Fuel tank filling installation having a retaining apparatus comprising - a filter element in a fluid line, ∘ wherein the fluid line of the retaining apparatus is a fuel line, ∘ wherein a fluid containing a foreign fluid can flow through the fluid line in a flow direction, and ∘ wherein the filter element is a coalescence element or a microfilter and is designed in such a way that retention and/or separation of the foreign fluid occurs at the filter element, - a volume flow control device having a controllable pump and/or a control valve, wherein the volume flow through the fluid line can be controlled by way of the volume flow control device, - a sensor device by way of which the fraction of foreign fluid in the fluid can be determined, ∘ wherein the sensor device has a sensor element for determining free water as foreign fluid in a hydrocarbon or oil as main constituent of the fluid, and - a control device by way of which the volume flow can be controlled by way of the volume flow control device in a manner dependent on the fraction of foreign fluid in the fluid as determined by way of the sensor device, characterized in that - the control device has a limit value stabilizing program, specifically a control loop, with an upper limit value (Xmax) for the fraction of foreign fluid in the fluid, wherein the control device has a PID controller with proportional, integral and differential behaviour; - the volume flow is controlled by way of the volume flow control device in a manner dependent on the fraction of foreign fluid in the fluid as determined by way of the sensor device in such a way that ∘ it is reduced if the fraction of foreign fluid in the fluid overshoots a target value, and ∘ it is increased if the fraction of foreign fluid in the fluid undershoots a target value. - a filter element in a fluid line, ∘ wherein the fluid line of the retaining apparatus is a fuel line, ∘ wherein a fluid containing a foreign fluid can flow through the fluid line in a flow direction, and ∘ wherein the filter element is a coalescence element or a microfilter and is designed in such a way that retention and/or separation of the foreign fluid occurs at the filter element, ∘ wherein the fluid line of the retaining apparatus is a fuel line, ∘ wherein a fluid containing a foreign fluid can flow through the fluid line in a flow direction, and ∘ wherein the filter element is a coalescence element or a microfilter and is designed in such a way that retention and/or separation of the foreign fluid occurs at the filter element, - a volume flow control device having a controllable pump and/or a control valve, wherein the volume flow through the fluid line can be controlled by way of the volume flow control device, - a sensor device by way of which the fraction of foreign fluid in the fluid can be determined, ∘ wherein the sensor device has a sensor element for determining free water as foreign fluid in a hydrocarbon or oil as main constituent of the fluid, and ∘ wherein the sensor device has a sensor element for determining free water as foreign fluid in a hydrocarbon or oil as main constituent of the fluid, and - a control device by way of which the volume flow can be controlled by way of the volume flow control device in a manner dependent on the fraction of foreign fluid in the fluid as determined by way of the sensor device, characterized in that - the control device has a limit value stabilizing program, specifically a control loop, with an upper limit value (Xmax) for the fraction of foreign fluid in the fluid, wherein the control device has a PID controller with proportional, integral and differential behaviour; - the volume flow is controlled by way of the volume flow control device in a manner dependent on the fraction of foreign fluid in the fluid as determined by way of the sensor device in such a way that ∘ it is reduced if the fraction of foreign fluid in the fluid overshoots a target value, and ∘ it is increased if the fraction of foreign fluid in the fluid undershoots a target value. ∘ it is reduced if the fraction of foreign fluid in the fluid overshoots a target value, and ∘ it is increased if the fraction of foreign fluid in the fluid undershoots a target value.