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Ảnh hưởng của bọt nước nano lên quá trình sản xuất khí metan từ lignin

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Nghiên cứu này nhằm mục đích điều tra ảnh hưởng của nước nanobubble (NBW) đối với việc phân hủy lignin và tác động của nó lên quá trình sản xuất khí metan từ lignin. Các thí nghiệm được thực hiện trong ba giai đoạn, bao gồm việc xác định các loại nanobubble, so sánh giữa quá trình tiêu hủy đơn và đồng tiêu hủy, và tỉ lệ của lignin và axit axetic mà có thể đạt được sản lượng mêtan và sự tiêu hủy lignin cao nhất. Mời các bạn cùng tham khảo!

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  1. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 EFFECT OF NANO-BUBBLE WATER ON ANAEROBIC METHANE FERMENTATION OF LIGNIN Ho Thi Hanga,b*, Zhang Xiaojingb, Lei Zhongfangb, Zhang Zhenyab a Falculty of Environment and Natural Resource, Dalat University, Dalat City, Vietnam b School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan * Corresponding author: Email: hanght@dlu.edu.vn Abstract Vietnam is an agricultural country that generates a huge amount of crop residues (lignocellulosic biomass). However, the improper disposal of crop residues in Vietnam brings about many environmental problems. Anaerobic digestion (AD) is regarded as a suitable technology for the conversion of lignocellulosic biomass into methane that can be used for heating and electricity generation. However, lignin, the major barrier to the bioconversion of lignocellulosic materials, can resist the hydrolysis of lignocellulose, thus limiting its AD process. In recent years, nanobubble technology is paid much attention in environmental field. Previous researches showed that nanobubbles have some unique properties such as stable existence in water for a long time, and large generation of highly reactive free radicals that can be applied for fermentation to achieve enhanced organic degradation. Therefore, this study aimed to investigate the effect of nanobubble water (NBW) on lignin reduction and its methane production during AD. The experiments were carried out in three phases including the determination of nanobubble categories, comparison between mono-digestion and co-digestion, and optimization of lignin to acetic acid ratio that can achieve the highest methane production and lignin reduction. In all the experiments, the fermentation reactors were mixed with their initial pHs adjusted to 7. All the reactors (80 ml of working volume) were sealed with silica gel stoppers and their headspace air was flushed with N2 to create anaerobic conditions. Finally, they were placed in an incubator maintained at 35 ± 2oC. The results showed that N2-NBW was the suitable NBW among the tested ones, yielding enhancing methane production by 22% compared to the control. Co-digestion of lignin and acetic acid showed higher methane production potential in comparison to the mono-digestion of lignin. The mixture with 95% of acetic acid to 5% of lignin could achieve the highest methane production and lignin reduction. Keywords: Anaerobic digestion; Methane production; Lignin reduction; Nanobubble water. 105
  2. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 ẢNH HƯỞNG CỦA BỌT NƯỚC NANO LÊN QUÁ TRÌNH SẢN XUẤT KHÍ METAN TỪ LIGNIN Ho Thi Hanga,b*, Zhang Xiaojingb, Lei Zhongfangb, Zhang Zhenyab a Khoa Môi trường và Tài nguyên, Trường Đại học Đà Lạt, Lâm Đồng, Việt Nam b Khoa Khoa học Môi trường và Sự sống, Trường Đại học Tsukuba, Ibaraki, Nhật Bản * Tác giả liên hệ: Email: hanght@dlu.edu.vn Tóm tắt Việt Nam là một nước thải ra một lượng lớn phế phẩm nông nghiệp (được gọi là sinh khối lignocellulosic). Tuy nhiên, phế phẩm nông nghiệp ở Việt Nam không được xử lý hợp lý gây ảnh hưởng tiêu cực tới môi trường. Tiêu hủy kỵ khí (AD) được coi là một công nghệ thích hợp cho việc chuyển đổi sinh khối lignocellulosic thành khí metan. Khí metan thường được dùng để sưởi ấm và phát điện. Tuy nhiên, lignin, một trong những thành phần chính của sinh khối lignocellulosic, là rào cản lớn đối với sự biến đổi sinh học của sinh khối lignocellulosic. Nó có thể chống lại sự thủy phân của lignocellulose, do đó hạn chế quá trình tiêu hủy kị khí sinh khối lignocellulosic. Trong những năm gần đây, công nghệ bọt khí nano được quan tâm nhiều trong lĩnh vực môi trường. Các nghiên cứu trước đây cho thấy rằng các bọt khí nano có một số đặc tính độc đáo như nó có thể tồn tại ổn định trong nước trong một thời gian dài, và có thể sản xuất một lượng lớn các gốc tự do có khả năng phản ứng cao làm tăng cường quá trình phân hủy các chất hữu cơ. Do đó, nghiên cứu này nhằm mục đích điều tra ảnh hưởng của nước nanobubble (NBW) đối với việc phân hủy lignin và tác động của nó lên quá trình sản xuất khí metan từ lignin. Các