Effect of mixing on the kinetics of polymer-aided flocculation

Abstract

Polymer-aided flocculation is defined in this study as the coagulation and flocculation process which is accomplished by using alum in conjunction with a small dosage of polymer. Polymer is commonly used to improve flocculation performance; however, few in-depth studies of mixing requirements when using polymer as an aid have been reported. Presented is an evaluation of the effect of mixing on the kinetics of polymer-aided flocculation. The study found that the optimum G̅_f-values occurred at 70 to 100 s⁻¹, such that G̅_f t_b-values (Camp Number) were in the range of 113,000 to 144,000. The limited improvement in kinetics achieved by increasing the G̅_f-value beyond 100 s⁻¹ will probably not justify the higher energy consumption. Moreover, experimentation has shown that the maximum G̅_f-value recommended for the design is G̅_f=200 s⁻¹ (G̅_f t_b=252,000), and the critical mixing rate occurred at G̅_f=400 s⁻¹ (G̅_f t_b=432,000), beyond which significant floc break-up will occur. The optimum G̅_f-value determined from the study suggests that the currently designed flocculators using alum alone can be used when polymer is used as a coagulant aid. Thus, the use of polymer as a coagulant aid can improve the coagulation and flocculation efficiency in existing systems.