We monitored the dynamic responses of the blankets in the full-scale floc blanket clarifiers at PingTsan Water Works with dosed polyaluminium chloride (PACl) subjected to step-changes. With the raw water turbidity ranging 7-13 NTU, the sudden drop in PACl dose from 1.86 to 0.95 ppm as Al in conventional coagulation-clarification process led to a wave-like spatial instability in distributions of both solids fraction and floc size of the blanket. Adding a pre-coagulation and pre-sedimentation prior to the conventional stage (two-stage process) produced rather stable blanket that was only mildly deteriorated at insufficient coagulant dose. Jar tests with single or two-stage coagulation-sedimentation revealed that the organic matter readily interacted with dosed PACl and might be responsible for the observed blanket structure deterioration. Dynamic flocculation tests using small-angle light scattering technique support this speculation. Linear stability analysis was conducted to demonstrate the blanket stability at infinitesimal process parameter change. The blanket would become linearly unstable at an upflow velocity exceeding a threshold value. Two-stage process produced large flocs, which exhibit high settling velocity, hence stabilizing the blanket clarification process.