thí nghiệm được thực hiện trong ba giai đoạn, bao gồm việc xác định các loại nanobubble, so sánh giữa quá trình tiêu hủy đơn và đồng tiêu hủy, và tỉ lệ của lignin và axit axetic mà có thể đạt được sản lượng mêtan và sự tiêu hủy lignin cao nhất. Tất cả các mẫu thí nghiệm được điều chỉnh pH=7. Các mẫu được chứa trong bình kín với thể tích mỗi bình là 80ml. Tất cả các bình này được duy trì ở nhiệt độ 35 ± 2 oC. Kết quả cho thấy rằng N2-NBW là loại NBW thích hợp, cho năng suất sản xuất khí metan tăng 22%. Quá trình đồng tiêu hủy lignin và axit axetic cho thấy tiềm năng sản xuất metan cao hơn so với tiêu hủy đơn lignin. Tỉ lệ 95% axit axetic : 5% lignin đạt được sản lượng mêtan và loại bỏ lignin cao nhất. Từ khóa: Tiêu hủy kị khí; Khí metan; Lignin; Bọt nước nano. 106
  3. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 1. INTRODUCTION 1.1. Environmental pollution from agricultural wastes disposal in Vietnam Vietnam is a country that produces a considerable amount of agricultural residues, i.e. lignocellulosic biomass. According to the National State of Environment 2011-2015 of Vietnam, Vietnam disposes over 76 million tons of crop residues annually with the main components being rice straw and rice stubble. Some crop straws are used for cooking in the countryside areas, while other agricultural wastes are used for growing mushrooms. However, recently, crop straws are not the main source for cooking in rural areas owing to other fuels being available such as electricity, gas, and coal. Consequently, most crop residues are burned on the field or disposed directly into the roads or lakes. These activities have posed environmental problems such as greenhouse gas (GHG) emission, air and water pollution. Therefore, it is requisite to use environmentally friendly technologies to treat lignocellulosic wastes. 1.2. Anaerobic digestion technology for lignocellulosic wastes treatment Currently, studies about lignocellulosic biomass are paid much attention because it is a cheap, abundant and renewable material with relatively high yield (Anwar, Gulfraz, & Irshad, 2014; Asgher, Ahmad, & Iqbal, 2013; Wu & He, 2013). Numerous studies have shown that lignocellulosic biomass can be transformed into many available forms of energy, such as heat, electricity, steam, methane, hydrogen, and other biofuels (Song et al., 2014). Among them, methane production via anaerobic process is growing worldwide because it can not only produce bioenergy but also treat organic waste for environmental protection (Achinas, Achinas, & Euverink, 2017; Ge, Xu, & Li, 2016; Song et al., 2014). 1.2.1. Advantages of anaerobic digestion of lignocellulosic biomass Anaerobic digestion (AD) is a process that organic materials are converted into biogas by microorganisms under free oxygen conditions. The major components of biogas are methane (CH4) and carbon dioxide (CO2) with other trace gases like hydrogen sulfide (H2S), ammonia (NH3), and water vapor (Ge et al., 2016). AD is a suitable technology for conversion of lignocellulosic biomass into biogas. Firstly, it can bring ec6onomic benefits. This technology, with energy output/input ratio (28.8 MJ/MJ), can produce energy more efficient than other energy generation technologies such as thermal- chemical technology. The biogas produced from AD also doesn’t need oil or natural gas supplies (Dieter Deublein & Steinhauser, 2008; Zheng, Zhao, Xu, & Li, 2014). Secondly, it brings environmental benefits. Organic wastes can cause environmental pollution and GHG emission if it is discarded directly into the environment. Therefore, methane production from lignocellulosic wastes can keep the environment clean from organic wastes. Moreover, biogas burning has less GHG emissions than other fuels like gasoline, diesel fuel and natural gas (Chandra, Takeuchi, & Hasegawa, 2012). Also, AD residue can be used as organic fertilizer for agriculture (Chandra et al., 2012). 107
  4. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 1.2.2. Disadvantages of anaerobic digestion of lignocellulosic biomass Although lignocellulosic wastes are attractive materials for methane generation, the AD efficiency from them is low due to the various refractory components contained(Taherzadeh & Karimi, 2008). Lignocellulosic wastes are composed of three main components that are cellulose (40–50%), hemicellulose (10–25%), and lignin (25– 40%) (Collinson & Thielemans, 2010). Cellulose and hemicellulose are carbohydrate polymers that are fermentable for bioenergy generation. However, lignin is a complex aromatic polymer that is considered as a major barrier to degrade lignocellulosic biomass for methane production (Zheng et al., 2014). The methane yield of lignocellulosic biomass is usually lower than 60% of the theoretical value (Frigon, Mehta, & Guiot, 2012). In general, AD process involves four steps: hydrolysis, acidogenesis, and methanogenesis (Zheng et al., 2014). Hydrolysis is a step to hydrolyze complex organic materials into soluble organic materials (Yebo Li, Park, & Zhu, 2011). In AD of lignocellulosic biomass, hydrolysis is regarded as the rate-limiting step due to the presence of lignin (Koyama, Yamamoto, Ishikawa, Ban, & Toda, 2017; Taherzadeh & Karimi, 2008). Lignin has a very complex molecular structure. It is regarded as amorphous polymers that are mainly composed of three phenylpropane monomers including p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Z. Chen & Wan, 2017). Lignin covers cellulose and hemicellulose to make rigidity to plant cell wall and forms a hindrance against digestion of bacteria. Consequently, lignin restricts approach of bacteria to cellulose and hemicellulose (Eva Palmqvist & Hahn-Hagerdal, 2000). These characteristics of lignin render lignocellulosic biomass extremely recalcitrant for anaerobic digestion. Owing to the persistent structure of lignin, studies about lignin degradation methods are necessary. Until recently, lignin degradation has been studied by some researches using enzymatic degradation (Q. Chen, Marshall, Geib, Tien, & Richard, 2012), thermal-chemical method (Guo et al., 2014), hydrothermal conversion (Barbier et al., 2012), electrochemical oxidation (Tolba, Tian, Wen, Jiang, & Chen, 2010), biological degradation (Chang, Choi, Takamizawa, & Kikuchi, 2014), photocatalytic degradation (H. Li, Lei, Liu, Zhang, & Lu, 2015), etc. These methods can enhance lignin degradation. However, there are some disadvantages of these methods such as their difficulty in full-scale application (biological degradation), toxic waste disposal, high cost (electrochemical oxidation) (Tolba et al., 2010) or energy consumption and a complex process. 108
  5. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 1.3. Nanobubbles application Recently, nanobubbles is paid attention in environmental engineering. Nanobubbles are extremely small gas bubbles with a diameter of less than 200 nm (Agarwal, Ng, & Liu, 2011). They are too tiny to be seen by bare eyes or standard microscopes (Azevedo, Etchepare, Calgaroto, & Rubio, 2016). Nanobubbles can do not collapse at once and remain stable in liquid for months (Agarwal et al., 2011; P. Li, Takahashi, & Chiba, 2009). Especially, nanobubble water can produce reactive oxygen species (ROS, also called oxygen free radicals). OH radical is one of the specific ROS produced by NB water (Liu, Oshita, Kawabata, Makino, & Yoshimoto, 2016). This finding is important to lignin degradation because OH radicals show a higher standard redox potential (2.80 V) than other oxidants such as ozone and hydrogen peroxide (2.07 and 1.77 V, respectively), and they can react rapidly with various organic compounds (P. Li et al., 2009; von Gunten, 2003). Moreover, NBs can improve the activity of anaerobic microorganisms and can be applied for not only water treatment but also fermentation for human waste treatment (Agarwal et al., 2011). With these unique characteristics, recently, NBs has been widely applied in water treatment engineering such as floatation, aeration, disinfection and advanced oxidation processes (Temesgen, Bui, Han, Kim, & Park, 2017). However, up to the present, its application for enhanced AD of lignin has not been documented yet. 1.4. Research objectives The objective of this study was to investigate the effect of nanobubble water (NBW) on AD of lignin for methane production. However, lignin is a low biodegradable material. And some studies have demonstrated that co-digestion of easily degradable materials with hardly degradable materials can have positive results such as high methane production and stable digestion process (Rodriguez-Chiang, Llorca, & Dahl, 2016). Thus, acetic acid (HAc) was used along with lignin as carbon source for AD in this study. This study also investigated the effect of NBW on mono-digestion and co-digestion of lignin and HAc. Also, the optimal ratio of HAc/lignin was examined. Methane production potential and lignin reduction were the two main indicators to evaluate the effect of NBW on AD of lignin. Besides these, volatile fatty acids (VFAs) consumption, and total organic carbon (TOC) removal were also analyzed to evaluate the efficiency of AD process. 2. MATERIALS AND METHODS 2.1. Nanobubble water generation Nanobubbles water was fabricated by the Micro-Nano Bubbles generator (HACK UFB Co., Ltd, Ushiroyacho, Kofu-shi, Yamanashi, Japan). Deionized water and some specific gas were pumped into the generator and recycling for 30 min. The water produced after nanobubbles generation was referred to nanobubble water (NBW). 109
