Flux decline in pressure-driven membrane processes is caused by concentration polarization and fouling. Application of the surface renewal concept provides an opportunity to describe these phenomena. To this end, the Danckwerts surface-age distribution function has to be modified by assuming that the age of the oldest surface element cannot exceed the duration of the process. In this manner the variation of surface-age distribution during the process has been included to correspond to the effect of the building up of the concentration polarization layer during the initial period of ultrafiltration. When the modified version of surface-age distribution is applied to determine the average flux, the relationship obtained takes into account flux decline as the effect of concentration polarization layer development. The practical significance of the model resides in its ability to describe the dynamic behavior of the plant : it can be applied to the optimization of operation modes, thus enabling the membrane cleaning strategy to be determined (i.e. process duration, back-flushing, or pulsation frequency, etc.). In particular, it is of great importance in membrane large-scale continuous plants.