A liquid storage system comprising a flexible, closed and freely expandable reservoir for storing liquid medium-containing material. A supply conduit conducts liquid, for example, from a reverse osmosis module, to the reservoir to expand the reservoir. A control valve is movable between an open position in which the control valve allows liquid to flow through the supply conduit to the reservoir and a closed position in which the valve blocks flow of liquid through the supply conduit to the reservoir. A discharge conduit is provided for delivering liquid from the reservoir, and a pump may be used for pumping the liquid from the reservoir through the discharge conduit. The control valve is controlled by a level sensor operable by expansion of the reservoir.