固定生物膜脫硝反應與電化學離子選擇性分離於提升含氮廢水處理效能之研析

Abstract

隨著人口增加以及產業活動蓬勃發展，過多的氮營養鹽排放到自然水體將造成水質優養化，並可能進一步造成水質惡化與生態危害。而隨著各國對於污水廠含氮物質放流水的排放標準逐漸加嚴，現有污水廠之生物處理技術則需要大幅地增加在曝氣的電力供應與額外添加額外碳源等操作成本，以達到含氮物質的排放標準。因此如何提升現有污水處理廠對於含氮汙染物質的去除效能，成為現今重要的議題。本研究提出以固定生物膜技術以及電化學離子分離技術來提升對於含氮物質的去除與分離效率。 本研究以固定生物膜技術做為同步硝化脫硝(Simultaneous nitrification and denitrification, SND)之除氮程序，此程序有助於減少傳統生物除氮程序所需要的反應體積，並能減少外加碳源的使用需求。本研究以三醋酸纖維將污水廠之活性污泥進行包埋，建立固定生物板反應器，並透過連續曝氣與間歇曝氣(一小時曝氣/一小時不曝氣)兩種操作模式，探討溶氧傳輸對於同步硝化脫硝效率之影響。由實驗結果顯示兩種曝氣模式具備相似的銨氮去除率約為76%，而間歇曝氣的SND效率約71%，與連續曝氣模式相比，能增加固定生物反應槽18%的SND效率。而分析反應槽內部的反應，可以觀察到在接近反應槽進流端主要以有機物的降解反應為主，而接近出流端則是以硝化反應與脫硝反應為主。透過16S rDNA做微生物定序，可以觀察到硝化菌與脫硝菌共同存在於固定生物板的證據，而各固定生物板上的微生物菌種會隨著反應槽內部的溶氧、有機物、含氮物質等受質濃度的變化而改變。 另一方面，本研究以薄膜電容去離子(Membrane capacitive deionization, MCDI)與電池去離子(Battery deionization, BDI)技術發展出分別對於硝酸鹽氮與銨氮具備離子選擇性的電化學離子分離技術。針對硝酸鹽氮的選擇性去除，本研究首先比較以電容去離子系統(CDI系統，一對多孔碳電極)與MCDI系統(一對披覆商用均相離子交換膜之多孔碳電極)對於硝酸鹽離子與氯離子的電吸附表現，以探討離子交換膜在離子電吸附過程中對離子選擇性所造成的影響。在1.2 V的操作電壓下，可以觀察到在硝酸根離子與氯離子的競爭電吸附過程主要可以分為充電初期(動態)與充電後期(接近平衡)兩階段。在充電初期，水合半徑越小的氯離子會越快吸附於孔洞材料上，在充電後期，具備較小水合比且與氮電極有較高作用力的硝酸根離子則逐漸競爭氯離子的吸附位點。當以商用均相膜披覆於碳電極時，透過減少同離子效應(Co-ion effect)可以大幅增加電極去離子容量，然而離子在薄膜的移動速度則主導電吸附選擇性，具備相同價數與擴散速率的硝酸鹽離子與氯離子於MCDI系統中表現相似的電吸附選擇性。為了提升MCDI的電吸附選擇性，本研究利用硝酸根離子具備較低水合能(Hydration energy)的特性，透過增加陰離子交換膜胺基疏水性的方式，增加硝酸根離子在薄膜中的傳輸速率，有效地將MCDI系統之電吸附選擇性係數提升至2，而電極的硝酸鹽去離子容量為0.22 mmol/g，為CDI系統硝酸鹽去離子容量的1.8倍。 針對銨氮的選擇性去除，本研究以陽離子嵌合型電池材料-鐵氰化鎳(Nickel hexacyanoferrate, NiHCF)作為銨離子之選擇性電極材料，並建立NiHCF//活性碳之非對稱式BDI系統。本研究首先以循環伏安法分析NiHCF電極對於不同陽離子的氧化還原特性，實驗結果顯示，NiHCF對於各陽離子的反應活性大小順序為：銨離子>鈉離子>鈣離子。而利用銨離子與鈉或鈣離子之混合溶液在0.8 V的操作電壓進行離子嵌合實驗，可以觀察到銨離子隨著充電時間增加，逐漸將嵌入在NiHCF晶格中的鈉離子或鈣離子取代掉，銨離子相對於鈉離子與鈣離子之嵌合選擇性分別為4.9和9.5，NiHCF對於銨離子的去離子容量為1.9–2 mmol/g。利用電池去離子系統將生活污水廠銨氮超標之放流水進行處理，結果顯示鐵氰化鎳電極對於陽離子之選擇性為：銨離子>鉀離子>鈉離子>鈣離子>鎂離子。 本研究成功地驗證固定生物膜與電化學選擇性離子分離技術的學理機制以及技術可行性，未來待技術成熟時，則有望將上述技術結合於現有污水處理系統以符合更嚴苛的含氮物質排放標準，同時降低污水處理流程之操作成本。而透過電化學選擇性濃縮分離之氮營養鹽則能進一步循環再利用，以促進達成污水資源化之永續發展目標。

