Effect of nano-Bubble water on anaerobic methane fermentation of lignin
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
aFalculty of Environment and Natural Resource, Dalat University, Dalat City, Vietnam
bSchool of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan
*Corresponding author: Email: [email protected]
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.
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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
aKhoa Môi trường và Tài nguyên, Trường Đại học Đà Lạt, Lâm Đồng, Việt Nam
bKhoa Khoa học Môi trường và Sự sống, Trường Đại học Tsukuba, Ibaraki, Nhật Bản
*Tác giả liên hệ: Email: [email protected]
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 ± 2oC.
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.
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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).
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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.
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Nanobubbles application
1.3.
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).
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Medium and inoculum
2.2.
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±2C 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
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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±2C. 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).
ꢀ
ꢅꢀ
ꢄ
ꢂ
ꢆ
ꢁꢂꢃ
TS =
VS =
(1)
(2)
ꢀ
ꢅꢀ
ꢆ
ꢆꢇꢈ
ꢀ
ꢅꢀ
ꢉꢂꢂ
ꢂ
ꢂ
ꢄ
ꢁꢂꢃ
ꢄ
ꢀ
ꢅꢀ
ꢆ
ꢆꢇꢈ
where mꢊ is the weight of dish, mꢊꢋꢌ is the weight of dish and initial sample,
mꢍꢎꢏ ꢐ is the constant weight of dish and sample after heating at 105ꢎC, and mꢑꢎꢎ ꢐ is
the weight of dish and sample after burning at 600ꢎC.
ꢂ
ꢂ
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)
ꢐ ꢅꢐ
ꢒ
ꢓ
ꢐ
ꢒ
where Ci (mg/l) is the initial concentration of TOC and Cf (mg/l) is the final
concentration of TOC.
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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 (80C) and Porapak
Q column (60C) employing N2 as a carrier gas.
ꢔ
(ꢀꢙ)
ꢕꢒꢖꢗꢘꢈ
Biogas yield (ml Biogas/g TOCremoval ) = ꢚꢗꢛꢜꢄ
×1000
(ꢙ)
(4)
× ꢔ
ꢞꢟꢘꢆꢠꢖꢞ
ꢝ
Methane yield (ml CH4 /g TOCremoval ) = (
) × Biogas yield
(5)
(6)
ꢡꢢꢣꢤꢅꢥꢍꢍ
ꢑꢦꢧꢦꢑꢨ
Methane content (%) = ꢩꢣꢪꢫꢤꢬꢣ ꢭꢮꢣꢙꢯ x100
ꢰꢮꢱꢲꢤꢌ ꢭꢮꢣꢙꢯ
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 Abs280 of lignin
solution
Lignin reduction and lignin reduction rate were calculated according to the
following equations: Lignin reduction (%) = (
) × 100
(7)
ꢐ ꢅꢐ
ꢒ
ꢓ
ꢐ
ꢒ
ꢳ ꢅꢳ
ꢴ
ꢵ
Lignin reduction rate (mg/L/d) =
(8)
ꢶꢷꢸꢹꢺꢻꢼꢽꢾ ꢿꣀꢹꢺꢻꢼꢽꢾ (ꢿꢺꣁ)
where Ci (mg/l) is the initial concentration of lignin and Cf (mg/l) is the final concentration
of lignin.
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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
3775
95
95:5
3679
190
90:10
3485
378
80:20
3099
754
100%HAc
100%lignin
HAc (mg/l)
Lignin (mg/l)
3872
0
0
3766
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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 180C and 150C, 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.
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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
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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
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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 R1-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 (R1 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 CH4/gTOCremoval in R3-DW and 40
mL CH4/gTOCremoval in R3-NBW through the mono-digestion of lignin (R3 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.
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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 R3 (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), R3 (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,
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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)
Lignin reduction
(%)
Lignin reduction rate
(mg/l/d)
Initial VFA
(mgC/l)
Reactor
R1+DW 100:0
R1+NB
NA
NA
628.34
W
100:0
NA
NA
615.73
557.66
R2+DW 90:10
R2+NB
16.33 ± 5.37
2.02 ± 0.7
W
90:10
23.75 ± 8.03
14.25 ± 2.64
3.45 ± 1.28
583.32
NA
R3+DW 0:100
32.52 ± 6.24
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 R3-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.
