線上混凝去除二氧化矽及對超過濾薄膜積垢之影響

In-line Coagulation/Ultrafiltration for Silica Removal from Brackish Water

楊淑如(Shu-Ru Yang)

指導教授 ： 陳孝行

國立臺北科技大學/工程學院/環境規劃與管理研究所/碩士(2007年)

摘要

二氧化矽為一種薄膜積垢物，淨水場中通常可以軟化或混凝方式去除。一般而言，軟化程序較有效去除二氧化矽，但軟化程序須採高pH值狀態、水中硬度必須足夠，且污泥產量多，加上成本花費較為昂貴。而混凝方式雖然經濟，但是混凝/膠凝/沉澱卻須大量土地面積。因此，一般小型淨水場中，採用線上混凝方式結合薄膜程序(MF/UF)可有效解決土地面積、又能同時達到去除效率且節省空間之程序。
本研究共討論四大部份，1.探討二氧化矽去除率、飽和度和薄膜回收率之關係；2.探討線上混凝對於二氧化矽之去除效果；3.探討線上混凝結合薄膜程序對於二氧化矽之去除效果；4.探討線上混凝對薄膜積垢的影響。
本研究之線上混凝對於二氧化矽去除結果，其去除率皆可達到40%以上，由二氧化矽去除率、飽和度和薄膜回收率之關係可知，當二氧化矽去除率至少40%時，此時薄膜回收率控制在85%即不會有二氧化矽積垢物產升之虞。於線上混凝結合薄膜程序中，以G值為2000s-1且 PACl加藥量為30mg/L為最佳，此時水中二氧化矽粒子與PACl達到完全混凝狀態，並因水中顆粒電性中和達到去穩定化區段，若此時隨著加藥量增加，水中氫氧基鋁錯鹽也增加，達到高濁度狀態，但又以加藥量為90mg/L時發生沉澱拌除。而在薄膜積垢方面，短期間 (120min) 操作薄膜程序，未有積垢物的產生，推估二氧化矽受到G値、混凝劑和pH影響，去除效果高達60%以上，故在短期間內，其二氧化矽積垢物對於阻塞薄膜孔徑並無太大結果顯示；但以長時間操作 (2000min) 薄膜程序，二氧化矽積垢物緩慢速度在阻塞薄膜孔徑，其薄膜表面比起未經使用之薄膜表面也僅增加了3.5*106 1/m 的阻力值。故本研究以線上混凝結合薄膜程序，在中性pH狀態下，求得最佳混凝效果，達到有效降低水中二氧化矽。

關鍵字

線上混凝 ； UF薄膜 ； 二氧化矽積垢 ； G値

並列摘要

Silica is a scaling species for membranes. The most common method of removal of silica is by softening or coagulation in water treatment. Generally, softening could be effective in removing silica, with higher pH and hardness but created a lot of sludge, and the cost expenditure is very high. Although coagulation is more cheaper than softening, the coagulation/flocculation/ precipitation actually require huge land space. Consequently, using in-line coagulation/ultrafiltration could save space and have silica removed.
In this study, a brackish water source containing silica concentration was utilized for silica removal and the four objectives are as follows：(1) Development of a removal-saturation -recovery curve to determine the threshold limits of RO recovery and silica removal requirement (2) Effects of pH, coagulant dosing and mixing intensity on silica removal for in-line coagulation (3) Effects of pH, coagulant dosing and mixing intensity on silica removal for in-line coagulation/ultrafiltration, and (4) Impact on membrane fouling due to preceding in-line coagulation.
Threshold limit for RO recovery and required silica removal were, first of all, determined by a removal-saturation-recovery curve. The result shows, in raw water (SiO2 = 30 mg/L) or 40% silica removal, that if the water recovery is high, the 85% saturation will be exceed 100% and silica scaling will occur. With velocity gradient (G) of 2000s-1and initial alum dose of 30mg/L, the water achieved destabilized particles, but the further increased in coagulation dose resulting in destabilization of the particles because of a near complete coverage of the particle with aluminum hydrolysis product. The further increasing of coagulation to the degree of high doses (90mg/L) could make the formation of a precipitate of Al(OH)3(s). For membrane fouling, the fouling will not be occurred in short time (120 min). Silica may receive G value, coagulation dosage and pH the influence, and its removal achieves as high as above 60%. Therefore, silica has not created membrane fouling. But after 2000min operation, silica accumulated particles with slow speed into pore size membrane, and the resistance only had increased the 3.5*106 1/m. This research with in-line coagulation/ ultrafiltration and in the neutral pH condition, obtains the best coagulation effect and achieves in removing silica.

並列關鍵字

In-line coagulation ； UF membrane ； Silica fouling ； G value

參考文獻

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