陰極生物膜於無膜式微生物電化學污水處理系統之應用發展

Development of cathode biofilms in membrane-less bioelectrochemical wastewater treatment systems

張朝欽(Chao-Chin Chang)

指導教授 ： 于昌平

國立臺灣大學/工學院/環境工程學研究所/博士(2021年)

https://doi.org/10.6342/NTU202101162

摘要

近年來，微生物燃料電池(Microbial Fuel Cell, MFC)被認為是一種新興的生物電化學技術，可以透過陽極/陰極(缺氧/好氧)槽行氧化還原作用，將污水中的營養鹽污染轉化為電能，達到同時淨化水質與產能的效益。其中陰極性能對於整個系統的功率輸出至關重要。除了將各種化學催化劑摻入陰極來提高性能外，微生物被觀察到可以促進陰極氧氣的還原、提高陰極輸出效能，微生物作為一種催化劑相較於化學催化劑更加環保。但是，仍需要探索生物催化陰極的長期活性和穩定性，並進一步證明其驅動真實電子元件的應用可行性。本研究第一部份中，我們開發了低成本的三槽無膜式MFC。著重比較生物催化陰極(生物陰極)和酞菁鐵(Iron Phthalocyanine, FePc)催化陰極(化學陰極)系統的長期性能。透過評估廢水的出流水質，系統的發電量進行效能評估，並結合電化學交流阻抗分析(Electrochemical Impedance Spectroscopy, EIS)以及16S rRNA基因高通量測序進行生物膜的電活性分析及微生物群落分析。此外，設計了一套電能管理系統(Power Management System, PMS)對MFC產生的電能進行能量收集。結果顯示在連續運行過程中，兩種系統(化學與生物陰極)均顯示出較高的化學需氧量去除率和氨氮去除率，但生物陰極MFC的總氮去除率明顯高於化學陰極MFC。即使在生物膜生長的情況下，化學陰極在整個操作期間仍顯示恆定的陰極電勢。與標準氫電極相比，生物陰極則顯示出波動但逐步增加的陰極電勢，能提供超過500 mV的電壓，這可能是由於生物陰極生物膜於電極表面的逐漸富集所導致。運行6個月後，生物陰極MFC的最大功率密度可以達到化學陰極MFC的5倍。 EIS分析的結果顯示，生物陰極比裸碳氈陰極(未生長微生物與塗佈鈦菁鐵)具有更高的歐姆電阻，但是微生物生物膜會大大降低陰極的極化阻抗。微生物群落分析結果顯示，Nitrospira是所有陰極生物膜中最主要的屬，但生物陰極中Zooglea的含量遠低於化學陰極和開路下生物陰極的樣本。連接PMS時，生物陰極和化學陰極MFC都成功地為電容器充電以驅動溫度傳感器。但是，生物陰極MFC電壓在充電91小時後顯著下降至低於100 mV，但在斷開PMS時逐漸恢復。
在本研究的第二部分中，我們沿續第一部份使用的模組(三槽無膜式MFC)進一步探索了生物陰極的串聯放大可行性。這部分評估了生物陰極MFCs在長期串聯放大下的電化學和污水處理效能。結果顯示，基於生物陰極的MFCs能不受串聯放大的影響，提供穩定的廢水處理性能。本研究對生物陰極的分析顯示，在生物陰極中也會發生電壓反轉，陰極電位大幅下降。我們的研究結果進一步證實，操作大約一個月後，生物陰極的電壓可以自動恢復，相較於生物陽極的回復期長得多。對於從電壓反轉恢復的生物陰極樣本進行電化學交流阻抗分析，結果顯示由於電容效應，極化阻抗顯著增加。串聯放大下的生物陰極系統亦成功為電源管理系統中的電容進行充電，進而驅動給小型傳感器。生物陰極的16S rRNA基因高通量測序的結果顯示Thiothrix (40.9％)被大量發現存在於具有電位回復功能的生物陰極樣本中。總體而言，這些結果表明，生物陰極的微生物群落可能轉移成具有電容特性的生物膜組成，從而克服串聯連接過程中的電壓反轉衝擊。
在本研究的第三部分中，我們進一步探索了陰極生物膜於不同模組的並聯放大可行性。這部分使用無膜式空氣陰極MFCs作為模組，探討其在並聯放大下的電化學和污水處理效能，並仔細探討了輸出功率放大損失的成因。與第一種模組(三槽無膜式MFC)不同，陰極生物膜於空氣陰極MFCs中時，雖可能促進污水去除效果，但反而對輸出功率帶來負面影響。陰極輸出電位、極化曲線和功率密度結果顯示，生物膜對陰極電位產生負面影響，導致放大時輸出功率損失增加。循環伏安法(Cyclic Voltammetry, CV)和EIS結果顯示陰極表面因生物膜附著導致起始電位下降、歐姆阻抗大幅增加，極化電阻些微降低。微生物群落分析結果顯示變形菌門與擬桿菌門為主要群落，其中與氮循環(Pseudomonas、Thiobacillus、Flavobacterium)、硫循環(Chlorobium)與氯循環(Dechlorobacter)相關的菌株為主要優勢菌屬。
本研究第一部份探索了生物陰極MFC在三槽無膜式MFC中長期運行的種種特性，證明了其廢水處理和能量收集的潛力，並揭露穩定性為生物陰極系統於長期操作時的關鍵因子；第二部份延續相同的模組證實了生物陰極MFC於串聯電壓反轉下的電能回復與生物電容特性，顯示本系統於串聯放大的應用潛力；第三部份探究了陰極生物膜於無膜式空氣陰極模組中對於並聯放大時的功率損失、電化學與微生物群落特性，替該系統於並聯放大的生物膜控制策略提供進一步的解析。未來有必要進一步擴大基於陰極生物膜的MFC放大化與控制策略的研究。

