Extracorporeal blood treatment apparatus, comprising: a blood treatment device comprising a blood chamber and a fluid chamber separated from one another by a semipermeable membrane; an extracorporeal blood circuit comprising a blood withdrawal line connected to an inlet port of the blood chamber and a blood return line connected to an outlet port of the blood chamber; a blood pump configured to be coupled to the extracorporeal blood circuit to circulate blood in the extracorporeal blood circuit, in particular configured to be coupled to the blood withdrawal line; a hydraulic circuit including a main line connectable to an inlet port of the fluid chamber and/or to the extracorporeal blood circuit and an effluent line connected to an outlet port of the fluid chamber, the main line being connectable to a water network to receive water, wherein the hydraulic circuit comprises a fluid preparation device configured to dilute concentrates in water to prepare a treatment fluid having set conductivity and/or set ion concentration; an ultrafiltration device (25, 26) to ultrafilter liquid from the blood chamber towards the fluid chamber of the blood treatment device and to the effluent line; a control unit connected to the preparation device, to the ultrafiltration device (25, 26) and to the blood pump; wherein the control unit is configured to execute the following procedure: ▪ setting the hydraulic circuit so that a fluid prepared through the fluid preparation device and not having the set conductivity and/or the set ion concentration bypasses the fluid chamber and is not infused in the extracorporeal blood circuit; ▪ controlling the fluid preparation device to change a conductivity and/or an ion concentration of the fluid not having the set conductivity and/or the set ion concentration to prepare the treatment fluid having the set conductivity and/or the set ion concentration while bypassing the fluid chamber and not infusing the fluid in the extracorporeal blood circuit; and ▪ simultaneously controlling the ultrafiltration device (25, 26) to perform pure ultrafiltration of fluid from the extracorporeal blood of a patient connected to the extracorporeal blood circuit. a blood treatment device comprising a blood chamber and a fluid chamber separated from one another by a semipermeable membrane; an extracorporeal blood circuit comprising a blood withdrawal line connected to an inlet port of the blood chamber and a blood return line connected to an outlet port of the blood chamber; a blood pump configured to be coupled to the extracorporeal blood circuit to circulate blood in the extracorporeal blood circuit, in particular configured to be coupled to the blood withdrawal line; a hydraulic circuit including a main line connectable to an inlet port of the fluid chamber and/or to the extracorporeal blood circuit and an effluent line connected to an outlet port of the fluid chamber, the main line being connectable to a water network to receive water, wherein the hydraulic circuit comprises a fluid preparation device configured to dilute concentrates in water to prepare a treatment fluid having set conductivity and/or set ion concentration; an ultrafiltration device (25, 26) to ultrafilter liquid from the blood chamber towards the fluid chamber of the blood treatment device and to the effluent line; a control unit connected to the preparation device, to the ultrafiltration device (25, 26) and to the blood pump; wherein the control unit is configured to execute the following procedure: ▪ setting the hydraulic circuit so that a fluid prepared through the fluid preparation device and not having the set conductivity and/or the set ion concentration bypasses the fluid chamber and is not infused in the extracorporeal blood circuit; ▪ controlling the fluid preparation device to change a conductivity and/or an ion concentration of the fluid not having the set conductivity and/or the set ion concentration to prepare the treatment fluid having the set conductivity and/or the set ion concentration while bypassing the fluid chamber and not infusing the fluid in the extracorporeal blood circuit; and ▪ simultaneously controlling the ultrafiltration device (25, 26) to perform pure ultrafiltration of fluid from the extracorporeal blood of a patient connected to the extracorporeal blood circuit. ▪ setting the hydraulic circuit so that a fluid prepared through the fluid preparation device and not having the set conductivity and/or the set ion concentration bypasses the fluid chamber and is not infused in the extracorporeal blood circuit; ▪ controlling the fluid preparation device to change a conductivity and/or an ion concentration of the fluid not having the set conductivity and/or the set ion concentration to prepare the treatment fluid having the set conductivity and/or the set ion concentration while bypassing the fluid chamber and not infusing the fluid in the extracorporeal blood circuit; and ▪ simultaneously controlling the ultrafiltration device (25, 26) to perform pure ultrafiltration of fluid from the extracorporeal blood of a patient connected to the extracorporeal blood circuit.