聚四氟乙烯薄膜生物反應槽之薄膜積垢特性

The Characterization of Fouling in PTFE Membrane Bioreactor

歐尚鑫(Shang-Hsin Ou)

指導教授 ： 游勝傑

中原大學/工學院/土木工程研究所/博士(2011年)

https://doi.org/10.6840/CYCU.2011.00141

摘要

由於薄膜程序具有良好的分離能力以及較佳的產水水質，薄膜生物反應槽已廣泛應用在都市及工業廢水處理程序中。然而，薄膜積垢仍為程序的最大缺點，隨著操作時間的增加，經由操作通量以及透膜壓力即可直接觀察到薄膜在處理效能上惡化的程度。因此，本研究以不同的方法評估臨界通量的值，避免過高的操作通量造成薄膜的嚴重積垢。此外，在長期的模廠操作中，亦探討薄膜生物反應槽在不同污泥停留時間下薄膜阻塞的機制。
本研究以經由不同膜面改質後之聚四氟乙烯薄膜進行臨界通量的測定， 結果顯示，在四種不同指標K、dP/dt、Hysteresis loop以及ΔTMP0的評估中，當污泥濃度低於6 gl-1時，以dP/dt所評估的臨界通量最高；當污泥濃度高於8 gl-1時，則dP/dt以及ΔTMP0所評估的臨界通量最高。同時也發現當污泥濃度高於8 gl-1時，各項指標所評估之臨界通量值則並無明顯變化。而以K以及dP/dt所評估之臨界通量於模廠之操作中，則可使薄膜有較長效的使用。
經由污泥濃度以及胞外聚合物成分的分析可得知，在長期的薄膜生物反應槽操作中，污泥停留時間對於薄膜積垢具有甚大的影響。而在不同膜面改質薄膜的比較中亦發現，疏水性薄膜在短污泥停留時間操作中具有較大的積垢速率。然而，當積垢物附著於膜面後，此差異即不存在。此外在各污泥停留時間的操作中，化學需氧量去除率與污泥粒徑大小、胞外聚合物組成、蛋白質與碳水化合物比例並無明顯關聯。在長污泥停留時間操作下，聚四氟乙烯薄膜操作通量即透膜壓力有較佳及較穩定的表現。因此適合做為未來薄膜生物反應槽的實廠應用。
在薄膜積垢機制的實驗，由胞外聚合物的分析中可知，附著性胞外聚合物為膜面上積垢物的主要組成。同時污泥在生長期之蛋白質與碳水化合物比明顯高於穩定期。而將薄膜阻抗以鬆散性污泥層、緻密性污泥層、內部積垢來區分，並在不同過濾時間的操作下發現，當過濾時間達三十分鐘後，濾餅層即快速累積形成於膜面上。此外碳水化合物為首要附著於膜面的物質，而蛋白質則隨時間逐漸增加於各部位之積垢物中。另外，在以純菌過濾的試驗中發現，聚四氟乙烯薄膜膜面上附著之蛋白質主要為疏水性，且其主要為細胞中負責轉譯的功能。

關鍵字

薄膜生物反應槽 ； 胞外聚合物 ； 透膜壓力 ； 臨界通量 ； 疏水性 ； 聚四氟乙烯 ； 薄膜積垢 ； 污泥停留時間

並列摘要

In recent years, membrane bioreactor (MBR) technologies have been extensively applied as the advanced municipal and industrial wastewater treatment process because of its excellent separation ability and production of good effluent quality. However, membrane fouling remains a major drawback of this process because it deteriorates the membrane performance with regard to operational flux and transmembrane pressure (TMP). In this study, the feasibility of critical flux was discussed by comparison with different approaches. The operation of long-term MBR with modified PTFE membrane was carried out through the adjustment of SRTs meanwhile investigated the relationship between operating condition and membrane fouling. The fouling mechanism was discussed by EPS transition in the MBR and batch tests as well.
For the critical flux detection with novel polytetrafluoroethylene (PTFE) flat membranes with different hydrophobicity (contact angle, CA=70°, 90°, 130°) and having a pore size of 0.22µm, the experimental results revealed that the critical flux decrease from 80 to 35 LMH(lm-2h-1), 50 to 30 LMH, 60 to 27.5 LMH, and 42 to 9.4 LMH by using K(permeability), dP/dt(TMP increasing rate), Hysteresis loop, and ΔTMP0(Initial TMP increase) approaches, respectively, with a increasing of MLSS form 1 to 20 gl-1. It was observed that when the MLSS concentration was less than 6 gl-1, the fouling rate method showed the highest critical flux among the four approaches. On the contrary, when the MLSS was higher than 8 gl-1, both the dP/dt and ΔTMP0 analysis showed the highest critical flux of the four approaches. All approaches there were no significantly reduction of membrane critical flux when MLSS concentration was higher than 8 gl-1. For the long-term operation, the critical flux determined using the K and dP/dt approaches showed better suitability at constant MLSS of 10gl-1.
The results from this study have clearly demonstrated that SRT is the key parameter that determines fouling propensity through MLSS and EPS fraction concentration. In the long-term bench scale MBR operation, the results revealed that hydrophobicities of PTFE membranes influenced TMP only at lower SRT (2 d) and the membrane with higher hydrophobicity (CA-130°) possesses higher and more rapid TMP rise than others slightly hydrophilic membranes (CA-70° and CA-90°). However, such differences were not observed as SRT increased in subsequent operation and the foulant easily attached on the membrane surface. In addition, the COD removal was not affected by the sludge properties such as the average particle size, EPS content and P/C ratio have changed with prolonging SRT. Furthermore, it was also observed that, as SRT increased, the EPS content and P/C ratio also decreased progressively. The PTFE membrane showed significantly higher and stable performance and low membrane resistance at longer SRT (30 d). This indicated that the variation of operational condition greatly influenced the changes in sludge characteristics and the fouling which in turn affect the membrane performance. Nevertheless, the MBR system with the modified PTFE membranes could offer a stable flux under operation of proper SRT and the excellent recovery on membrane performance also becomes the dominance in further development.
The characteristics of extracellular polymeric substance (EPS) and the mechanism of fouling formation were investigated by using a submerged membrane bioreactor system consisting of a polytetrafluoroethylene (PTFE) membrane. During 85 days of operation, the bound EPS observed as the main component, which contributed major part of resistance to fouling. The protein to carbohydrate ratio was higher in the growth phase than in the stationary phase. The fouling resistances contributed by each individual resistances, i.e. loose cake layer, dense cake layer and internal fouling evaluated by batch filtration test after rinse, wash, and backwash operations respectively. The results showed that fouling of membrane and transmembrane pressure increased very quickly within 30 min of operation mainly due to resistance offered by immediate cake layer formation. The analysis of EPS contributed by different fouling layer revealed that most of the carbohydrate adsorbed quickly on the PTFE membrane first and then protein concentration increased with increasing filtration time. The protein content of EPS gradually increased when the foulant approach the membrane, whereas carbohydrate content showed opposite trend. Furthermore, the filtration of pure bacteria (Ralstonia) showed that most of proteins adsorbed on the membrane surface were hydrophobic and identified its contribution as the biological functions of translation.

並列關鍵字

sludge retention time ； SRT ； EPS ； fouling ； polytetrafluoroethylene ； critical flux ； PTFE ； TMP ； Extracellular polymer substance ； MBR ； transmembrane pressure ； hydrophobic membrane ； membrane bioreactor

參考文獻

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