前氧化結合截留式微過濾處理含鐵錳地下水之研究

Abstract

前氧化結合微過濾單元為常見處理地下水之方式，首先將水體中之鐵、錳離子氧化為顆粒物後，再利用薄膜過濾以達淨水之效果。前氧化程序時，由於錳離子活性較低、氧化速率較鐵離子緩慢，導致使用薄膜過濾時未氧化之錳離子吸附於薄膜孔洞內，並持續氧化成顆粒物而造成嚴重之不可逆積垢。為降低不可逆積垢之發生，於薄膜單元前確保水體中錳離子完全氧化成顆粒物為關鍵之步驟。因此，本研究將探討於完全氧化條件下，鐵、錳氧化物顆粒特性對於MF薄膜過濾行為之影響。本研究使用粒徑分析儀即時監控不同攪拌強度及鐵錳濃度比條件下以評估氧化後所形成之鐵、錳氧化物顆粒特性，並探討鐵、錳氧化物顆粒特性對於薄膜過濾通量衰減及濾餅阻抗之影響。 本研究結果顯示於相同鐵錳濃度比值下，攪拌強度增加會使得鐵、錳氧化物顆粒粒徑變小；顆粒之碎形維度隨攪拌強度增加而變大。於相同攪拌強度時，氧化物顆粒粒徑、碎形維度隨鐵錳濃度比值增加而變大。然而影響氧化物顆粒生成之主要因子為攪拌強度而非鐵錳濃度比值。另一方面，濾餅所造成之阻抗為影響薄膜過濾通量之關鍵，若形成濾餅之顆粒物質粒徑較大、濾餅結構較為鬆散且較容易壓縮，將造成較高之濾餅阻抗使得通量降低；反之當顆粒物質之粒徑較小、濾餅結構較為緊實者濾餅阻抗較低。此外，鐵錳濃度比值對於濾餅阻抗具有顯著之影響，當鐵錳濃度比值較高時，所形成之鐵錳氧化物濾餅表面因具有較高之疏水性，使得平均通量降低、造成更嚴重之不可逆積垢。

A hybrid oxidation-microfiltration process is commonly used to remove ferric and manganese ions from groundwater through the rejection of ferric and manganese oxide on membrane. In oxidation, the manganese ions are more difficult to be oxidized than iron because they are inactive in water. Then the inoxidized manganese ions are frequently adsorbed in the pore of membrane and then continuously form oxidized particles, which result in a heavy irreversible fouling for membrane filtration. To reduce the irreversible fouling during membrane filtration, the formation of oxidized manganese before membrane filtration is the key to improve the performance of membrane filtration. Therefore, the study aims to investigate the effects of the formation of ferric and manganese oxidized particles on the behavior of membrane filtration in a preoxidation-microfiltration process. Before membrane filtration, the formation of oxidized particles in the solution of different Fe/Mn ratio at various mixing intensity was continuously monitored by a particle sizer to evaluate the effects of mixing on the characteristics of oxidized particles. After that, various solution containing oxidized particles was filtrated by microfiltration to determine the effects of particles properties on flux decline and cake resistance of membrane filtration. The result showed that the size of oxidized particles in solution containing the same Fe/Mn ratio decrease but the fractal dimension increase when mixing intensity was increased. By contrast, the size and fractal dimension of oxidized particles increased with increasing Fe/Mn ratio of solution under a constant mixing intensity. However, the particle formation is mainly affected by mixing intensity rather than the Fe/Mn ratio of solution. On the other hand, the cake resistance dominates the flux of membrane filtration. The structure of large oxidized particles is looser and easily compressed, which results in higher specific cake resistance. On the opposite, the small oxidized particles are more compact, which lead to the lower specific cake resistance. Furthermore, the Fe/Mn ratio of solution can influence significantly the cake resistance during membrane filtration. When the oxidized particles with more hydrophobic surfaces in solution of higher Fe/Mn ratio were filtrated by membrane, the average flux was lower. At such condition, the more severe irreversible fouling in membrane filtration also occurred.