Comparison of submerged coagulation and traditional coagulation on membrane fouling: Effect of active flocs

► The TMP development in sub-CUF was much lower compared with that in CUF. ► Continuous precipitate of alum caused dramatically higher growth ability of flocs. ► Flocs in sub-CUF were larger than those formed in CUF, and small flocs decreased. ► The fractal dimension of flocs formed in sub-CUF was lower than that in CUF. ► Fresh alum precipitate re-activated the aged flocs causing lower increase of TMP.