In this study, a dynamic membrane (DM) was formed by flocs produced by river water pollutants reacting with polyaluminum chloride (PAC). The DM was used to separate the flocs from the mixed liquor at constant pressure. Two support media for the DM were tested: non-woven fabric filter and dacron mesh. The formation time of the DM was determined according to the critical flux. The formation process and filtration mechanisms were evaluated based on typical crossflow membrane fouling models and combinations of them. When a non-woven fabric filter was used as the support media, the formation time of the DM was 70 min at 4 cm water head drop (WHD) and 20 min at 8 cm WHD. When dacron mesh was used as the support media, the membrane formation required 100 min at 4 cm WHD and 70 min at 8 cm WHD. During DM formation on a non-woven fabric filter, the internal pore blocking plays the dominant role. But when dacron mesh is used as the support media, complete blocking and intermediate blocking dominate. After the formation of a DM, the combined models have good agreement with the experimental data compared with the typical filtration models.