Method and appliance for treating water

A multistage process of treating water in which a raw water stream is first fed into a membrane separation stage and then further purified in a deionization stage downstream thereof, wherein

• the membrane separation stage comprises a first and a second membrane separation device and the deionization stage comprises an electrodeionization appliance having at least one concentrate chamber and at least one diluate chamber,
• the raw water stream in the first membrane separation device is separated into a first concentrate stream and a first permeate stream,
• the first concentrate stream is separated at least in part in the second membrane separation device into a second concentrate stream and a second permeate stream,
• the first permeate stream is fed into the at least one diluate chamber of the downstream electrodeionization appliance, from which it exits again as product stream,
• the second permeate stream is fed into the at least one concentrate chamber of the downstream electrodeionization appliance, from which it exits as third concentrate stream, and
• the second and the third concentrate streams are degassed in a device that degasses water and are added to the raw water stream before the stream is fed into the membrane separation stage.