A water treatment system (1010, 1210) for softening water by adding a water treatment composition to water, said system comprising: a) a supply of water (1012, 1212) divided into a main stream (1014, 1214) and a subsidiary stream (1016, 1216); b) a water treatment device (1018, 1218) comprising: i) an inlet (1024, 1224) and an outlet (1026, 1226), the inlet (1024, 1224) for receiving the subsidiary stream (1016, 1216) into the device (1018, 1218); 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 (1020, 1220) onto which the inlet (1024, 1224) and the outlet (1026, 1226) are mounted, the head portion (1020, 1220) comprising a first channel for directing the subsidiary stream (1016, 1216) through the water treatment device (1018, 1218), from the inlet (1024, 1224) into the first compartment and then into the second compartment, so as to dissolve a portion of the water treatment composition (1032, 1132) into the subsidiary stream (1016, 1216), and out the outlet (1026, 1226), and a second channel for directing the subsidiary stream (1016, 1216) only through the head portion (1020, 1220) between the inlet (1024, 1224) and the outlet (1026, 1226) thereby bypassing the first compartment and the second compartment; and c) a flow control device (1054, 1255) controlling the flow rate of the subsidiary stream (1016, 1216) through the water treatment device (1018, 1218); wherein the main (1014, 1214) and subsidiary streams (1016, 1216) are divided and recombined outside of the water treatment device (1018, 1218), wherein the second compartment is a cartridge having a first and a second opening (1046, 1047), the first opening for receiving the subsidiary stream (1016, 1216) into the cartridge, and the second opening for the subsidiary stream (1016, 1216) to exit the cartridge, the first opening at a bottom portion adjacent to the bottom, the second opening adjacent to the head portion (1020, 1220), and the cartridge encloses a water-permeable bag (1030, 1100) containing the water treatment composition. a) a supply of water (1012, 1212) divided into a main stream (1014, 1214) and a subsidiary stream (1016, 1216); b) a water treatment device (1018, 1218) comprising: i) an inlet (1024, 1224) and an outlet (1026, 1226), the inlet (1024, 1224) for receiving the subsidiary stream (1016, 1216) into the device (1018, 1218); 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 (1020, 1220) onto which the inlet (1024, 1224) and the outlet (1026, 1226) are mounted, the head portion (1020, 1220) comprising a first channel for directing the subsidiary stream (1016, 1216) through the water treatment device (1018, 1218), from the inlet (1024, 1224) into the first compartment and then into the second compartment, so as to dissolve a portion of the water treatment composition (1032, 1132) into the subsidiary stream (1016, 1216), and out the outlet (1026, 1226), and a second channel for directing the subsidiary stream (1016, 1216) only through the head portion (1020, 1220) between the inlet (1024, 1224) and the outlet (1026, 1226) thereby bypassing the first compartment and the second compartment; and i) an inlet (1024, 1224) and an outlet (1026, 1226), the inlet (1024, 1224) for receiving the subsidiary stream (1016, 1216) into the device (1018, 1218); 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 (1020, 1220) onto which the inlet (1024, 1224) and the outlet (1026, 1226) are mounted, the head portion (1020, 1220) comprising a first channel for directing the subsidiary stream (1016, 1216) through the water treatment device (1018, 1218), from the inlet (1024, 1224) into the first compartment and then into the second compartment, so as to dissolve a portion of the water treatment composition (1032, 1132) into the subsidiary stream (1016, 1216), and out the outlet (1026, 1226), and a second channel for directing the subsidiary stream (1016, 1216) only through the head portion (1020, 1220) between the inlet (1024, 1224) and the outlet (1026, 1226) thereby bypassing the first compartment and the second compartment; and c) a flow control device (1054, 1255) controlling the flow rate of the subsidiary stream (1016, 1216) through the water treatment device (1018, 1218);