  6. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 2.2. Medium and inoculum Medium consisted of lignin and HAc. Lignin material (alkali) and HAc were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). The inoculum used in this study was the digested sludge sampled from the municipal wastewater treatment plant in Tsukuba, Japan. Fresh inoculum was stored in an incubator at a temperature of 35±2C for 7 days before starting the experiment, in order to eliminate the influence of residual organics in the inoculum. 2.3. Batch AD experiments The study was divided into three phases for the determination of the effect of NBW on methane production of lignin. Fig. 2-1 described all the three phases. In Phase 1, nitrogen nanobubble water (N2-NBW) and carbon dioxide nanobubble water (CO2- NBW) were added individually into the reactors to examine their appropriateness for AD of lignin. Fig. 2- 1 Three phases of this study Deionized water (DW) was added into the control reactor. The medium added in Phase 1 was 10% of lignin, and 90% of HAc (base on their total organic carbon, TOC). Three groups in this phase were named DW, CO2-NBW, N2-NBW. In Phase 2, the suitable NBW obtained in Phase 1 was further used to determine the effect of NBW on mono-digestion of lignin and co-digestion of lignin with HAc. Three groups in Phase 2 included R1 (mono-digestion of HAc), and R2 (co-digestion of 10% lignin with 90% HAc), and R3 (mono-digestion of lignin). In R1, R2, and R3 groups, NBW and DW were added individually into the reactors to compare the effect of NBW sample with the control 110
  7. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 sample in each group. Phase 3 analyzed the appropriate ratio of HAc to lignin that could enhance lignin reduction and methane production. In this phase, the effect of NBW on methane production from four different ratios of HAc to lignin was analyzed. The ratios consisted of NBW1 (97.5:2.5), NBW2 (95:5), NBW3 (90:10), NBW4 (80:20) (TOC basis). In all the phases, the bottles with a working volume of 80 mL were used. All the reactors in each phase received the same amount of medium and inoculum. The fermentation reactors were mixed and adjusted to pH 7 by using 1.0 M NaOH or HCl. All reaction bottles were sealed with silica gel stoppers and their headspace air was flushed with N2 to create anaerobic conditions. Finally, they were placed in an incubator maintained at 35±2C. All determinations were conducted in duplicate with average values being used. 2.4. Analytical methods and calculations 2.4.1. Total solids (TS) and volatile solids (VS) TS and VS were determined according to the standard methods (APHA, 2012). TS = (1) VS = (2) where m is the weight of dish, m is the weight of dish and initial sample, m is the constant weight of dish and sample after heating at 105 C, and m is the weight of dish and sample after burning at 600 C. 2.4.2. pH value pH value was measured with a METTLER TOLEDO pH meter. 2.4.3. TOC TOC was analyzed by using total carbon analyzer (TOC-VCNS with ASI-V autosampler, Shimadzu, Japan) after centrifuging and filtrating the samples through 0.45 µm membrane. TOC removal rate was calculated as the following equation: ( ) TOC removal (%) = x 100 (3) where Ci (mg/l) is the initial concentration of TOC and Cf (mg/l) is the final concentration of TOC. 111
  8. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 2.4.4. Biogas composition or content Biogas composition or content was analyzed by using gas chromatograph (GC- 8A, Shimadzu, Japan) equipped with a thermal conductivity detector (80C) and Porapak Q column (60C) employing N2 as a carrier gas. ( ) Biogas yield (ml Biogas/g TOCremoval ) = ×1000 (4) × () ( ) Methane yield (ml CH4 /g TOCremoval ) = × Biogas yield (5) Methane content (%) = x100 (6) 2.4.5. Lignin reduction and lignin reduction rate Concentration of lignin was analyzed by spectrophotometry at 280 nm (UV-1800, Shimadzu, Japan) after centrifuging and filtrating the samples through 0.45 µm membrane and calculated based on the standard curve (Fig. 2-2). Fig. 2- 2 Linear relationship between lignin concentration and Abs 280 of lignin solution Lignin reduction and lignin reduction rate were calculated according to the ( ) following equations: Lignin reduction (%) = × 100 (7) Lignin reduction rate (mg/L/d) = (8) ( ) where Ci (mg/l) is the initial concentration of lignin and C f (mg/l) is the final concentration of lignin. 112
  9. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 2.4.6 Calculation of lignin and HAc amount added to the reactors The amount of lignin and HAc added in three phases (Table 2-1) were calculated based on TOC (Fig. 2-3, Fig. 2-4). The total TOC added in the experiments was controlled at 2000mg/l. Fig. 2- 3 Linear relationship between HAc concentration and TOC Fig. 2- 4 Linear relationship between lignin concentration and TOC Table 2- 1 Concentrations of lignin and HAc added in the experiments TOC ratio 97.5:2.5 95:5 90:10 80:20 100%HAc 100%lignin HAc (mg/l) 3775 3679 3485 3099 3872 0 Lignin (mg/l) 95 190 378 754 0 3766 113
