As membrane filtration systems are more commonly used in the water treatment industry, the call for a reliable, fast and on-line integrity testing system becomes more urgent. Especially where membrane filtration is used in potable water production for the removal of pathogenic microorganisms, the integrity of such a system is of the utmost importance. Membrane integrity testing can be performed in a number of ways, where the pressure-hold or vacuum-hold test and the diffusive airflow test are well known. Although relatively widely applied in membrane plants worldwide, all of these methods exhibit major drawbacks like off-line testing and the lack of a direct relationship between the measured data and the removal efficiency (log-removal). To overcome the above mentioned drawbacks of conventional integrity testing systems, a new kind of integrity test, the Spiked Integrity Monitoring System or SIM®-System was developed by NORIT Membrane Technology in close co-operation with Water Supply Company North Holland and IWW Rhenish-Westphalian Institute for Water Research, which combines the accuracy of a challenge test with the speed of a pressure test, while keeping the system under test in operation.