Solid- State Anaerobic Digestion of Lignocellulosic Biomass for Biogas Production
Thesis
Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in
the Graduate School of The Ohio State University
By
Lo Niee Liew, B.S.
Graduate Program in Food, Agricultural and Biological Engineering
The Ohio State University
2011
Thesis Committee:
Dr. Yebo Li, Advisor
Dr. Jay F. Martin
Dr. Frederick C. Michel
i
Copyright by
Lo Niee Liew
2011
ii
Abstract
The anaerobic digestion (AD) process can generally be classified into two
categories based on the total solids (TS) content of material in the digester: solid-state
anaerobic digestion (SS-AD) which has TS content of 20% and higher, and liquid AD
which has TS content of 15% and lower. SS-AD has the advantages of using a smaller
reactor volume and less moving parts as agitation is generally not required. Energy
demand for heating the material in digester is therefore reduced with a smaller volume of
materials to be heated.
Lignocellulosic biomass has been considered as one of the suitable feedstock for
SS-AD as it is easily available and issues encountered in liquid AD such as stratification
can be avoided. The challenge of utilizing lignocellulosic biomass in AD is the
recalcitrant properties of such material. Pretreatment is therefore necessary to improve
the biodegradability of lignocellulosic biomass.
In this study, SS-AD of four types of feedstocks was tested under batch operation.
The SS-AD was conducted under 37C for 30 days. The feedstocks selected were corn
stover, wheat straw, fallen tree leaves, and yard waste. As inoculation is essential in SSAD, effect of substrate to inoculum (S/I) ratio (volatile solids of substrate to volatile
solids of inoculum) on methane yield was investigated. For all feedstocks tested, the
highest methane yield was obtained at S/I ratio of 2. The highest methane yield of 81.2
L/kg volatile solids (VS) was obtained from SS-AD of corn stover followed by wheat
straw (66.9 L/kg VS), leaves (55.4 L/kg VS) and yard waste (40.8 L/kg VS). An inverse
linear relationship was obtained between the methane yield and lignin content of
iii
lignocellulosic biomass. The methane production of the feedstock in SS-AD fits the
simple first-order kinetic model.
SS-AD of leaves with simulatanoues alkaline (NaOH) treatment was further
studied to improve the methane production. The highest methane yield of 81.8 L/kg
volatile solids (VS) was obtained at NaOH loading of 3.5% and S/I ratio of 4.1. However,
it was not significantly different from that of control (without NaOH addition). At S/I
ratio of 6.2, NaOH loading of 3.5% enhanced the methane yield by 24-fold over the
control. The AD process at S/I ratio of 8.2 were failed. In addition, increasing the total
solid (TS) content from 20% to 26% reduced biogas yield by 35% in reactors at S/I ratio
of 6.2 and NaOH loading of 3.5%. Cellulose and hemicellulose degradation and methane
yields during the 30-day AD process are highly related.
The results obtained from this study showed the feasibility of utilizing
lignocellulosic biomass as feedstock in SS-AD for biogas production. In addition,
alkaline pretreatment with NaOH was also shown to enhance methane production from
leaves in SS-AD.
iv
Dedication
This document is dedicated to my family
v
Acknowledgments
My most sincere appreciation and gratitude to my advisor, Dr. Yebo Li for his
infinite support and guidance provided throughout my graduate study. I would also like to
gratefully thank professors serving as my dissertation committee: Dr. Jay F. Martin and
Dr. Frederick C. Michel for their time.
I would also like to express thanks to the staff members of Food, Agricultural and
Biological Engineering Department: Mike Klingmans for his dedication in providing the
engineering assistance needed whenever required, Mary wicks for her enthusiasm in
proof reading the thesis and both Peggy Christman and Candy McBride for their prompt
response to all the administrative supports I needed.
