a head device (2) for receiving the replaceable liquid treatment cartridge (3), the head device (2) including at least one liquid inlet (4) and at least one liquid outlet (5) and being arranged to conduct at least a fraction of liquid received through the at least one liquid inlet (4) to at least one of the liquid outlets (5) via the liquid treatment cartridge (3) such as to pass through at least a certain one of the liquid treatment parts (12). A method of monitoring the operation of the liquid treatment apparatus (1) includes obtaining first data ( M init ,RC ), the first data ( M init ,RC ) being representative of a remaining capacity of the certain liquid treatment part (12) in terms of an amount of substance. Second data (CH) including data representative of a value at at least one point in time of a measure of a concentration of components removable by the liquid treatment part (12) in liquid received through the at least one liquid inlet (4) and representative of a value of the measure at at least one point in time is obtained. Output data ( RC,RV,RT,OT ) is obtained. A device (20) for implementing a human-machine interface is enabled to provide perceptible output pertaining to replacement of the liquid treatment cartridge (3) and based on at least some of the output data ( RC,RV,RT,OT ). Obtaining the output data ( RC,RV,RT,OT ) includes using the first and second data ( CH ) to calculate a value of at least one parameter (RV,RT 2 ) such that the value is proportional to the remaining capacity and inversely proportional to the measure of the concentration of components removable by the liquid treatment part.