光觸媒氧化及不織布薄膜過濾複合系統:分解特性及過濾行為之探討

Abstract

懸浮式光觸媒氧化系統具有提供大量有效表面積、簡化光源配置及無需固定光觸媒等優點，但如何有效且經濟地分離並回用光觸媒，仍是環保界亟待解決的重要課題。本研究從材料選擇、系統建立及驗證等方面著手，希望發展一種新穎性的光觸媒及薄膜複合系統。 本研究以不織布薄膜（或稱巨孔薄膜）取代一般使用微孔薄膜的光觸媒薄膜複合系統(photocatalytic membrane reactor, PMR)，由其材質特性分析進而探討不織布薄膜之過濾行為。並就不同目標污染物如亞甲基藍、4-氯酚及二級放流水等進行污染物光分解特性研究及應用可行性驗證，以探討光觸媒及不織布薄膜複合系統建立之可能與效能。 首先，藉由不織布材質結構特性分析，並進行最佳不織布薄膜篩選的研究。採用0.2、2.0及20.0 µm等三種不同孔洞之不織布薄膜，並以比通量及濾液殘留濁度為指標。實驗發現，選用2.0 µm不織布薄膜，使其孔洞大小略等於或小於凝聚後光觸媒顆粒(或稱二級顆粒)（約2-8 µm）時，能有效地分離光觸媒，並同時可以符合上述兩項指標需求。 其次，由於不織布薄膜的過濾阻抗(filtration resistance)會受操作條件如光觸媒濃度、pH值、空氣強度及操作通量等影響，尤其操作通量達3 m3/m2/day或更高時，過濾阻抗更為明顯。由於不織布薄膜孔洞較大，且薄膜本身過濾阻抗極低，故其過濾阻抗以多孔性濾餅(cake formation)為主，約佔80％，而可逆及不可逆之孔洞阻塞(pore blocking)或孔洞縮小(pore narrowing)之過濾阻抗相對較低，分別各佔10％左右。本研究結果顯示，上述過濾阻抗經由適當操作條件控制，可維持多孔性濾餅之厚度，進而獲得較低的過膜壓力及穩定操作通量。 接著，使用亞甲基藍為目標污染物進行批次及連續實驗以探討光觸媒及不織布薄膜複合系統的建立。在批次實驗中，由於光分解目標污染物濃度低，可適用Langmuir -Hinshelwood反應動力之擬一階反應動力之描述。連續實驗中，槽中SS濃度隨操作流量（或通量）增加而降低，且濾液SS濃度低於偵測極限值，被攔截的光觸媒顆粒將在不織布薄膜表面形成多孔性濾餅。當過濾層形成後，配合適當操作條件，可獲得相當低過膜壓力（約3.5 kPa）及維持穩定操作通量。 最後，以4-氯酚及二級放流水為對象進行複合系統之應用研究，主要訴求為光觸媒鈍化及積垢物質之去除。在4-氯酚光分解過程中，發現氯離子生成並不會造成光觸媒鈍化或毒化現象，此與懸浮式光觸媒系統可以提供大量表面積及適當操作條件如pH值及溶氧有關。另外，複合系統可以有效去除二級放流水之積垢物質如生物高分子及腐植酸等，進而大幅改善UF薄膜過濾性，有助於提升本複合系統之應用潛力。 綜合上述討論，本研究提出之光觸媒氧化及不織布薄膜分離複合系統可有效分離懸浮光觸媒顆粒及分解污染物，配合最佳孔洞不織布薄膜選用及其表面形成多孔性濾餅之過濾方式，可以獲得低過膜壓力與穩定操作通量，進而避免積垢現象發生。

A suspended photocatalytic oxidation system offers lots of active sites for photocatalytic oxidation, simplifies UV light arrangement, and obviates the complexities of fixing a photocatalyst onto a substrate. However, effective and economic separation of photocatalyst from the slurry system is still a problem to be addressed. In this study, a novel photocatalytic membrane reactor (PMR) has been developed based on material selection, system development, and application study. The non-woven membrane (or called macroporous membrane) instead of microporous membranes, e.g. MF or UF, was proposed in a PMR to separate photocatalysts from slurry and to photodegrade model compounds such as methylene blue, 4-chlorophenol, and secondary effluent. Three major parts including material characteristics of non-woven membrane and their filtration behavior, the development of the hybrid system from batch and continuous tests and the application studies of the hybrid system are emphasized in this study. Three different pore sizes of non-woven membrane, 0.2, 2.0, and 20.0 µm, were applied to determine the most optimum filtration performance in terms of specific flux and residual turbidity in permeate. The results showed that when the pore size of the non-woven membrane was equal to or smaller than that of secondary particles of photocatalyst, a high specific flux and low turbidity in permeate were obtained simultaneously. Therefore, a 2.0 µm pore size of the non-woven membrane was selected for further study. The filtration resistance in non-woven membrane was determined by the operating conditions, e.g. concentration of photocatalyst, pH value, air intensity and applied flux. The filtration resistance was more and more significant when applied flux was 3.0 m3/m2/day or more. Because the pore size of the non-woven membrane was large (i.e. 2.0 µm) and the filtration resistance from membrane itself was minor, the filtration resistance was dominated by porous cake formation, about 80%, and the rest of 20% was shared equally by reversible and irreversible pore blocking or pore narrowing. Under the circumstances, the thickness of porous cake formation was controlled by optimum operating conditions to obtain stable applied flux and low TMP. The photodegradation characteristics of methlyene blue and filtration behavior using a 2.0 µm pore size of non-woven membrane were demonstrated in batch and continuous modes. The photodegradation of methlyene blue followed by pseudo-first-order reaction kinetics was proposed, because a low concentration of model compound was used. In continuous mode, the concentration of SS in hybrid system was decreased with increasing flow rate (or applied flux) resulting in the formation pf porous cake layer on the surface of non-woven membrane. Then, TMP was maintained around 3.5 kPa under optimum air intensity to induce a crossflow velocity after cake layer was formed. The inactivation of photocatalyst and the photodegradation performance of organic fouling materials were focused on the application study of this hybrid system, when 4-cholophenol and secondary effluent were used, respectively. The inactivation or toxicity of photocatalyst was no obvious when the chloride ion was formed after photodegradation of 4-chlorophenol, because the tremendous active sites onto the surface of the suspended photocatalyst could offer and suitable operating conditions, e.g. pH value and dissolved oxygen, were performed. Moreover, organic fouling materials, e.g. biopolymer and humic acids, in the secondary effluent were photodegraded using this hybrid system to improve the filtration ability of UF membrane by a batch stirred cell test. Therefore, the proposed hybrid system in which a non-woven membrane could replace microporous membrane in a PMR system was used to separate completely suspended photocatalysts and to photodegrade effectively model compounds. At the same time, low TMP and stable applied flux in this hybrid system was expected to reduce dramatically membrane fouling potential after the optimum pore size of non-woven membrane was selected and the porous cake layer was formed on the surface of non-woven membrane.