I am also indebted to my lab members: Dr. Ellen Wan, Stephen Park, Dr. Jian Shi,
Dr. ZhiFang Cui, Dr. Guiming Fu and Stephanie Xu, for their attentiveness in providing
the supports and encouragements throughout my stay in Wooster, OH
Lastly, I would like to express my deepest gratitude to my family members for
their constant understanding, patience and encouragement in all the undertakings I
pursue.
vi
Vita
March, 1981 …...…………………………..Born – Johor, Malaysia.
December, 2004 .............................................B.S. in Agricultural and Biological
Engineering, Purdue University.
December, 2006 .............................................Food Technologist, Cadbury
Confectioneries, Shah Alam, Malaysia.
April, 2008 .....................................................Operational Auditor, Nestlé, Petaling Jaya,
Malaysia.
July, 2009 .......................................................Project Engineer, Nestlé, Petaling Jaya,
Malaysia.
August, 2009 to present .................................Graduate Research Associate, Department
of Food, Agricultural and Biological
Engineering , The Ohio State University.
vii
Table of Content
Abstract iii
Acknowledgments.............................................................................................................. vi
Vita……………….. .......................................................................................................... vii
List of Tables ..................................................................................................................... xi
List of Figures ................................................................................................................... xii
Chapter 1 Introduction ........................................................................................................ 1
Chapter 2 Literature Review ............................................................................................... 6
2.1. Lignocellulosic Biomass as Feedstock for Anaerobic Digestion ......................... 6
2.1.1. Crop residues ............................................................................................. 8
2.1.2. Yard waste ............................................................................................... 12
2.2. Pretreatment of Lignocellulosic Biomass for Anaerobic Digestion ................... 17
2.3. Operation of SS-AD ........................................................................................... 39
Chapter 3 Effect of Substrate to Inoculum Ratio on Methane Production in Solid-State
Anaerobic Digestion of Lignocellulosic Biomass ............................................ 42
3.1. Introduction ........................................................................................................ 42
3.2. Materials and Methods ....................................................................................... 44
3.2.1. Feedstock and inoculum .......................................................................... 44
3.2.2. Solid-state anaerobic digestion ................................................................ 44
3.2.3. Analytical methods .................................................................................. 46
3.2.4. Statistical analysis .................................................................................... 47
3.3. Results and discussion ........................................................................................ 48
3.3.1. Composition analysis of inoculum and lignocellulosic biomass ............. 48
3.3.2. Biogas production .................................................................................... 49
3.3.3. Relationship between biogas production and lignin content ................... 57
3.3.4. Degradation of holocellulose and extractives .......................................... 58
3.4. Conclusion .......................................................................................................... 60
ix
Chapter 4 Enhancing the Solid-state Anaerobic Digestion of Fallen Leaves through
Simultaneous Alkaline Treatment .................................................................... 62
4.1. Introduction ........................................................................................................ 62
4.2. Materials and methods........................................................................................ 65
4.2.1. Feedstock and inoculum .......................................................................... 65
4.2.2 Solid-state anaerobic digestion with simultaneous NaOH treatment ....... 66
4.2.3. Analytical methods .................................................................................. 67
