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. This device precludes water-hammering in the system and keeps the flow within the pressure tolerances of the system so that the tanks and many elements can be made of relatively light plastic. A pair of electromagnetic valves which are closed when de-energized are disposed on the modular coupling on the liquid-treating tank. The electromagnetic valves are energized to open them by application of an electrical signal applied when the level of treated water is detected to be below the predetermined level. Outflow of treated water from the storage tank is automatically controlled by detecting when the treated water level exceeds a predetermined level. The storage tank is vented to allow entry of filtered air and venting of air to the atmosphere as the water level therein varies and avoids contact with the atmosphere.