Determination of Rapid-Mixing Parameters in Coagulation: Destabilization Mechanisms and Aggregation Kinetics Approach
|關鍵字:||混凝;膠凝;快混;淨水;聚集動力;光纖膠羽偵測儀;膠羽;碎形維度;coagulation;flocculation;rapid-mixing;water treatment;aggregation kinetic;Photometric Dispersion Analyzer;floc;fractal dimension|
混凝/膠凝為淨水程序中廣泛運用於去除水中微粒的的物化程序，混凝可概分為三個階段，首先混凝劑加入水中首先進行一連串的化學反應之後形成有作用性的混凝作用基，接下來混凝劑作用基透過 ”快混”的操作 與水中穩定的微粒接觸，並進行膠體去穩，最後階段為已去穩的膠體微粒進行有效碰撞之後，形成粒徑較大的膠羽，形成膠羽可在後續沈澱與過濾處理單元之中被去除。快混單元的主要目的是在短時間內均勻地分佈混凝劑使其與水中顆粒碰觸且作用，進而達到顆粒去穩的作用，然而時至今日，此單元的操作仍是以經驗為主，沒有具體的準則可供操作或設計人員參考。
ABSTRACT Coagulation/flocculation is an essential physicochemical process for particle removal in water treatment. The process involves three continuous sequential steps. The first step involves the addition and the activation of the coagulant in water through a series of chemical reactions to form active coagulant species. Next, the coagulant species react with the suspended colloidal particles to destabilize the colloids with the aid of rapid-mixing. Finally, the destabilized colloidal particles collide form aggregates which can then be removed in the subsequent sedimentation and filtration. The purpose of rapid-mixing is to disperse the coagulants in the reactor within a short period of time. Since no concrete guideline concerning the engineering design of the operation of rapid-mixing in water treatment can be offered, the practice of rapid-mixing relies mostly on experiences. This study investigated the effects of rapid-mixing parameters on coagulations of various destabilization mechanisms. Three synthetic turbid water samples from low to high turbidity were prepared from Ball clay for this study. Alum and PACl were employed as coagulants, and their predominant coagulation mechanisms under different dosages were determined. Two mechanisms, the ACN and the sweep coagulation, dominated the destabilization of the colloid under such experimental conditions. Although both mechanisms were effective in removing particles from high and medium turbid waters, only sweep coagulation was effective for the coagulation of low turbid water. The effects of operational parameters of rapid-mixing on particle destabilization and removal were studied by monitoring the changes in turbidity and particle count. Parameters investigated were concentration of the coagulant working solution, dosing site, mixer type, mixing intensity and mixing time. And the coagulation was performed on high turbid water. The concentration of the working solution, dosing site and mixer type had no significant effect on particle removal. Mixing intensity and time, on the other hand, exhibited great impact on coagulation efficiency. For the ACN coagulation, the transportation of the coagulant species was important. For the sweep coagulation, prolonged mixing at high intensity had adverse effect on particle removal. These were further studied by the aggregation kinetics approach. For ACN coagulation, the effects of local overdosing on the subsequent flocculation and sedimentation were studied. The residual turbidity, particle count, dynamics of the particle aggregation, flocs characteristics such as size, density and fractal dimension were determined for this purpose. The results showed that insufficient rapid-mixing were unable to induce aggregation to effectively coagulate small particles. The flocculation induced was cluster-particle aggregation, which formed denser and more settleable flocs. Sufficient rapid-mixing induced cluster-cluster aggregation which effectively coagulated most particles. The flocs formed were looser and less settleable. An aggregation and sedimentation model was proposed to illustrate the significance of rapid-mixing intensity on coagulation. For both sweep and ACN coagulation, the effects of rapid-mixing time on the turbidity and the particle counts of the supernatant were investigated. The dynamics of the particle aggregation was evaluated by monitoring the PDA ratio output of the coagulated suspension. The size of the microflocs was examined with a particle sizer (PS 2400 PC). Floc strength and its reaggregation capacity under sweep and ACN coagulations were also studied. The results showed that in sweep coagulation longer mixing time resulted in higher residual turbidity, while in ACN coagulation the same or lower residual turbidity. The flocs formed from prolonged rapid-mixing under sweep coagulation contained low reaggregation capacity. The flocs were small and hard to settle after flocs breakup and reaggregation. Finally, an optical technique (modified PDA system) was explored to determine the performance of the rapid-mixing. For each coagulation mechanism and turbid condition, the aggregation index of PDA was measured and compared with the residual turbidity of the supernatant under various rapid-mixing times. The residual turbidity and the PDA ratio output demonstrated a significantly inverse relationship. The optimal rapid-mixing times were determined from the jar tests, the PDA method and an empirical model. Consistent results were discovered, which suggested the feasibility of the modified PDA system in optimizing the coagulation operation.