4.2.4. Statistical analysis .................................................................................... 68
4.3. Results and discussion ........................................................................................ 68
4.3.1. Biogas production .................................................................................... 68
4.3.2 Variation of pH, total volatile fatty acids (TVFAs) and alkalinity ........... 73
4.3.3. Degradation of cellulose and hemicellulose ............................................ 76
4.4. Conclusions ........................................................................................................ 78
Chapter 5 Conclusions and Suggestions for Future Research .......................................... 80
Reference…………. ......................................................................................................... 82
x
List of Tables
Table 2.1 Methane yield from various crop residues tested with anaerobic digestion
Table 2.2 Methane yield from various wood and forestry waste tested with anaerobic
digestion
Table 2.3 Various pretreatment methods applied on lignocellulosic biomass in anaerobic
digestion
Table 2.4 Alkaline pretreatment of lignocellulosic biomass in anaerobic digestion
Table 3.1. Design of reactors using four different feedstocks
Table 3.2 Characteristics of inoculum and lignocellulosic biomass
Table 3.3 Correlation coefficients of lignocellulosic biomass using first-order kinetic
Table 4.1 Characteristics of leaves and inoculum
xi
List of Figures
Figure 1.1 Phases in anaerobic digestion
Figure 2.1 Anaerobic digestion of lignocellulosic biomass
Figure 2.2 Effect of pretreatment on lignocellulosic biomass
Figure. 3.1 Methane production obtained from SS-AD of lignocellulosic biomass in 30
days
Figure. 3.2 Daily methane yield of lignocellulosic biomass in 30-day SS-AD
Figure. 3.3 Logarithmic plot for methane production from lignocellulosic biomass with
S/I ratio of 2
Figure. 3.4 Correlation between total methane yield (expressed in L/kg VSfeedstock) and
lignin content of lignocellulosic biomass
Figure 3.5 Degradation of holocellulose and extractives during 30 days of SS-AD, based
on initial loading of 100g TS
Figure 4.1 Effect of S/I ratio and NaOH loading on total methane yield (digestion time:
30 days,
TS: 20%)
Figure 4.2 Effect of NaOH loading on daily methane yield at S/I ratios of a) 4.1 and b)
6.2 (digestion time: 30 days, TS: 20%)
Figure 4.3 Effect of NaOH loading and TS content on total methane yield (digestion
time: 30 days, S/I ratio: 6.2)
xii
Figure 4.3 Effect of NaOH loading and TS content on total methane yield (digestion
time: 30 days, S/I ratio: 6.2)
Figure 4.4 Initial and final a) pH and b) TVFA/alkalinity ratios for reactors with different
NaOH loading and S/I ratio (digestion time: 30 days, TS: 20%)
Figure 4.5 Effect of NaOH loading on reduction of a) cellulose and b) hemicellulose (digestion
time: 30 days, TS: 20%, S/I ratio: 6.2)
xiii
Chapter 1 Introduction
Anaerobic digestion involves decomposition of organic matter by a microbial
consortium in an oxygen free environment. It is a natural process occurring under
anaerobic environment in places such as ocean, lake sediments, waterlogged soil,
digestive tracts (Chynoweth and Isaacson, 1987). Biogas consisting mainly methane
(CH4, 60%) and carbon dioxide (CO2, 40%) is released as final product. The synergistic
process by the consortium of anaerobic microorganism can generally be classified into 4
phases: hydrolysis, acidogenesis, acetogenesis and methanogenesis (Gerardi, 2003; Yu
and Schanbacher, 2010). Each phase is facilitated by a distinct group of microorganism
(Yu and Schanbacher, 2010). As depicted in Figure 1.1, hydrolysis is the first step in AD
process where disintegration of polymeric complex substrates such as carbohydrates,
lipids and protein into simpler substrates (glucose, fructose, amino acids and long-chain
fatty acids) occurs. The hydrolysis process is followed by the acidogenic phase which is
carried out by the acidogenic bacteria in converting the monomers and oligomers to
short-chain fatty acids or volatile free fatty acids (VFA), carbon dioxide, hydrogen and
alcohols (Li et al., 2011b; Yu and Schanbacher, 2010). Acetogenic bacteria that presents
in a symbiotic relationship with methanogens then degrade VFA to acetate and hydrogen.
The final stage involves utilization of acetates and hydrogen by methanogen to produce
methane gas (Gerardi, 2003).
1
Figure 1.1. Phases in anaerobic digestion (adapted from Yu and Schanbacher, 2010).
Nomenclatures: AA = amino acids, LCFA = long chain fatty acids, SCFA = short chain
fatty acids, HCOO- = formic acid and CH3-COO- = acetic acids.