  10. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 2.4.7. VFAs VFAs were measured by using gas chromatograph (GC-8A) equipped with Unisole F200 30/60 column and flame ionization detector (FID). All samples were centrifuged and filtrated through 0.45 µm membrane. Afterward, they were added 3% phosphoric acid solution to acidify samples at a volume of 0.1mL phosphoric acid (3%) and 0.9 ml sample for VFA analysis. Gas chromatograph (GC-8A, Shimadzu) equipped with Unisole F-200 30/60 column and flame ionization detector (FID) were used for analysis of VFAs, including HAc (HAc), propionic acid (HPr), iso-butyric acid (iso- HBu), n-butyric acid (n-HBu), iso-valeric acid (iso-HVa) and n-valeric acid (n-HVa) (Huang et al., 2016). 1µL of the prepared sample was injected into GC-FID directly with a retention time of 12 min. Nitrogen was employed as the carrier gas. The temperatures of injection port and column were maintained at 180C and 150C, respectively. 3. RESULTS AND DISCUSSION 3.1. Effect of NBW on methane production from lignin 3.1.1 Methane production potential To investigate the effects of different kinds of NBW on AD of lignin, the experiment was carried out with CO2-NBW and N2-NBW addition. Fig. 3-1 plots the time course of the cumulative methane yield under different kinds of NBW addition. The highest methane yield was noted in the N2-NBW reactor (728 mL CH4/g-TOCremoval), followed by the CO2-NBW (703 mL CH4/g-TOCremoval) and DW (599 mL CH4/g- TOCremoval) reactors. The increase in methane production followed a descending order as N2-NBW (22% increase) > CO2-NBW (17% increase) > the control (DW group). This result indicated that the addition of NBW showed a positive effect on methane production, and N2-NBW addition achieved the highest methane yield. (Barakat, Monlau, Steyer, & Carrere, 2012) reported that methane potential from natural and synthetic lignin was small owing to their low biodegradability. Besides, during the time of most biogas production, around 64.15% to 70.86% of methane content was determined in all the gaseous samples. It demonstrated that the addition of NBW didn't influence the methane content. However, the maximum cumulative methane yield reached the highest on day 4 in the NBW added reactors, while the DW reactor reached its highest methane yield on day 6. It denoted that a faster methane production rate was attained in the NBW added reactors. This observation is important to AD application at large scale. 114
  11. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 Fig. 3- 1 Cumulative methane yield from AD reactors with different kinds of NBW addition in Phase 1 3.1.2 Control parameters TOC removal was used to measure organic carbon removal efficiency through the AD, which was 46%, 42%, 44% in the DW, CO2-NBW and N2-NBW reactors, respectively (Fig. 3-2). This result showed that the TOC removal efficiency in the NBW reactors was slightly lower than the control reactor. Fig. 3- 2 TOC removals in the different NBW reactors during Phase 1 During this experiment, lignin and HAc were the main organic components for biogas production. Hence, lignin reduction and VFA consumption rates were determined. Fig. 3-3 presents the lignin reduction in the AD process. Lignin reduction showed the 115
  12. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 same tendency as the cumulative methane yield. The highest lignin reduction occurred in the N2-NBW reactor (19.3%), followed by the CO2-NBW (17.4%) and the control (DW, 6.4%) reactors after AD for 12 days. It illustrated that NBW can enhance the lignin reduction. Probably, this is one of the reasons leading to the higher methane production in the NBW reactors. However, a low lignin reduction was noted, most probably due to that lignin is difficult to degrade under AD condition (Lin, Ge, & Li, 2014). Fig. 3- 3 Lignin reduction in different NBW reactors during Phase 1 VFAs consumption is shown in Fig. 3-4. The initial VFAs were only HAc detected in all the samples due to only HAc as VFAs was added in all the samples. Also, it can be observed that the initial VFAs in all the samples were similar because the same amount of HAc was added into all the samples. Moreover, almost all the HAc was converted to biogas during the AD process. This result manifested that NBW addition didn’t influence VFAs consumption capacity. Fig. 3- 4 VFAs consumption in the different NBW reactors during Phase 1 116
