The effect of fast mixing conditions on the coagulation–flocculation process of highly turbid suspensions using liquid bittern coagulant

Abstract

The effect of fast mixing on floc formation and pollutant removal, using magnesium hydroxide as a coagulant, was investigated through characterization of relative strength and size of the formed flocs while operating at different mixing speeds and mixing times using a dynamic optical monitoring apparatus, and photometric dispersion analyzer (PDA2000). The parameters investigated included fast mixing speed (80, 100, and 120 rpm) and time (20, 40, and 60 s). Highly turbid kaolin clay suspensions (1213 ± 36 NTU) were alkalized using sodium hydroxide (NaOH) to pH values of 10.51 ± 0.02 at temperatures 20.7 ± 0.1°C, and liquid bittern (LB) was used as a coagulant. Fast mixing time had a clear effect on the flocs resistance to applied shear during the slow mixing phase. For all fast mixing times, 120 rpm caused the formation of largest flocs. Stronger flocs, indicated by the least change in flocculation index with time, required 60 s to form at all fast mixing speeds. Turbidity and TSS removal efficiencies are not only dependent on fast mixing speed but also on fast mixing time, such that higher fast mixing speeds were required for shorter mixing time