AD has been applied in the processing of municipal and industrial sludge for
nearly 100 year (Chandler et al., 1890). The initial applications of AD were for
stabilization and treatment of waste sludge (Yu and Schanbacher, 2010). AD process has
received tremendous attention lately due to rising of fuel prices and the needs for
2
renewable energy and fuels. Biogas, as a promising alternative source of energy, could
be used for the following applications: generation of heat and/or electricity from burned
biogas (Sims, 2003; Ghosh et al., 2000), liquefaction of biogas into methanol and
chemical feedstock to replace hydrocarbon and coal (Kasali and Senior, 1988),
compression of biogas to be used as a source of car fuel similar to that of compressed
natural gas (CNG) and purification of biogas to be fed into gas distribution grids.
Anaerobic digesters are categorized as liquid anaerobic digestion/wet
fermentation and solid-state anaerobic digestion (SS-AD)/dry fermentation based on the
TS content of material contained in the digester. Currently no official standard is
available for the cut-off between wet and dry fermentation but wet fermentation generally
refers to substrate with total solids lower than 15% whereas dry fermentation denotes
substrate with total solids higher than 15% (De Baere and Mattheeuws, 2010; Yebo Li et
al., 2011).
SS-AD is normally chosen over liquid anaerobic digestion because the digestate
can be easily composted to be used as fertilizer or soil conditioner as no or minimal
dewatering of digestate is required. Feedstock contains small amount of nonbiodegradable material such as plastic, glass and stones does not cause major issue in SSAD. Furthermore, there is no moving part such as shaft and impeller in SS-AD thus
reducing the maintenance of mechanical device. In addition, increase in TS content of
feedstock used in digester implies a smaller volume reactor thus reduces material cost
and energy needed for heating. Furthermore, reduction in working volume also implied a
higher methane production per unit volume. Problem faced in liquid AD such as floating
3
and stratification of solid can be avoided in SS-AD (Chanakya et al., 1993; Chanakya et
al., 1999; Nordberg an Edström, 1997).
Europe, specifically Germany, is leading AD technology. In recent years, number
of SS-AD plant has increased and accounts for 63% of the new installations in Europe
over the past five years (De Baere Luc and Mattheeuws Bruno, 2010). However, there is
currently no commercial scale SS-AD operating in USA, although the first SS-AD
system is scheduled to be installed at the University of Wisconsin-Oshkosh in 2010
(University of Wisconsin, 2010).
Based on a report published by International Energy Agency (IEA) (www.iea.org,
2008), there were a total of 237 commercial digesters worldwide treating 3,000 to
497,000 tons of waste annually at each plant. A digester treating more than 2500 tons of
waste annually was regarded as commercial digester in this survey. Germany led by the
number of commercial plants (82) followed by Austria (27). Of the 237 commercial
digesters reported, 64 of the digesters are of SS-AD system. In recent years, number of
SS-AD plant has increased and accounts for 63% of the new installations in Europe over
the past five years (De Baere Luc and Mattheeuws Bruno, 2010). As of April 2011, there
were at least 167 digesters operating in commercial livestock farms in the US (EPA,
2011). Most of these digesters use animal manure or food waste as feedstock. However,
currently there is no commercial scale SS-AD operating in USA, although the first SSAD is scheduled to be installed at the University of Wisconsin-Oshkosh in 2010
(University of Wisconsin Oshkosh, 2011).
4
SS-AD is normally chosen over liquid anaerobic digestion because the digestate
can be easily composted to be used as fertilizer or soil conditioner as no or minimal
dewatering of digestate is required. Feedstock contains small amount of nonbiodegradable material such as plastic, glass and stones does not cause major issue in SSAD. Furthermore, there is no moving part such as shaft and impeller in SS-AD thus
reducing the maintenance of mechanical device. In addition, increase in TS content of
feedstock used in digester implies a smaller volume reactor thus reduces material cost
and energy needed for heating. Furthermore, reduction in working volume also implied a
higher methane production per unit volume. Problem faced in liquid AD such as floating
and stratification of fibers can be avoided in SS-AD (Chanakya et al., 1993; Chanakya et
al., 1999; Nordberg an Edström, 1997).
