Add-on module for an apparatus for extracorporeal treatment of blood, wherein the apparatus comprises: a blood treatment unit; an extracorporeal blood circuit coupled to the blood treatment unit and comprising a blood withdrawal line and a blood return line connectable to a vascular access of a patient; a blood pump configured to be coupled to a pump section of the extracorporeal blood circuit; wherein the add-on module is configured to be placed in-line between a main portion of the apparatus and the vascular access of the patient and comprises: - a substantially H-shaped conduits assembly comprising a withdrawal conduit, a return conduit and a bridging conduit connecting the withdrawal conduit to the return conduit; wherein the withdrawal conduit is connectable upstream and downstream to the withdrawal line, wherein the return conduit is connectable upstream and downstream to the return line; - a plurality of valves (26, 27, 28, 29, 30, 31) operating on the withdrawal conduit, on the return conduit and on the bridging conduit and configured to divert a flow of liquid and/or blood without disconnecting the patient; wherein the plurality of valves (26, 27, 28, 29, 30, 31) comprises at least a withdrawal valve (26, 27) operating on the withdrawal conduit, at least a return valve operating on the return conduit and at least a bridging valve (29, 30) operating on the bridging conduit; characterized in that said at least a withdrawal valve (26, 27) comprises a first withdrawal valve and a second withdrawal valve operating on the withdrawal conduit, wherein the first withdrawal valve and the second withdrawal valve are placed on opposite sides with respect to a junction of the bridging conduit to the withdrawal conduit, wherein the first withdrawal valve is placed upstream, with respect to a flow of blood during treatment, the junction of the bridging conduit to said withdrawal conduit, wherein the second withdrawal valve is placed downstream, with respect to the flow of blood during treatment, the junction of the bridging conduit to said withdrawal conduit. a blood treatment unit; an extracorporeal blood circuit coupled to the blood treatment unit and comprising a blood withdrawal line and a blood return line connectable to a vascular access of a patient; a blood pump configured to be coupled to a pump section of the extracorporeal blood circuit; wherein the add-on module is configured to be placed in-line between a main portion of the apparatus and the vascular access of the patient and comprises: - a substantially H-shaped conduits assembly comprising a withdrawal conduit, a return conduit and a bridging conduit connecting the withdrawal conduit to the return conduit; wherein the withdrawal conduit is connectable upstream and downstream to the withdrawal line, wherein the return conduit is connectable upstream and downstream to the return line; - a plurality of valves (26, 27, 28, 29, 30, 31) operating on the withdrawal conduit, on the return conduit and on the bridging conduit and configured to divert a flow of liquid and/or blood without disconnecting the patient; - a substantially H-shaped conduits assembly comprising a withdrawal conduit, a return conduit and a bridging conduit connecting the withdrawal conduit to the return conduit; wherein the withdrawal conduit is connectable upstream and downstream to the withdrawal line, wherein the return conduit is connectable upstream and downstream to the return line; - a plurality of valves (26, 27, 28, 29, 30, 31) operating on the withdrawal conduit, on the return conduit and on the bridging conduit and configured to divert a flow of liquid and/or blood without disconnecting the patient; wherein the plurality of valves (26, 27, 28, 29, 30, 31) comprises at least a withdrawal valve (26, 27) operating on the withdrawal conduit, at least a return valve operating on the return conduit and at least a bridging valve (29, 30) operating on the bridging conduit; characterized in that said at least a withdrawal valve (26, 27) comprises a first withdrawal valve and a second withdrawal valve operating on the withdrawal conduit, wherein the first withdrawal valve and the second withdrawal valve are placed on opposite sides with respect to a junction of the bridging conduit to the withdrawal conduit, wherein the first withdrawal valve is placed upstream, with respect to a flow of blood during treatment, the junction of the bridging conduit to said withdrawal conduit, wherein the second withdrawal valve is placed downstream, with respect to the flow of blood during treatment, the junction of the bridging conduit to said withdrawal conduit.