關鍵字

微生物燃料電池 ； 生物陰極 ； 電壓反轉 ； 電化學分析 ； 16S rRNA高通量測序

並列摘要

The microbial fuel cell (MFC) as an emerging bioelectrochemical wastewater treatment technology, can convert nutrient pollution to electrical energy through oxidation/reduction reaction (i.e. oxidation on the Anode, reduction on the Cathode). The cathode performance is critical for the power output. Apart from various chemical catalysts being incorporated onto the cathode, microorganisms have been observed to facilitate cathodic oxygen reduction, which can improve cathode performances in MFCs more eco-friendly and sustainable. However, the long-term activities and stabilities of biofilms on the cathode still need to be investigated, and further proving their applicability in a large scale to drive a practical device should be considered. Therefore, this dissertation aims to investigate the development of cathode biofilms in low-cost membrane-less bioelectrochemical wastewater treatment systems, including three main parts.
First of all, characteristics of cathodic biofilms under various scenarios are comprehensively investigated in three-chamber membrane-less MFCs by continuous treatment, electrochemical analysis and microbial community analysis. The long-term performance of biocathode (i.e. electrode + biofilm) and bio/iron(II) phthalocyanine (FePc)-cathode (i.e. electrode + FePc + biofilm) MFCs was evaluated through effluent water quality, electricity production, electrochemical impedance spectroscopy (EIS) analysis, and 16S rRNA gene Illumina sequencing. During the continuous operation, both systems demonstrated high chemical oxygen demand (COD) and ammonium removal, but biocathode MFCs could achieve significantly better total nitrogen removal (RTN) than bio/FePc-cathode MFCs. The bio/FePc-cathode showed constant cathode potential during the entire operation period, but the biocathode showed varied but step-wise increasing cathode potential that achieved a level higher than 500 mV versus the standard hydrogen electrode, likely due to a gradual enrichment of biocathode biofilm. EIS analysis revealed that biocathode had higher ohmic resistance than bare carbon felt cathode but the microbial biofilm could significantly impair the polarization resistance of a cathode material. Microbial community analysis showed the presence of nitrifying and denitrifying bacteria in the biocathode biofilm. When connecting power management system (PMS), both biocathode and bio/FePc-cathode MFCs successfully charged a capacitor, but the biocathode MFC voltage significantly dropped to less than 100 mV after a 91-h charging, but gradually recovered to 500 mV after disconnecting PMS. This study has demonstrated the potential application of oxygen reduction biocathode MFCs for continuous wastewater treatment that could harvest energy for a long period of time.
Subsequently, in the second part, the scale-up evaluation of biocathode systems in a serial connection are carried out in the same module (i.e. three-chamber membrane-less MFCs). The electrochemical and treatment performance of long-term serial connection of oxygen reducing biocathode-based MFCs was evaluated. Results showed that biocathode-based MFCs could reach stable wastewater treatment performance without influenced by serial connection. Voltage reversal shock was previously observed in bioanodes during serial connection, but our analysis of cathode potential demonstrated that voltage reversal would also occur in biocathodes. Furthermore, our results revealed that the voltage reversal of biocathodes could be self-recoverable after ca. one month of operation. Electrochemical impedance spectroscopy analysis of the biocathode recovered from voltage reversal revealed the substantial increase of polarization resistance due to the capacitance effect. The 16S rRNA gene high throughput sequencing of biocathode samples revealed the presence of abundant nitrifying and denitrifying bacteria and moreover, the unique microbial composition in the biocathode recovered from voltage reversal showed highly enriched Thiothrix (40.9%). Electrical energy from the serially connected biocathode-based MFCs was successfully harvested using the PMS containing the DC-DC converter and capacitor. Overall, these results suggested microbial community of biocathodes could shift to gain biofilm capacitance to overcome the voltage reversal shock during serial connection and warranted the need for further research on the scaled-up of biocathode-based MFCs.
With regard to the third part, the scale-up evaluation of biocathode systems in parallel connection is carried out in different module. The scale-up power loss and electrochemical performance of long-term parallel connection in membrane-less air-cathode MFCs was evaluated. Different from systems connected in three-chamber membrane-less MFCs, when cathodic biofilm was grown in the air-cathode MFC, it not only posed positive impact but also negative impact on the cathodic performance. Electrical performance, power density curves and EIS indicated cathode biofilms may pose negative impact on the cathode potential in closed circuit, contributing to the increasing ohmic resistance but decreasing polarization resistance, reducing the onset potential, responsible for the scaling up power loss. The results of microbial community analysis demonstrated Proteobacteria and Bacteroides were the main phyla, and the populations related to the nitrogen cycle (Pseudomonas, Thiobacillus, Flavobacterium), the sulfur cycle (Chlorobium) and the chlorine cycle (Dechlorobacter) were the main dominant genera.

並列關鍵字

microbial fuel cells ； biocathode ； voltage reversal ； electrochemical analysis ； 16S rRNA gene Illumina sequencing

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