An apparatus for the treating of a fluid, in particular for the treating of water for dialysis, which comprises a first hollow fiber membrane filter, wherein the first hollow fiber membrane filter further comprises a plurality of hollow fiber membranes which form a retentate chamber (F 1R) and a filtrate chamber (F 1F), wherein the retentate chamber (F 1R) and the filtrate chamber (F 1F) are separated from each other by the semipermeable membrane walls (F 1M) of the hollow fiber membranes, a fluid port for the supplying of fluid, in particular water, to the retentate chamber, a fluid port for the draining of fluid, in particular water, from the retentate chamber, at least one fluid port (103a I 103b) for the draining of fluid, in particular water, from the filtrate chamber, wherein the apparatus further comprises a fluid line in fluid communication with the fluid port for supplying fluid, in particular water, to the retentate chamber (F 1R) of the first hollow fiber membrane filter, a fluid line in fluid communication with the fluid port for draining fluid, in particular water, from the retentate chamber (F 1R) of the first hollow fiber membrane filter, a fluid line in fluid communication with the fluid port for draining fluid, in particular water, from the filtrate chamber (F 1F) of the first hollow fiber membrane filter, a first valve connection in engagement with the fluid line for draining fluid, in particular water, from the retentate chamber, wherein the valve connection has at least two valve positions, by means of which the flow of fluid through the fluid line for draining fluid from the retentate chamber can be blocked or conducted, optionally further comprising a first fluid branching point in the fluid line for draining fluid, in particular water, from the retentate chamber (F 1R) of the first hollow fiber membrane filter in fluid communication with a first branching fluid line, a second valve connection in engagement with the first branching fluid line, wherein the second valve connection has at least two valve positions, by means of which the flow of fluid, in particular water, through the first branching fluid line can be blocked or conducted, characterized in that a sampling point is arranged downstream the first valve connection in fluid communication with the fluid line for draining fluid, in particular water, from the retentate chamber (F 1R), and/or that a second sampling point is arranged downstream the second valve connection in fluid communication with the first branching fluid line. which comprises a first hollow fiber membrane filter, wherein the first hollow fiber membrane filter further comprises a plurality of hollow fiber membranes which form a retentate chamber (F 1R) and a filtrate chamber (F 1F), wherein the retentate chamber (F 1R) and the filtrate chamber (F 1F) are separated from each other by the semipermeable membrane walls (F 1M) of the hollow fiber membranes, a fluid port for the supplying of fluid, in particular water, to the retentate chamber, a fluid port for the draining of fluid, in particular water, from the retentate chamber, at least one fluid port (103a I 103b) for the draining of fluid, in particular water, from the filtrate chamber, wherein the apparatus further comprises a fluid line in fluid communication with the fluid port for supplying fluid, in particular water, to the retentate chamber (F 1R) of the first hollow fiber membrane filter, a fluid line in fluid communication with the fluid port for draining fluid, in particular water, from the retentate chamber (F 1R) of the first hollow fiber membrane filter, a fluid line in fluid communication with the fluid port for draining fluid, in particular water, from the filtrate chamber (F 1F) of the first hollow fiber membrane filter, a first valve connection in engagement with the fluid line for draining fluid, in particular water, from the retentate chamber, wherein the valve connection has at least two valve positions, by means of which the flow of fluid through the fluid line for draining fluid from the retentate chamber can be blocked or conducted, optionally further comprising a first fluid branching point in the fluid line for draining fluid, in particular water, from the retentate chamber (F 1R) of the first hollow fiber membrane filter in fluid communication with a first branching fluid line, a second valve connection in engagement with the first branching fluid line, wherein the second valve connection has at least two valve positions, by means of which the flow of fluid, in particular water, through the first branching fluid line can be blocked or conducted, characterized in that a sampling point is arranged downstream the first valve connection in fluid communication with the fluid line for draining fluid, in particular water, from the retentate chamber (F 1R), and/or that a second sampling point is arranged downstream the second valve connection in fluid communication with the first branching fluid line.