With the increase in population and the rapid development of industrial activities, the discharge of excessive nitrogen nutrients into natural water bodies could lead to eutrophication and may further cause water quality deterioration and ecological impacts. As the discharge standards for nitrogen from wastewater treatment plants (WWTPs) become gradually stringent, developing more efficient nitrogen removal and separation technologies has become a critical issue. This study proposes to use immobilized biomass technology and electrochemical ion separation technology to improve nitrogen removal and separation efficiency. In this study, immobilized biomass technology was used to achieve simultaneous nitrification and denitrification (SND). In this study, waste sludge from municipal wastewater treatment plants was immobilized by cellulose triacetate as bioplates, and an immobilized bioplate reactor (IBPR) was successfully established for nitrogen removal tests. The SND efficiency of the IBPR was increased 18% under the intermittent aeration (IA) mode compared with that under the continuous aeration (CA) mode. During IA operation, the IBPR achieved 96% COD removal and 76% NH4+-N removal, with 71% SND. The results of microbial community analysis by high-throughput sequencing showed that nitrogen-related functional bacteria were more abundant in the bioplates than in the attached biofilms. The colocalization of nitrifiers and denitrifiers in the bioplates was first uncovered, and the microbial community of nitrogen-related functional bacteria clearly shifted with the substrate concentration gradients. On the other hand, electrochemical ion separation technology including membrane capacitive deionization (MCDI) and battery deionization (BDI) technologies were utilized to selectively separate NO3− and NH4+, respectively. To investigate the role of anion exchange membranes (AEMs) in selective NO3− electrosorption in MCDI, this study first compared the competitive electrosorption behaviors of NO3− and Cl− in a capacitive deionization system (CDI system, a pair of porous carbon electrodes) and in an MCDI system (a pair of porous carbon electrodes coated with a commercial homogeneous ion exchange membrane). Our results indicated that the electrosorption selectivity for NO3− ions in CDI was determined by the contributions of ion-carbon affinity and electrical force during the charging period. In comparison, the NO3− selectivity was found to be significantly decreased in MCDI with the commercial AEM due to the presence of an ion-exchange membrane that controlled ion kinetics. To improve the electrosorption selectivity of MCDI, this study fabricated heterogeneous AEMs (hAEMs) with different hydrophobicities on the amine groups. The results showed that hAEM with more hydrophobic amine functional groups exhibited higher NO3− selectivity. The NO3− selectivity of MCDI with selective hAEM was increased to 2, and the deionization capacity of NO3− was 0.22 mmol/g. To achieve selective NH4+ separation from wastewater, this study utilized a hexacyanoferrate (NiHCF) electrode as a selective NH4+ electrode. To assess the competitive intercalation between NH4+ and other common cations (Na+, Ca2+), a NiHCF//activated carbon (AC) hybrid BDI cell was established to treat mixed-salt solutions. The results of cyclic voltammetry (CV) analysis showed a higher current response of the NiHCF electrode toward NH4+ ions than toward Na+ and Ca2+ ions. In a single-salt solution with NH4+, the optimized operating voltage of the hybrid BDI cell was 0.8 V, with a higher salt adsorption capacity (51.2 mg/g) than those obtained at other voltages (0.1, 0.4, 1.2 V). In a multisalt solution containing NH4+, Na+, and Ca2+ ions, the selectivity coefficients of NH4+/Ca2+ and NH4+/Na+ were 9.5 and 4.9, respectively. The feasibility of selective NH4+ capture using the NiHCF electrode in a hybrid BDI cell was demonstrated by treating the effluent from a municipal wastewater treatment plant (WWTP). The intercalation preference of the NiHCF electrode with the WWTP effluent was NH4+>K+>Na+>Ca2+>Mg2+, and NH4+ showed the highest salt adsorption capacity among the cations during consecutive cycles. These resulted showed that NiHCF electrode has higher intercalation selectivity towards cations with smaller size and lower (de)hydration energy. The results of this thesis successfully verified the mechanism and technical feasibility of immobilized biomass and electrochemical selective ion separation technologies. It is expected that these technologies can be combined with the existing wastewater treatment system to meet more stringent nitrogen discharge standards, reducing the operating costs of nitrogen removal and recovery. The concentrated nitrogen nutrients separated by electrochemical processes can be further recycled and reused for sustainable development.