Abstract: A modular liquid-treatment system for treating water having a liquid-treating tank with a liquid-treatment cartridge therein for treating untreated water and a similar storage tank which receives the treated water and stores it for supply on demand. The detection of variation of the water level of the storage tank below a predetermined level is used to automatically control supply of untreated water to the liquid-treating tank. A modular liquid-flow control unit is removably mounted on the two alike tanks. The unit has two similar symmetrically mounted couplings each removably mounted on a corresponding one of the tanks. A supply valve on the unit controls supply of water from an untreated water supply main and supplies the untreated water to a flow-control device which maintains the flow substantially constant in the presence of pressure variations in the supply main.