  13. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 3.2. Effect of NBW on mono-digestion of lignin and co-digestion of lignin with HAc 3.2.1 Methane production potential Cumulative methane yields from the three groups including R 1-DW and R1-NBW (mono-digestion of 100% HAc), R2-DW and R2-NBW (co-digestion of 10% lignin with 90% HAc), and R3-DW and R3-NBW (mono-digestion of 100% lignin) are presented in Fig. 3-5. The cumulative methane production from the mono-digestion of HAc (R 1 group) was 950 mL CH4/gTOCremoval in R1-DW and 985 mL CH4/gTOCremoval in R1-NBW, while the cumulative methane production was only 23 mL CH 4/gTOCremoval in R3-DW and 40 mL CH4/gTOCremoval in R3-NBW through the mono-digestion of lignin (R 3 group). Fig. 3- 5 Cumulative methane yields from the mono-digestion and co-digestion in Phase 2 This result indicated that HAc possesses much higher degradability than lignin, and lignin is an extremely hardly degradable material. The accumulative methane yield of co-digestion of lignin with HAc (R2 group) was 837 mL CH4/gTOCremoval in R2-DW and 1061 mL CH4/gTOCremoval in R2-NBW. This result showed that the methane production from co-digestion of lignin with HAc was much higher than the mono- digestion of lignin. This phenomenon implied that AD using lignin as a sole carbon source was ineffective, while co-digestion of lignin with HAc was successful with improved methane production. Rodriguez-Chiang et al. (2016) also revealed that methane yield from the co-digestion of acetate-rich with lignin-rich wastewater was much higher than the mono-digestion of lignin-rich wastewater. 117
  14. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 Regarding the effect of NBW in each group, a significant difference in the cumulative methane yield between NBW and DW reactors was noticed in the co- digestion of lignin with HAc (R2 group). In R2, the cumulative methane yield in the NBW reactor (1061 mL CH4/gTOCremoval) was 21% higher than that in the DW reactor (837 mL CH4/gTOCremoval). This result demonstrated that NBW addition has positive effect on co- digestion of lignin with HAc. In R1, HAc can be converted directly into methane by methanogenesis bacteria. Therefore, digestion from HAc can be used to indicate the effect of NBW on methanogenesis step. As it can be seen, the difference of cumulative methane yield between NBW reactor and DW reactor in R1 were negligible. This implies that NBW has little effect on methanogenesis. In R 3 (mono-digestion of lignin), methane yield attained in the DW reactor and NBW reactor were extremely low. According to the results obtained, it can be inferred that the addition of NBW showed better performance in the co-digestion of lignin with HAc compared to the mono-digestion of lignin. 3.2.2 Other parameters Fig. 3-6 presents the TOC removal rate in R1 (mono-digestion of HAc), R2 (co- digestion of 10% lignin with 90% HAc), R 3 (mono-digestion of lignin). Fig. 3- 6 Cumulative methane yields from the mono-digestion and co-digestion in Phase 2 It is noteworthy that the highest TOC removal rate was attained in the mono- digestion of HAc (70% in R1-DW, 61% in R1-NBW), followed by the co-digestion of lignin with HAc (55% in R2-DW, 51% in R2-NBW) and the mono-digestion of lignin (6% in R3-DW, 8% in R3-NBW). These results demonstrated that higher HAc concentration brought about higher TOC removal, indicating its higher biodegradability efficiency. This result was consistent with the difference in methane yield among these groups. Interestingly, TOC removal in R1-NBW (61%) was lower than R1-DW (71%). Similarly, 118
  15. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 TOC removal in R2-NBW (51%) was lower than R2-DW (55%). This phenomenon indicated that TOC removal in the NBW reactors was lower than the control reactors, while the methane production from the NBW reactors was higher than the control reactors. Probably, NBW addition can enhance hydrolysis process that can solubilize the organic compounds in inoculum into soluble organics. Table 3- 1 Major results in Phase 2 Mix ratio (HAc: Lignin reduction Lignin reduction rate Initial VFA Reactor Lignin) (%) (mg/l/d) (mgC/l) R1+DW 100:0 NA NA 628.34 R1+NB W 100:0 NA NA 615.73 R2+DW 90:10 16.33 ± 5.37 2.02 ± 0.7 557.66 R2+NB W 90:10 23.75 ± 8.03 3.45 ± 1.28 583.32 R3+DW 0:100 14.25 ± 2.64 32.52 ± 6.24 NA R3+NB W 0:100 9.35 ± 4.26 21.35 ± 10.33 NA Note: NA: not available Lignin reduction and lignin reduction rate during mono-digestion and co- digestion are presented in Table 3-1. R1 samples (mono-digestion of HAc) were used as the control to test lignin reduction. Higher lignin reduction was found in the co-digestion with 16% in R2-DW and 24% in R2-NBW compared to the mono-digestion of lignin with 14% in R3-DW and 9% in R3-NBW. This result showed that