Device for disinfecting a hydraulic circuit of an apparatus for processing liquids for dialysis, wherein the device comprises the following: at least one valve arrangement; at least one source for disinfectant; at least one first line, which is or can be connected to the source for disinfectant and to the hydraulic circuit; at least one second line, which branches off from the first line and is connected to the ambient air or to an air reservoir; wherein the valve arrangement has at least one first and at least one second valve, which are located in the first line before and after the point at which the second line branches off from the first line, wherein the valve arrangement furthermore has at least one third valve, which is located in the second line, wherein the first and the second line are arranged such that when the first and third valve are open, the second line and the first line are ventilated between the second valve and the source, so that the disinfectant located therein drains off to the source, wherein at least one pump is located in the hydraulic circuit for generating a vacuum in the hydraulic circuit and the first line opens on the suction side of the pump into the hydraulic circuit, and wherein located in the hydraulic circuit is at least one fourth valve, which is arranged in the flow direction of the liquid in the hydraulic circuit upstream of the opening of the first line into the hydraulic circuit, wherein at least one control or adjustment unit is present, which is designed so that it operates the valve arrangement in at least a first operating mode in which no disinfectant is supplied to the hydraulic circuit, and in at least a second operating mode in which disinfectant is supplied to the hydraulic circuit, wherein in the first operating mode the first and the second valve are closed and the third valve is open and wherein in the second operating mode the first and the second valve are open and the third valve is closed, characterised in that the control or adjustment unit is designed so that after completion of the second operating mode, i.e. following the supply of disinfectant to the hydraulic circuit, it operates the valve arrangement such that first the second valve is closed and the third valve is opened, and that then the first valve is closed. at least one valve arrangement; at least one source for disinfectant; at least one first line, which is or can be connected to the source for disinfectant and to the hydraulic circuit; at least one second line, which branches off from the first line and is connected to the ambient air or to an air reservoir; wherein the valve arrangement has at least one first and at least one second valve, which are located in the first line before and after the point at which the second line branches off from the first line, wherein the valve arrangement furthermore has at least one third valve, which is located in the second line, wherein the first and the second line are arranged such that when the first and third valve are open, the second line and the first line are ventilated between the second valve and the source, so that the disinfectant located therein drains off to the source, wherein at least one pump is located in the hydraulic circuit for generating a vacuum in the hydraulic circuit and the first line opens on the suction side of the pump into the hydraulic circuit, and wherein located in the hydraulic circuit is at least one fourth valve, which is arranged in the flow direction of the liquid in the hydraulic circuit upstream of the opening of the first line into the hydraulic circuit, wherein at least one control or adjustment unit is present, which is designed so that it operates the valve arrangement in at least a first operating mode in which no disinfectant is supplied to the hydraulic circuit, and in at least a second operating mode in which disinfectant is supplied to the hydraulic circuit, wherein in the first operating mode the first and the second valve are closed and the third valve is open and wherein in the second operating mode the first and the second valve are open and the third valve is closed, characterised in that the control or adjustment unit is designed so that after completion of the second operating mode, i.e. following the supply of disinfectant to the hydraulic circuit, it operates the valve arrangement such that first the second valve is closed and the third valve is opened, and that then the first valve is closed.