Flocculation kinetics using dual coagulants: effects of temperature, addition sequence, rapid mixing, and sulfate

The effects of temperature, rapid mixing time and intensity, sulfate, and coagulant addition sequence of dual coagulants on the flocculation kinetics were studied. Either ferric nitrate alone or a cationic polymer alone or in combination as dual coagulants were used. The flocculation kinetics were monitored using mainly a Photometric Dispersion Analyzer (PDA). An Automatic Image Analyzer was also used in a limited number of experiments to monitor the flocculation kinetics. Kaolinite clay (with four concentrations: 5, 25, 50, and 100 mg/L) suspended in distilled water with added salinity was used at two temperatures (5 or 23°C) and two pH values (6.0 or 7.8) to simulate natural water. Sulfate effects were also studied using either zero concentration or 0.001 M concentration. The following conclusions were drawn from this work: (1) Extended rapid mixing is an important step for particle growth when inorganic coagulants or dual coagulants were used, but not when cationic polymers alone were used. The optimum extended rapid mixing time is a function of the suspended matter concentration. (2) Rapid mixing at a low mixing intensity (G ≅ 450 s[superscript]-1) and longer mixing times gave better flocculation kinetics than high mixing intensity (G ≅ 900 s[superscript]-1) with shorter mixing times for the same Gt product value. (3) Dual coagulants outperformed the sole use of each coagulant alone when the inorganic coagulant dose was high enough to form metal hydroxide precipitate. When the mechanism of coagulation of the inorganic coagulant was completely charge neutralization, the use of dual coagulants was inferior to the use of the cationic polymer alone. (4) The cationic polymer should be added simultaneously with the inorganic coagulant. Simultaneous addition was found to produce the strongest flocs. (5) Low temperature has a detrimental effect on the flocculation process for low turbidity waters regardless of the coagulant type. This detrimental effect is reduced (for inorganic coagulants) and almost eliminated (for cationic polymers alone or for dual coagulants) for higher turbidity waters