As lignocellulosic biomass is still not a common feedstock for the AD system in
the US, the aim of this study was to investigate the methane potential of lignocellulosic
biomass especially in SS-AD system. Lignocellulosic biomass with different composition
was selected in this study. Due to the recalcitrant properties of lignocellulosic biomass,
alkaline pretreatment can be conducted on lignocellulosic biomass to enhance the
methane potential in anaerobic digestion. Feasibility of utilizing a simpler process,
simultaneous alkaline treatment with SS-AD was also studied. Lignocellulosic biomass
which is high in lignin content was selected for the simultaneous alkaline treatment study.
The outcome from this study is expected to aid the commercialization of the solid-state
anaerobic digester.
5
Chapter 2 Literature Review
Many organic wastes such as food waste, animal manure and municipal solid
waste have been shown promising methane yield from AD. Lignocellulosic biomass such
as energy crop and crop residues has gained much attention lately due to their abundance
of available. However, the recalcitrant nature of lignocellulosic biomass poses challenges
to SS-AD such as lower methane yield. Pretreatments - physical, chemical or biological,
are therefore necessary to improve the biodegradability of lignocellulosic biomass for
methane production.
2.1. Lignocellulosic Biomass as Feedstock for Anaerobic Digestion
A wide range of organic materials, such as food waste, municipal solid waste and
animal manure have been commonly used as feedstocks in anaerobic digestion. The
selection of feedstocks is mainly influenced by the feedstock availability. Moreover, each
feedstock differs in characteristic (composition and texture) thus handling of each
feedstock in AD process varies. In addition to the aforementioned feedstock,
lignocellulosic biomass has lately gained more attention as a suitable substrate for
anaerobic digestion.
According to research studies on using energy crops and crop residues in AD to
generate renewable biogas, it was estimated that the energy potential of biomass and
wastes in US was about 29.5 EJ per year, excluding marine based biomass (Chynoweth
and Isaacson, 1987; Legrand and Warren, 1987); However, this estimation is likely to be
6
on the high side as cultivation of energy crop was expected to account for the major
source of energy, 74.6% (22 EJ/year) followed by crop residue (4.1 EJ/year).
Unit operations of an AD reactor with lignocellulosic biomass include: size
reduction of feedstocks, anaerobic digestion of feedstocks in batch or continuous mode,
and biogas utilization. General utilization of lignocellulosic biomass in AD process is
depicted in Figure 2.1.
Industry
Food, fibre and
woods process
residues
Agriculture
Energy and short
rotation crops,
crop residue and
animal waste
Forestry
Forest harvesting
and agroforest
residue
Sewage sludge
and animal
manures
Lignocellulosic biomass
feedstock
Digestatate
Biofertiliser
Waste
Urban wood and
domestics
wasterwater
Anaerobic digestion
Biogas
50-60% Methane,
40-50% Carbon dioxide
Heat
Methane
Direct
combustion
Gas turbine
Methanol
Electricity
Cogeneration
Figure 2.1 Anaerobic digestion of lignocellulosic biomass (adapted from IEA, 2007)
Feedstock composition (lignin, lignocellulose and extractive), harvesting season,
particle size, pretreatment methods substantially affect the methane production. The
7
amount and type of inoculum, digestion duration (ranged from 20 to 150 days), pH
adjustment, nutrients (macronutrients and micronutrients) supplementation and reactor
system configurations are operating parameters affecting of the perfromance of AD.
Biochemical methane potential (BMP) discussed in this chapter mostly refers to the batch
technique developed by Owen et al. (1979), although some degree of modications were
likely performed by the each researcher.
2.1.1. Crop residues
Lignocellulosic biomass generated world wide from agricultural residue alone
comes to an average of 1091 million tons per year (Sánchez, 2005). In the US, the
biomass resources totaled at 409 million tons per year which crop residue accounts for
about 41% of the total biomass yield (IEA, 2007). Reported availablity of corn stover and
wheat straw is 82 and 33 million dry tons per year, respectively (Kadam and McMillan,
2003).
