Abstract: A method and an apparatus for withdrawing liquid from a reservoir, such as concentrate container, by means of a water jet device received in a clamping device. The reservoir is provided with an outlet opening having a valve mechanism fitted with a lip seal. The water jet device connects on the one hand to a water supply hose and on the other hand to a drain spout. The method and apparatus are characterized in that the reservoir is a flexible bag fitted with a connector or insert. The bag is received in a reservoir box whose underside has an opening for the passage of the connector or insert, as well as a plurality of passages for receiving the attachment elements of the clamping device.