Effects of initial stirring conditions on humic floc morphological structure during coagulation process

The coagulation jar tests, with aluminum sulfate as the coagulant and humic acid as the research object, were conducted under different initial stirring conditions. Based on analyzing and evaluating the floc structure characteristics through PDA online monitoring, image analysis and particle image velocimetry, effects of initial stirring conditions on floc morphological structure was discussed. The results show that stirring conditions have a significant impact on floc forming process and its morphological characteristics. The optimum fast mixing condition is stirring 1 min with 300 r·min^-1. This indicates that flocs formed under this condition (1 min, 300 r·min^-1) have a compact structure, a strong resisting shearing ability, and the optimal overall properties, with floc fractal dimension of 1.8842, strength of 0.164 N·m^-2, and size of 0.43 mm. Formation of humic flocs includes the precipitation of insoluble particles and the production and combination of primary particles, in which the precipitation of insoluble particles and the densification of initial grain are the most important steps that influence floc structure. However, the initial fast stirring condition affects the diffusion mixed effect of coagulant, the formation rate and densification degree of primary particles. This is the key factor of the significant influence of initial stirring condition on humic acid coagulation process.