The role of ozone for reducing foulants in the membrane process has been studied. In this study, Ibuprofen (IB, a micropollutant) and MS2 bacteriophage (MS2, a microorganism) were tested as two different types of foulant probes to investigate the membrane fouling mechanisms during microfiltration (MF). The extent of fouling in the tested MF process was adequately predicted by the adsorptive fouling model for IB and the internal pore plugging model for MS2 as compared to other classical fouling models. The fouling contributed by both foulants was found to be almost irreversible. In order to reduce the fouling problem during the MF process, several ozone experiments have been conducted: First, to investigate the preventive effect of ozone destructing foulants in the aqueous phase prior to the foulant accumulation on MF; Second, to investigate the treatability of a fouled membrane with ozone. The role of MF filtration in the MF-ozonation hybrid scheme has also been investigated. A slower ozone decay rate, which is a favorable condition from the disinfection perspective, but a lower IB removal rate, which is negative from the oxidative standpoint, was observed during ozonation after the MF process.