anaerobic co-digestion of lignin with HAc can improve lignin reduction. Although lignin reduction in the mono- digestion of lignin group (R3) was lower than the co-digestion of lignin with HAc group (R2), the lignin reduction rate in the mono-digestion of lignin (32.52 mg/l/d in R 3-DW and 21.35 mg/l/d in R3-NBW) was much higher than that in the co-digestion of lignin with HAc (2.02 mg/l/d in R2-DW and 3.45 mg/l/d in R2-NBW). This was due to the amount of lignin addition in the mono-digestion of lignin (100% lignin) was much higher than the co-digestion scenario (10% lignin). In R2, it also can be observed that lignin reduction in the NBW reactor (24%) was higher than that in the DW reactor (16%). It agreed with higher methane production in R2-NBW compared to R2-DW. Regarding VFA consumption, the initial VFA in R1 was higher than that in R2 because HAc concentration added to R1 (100%) was greater than that to R2 (90%) (Table 3-1). It was not detected in R3 because no HAc was added to this group. VFA at the end of the experiment could not be detected in all the samples, indicating that VFA was almost consumed in the AD process. 119
  16. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 3.3. Appropriate ratio of HAc to lignin in the co-digestion with NBW addition 3.3.1. Methane production potential The ultimate methane potential with NBW addition was examined in the reactors with different HAc to lignin ratios (Fig. 3-7). The ratios consisted of NBW1 (97.5/2.5), NBW2 (95/5), NBW3 (90/10), and NBW4 (80/20) based on TOC. It can be observed that the methane production from the NBW4 reactor (824 mL CH 4/gTOCremoval) was significantly lower than those from other reactors (1054, 1028, and 1061 mL CH4/gTOCremoval for NBW1, NBW2, NBW3, respectively). The total volume of methane (mL CH4) produced in all the reactors also showed the same tendency as methane production based on TOC removed (mL CH4/gTOCremoval). The total methane production from NBW4 (18.79 mL CH4) was lower than those from NBW1 (25.73 mL), NBW2 (24.68 mL), and NBW3 (24.96 mL). Fig. 3- 7 Methane production from different ratios of lignin to HAc in Phase 3 These results demonstrated that high lignin concentration could have negative effect on methane production. It was consistent with the report of Rodriguez-Chiang et al. (2016) that there was a negative correlation between the total lignin concentration and methane production of co-digestion of acetate-rich with lignin-rich wastewater. Y. Li et al. (2013) also found a similar negative correlation between lignin concentration and the methane potential of lignocellulosic and manure wastes. 3.3.2. Other parameters TOC removal efficiency in different reactors with different acetate to lignin ratios is shown in Fig. 3-8. TOC removal was 55%, 54%, 51%, and 46% in NBW1, NBW 2, NBW3, and NBW4, respectively. It can be observed that TOC removal decreased with 120
  17. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 the increase of lignin concentration. This result was consistent with a previous research that the lignin is an indicator of anaerobic biodegradability degree of substrates, and a high lignin content is a negative indicator for methane production (Candia-García, Delgadillo-Mirquez, & Hernandez, 2018). Fig. 3- 8 TOC removal in different reactors with different ratios of lignin to HAc in Phase 3 Table 3-2 displays the lignin reduction and lignin reduction rate in the different reactors with different acetate to lignin ratios. The highest lignin reduction was detected in NBW2 (43%), followed by NBW3 (24%), NBW4 (18%), and NBW1 (13%). The trend of lignin reduction was similar to that of TOC removal, apart from lignin reduction in the ratio of 97.5:2.5. It seems that high lignin concentration caused low lignin reduction. At a ratio of 97.5:2.5, lignin reduction was the lowest, which had the lowest concentration of lignin. The results indicated that ratio of 95:5 was the most favorable for lignin reduction. However, D.C . Peng and Jin (1993) revealed that lignin could be partly adsorbed by anaerobic bacteria. Therefore, future study is necessary to identify whether lignin reduction is caused by lignin degradation or lignin adsorption. Table 3-2 also presents the VFA concentration in the different reactors with different ratios of HAc to lignin. The initial VFA concentration in NBW1, NBW2, and NBW3 was similar due to no significant difference in HAc added, while the initial VFA in NBW4 was lower because of the lower amount of HAc added. VFA at the end of the experiment was not detected in all the samples. This result agreed with the lower methane production in NBW4 compared to other reactors. 121