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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 CH4/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
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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.
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Table 3- 2 Major results in Phase 3
Reacto Mix ratio
Lignin removal
(%)
Lignin removal rate
(mg/l/d)
Initial VFA
(mgC/l)
r
(HAc:Lignin)
NBW1 97.5:2.5
NBW2 95:5
NBW3 90:10
NBW4 80:20
12.55 ± 13.43
42.69 ± 11.31
23.75 ± 8.03
18.20 ± 5.63
0.45 ± 0.49
3.03 ± 0.78
3.45 ± 1.28
6.06 ± 1.90
617.24
630.10
583.32
474.87
4.
4.1.
CONCLUSIONS
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
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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
KỶ YẾU HỘI NGHỊ KHOA HỌC THƯỜNG NIÊN TRƯỜNG ĐẠI HỌC ĐÀ LẠT NĂM 2018
Azevedo, A., Etchepare, R., Calgaroto, S., & Rubio, J. (2016). Aqueous dispersions of
nanobubbles: Generation, properties and features. Minerals Engineering, 94, 29-
37. doi: 10.1016/j.mineng.2016.05.001
Barakat, A., Monlau, F., Steyer, J. P., & Carrere, H. (2012). Effect of lignin-derived and
furan compounds found in lignocellulosic hydrolysates on biomethane
production. Bioresour Technol, 104, 90-99. doi: 10.1016/j.biortech.2011.10.060
Barbier, J., Charon, N., Dupassieux, N., Loppinet-Serani, A., Mahé, L., Ponthus, J., . . .
Cansell, F. (2012). Hydrothermal conversion of lignin compounds. A detailed
study of fragmentation and condensation reaction pathways. Biomass and
Bioenergy, 46, 479-491. doi: 10.1016/j.biombioe.2012.07.011
Candia-García, C., Delgadillo-Mirquez, L., & Hernandez, M. (2018). Biodegradation of
rice straw under anaerobic digestion. Environmental Technology & Innovation,
10, 215-222. doi: 10.1016/j.eti.2018.02.009
Chandra, R., Takeuchi, H., & Hasegawa, T. (2012). Methane production from
lignocellulosic agricultural crop wastes: A review in context to second generation
of biofuel production. Renewable and Sustainable Energy Reviews, 16(3), 1462-
1476. doi: 10.1016/j.rser.2011.11.035
Chang, Y. C., Choi, D., Takamizawa, K., & Kikuchi, S. (2014). Isolation of Bacillus sp.
strains capable of decomposing alkali lignin and their application in combination
with lactic acid bacteria for enhancing cellulase performance. Bioresour Technol,
152, 429-436. doi: 10.1016/j.biortech.2013.11.032
Chen, Q., Marshall, M. N., Geib, S. M., Tien, M., & Richard, T. L. (2012). Effects of
laccase on lignin depolymerization and enzymatic hydrolysis of ensiled corn
stover. Bioresour Technol, 117, 186-192. doi: 10.1016/j.biortech.2012.04.085
Chen, Z., & Wan, C. (2017). Biological valorization strategies for converting lignin into
fuels and chemicals. Renewable and Sustainable Energy Reviews, 73, 610-621.
doi: 10.1016/j.rser.2017.01.166
Collinson, S. R., & Thielemans, W. (2010). The catalytic oxidation of biomass to new
materials focusing on starch, cellulose and lignin. Coordination Chemistry
Reviews, 254(15-16), 1854-1870. doi: 10.1016/j.ccr.2010.04.007
D.C . Peng, & Jin, Q. T. (1993). Anaerobic digestion of alkaline black liquor using an up-
flow anaerobic sludge blanket reactor. Journal of Chemical Technology &
Biotechnology, 58, 89-93.
Dieter Deublein, & Steinhauser, A. (2008). Biogas from waste and renewable sources: an
introduction Weinheim: WILEY-VCH Verlag GmbH & Co. KGaA.
Eva Palmqvist, & Hahn-Hagerdal, B. (2000). Fermentation of lignocellulosic
hydrolysates. II: inhibitors and mechanisms of inhibition. Bioresour Technol, 74,
25–33. doi: 10.1016/S0960-8524(99)00161-3
124
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