As summarized in Table 2.1, most of the studies conducted on anaerobic digestion
of crop residues were in batch mode and at low total solid (TS) content. Several forms of
corn residue including fodder maize, corn stover and ensilaged corn stover have been
tested as feedstocks in anaerobic digestion. Most of the studies observed methane yield
ranges from 233 to 330 L/kg VS. However, several studies on methane production of
corn stover showed lower methane yields which could be attributed to larger particle
sizes (Zheng et al., 2009). Molnar (1988) reported a combined aerobic and anaerobic
digestion system at high TS content (27%) using a mixture of corn stalk and leaves (75%)
and beef manure (25%). No heating was provided throughout the process thus energy
8
demand for heating was eliminated. Substantial methane productivity of 24.8 L/L was
reported in this study, methane yield (L/kg VS) was however not reported in this study.
Another crop residue that has been commonly studied is wheat straw. Overall,
methane yield obtained from BMP tests was generally higher compared to non-BMP
tests, it ranged from 302 to 333 L/kg VS. As shown by studies conducted by Møller et al.
(2004) and Sharma et al. (1988), methane yield decreased with increase in particle size.
Co-digesting cattle dung with wheat straw in study conducted by Somayaji and Khanna
(1994) did not show a significant increase in daily methane yield.
Other crop residues that have been tested include rice straw, vegetable and fruit
crop residue. Methane yield obtained during anaerobic digestion of rice straw varied from
241 to 365 L/kg VS (Sharma et al., 1988).However, lower methane yield (107 to 195
L/kg VS) was obtained when it was co-digested with cow dung (Somayaji and Khanna,
1994). This could be attributed to the higher total TS content or ammonia inhibition that
was likely caused by high level of cow dung. Methane yields obtained from anaerobic
digestion of fruit and vegetable crop residue ranging from 341 to 530L/kg VS were
higher than that of other crop residues such as corn stover, wheat straw and rice straw.
9
Table 2.1. Methane yield from various crop residues tested with anaerobic digestion.
Feedstock
AD System
Maize,
ensilaged
CSTR
Corn stover
BMP
Corn stover
Particle size
(mm)
0.17
Temperature,
(C)
39
Total
solid
< 20%
Between
0.25 and
0.60
35
< 20%
Batch
5-10
35
Corn stalk and
leaves (75%)
and beef cattle
manure (35%)
Batch
(composting +AD)
Not reported
Wheat straw
Batch
Wheat straw
(2 varieties)
Substrate
loading
1.3 g TS/L. day
Methane
production
330 L/kg VS
Reference
70
2.3 – 3.8 g TS/L
360 L/kg VS
Tong et al., 1990
< 20%
75
50 – 80 g TS/L
125-160 L/kg VS
Zheng et al.,
2009
35
~ 27%
30
Not reported
24.8 L/L
Molnar, 1988
10
35
< 20%
150
S/I ratio
0.1 – 4.0
299 331 L/kg VS
Hashimoto, 1989
BMP
0.60
35
< 20%
60 - 69
2.3 – 3.8 g TS/L
302 - 333 L/kg VS
Tong et al., 1990
Wheat straw
Batch
1-30
35
< 20%
60
S/I ratio of 1.4
145 – 161 L/kg VS
Møller et al.,
2004
Wheat straw
Batch
0.1 - 6 and
30 x 5
37
< 20%
56
8% (w/v)
feedstock
162-249 L/kg VS
Sharma et al.,
1988
Wheat straw
Batch
Not reported
37
4%
30
S/I ratio
(TS basis)
of 0.5
183 L/kg VS
Müller and
Trösch, 1986
10
Digestion
time (days)
60
Klimiuk et al.,
2010
Table 2.1. Methane yield from various crop residues tested with anaerobic digestion (Cont.)
Feedstock
AD System
Cattle dung
and wheat
straw (codigestion)
CSTR
Particle size
(mm)
Not reported
Rice straw
Batch
.
