Evaluation of floc strength based on morphological analysis and optical online monitoring

Based on morphological analysis and PDA online monitoring, a method was established for the evaluation of floc strength. In order to calculate the binding force of flocs formed under a given flocculation condition, a jar test was conducted. The morphological characteristics of flocs were studied by image analysis and the particle image velocimetry technology. A shear force was exerted on the flocs by introducing the flocculated suspension through a transparent tube where the velocity gradient value of the flow could be accurately controlled. As the tube was connected with a particle dispersion analyzer (PDA), the condition of floc breakage was monitored online and the critical condition was identified by analyzing the PDA outputs (FI curves). According to the equivalent relationship among shear force, breaking force and binding force under the critical breaking condition, the binding force coefficient k₁ could thus be determined, and the binding force B which represented the floc strength was evaluated. The validity of this method was proved by a series of experiments using aluminium sulphate as the coagulant for the flocculation of humic acid.