Analysis of the Impacts of Flocculation Modes and Coagulant Dose on Dissolved Air Flotation

Polyaluminium chloride (PAC) was adopted as coagulant to treat the high algae-laden water containing lots of cells of Microcysits aeruginosa (MA) by jar test of cogulation/flocculation/dissovled air flotation (C/F/DAF) process. The constant flocculation and the tapered flocculation with two steps were used in C/F stage. Two-dimension fractal dimension (D₂) and strength factor (SF) were respectively introduced to characterize the floc form and strength. The impacts of flocculation mode and coagulant dose on C/F/DAF were analysed by studying the floc structure, size and strength. The impacts of flocculation mode and coagulant dose on C/F/DAF process were discussed, and the causes of these impacts were analyzed by investigating floc sizes, structures and strengths formed in different conditions. The experimental results showed that at the PAC dose of 5.6~9.8 mg Al₂O₃/L, the flocs formed under bridging flocculation had larger size and highly branched structure, and they were more easily removed by flotation. Comparing the tapered flocculation with the constant flocculation, the former mode could obtain the satisfactory effect of flotation separation at a less dose, because the floc strength formed by this flocculation mode was greater. On the condition of the same energy consumption, the tapered flocculation is more advantageous than the constant flocculation in cost reduction for saving coagulant dose.