0.1 - 6 and
30 x 5
Cattle dung
and rice straw
(co-digestion)
CSTR
Not reported
Barley straw
Batch
(with leachate
percolation)
n/a
Cauliflower
leaves
Batch
0.1 - 6 and
30 x 5
Vegetable crop
residue
BMP
1
Temperature,
(C)
Not reported
Total
solid
10%
Digestion
time (days)
40
Substrate
loading
Not reported
Methane
production
107 –
113 L/kg TS. day
Reference
37
< 20%
56
8% (w/v)
feedstock
241 – 367 L/kg TS
Sharma et al.,
1988
10%
40
Not reported.
107 –
195 L/kg TS. day
Somayaji and
Khanna, 1994
35 to 37%
110
S/I ratio of
6.7 to 50.
151 to
226 L/kg VS
Torres-Castillo
et al., 1995
37
< 20%
56
8% (w/v)
feedstock
407 – 423 L/kg VS
Sharma et al.,
1988
35
< 20%
2 g VS/L
80 - 530 L/kg VS
Shiralipour and
Smith, 1984
Not reported
25 and 35
11
Until biogas
production
ceased.
Somayaji and
Khanna, 1994
2.1.2. Yard waste
In addition to studies on anaerobic digestion of agricultural residues, other sources
that are worth looking into include municipal soild waste such as yard waste consisting of
mainly grass clippings and leaves (Haug, 1993). Yard waste was once ranked as the
second largest amount among the U.S. solid waste stream (Tchobonoglous, 1977). Based
on report published by International Energy Agency, U.S. produces 30.9 million tons of
yard waste annually (IEA, 2007). Yard waste is usually composted. The stored energy is
however lost as heat during the composting process (Koch et al., 2009). Yard waste is an
attractive feedstock for AD since tipping fees are normally charged for collection. The
overall economics of AD operations can be improved if low or negligible cost feedstocks
are used.
Table 2.2 shows the methane yield of various wood and forestry wastes that have
been reported in the literature. Methane yields during anaerobic digestion of these wastes
were generally lower as compared to crop residues, which ranged from 42 to 410 L/kg
VS. This was likely attributed to higher lignin content found in the lignocellulosic
biomass; such as white fir that has 29% lignin content compared to corn stover and wheat
straw that has 10.3% and 17%, respectively (Tong et al., 1990). Other factors that could
cause such observation include cellulose crystallinity and structure of lignin.
Shiralipour and Smith (1984) investigated six groups of plant material (total of
603 samples) and concluded that methane production from woody biomass (20 to 270
L/kg VS) is generally lower than other plant groups such as freshwater aquatics (70 to
12
430 L/kg VS), forage and grasses (70 to 410 L/kg VS), roots and tubers (190 to 430 L/kg
VS), marine (80 to 380 L/kg VS) and crop residues (80 to 530 L/kg VS).
Jerger et al. (1982) studied the conversion of woody biomass to methane in
anaerobic digestion; six types of woody biomass (black alder, cottonwood, eucalyptus,
hybrid poplar, loblolly pine and sycamore) were selected. Loblolly pine had the highest
lignin content (34.5% of extractive-free dry matter basis) followed by black alder
(28.1%). Lignin content in other woody biomass ranged from 24.7% to 25.6%. In this
test, higher biodegradability (expressed as % VS reduction) of 3 to 44% was noted with
batch mode reactor compared to semi-continuous fed reactor (1.6% to 8.5%). Batch mode
was therefore selected to study the biodegradability of woody biomass in anaerobic
digestion. The highest methane yield was obtained for hybrid poplar and sycamore (320
L/kg VS, respectively) followed by black alder. Although eucalyptus had relatively lower
lignin content, the methane yield was however the lowest (0.014 L/kg VS) among the
biomass feedstocks tested. This study therefore shows no clear correlation between lignin
content and methane yield. However, it is worth noting that the composition (lignin and
holocellulose) was expressed as percentage of extractive-free dry matter in this study
instead of total solid basis. Therefore, the level of lignin content among the biomass
tested is likely to be different when expressed as percent of total solids.
