Analysis And Control Technology On Floc Morphological Characteristics In Micro-Flocculation Stage

Conventional water-treatment processes consist of coagulation- sedimentation- filtration-disinfection, among which, flocculation, one of the most important units, has a significant impact on downstream treatment proccesses and the final effluent quality. Sedimentation and filtration processes can only remove particles of a certain size, and the removal efficiency of micro-particles (particle size <5μm) is not very significant. Flocculation can change size distribution of particles in water. During this process, small particles are expected to be removed to a maximized extent, producing more excellent quality of settled/filtered water. Micro-flocculation stage is the stage when micro-particles change dramatically, and the dynamic changes of floc size at this stage have a significant impact on the whole water-treatment process.In this thesis, the suspension of kaolin and humic acid was used as raw water for experiments, and polyaluminium chloride (PAC) was taken as flocculant. The on-line turbidimeter and on-line particle counter were used to monitor the dynamic change of turbidity and particles. A series of experiments were carried out to study the effect on the characteristics of floc size distribution in micro-flocculation stage when dosage and water quality were changed. Meanwhile, fractal theory was introduced here to reflect the change in particle size at this stage. This methodology could provide a useful tool to characterize macroscopic efficiency of flocculation by microscopic mophology of flocs. The results showed that, during flocculation, the higher the change rate of micro-particle number (particle size <5μm), the better the flocculation efficiency, and vice versa; during micro-flocculation, the higher the increase rates of fractal dimensions of both floc size distribution and growth, the faster the flocs formed with larger size and more compact structure. Fractal dimention of floc growth can be used to describe this process accurately.The experimental results also showed that floc morphological characteristics in micro-flocculation stage had a certain effect on the subsequent units of water treatment. The floc morphological characteristics were reported in terms of the fractal dimension of floc size distribution and fractal dimention of floc growth in micro-flocculation stage. There was a good relationship between these indexes and turbidity/total particle number for settled water. Also, the two kinds of fractal dimensions were directly related to filtered water quality. Therefore, the morphological characteristics of flocs formed at micro-focculation stage can be cosidered as effective indexes to evaluate the efficiency of flocculation, and reflect and control the operational condition of the whole process.Furthermore, experiments were conducted to investigate control effect on floc morphological characteristics in micro-flocculation stage using hydrous manganese dioxide. The results showed that the coagulant effect of hydrous manganese dioxide could greatly encourage the change of micro-particles and accelerate the growth rate of flocs, resulting in the increase of change rate of two fractal dimensions mentioned above with time and their values. These indicated that the presence of hydrous manganese dioxide could obtain wonderful efficiency of water treatment, strong abilities to removal turbidity and particles, and a high removal rate of UV₂₅₄, which mainly occurred during the micro-flocculation stage.