| Summary: | 碩士 === 國立臺北科技大學 === 環境規劃與管理研究所 === 94 === The objective of this study was to investigate the removal of silica from brackish water by the pretreatment of softening and coagulation. This study was conducted by the following four steps：(1) Development of a removal-saturation-recovery curve to determine the threshold limits of RO recovery and silica removal (2) Determining the enhanced effect for silica removal by pretreatment (3) Determining the adsorption isotherm simulation and the silica removal efficiency test in a pilot test, and (4) Qualitatively determining the effects of Ca and Mg for silica removal and verifying removal mechanism by XRD.
Threshold limit for RO recovery and required silica removal were firstly determined by a removal-saturation-recovery curve. The result shows, in raw water (SiO2 = 21 mg/L) or 20% silica removal case, if the water recovery is high, the % saturation will be exceed 100% and silica scaling will occur. High pH was more effective for silica removal when pH>11 and the silica removal ratio exceeded 80%. The precipitation of Mg(OH)2 has assisted more for silica removal than that of CaCO3 and the ratio of 0.045 mg SiO2/mg Mg(OH)2 and 0.028 mg SiO2/mg CaCO3 were statistically determined.
Silica removal could be enhanced by coagulation. Experimental results show the silica removal was increased as pH was increased. The addition of PACl was capable of enhancing silica removal for all the pHs other than 11.Therefore, the addition of PACl for silica removal can not be applied for pH higher than 10, causing adverse effect for silica removal.
The adsorption pilot tests were performed to assess the rate of silica adsorption by Mg(OH)2, using Langmuir and Freundlich isotherm respectively. Adsorption of silica by Mg(OH)2 was better fitted by Freundlich isotherm, According to the XRD database from the pilot tests, the most prevalent species were identified as Mg(OH)2 and CaCO3. Therefore, no Mg2SiO4 and Ca2SiO4 were observed and were verified by XRD analysis.
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