| Summary: | 碩士 === 國立交通大學 === 環境工程系所 === 94 === Polyaluninum chloride (PACl) has been widely used in water and wastewater treatments, substituting alum and AlCl3. It is superior to other coagulants in its good performance including the low dosage demand and low sludge volume, less alkalinity consumption and independency of temperature. PACl is produced by prehydrolyzation of AlCl3 and the content of Al13 varies with the condition of manufacturing. Many manufacturing processes and purification methods of PACl have been developed by researchers. They have concentrated on production of PACl with stable and high Al13 content at low energy cost. The high Al13- containing PACl has been used in the study of coagulation to clear the individual role of monomeric Al, polymeric Al and colloidal Al during coagulation process. This study assessed the coagulation and sedimentation of silica particles by using four PACls with different aluminum speciation and transformation characteristics. The effect of pH on coagulants speciation and the coagulation efficiencies were investigated. The quality of the treated water, particle aggregation dynamics and sludge characteristics were evaluated to investigate the coagulation mechanisms of individual Al species on silica particles.
Experimental results proved that the Al13 tridecamer produced in high degree of neutralization (γ value) was more stable in the solution and less transformable with the variation of solution pH and aging time. The coagulation performance of each PACls was also improved with the increase in degree of neutralization. Alum and PACl14, both with low γ value, performed best in neutral and acidic pH ranges, while PACl21 and PACl-Al13 worked most efficiently in alkali pH. The coagulation by four coagulants demonstrated that adsorption and charge neutralization were the most effective coagulation mechanism in treating silica particles of extremely negative zeta potential. However, under certain circumstances, sweep flocculation could occur in the coagulation of Alum and PACl14. On the other hand, the coagulation of PACl21 could be supplemented by electrostatic patch effect (EPE), while that of PACl-Al13 might be effected by the combination of EPE and inter-particle bridging.
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