  18. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 Table 3- 2 Major results in Phase 3 Reacto Mix ratio Lignin removal Lignin removal rate Initial VFA r (HAc:Lignin) (%) (mg/l/d) (mgC/l) NBW1 97.5:2.5 12.55 ± 13.43 0.45 ± 0.49 617.24 NBW2 95:5 42.69 ± 11.31 3.03 ± 0.78 630.10 NBW3 90:10 23.75 ± 8.03 3.45 ± 1.28 583.32 NBW4 80:20 18.20 ± 5.63 6.06 ± 1.90 474.87 4. CONCLUSIONS 4.1. Conclusions Anaerobic digestion is an attractive technology for lignocellulosic wastes treatment. However, lignin, one of the main component of lignocellulosic waste, hinders the hydrolysis process that causes a low methane production from lignocellulosic biomass. Nanobubble application is a promising method for lignin degradation. This study mainly investigated the effect of nanobubble water on anaerobic digestion of lignin. This study also explored the effect of NBW on mono-digestion of lignin and co-digestion of lignin with HAc. Besides, the optimal ratio of HAc to lignin was examined for an enhanced methane production and lignin reduction. According to the results from these experiments, the conclusions could be summarized as follows:  The addition of NBW showed a positive effect on methane production from lignin. However, effect of NBW was different for different kinds of NBW. Methane yield in the N2-NBW addition reactor was the highest, 22% higher than the control reactor, followed by CO2-NBW addition (17 % higher than the control reactor). Besides, NBW also can enhance the lignin reduction. The highest lignin reduction occurred in the N2-NBW addition reactor was 19.3%, followed by CO2-NBW addition reactor (17.4%) and the control (DW, 6.4%) after AD for 12 days.  Addition of NBW showed better performance in the co-digestion of HAc with lignin compared to the mono-digestion of lignin. In the co-digestion of lignin with HAc, the accumulative methane yield from the NBW addition reactor (1061 mL CH4/gTOCremoval) was significantly higher than that from the control reactor (837 mL CH4/gTOCremoval). In the mono-digestion of lignin, the cumulative methane production was extremely low (only 23 mL CH4/gTOCremoval in the control reactor and 40 mL CH4/gTOCremoval in the NBW addition reactor). Also, anaerobic co-digestion of lignin with HAc can improve lignin reduction. Higher lignin reduction was detected in the co- digestion of lignin with HAc (16% in the control reactor and 24% in the NBW 122
  19. KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018 addition reactor) compared to the mono-digestion of lignin (14% in the control reactor and 9% in the NBW addition reactor).  The effect of different ratios of lignin to HAc on methane production and lignin reduction was further investigated. High lignin content showed a negative effect on methane production. Methane production in the reactor with a ratio of 80:20 (HAc:lignin) was 824 mL CH4/gTOCremoval, significantly lower than those from other reactors (1054, 1028, and 1061 mL CH4/gTOCremoval for the HAc/lignin ratios of 97.5:2.5, 95:5, and 90:10, respectively). Also, the highest lignin reduction was detected in the reactor with a HAc/lignin ratio of 95:5 (43%), followed by 90:10 (24%), 80:20 (18%), 97.5:2.5 (13%). Therefore, the 95:5 of HAc to lignin ratio is appropriate for the enhanced methane production and lignin reduction. 4.2. Further research plan This study clarified the positive effects of nanobubble water on anaerobic digestion of lignin. Nanobubble water can improve lignin reduction and methane production. However, further research is necessary to shed light on the mechanisms involved. Future search should quantify the amount of lignin adsorbed by the anaerobic biomass lignin degraded in the anaerobic digestion. Also, the effect of NBW on hydrolysis step should be further analyzed. REFERENCES Achinas, S., Achinas, V., & Euverink, G. J. W. (2017). A technological overview of biogas production from biowaste. Engineering, 3(3), 299-307. doi: 10.1016/j.eng.2017.03.002 Agarwal, A., Ng, W. J., & Liu, Y. (2011). Principle and applications of microbubble and nanobubble technology for water treatment. Chemosphere, 84(9), 1175-1180. doi: 10.1016/j.chemosphere.2011.05.054 Anwar, Z., Gulfraz, M., & Irshad, M. (2014). Agro-industrial lignocellulosic biomass a key to unlock the future bio-energy: A brief review. Journal of Radiation Research and Applied Sciences, 7(2), 163-173. doi: 10.1016/j.jrras.2014.02.003 APHA. (2012). Standard Methods for Examination of Water and Wastewater, twenty- second ed. . Washington DC American Public Health Association. Asgher, M., Ahmad, Z., & Iqbal, H. M. N. (2013). Alkali and enzymatic delignification of sugarcane bagasse to expose cellulose polymers for saccharification and bio- ethanol production. Industrial Crops and Products, 44, 488-495. doi: 10.1016/j.indcrop.2012.10.005 123
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