生物薄膜系統中污泥特性對薄膜積垢之影響及二氧化鈦複合薄膜減緩積垢之研究

Abstract

生物薄膜程序為近年廢水生物處理程序中一嶄新之研究，由於利用薄膜單元取代傳統的沉澱池，使其具有較小的佔地面積、較少的汙泥產量及優良的出流水質等優勢。然而，生物薄膜中薄膜的阻塞卻限制其廣泛的應用，因為薄膜的阻塞會導致出水通量衰減、增加薄膜的清洗和更換次數，使操作成本增加。儘管已有許多研究針對薄膜積垢的機制及預防進行廣泛地探討，但其研究結果卻往往顯示出不同的結果。因為污泥及進流水的複雜特性及來源不同，或是不相同的薄膜模組及操作情況往往會導致此不一致的結論。薄膜積垢肇因於污泥和薄膜間的交互作用而導致汙泥中不同的成份吸附或沉積於薄膜表面。因此，汙泥的特性、薄膜的特性及其水力情況往往為導致薄膜積垢的關鍵因素。 因此，本研究主要針對不同的污泥特性對薄膜積垢進行探討。研究利用一好氧性選種槽將污泥中的主要菌相由絲狀菌轉變為膠羽形成菌，藉此探討其不同污泥特性下的薄膜過濾特性。結果顯示，儘管污泥膨化嚴重，生物薄膜系統仍可維持優良的出流水水質。實驗過程中，總有機碳和氨氮的去除率皆分別維持98%和99%的高去除率。然而，即使絲狀菌比膠羽形成菌有較大的粒徑分布，當絲狀菌大量增生而導致污泥膨化時，薄膜積垢情況嚴重。研究發現，當絲狀菌大量增生時，污泥中的溶解性微生物產物大量增加，尤其是當中的多醣類含量。由傅立葉紅外線光譜結果顯示，溶解性微生物產物中的多醣類和蛋白質為導致此嚴重積垢發生的主因。污泥膠羽上的胞外聚合物並非造成此嚴重積垢的原因。 污泥停留時間的不同導致不同的污泥特性，進而對薄膜積垢造成重要的影響。本研究將污泥停留時間設定為10、30及60天，發現污泥停留時間越短薄膜積垢越嚴重。污泥停留時間為10天的生物薄膜系統，其薄膜積垢遠比操作於30及60天的嚴重許多，主要因為污泥於污泥停留時間為10天的生物薄膜分泌較多的溶解性微生物產物。然而，污泥停留時間30及60天的生物薄膜系統，其不同的積垢程度主要是因為膠羽中的胞外聚合物不同所導致。溶解性微生物產物於較短的污泥停留時間中，有較大的分子量分布。動態膜在過濾開始的1小時內便穩定形成於薄膜表面，進而過濾篩除污泥中的較大分子量的溶解性物質。而此形成於薄膜表面的動態膜並不會造成明顯的積垢現象。泥停留時間為10天的生物薄膜系統中，溶解性微生物產物多為親水性物質，且大部分被薄膜截留於反應槽中。此親水性物質多為多醣類，並為主要的積垢物。 為了減緩薄膜積垢的發生，本研究利用二氧化鈦合成複合薄膜，用以降低薄膜積垢的速率。研究以低溫中性共沉澱凝膠法方式合成奈米級二氧化鈦，並利用含浸法合成二氧化鈦複合薄膜。此外，並利用文獻中所採用的酸性二氧化鈦溶液來合成二氧化鈦複合薄膜，探討並比較以此方法所合成之複合薄膜的抗積垢能力。結果顯示，不管以何種二氧化鈦合成之複合薄膜皆可降低薄膜積垢速率，主要原因為增加薄膜表面的親水性。適當的二氧化鈦含量對複合薄膜的抗積垢能力有很大的影響，雖然較多的二氧化鈦可以增加薄膜的親水性，但過多的二氧化鈦卻會導致薄膜孔洞的堵塞，增加過濾的阻抗。由超音波清洗薄膜的結果顯示，此複合薄膜的二氧化鈦可牢固的附著於薄膜表面，不會因清洗而大量流失二氧化鈦。

Membrane bioreactor (MBR) processes, in which membrane filtration is combined with biological degradation for biomass separation, have attracted great attention recently because of their advantages over conventional activated sludge processes, including a smaller footprint, less sludge production and superior effluent quality. However, their wide applications have been hindered by their excessive operation cost due to membrane fouling. Membrane fouling reduces water production and membrane lifespan, thereby, increasing operation and maintenance costs. Many studies have been devoted to exam the mechanisms of fouling and fouling mitigation in MBRs. Despite such intensive efforts, no conclusion on the cause of MBR fouling has been agreed upon, which may be due to the complication of mixed liquor and influent, module design and operation conditions implemented in various studies. Membrane fouling is the interaction between sludge and membrane, which results in adsorption or deposition of components in sludge on membrane surface. As a result, characteristics of sludge and membrane and hydrodynamic conditions are vital factors affecting membrane fouling in MBRs. In this study, the impact of sludge characteristics on membrane fouling in a submerged MBR was investigated. Bulking sludge due to excessive growth of filamentous bacteria was changed to normal sludge by use of an aerobic selector. Excellent effluent quality was achieved in the MBR regardless of the quality of the sludge of the bioreactor. However, serious fouling was observed when bulking sludge occurred and filamentous bacteria were found dominant in the reactor despite the larger particle size distribution of the sludge. Filamentous bacteria were found to produce more SMP including soluble polysaccharides and soluble proteins in the mixed liquor, which contributed to severe fouling in bulking sludge, in particular, the release of high concentration of soluble polysaccharides. Bound extracellular bound substances (EPS) which was found similar in the normal sludge and bulking sludge was not the cause for membrane fouling in this case. Sludge retention time (SRT) has been found to alter sludge characteristics and therefore, have significant impact on membrane fouling. Shorter SRT (10 days) resulted in higher fouling propensity most likely due to higher SMP in the mixed liquor. Despite the similar amount of SMP in mixed liquor, fouling rate at SRT 30 days was higher than SRT 60 days. This may be due to the higher content of bound EPS in sludge flocs at SRT 30 days, resulting in higher specific cake resistance. SMP was found to have larger molecular weight at shorter SRT. Dynamic membranes which can reject small components such as solutes in mixed liquor were formed on the membrane in the beginning of the filtration. The dynamic membrane would not cause apparent membrane fouling but improved membrane rejection instead. Hydrophilic fraction which was accumulated in the mixed liquor was the dominant species in SMP. Hydrophilic carbohydrates were most likely the major foulants at SRT 10 days . To reduce membrane fouling, titanium dioxide (TiO2) composite membranes were prepared. A chemical copreciptization-peptization method was used to produce TiO2 nanoparticles in neutral sol. The composite membranes were prepared by dip-coating the membrane in the neutral TiO2 sol and also in acidic TiO2 suspension. Filtration tests showed that membrane fouling was reduced in both cases possibly due to the increase in hydrophilicity of the membrane. Results showed that optimal amount of coating is important in fouling mitigation. Too much TiO2 on membrane surface deteriorated membrane filtration due to the blocking of membrane pores. The ultrasonic washing test showed that most TiO2 particles were firmly coated on the surface of the TiO2 composite membrane.