Apparatus (100; 200; 300) for treating salt-containing water by means of reverse osmosis, in which the salt-containing water is separated into a permeate stream and a concentrate stream, having the features • the apparatus comprises an inlet (115; 215; 315) for the salt-containing water, • the apparatus comprises an outlet (116; 216; 316) for the permeate, • the apparatus comprises an outlet (117; 217; 317) for the concentrate, and • the apparatus comprises a reverse osmosis device (102; 202; 302), and the additional features that • the inlet and the outlets are integrated into a base unit (101; 201; 301) which is able to be installed in a positionally fixed manner, and • the reverse osmosis device (102; 202; 302) is provided as an exchangeable unit which is detachably connected to the base unit (101; 201; 301), • the exchangeable unit comprises a pressure vessel as part of the reverse osmosis device (102; 202; 302), and • the pressure vessel comprises a top part (103b; 203b) into which an entry opening (107; 207) for the salt-containing water, an exit opening (108; 208) for the permeate and an exit opening (109; 209; 309) for the concentrate are integrated, wherein the reverse osmosis device (102; 202; 302) and the base unit (101; 201; 301) are configured in such a way that • the entry opening (107; 207) is coupled to the inlet (115; 215; 315) for the salt-containing water, • the exit opening (108; 208) for the permeate is coupled to the outlet (116; 216; 316) for the permeate, and • the exit opening (109; 209; 309) for the concentrate is coupled to the outlet (117; 217; 317) for the concentrate when the reverse osmosis device (102; 202; 302) is connected to the base unit (101; 201; 301), characterized in that • the apparatus comprises a blending device (118; 218; 318) by way of which permeate exiting the reverse osmosis device (102; 202; 302) can be mixed with the salt-containing water and/or concentrate, wherein • the blending device (118; 218; 318) is in the form of a valve, • the blending device (118; 218; 318) comprises a channel, which is delimited by a wall, and a valve body, which is mounted in an axially displaceable and/or rotationally movable manner within the channel, and • the valve body is a constituent part of the base unit (101; 201; 301), and the channel is, together with the wall delimiting the channel, a constituent part of the exchangeable unit (102; 202; 302). • the apparatus comprises an inlet (115; 215; 315) for the salt-containing water, • the apparatus comprises an outlet (116; 216; 316) for the permeate, • the apparatus comprises an outlet (117; 217; 317) for the concentrate, and • the apparatus comprises a reverse osmosis device (102; 202; 302), • the inlet and the outlets are integrated into a base unit (101; 201; 301) which is able to be installed in a positionally fixed manner, and • the reverse osmosis device (102; 202; 302) is provided as an exchangeable unit which is detachably connected to the base unit (101; 201; 301), • the exchangeable unit comprises a pressure vessel as part of the reverse osmosis device (102; 202; 302), and • the pressure vessel comprises a top part (103b; 203b) into which an entry opening (107; 207) for the salt-containing water, an exit opening (108; 208) for the permeate and an exit opening (109; 209; 309) for the concentrate are integrated, • the entry opening (107; 207) is coupled to the inlet (115; 215; 315) for the salt-containing water, • the exit opening (108; 208) for the permeate is coupled to the outlet (116; 216; 316) for the permeate, and • the exit opening (109; 209; 309) for the concentrate is coupled to the outlet (117; 217; 317) for the concentrate • the apparatus comprises a blending device (118; 218; 318) by way of which permeate exiting the reverse osmosis device (102; 202; 302) can be mixed with the salt-containing water and/or concentrate, wherein • the blending device (118; 218; 318) is in the form of a valve, • the blending device (118; 218; 318) comprises a channel, which is delimited by a wall, and a valve body, which is mounted in an axially displaceable and/or rotationally movable manner within the channel, and • the valve body is a constituent part of the base unit (101; 201; 301), and the channel is, together with the wall delimiting the channel, a constituent part of the exchangeable unit (102; 202; 302).