UF even withstands episodic increase of particle numbers in the feed (e.g., caused by conditioning of ion-exchange resins) without negative effects for water quality and service lifetime. Such a particle challenge typically involves high concentrations of ~10-nanometer (nm) particles. On the other hand, current on-line particle counters approach their lower detection limit at about 10nm. Large numbers of particles nearby and below this limit remain undetected. Hence, the integrity of a UF membrane is essential to keep these small particles under control; it can hardly be evaluated by comparing particle counts upstream and downstream. Therefore, better test methods are needed to identify membrane damages, and to decide on service life and replacement of UF membranes.