In this paper the relationship between liquid velocity and the concentration of components in the premeate is described for capillary nanofiltration (NF) membranes. Capillary NF combines the advantages of NF - good retention of bivalent ions and DOC - with the advantages of capillary membranes - robust with respect to fouling and good cleaning possibilities. Tests were performed on Twente canal water in cooperation with the Overijssel Water Supply Company. The operation mode of the tested system is dead-end with recirculation. The concentration of components in the system is not constant in time but increases during filtration. Periodically concentrate is discharged from the system. The different permeate concentrations for different liquid velocities can be explained entirely by the effect of the concentration polarization. The first experiments confirm that retention is a membrane characteristic.