開發電驅動厭氧消化技術於農牧廢水高值化應用

Abstract

養豬廢水為臺灣主要畜牧廢水之一，其日排放量超過11萬立方公尺，由於養豬廢水含有高濃度有機物質與營養鹽類，若不當排放，恐會造成嚴重環境污染與生態危機。傳統上，臺灣對養豬廢水處理採三段式處理程序（固液分離、厭氧消化及活性污泥法），用來移除廢水中的有機物質，除此之外，還可以有效將有機物轉換為生物沼氣，殘留的固體也可經加工再利用製成土壤改良劑或是作為公共建設填充物（例如污泥餅）。在某些情況下，為了提升生物沼氣產量，可以採取共消化的方式，將不同的有機質（例如稻殼或稻桿等）加入同一消化槽內提升生物沼氣產量；然而，厭氧消化過程中的酸化現象與處理時間過長等問題，卻降低三段式廢水處理成效。有鑒於此，本研究開發新穎電驅動厭氧消化技術，利用微弱電流來刺激微生物活性，提升反應速率與生物沼氣產量。實驗數據方面利用動力學模型研析最大甲烷產量與微生物生產相關參數（例如停留時間、產率等），並以生命週期評估分析傳統厭氧消化與新興電驅動厭氧消化之環境衝擊（例如碳排放強度等）。本研究最後結合微生物菌相分析結果，提出電驅動厭氧消化中微生物群之反應，並提出放大工程設計於現存厭氧消化廠中示範之作法。 本研究以嘉義某養豬戶為對象，採集養豬廢水相關數據後進行試驗。研究結果發現，營養基質與污泥接種物的添加對於整體實驗有非常大的影響力，不但使產氣量超過110 mL/gVS，還使系統反應時間可持續超過60天。本研究證明電驅動厭氧消化技術具有較高之甲烷產量，相較於傳統厭氧消化最高可提升14.5%。實驗中，最高甲烷產值為純稻殼之電驅動厭氧消化，其產量達74.9 mL/gVS。關於其他氣體分析，氫氣最高產量為0.57 mL/gVS，二氧化碳最高產量約為13.4 mL/gVS，分別各約占了總產氣量0.3%與4.8%。另外，本研究針對電壓參數進行調整，發現電驅動厭氧系統在0.6 V下，不論是在總氣體、甲烷等，都比在1.2 V下來得高。共消化實驗結果顯示，養豬廢水與稻殼的揮發性固體比例（PW：RS）會影響其甲烷產量，並且在比例為3:1時會有最佳之共消化影響效益（共消化系數為45%，且該電驅動厭氧消化產氣為68.6 mL/gVS）。動力學分析上的結果則顯示，Modified Gompertz模型可以高度吻合電驅動厭氧消化系統（R2：0.965-0.997），不只如此，模擬結果也指出電驅動厭氧消化可提高生物甲烷的生產速率，最高為5.23 mL/gVS/day。另一方面，生命週期評估結果顯示在各項環境衝擊指數，在研究室規模的分析中，電驅動厭氧消化技術可降環境衝擊低約2.6-14.5%；同時，在放大設計模擬下，發現豬糞產生對於生態品質衝擊最為嚴重（約占88.1%），而在其他環境衝擊指數上，則是以處理廠為主要來源。菌相分析結果分別顯示，Clostridium sensu stricto 1、Turicibacter及Terrisporobacter等為主要優勢菌種，負責進行水解與產酸反應；而甲烷古細菌當中則以Methanosaeta比例最高，該菌主要以乙酸作為主要碳源使用，是一種常在厭氧消化中常見的古細菌。

In Taiwan, the discharge of piggery wastewater (PW) is higher than 110,000 m3. With containing high concentration COD, nitrogen and phosphorus, untreated PW can pose serious environmental and ecological crisis. One conventional way to deal with this issue is three-stage wastewater treatment (including solid-liquid separation, anaerobic digestion (AD) and activated sludge process), where organic waste could be transferred to green biofuel or biofertilizer. In some cases, to increase the biogas production, the different organic matters (such as rice husks and straws) will be introduced in the same digester to enhance the performance of AD. However, several problems of process, such as acidification and long retention time, inhibit the performance and efficiency of biogas production. In view of these issues, this study will develop a novel technique combined with AD and electrochemical system, as known as electricity-assisted anaerobic digestion (EAAD). The mechanism of EAAD is introducing the low current to stimulate and active the microorganisms to significantly improve the productivity of bio-products. The performances were evaluated through the kinetic model to determine the biological parameters (such as lag time and methane production rate). Additionally, life cycle assessment (LCA) was used to quantify the environmental impacts (e.g., carbon footprint) in different scenarios. Future research directions include elucidating the microbial reaction pathways in EAAD and scaling up the engineering of EAAD for application in wastewater treatment plants.In this study, experimental PW was produced in Chiayi, Taiwan. In Preliminary experiment, the adding of medium and sludge has significant effect on biogas production. The production could be more than 110.00 mL/gVS, and the system could be sustained for more than 60 days. The result showed that EAAD had 14.5% higher methane level than the conventional AD, and the best methane yield was 74.9 mL/gVS in the case of EAAD (0:1). The highest hydrogen yield and carbon dioxide were 0.57 mL/gVS and 13.4 mL/gVS, which was account for about 0.3% and 4.8%, respectively. Additionally, the results showed EAAD had better performance at 0.6 V than at 1.2 V. For the effect of co-digestion, it demonstrated that the PW:RH ratio would affect the performance of methane production, and when the ratio was 3:1, it had the best co-digestion effect (45%). The kinetic study showed that the Modified Gompertz model had significant fitness (R2: 0.965-0.997), compared to the first-order kinetic model (R2: 0.894-0.932). Moreover, EAAD was also indicated that could raise the methane productivity, which highest value was 5.23 mL/gVS in the case of EAAD (0:1). On the other hands, LCA was conducted to evaluate the environmental impacts. The results in a lab scale showed the EAAD could reduce the environmental impact by 2.6-14.5%, and demonstrated the optimization of voltage in EAAD was at 0.6 V. Furthermore, the results of the large scale showed the process of manure generation was the main contributor to ecosystem equality (88.1%), and the process of treatment plant was dominant contributor in other damage categories. Dominate bacteria in AD and EAAD were Clostridium sensu stricto 1, Turicibacter, and Terrisporobacter, which were contributed the hydrolysis and acid-forming. Methanosaeta had the highest proportion in kingdom of archaea, and it was known as an acetoclastic methanogen common found in the environment of AD.