A water treatment system for softening water by adding a water treatment composition to water, said system comprising: a) a supply of water divided into a main stream and a subsidiary stream ; b) a water treatment device comprising: i) an inlet and an outlet , the inlet for receiving the subsidiary stream into the device ; ii) a body portion comprising at least two compartments, including a first compartment and a second compartment , the second compartment containing a water treatment composition , the body portion having a bottom ; iii) a head portion onto which the inlet and the outlet are mounted, the head portion comprising a first channel for directing the subsidiary stream through the water treatment device , from the inlet into the first compartment and then into the second compartment , so as to dissolve a portion of the water treatment composition into the subsidiary stream , and out the outlet , and a second channel for directing the subsidiary stream only through the head portion between the inlet and the outlet thereby bypassing the first compartment and the second compartment ; and c) a flow control device controlling the flow rate of the subsidiary stream through the water treatment device ; wherein the main and subsidiary streams are divided and recombined outside of the water treatment device , wherein the second compartment is a cartridge having a first and a second opening , the first opening for receiving the subsidiary stream into the cartridge , and the second opening for the subsidiary stream to exit the cartridge , the first opening at a bottom portion adjacent to the bottom , the second opening adjacent to the head portion , and the cartridge encloses a water-permeable bag containing the water treatment composition . a) a supply of water divided into a main stream and a subsidiary stream ; b) a water treatment device comprising: i) an inlet and an outlet , the inlet for receiving the subsidiary stream into the device ; ii) a body portion comprising at least two compartments, including a first compartment and a second compartment , the second compartment containing a water treatment composition , the body portion having a bottom ; iii) a head portion onto which the inlet and the outlet are mounted, the head portion comprising a first channel for directing the subsidiary stream through the water treatment device , from the inlet into the first compartment and then into the second compartment , so as to dissolve a portion of the water treatment composition into the subsidiary stream , and out the outlet , and a second channel for directing the subsidiary stream only through the head portion between the inlet and the outlet thereby bypassing the first compartment and the second compartment ; and i) an inlet and an outlet , the inlet for receiving the subsidiary stream into the device ; ii) a body portion comprising at least two compartments, including a first compartment and a second compartment , the second compartment containing a water treatment composition , the body portion having a bottom ; iii) a head portion onto which the inlet and the outlet are mounted, the head portion comprising a first channel for directing the subsidiary stream through the water treatment device , from the inlet into the first compartment and then into the second compartment , so as to dissolve a portion of the water treatment composition into the subsidiary stream , and out the outlet , and a second channel for directing the subsidiary stream only through the head portion between the inlet and the outlet thereby bypassing the first compartment and the second compartment ; and c) a flow control device controlling the flow rate of the subsidiary stream through the water treatment device ; <EOS>