Properties of alum and polyaluminium chloride (PACl) flocs were analyzed in order to explain solid/liquid separation behavior of these aggregates in dissolved air flotation and gravity settling. PACl flocs settle better and are less sensitive to changes in water temperature than alum flocs. Therefore, PACl flocs may be more suited for gravity separation, especially in cold waters, and alum flocs may be preferred for flotation. At an optimum coagulant dose for dissolved air flotation the logarithmic mean size of alum flocs was close to the size of the air bubbles (30 μm) and the proportion of flocs smaller than 20 μm was about 30.5%.

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