A box-type dishwasher, wherein the dishwasher has a fresh-water connection with a feed valve, wherein a water-treatment system with a reverse-osmosis device is provided downstream of the feed valve in order to treat, in particular as required, as least some of the fresh water fed to the dishwasher via the fresh-water connection, wherein a backflow preventer is provided between the feed valve and the reverse-osmosis device in order to prevent water from flowing back from the dishwasher into a fresh-water-supply installation which is, or can be, flow- connected to the fresh-water connection.