Research On Flocculation Mechanisms And Transport Model Of Cohesive Fine Sediment

There is a large amount of cohesive fine sediment in estuary, lakes and reservoir, etc. Its existence changes the adsorption, sedimentation, transport and other characteristics of sediment, which cause abnormal phenomenon between these areas, such as, the formation of entrance bar in estuary, the deposition of sediment in reservoir, the intrinsic pollution of rivers and lakes water by bottom silt with high cohesive fine sediment content, the high water content of dredged silt, and so on. And these affect channel navigation, the lasting service of reservoir, water quality rehabilitation and the disposal of dredged silt, etc. The flocculation and electrochemi-cal characteristics of cohesive fine sediment are responsible for these phenomenons. Therefore, studying flocculation of cohesive fine sediment has important meaning to the regulation of channel and estuary, the forecast of scour and deposition, the disposal of dredged silt and the comprehensive management on water environment.However, the flocculation of cohesive fine sediment is a multi-interdisciplinary subject involving the field of colloidal chemistry, molecular dynamics, mud-sand dynamics, and hydraulics. Therefore, special researches on it are few. Integrating flocculation theory, fractal theory, image analysis theory and sediment transport theory, we studied three key problems from both macro and micro perspectives, which are flocculation process, settling performance and transport model of cohesive fine sediment, through laboratory experiment and numerical simulation. And then we discussed the variations of flocculation process, sediment floc structure and settling performance in different mechanism and conditions, and presented a relative better transport model of cohesive fine sediment. The adopted research tools and main conclusions are as follows:(1) Based on flocculation theory, we analyzed the range of cohesive fine sediment, electrification mechanism, collision and cohesion mechanisms from the standing point of the physical and chemical properties of cohesive fine sediment to provide the theory base for further research.(2) Integrating cluster-growth model, fractal and sediment theory, we proposed an advanced model(CSFG model) from micro aspects to simulate the flocculation process of cohesive fine sediment. The model consists of four modules:Brownian motion, floc settling, flow and floc breakage. Meanwhile, bonding probability is adopted to reflect the influence of London force and electrostatic repulsion between particles on flocculation. Finally, we discussed the influencing rules of collision mechanisms and sediment conditions on flocculation velocity, floc size distribution and floc structure through CSFG model. The results show that:1. Flocculating characteristics of cohesive fine sediment are not the same in different collision mechanisms. As collision mechanisms are Brownian motion and differential sedimentation(still water), in comparison to Brownian motion collosion mechanism, flocculation speeds up, fractal dimension varies larger, and fractal dimension in stationary phase(2.04,2.12) is bigger. Moreover, floc size distribution becomes wider with a greater heterogeneity. In flowing water, the effects of flow on flocculation include two aspects:low strength flow promotes the flocculation of cohesive fine sediment and the effects are similar to that of gravity; high strength flow not only improves the collision of sediment particles(flocs), but also makes sediment floc break up, and it will reach a stationary phase as formation rate equals to breakdown speed of flocs. Furthermore, the higher of flow strength, the earlier enter the stationary phase, the smaller of floc size, the narrower of floc size distribution. While fractal dimension of flocs is biggest in lower regions and smallest in middle regions.2. Flocculating characteristics of cohesive fine sediment are also distinct under different conditions. The higher of sediment initial concentration, the faster of sediment flocculation, the larger of fractal dimension. Moreover, there is an exponent relation between floc average size and sediment initial concentration. While sediment initial distribution influences the flocculation by altering the level of differential flocculation. The wider of sediment initial distribution, the faster of sediment flocculation, the larger of fractal dimension, and the more even of sediment floc size distribution.3. According to the simulated and theoretical analysis results, a new method is put forward to improve the settling and solidification properties of bottom sediment in urban lakes, in which sediment is treated with ethanol solution. We took silt from Nanhu lake as an example to verify the new method, and the experimental results answer to expectation.(3) Taking sediment concentration as a parameter, a series of settling experiments were carried out using a self-made rotary cylinder from macro aspects. Based on the results of single-factor experiments, the effects of multiple factors on the settling property of cohesive sediment were dissected. And image analysis was introduced to discuss the mechanism of the effects. The results reveal that the settling performance of cohesive fine sediment is closely related to flocculation, and factors affect the settling performance by changing the level of flocculation.According to the effect laws, the effects fall into three types:first, the settling is influenced through changing collision frequency, such as, initial concentration. Second, it is affected by changing bonding probability, such as, electrolyte and high molecular polymer with a best content. Third, settling is influenced by changing both collision frequency and bonding probability, such as, flow, with low strength promotes settling, while high strength retards settling.The results of variance analysis indicate that the influenced level of these factors is different in different stages. When the content of electrolyte in flow is high, at early stage, the effect of initial sediment concentration takes first, flow takes second, and electrolyte takes third, but at the remaining time, flow takes the first, electrolyte takes the second, and initial sediment concentration takes the third, moreover the influence of initial sediment concentration is on the wane with time passing by. As high molecular polymer exists in flow, at early stage, the effect of high molecular polymer is the strongest, flow is the weakest, and initial sediment concentration takes the middle. But flow is the first, high molecular polymer is the second, and initial sediment concentration is the weakest at the remaining time.(3) Based on flocculation theory, fractal theory and sediment transport theory, a model is developed to simulate the transport of cohesive fine sediment. The model is organized into three terms:flocculation, sedimentation and turbulent diffusion. flocculation term takes an advanced population balance equation including particle coagulation and floc breakage. An improved equation is established to compute the settling velocity of flocs in the sedimentation term and turbulent diffusion coefficient is computed by empirical equations in the diffusion term. The new transport model proves to be reasonable and has a certain precision through verification test. The vertical transport property of cohesive fine sediment in still water was studied through the model, as well as the effect laws of gravity, flocculation, and turbulent flow on the vertical transport of cohesive fine sediment.The results reveal that, in still water, the vertical transport of cohesive fine sediment makes up of two phases:first, the supply of sediment from upper regions is enough, and flocculation takes the lead. Second, collision frequency reduces with the decrease of sediment concentration, meanwhile, the supply from upper regions is few, and settling of sediment flocs has a major role. While in flowing water, the effect of gravity is more marked in upper region and later period, flocculation occupies a predominant position in lower region and early stage, and turbulent flow affects the sediment vertical transport in the whole areas. Moreover, the larger of the turbulent energy dissipation coefficient, the more notable of the effects.