Coagulation in submerged ultrafiltration tank (sub-CUF) with continuous low aeration was compared with traditional coagulation by alum in flocculation system and subsequent ultrafiltration (CUF), for treating synthetic humic acid water. The transmembrane pressure (TMP) in sub-CUF was much lower comparing with the one in CUF. Humic acid was removed significantly by coagulation process in two membrane systems, but there was no difference of removal efficiency between these two membrane tanks. Active flocs formed in sub-CUF were significantly larger than those formed in CUF, and the frequency of small flocs was also lower than that in CUF. The coagulation process in jar test confirmed that continuous precipitate of alum can significantly increase the activity of flocs, causing dramatically higher growth ability of flocs. The fractal dimension of flocs formed in sub-CUF (2.53) was lower than that in CUF (2.65) as the continuous new coagulant activated the surface of flocs and increased the branches of flocs, which may induce lower density of cake layer. The image of scanning electron microscope (SEM) further confirmed that the density of cake layer on the sub-CUF membrane surface was lower than that by CUF. Therefore, the characteristic of flocs and cake layer, changed by continuous freshly formed hydroxide precipitate of alum by re-activating the inactive surface sites of aged flocs, caused lower increase of TMP development. (C) 2012 Elsevier B.V. All rights reserved.