Chandler et al. (1980) studied the biodegradability of various lignocellulosic
biomass (including wheat straw, corn stalks, corn leaves, cattails, treated kelp, water
hyacinth, corn meal and newsprint) and manures in AD for 90 or 120 days. Lignin
content was found to be the major factor affecting the biodegradability of feedstocks.
13
Strong reciprocal correlation constants (r2) were obtained between 1) VS destruction
efficiency and lignin content (expressed as percent VS) (r2 = 0.94) and, 2) Cell wall
destruction efficiency and lignin content to cell wall ratio (r2 = 0.88). Correlation
between methane production and lignin
BMP discussed in this section was generally used to tdetermine the
biodegradability of a feedstock in anaerobic digestion by measuring the cumulative
methane yield observed under batch assay and anaerobic incubation in chemically
defined medium (Owen et al., 1979). Supplements such as micronutrients, trace elements
and buffering agents were added as part of the chemically defined medium. This method
therefore assumes no inhibition occurs in the AD and the inoculum was in excess so that
the methane yield observed was the ultimate methane yield that could be obtained from a
particular feedstock. The BMP tests normally last longer (up to 150) as the test is
terminated when biogas production ceased to almost zero.
14
Table 2.2. Methane yield from various forest and yard wastes tested with anaerobic digestion.
Feedstock
AD System
Miscanthus spp.
Silage
CSTR
Particle size
(mm)
0.17
Temperature,
(C)
39
Total
solid
< 20%
Digestion time
(days)
60
Forage and grass
(49 samples)
BMP
1
35
< 20%
Batch
0.1 - 6 and
30 x 5
37
< 20%
Until biogas
production
ceased.
56
Mirabilis leaves
White fir
BMP
0.42
35
< 20%
77
Wood grass
BMP
0.85
35
< 20%
Fresh Parthenium
hysterophorus L.,
terrestrial weed
Batch
Not reported
26
Dried Parthenium
hysterophorus L.,
terrestrial weed
Batch
0.5
Dhub grass,
commonly known
as “Bermuda grass”
Batch
0.1 - 6 and
30 x 5
Methane
production
100 –
190 L/kg
VS
70 –
410 L/kg
VS
241 –
367 L/kg
VS
Reference
2.3 – 3.8 g
TS/L
42 L/kg VS
Tong et al., 1990
104
2.3 – 3.8 g
TS/L
291 L/kg
VS
Tong et al., 1990
< 20%
35
S/I ratio
0.1 - 1.3
29 –
152 L/kg
VS
Gunaseelan, 1994
26
< 20%
21
S/I ratio
0.1 – 0.7
54 –
152 L/kg
VS
Gunaseelan, 1994
37
< 20%
56
8% (w/v)
feedstock.
137 –
228 L/kg
VS
Sharma et al., 1988
15
Substrate
loading
1.3 g TS/L.
day
2 g VS/L
8% (w/v)
feedstock
Klimiuk et al., 2010
Shiralipour and
Smith, 1984
Sharma et al., 1988
Table 2.2. Methane yield from various forest and yard wastes tested with anaerobic digestion (Cont.)
Feedstock
AD System
Grass silage
Loop reactor
Grass silage
Batch
(leach bed
reactor)
BMP
Temperature,
(C)
38
Total
solid
< 20%
Digestion time
(days)
50
Substrate
loading
3.5 g VS/ L.
day
Methane
production
260 L/kg
VS
30
35
< 20%
55
3.1 g VS/ L.
day
141 to 204
L/kg VS
Lehtomäki et al.,
2008
0.8
35
< 20%
60 – 100
S/I ratio of
0.5
210 –
390 L/kg
VS
Turick et al., 1991
Gliricidia leaves
Batch
1
32
< 20%
28
Not reported
Gunaseelan, 1988
Grass
BMP
1.5
35
< 20%
90
2 g VS/L
165 –
180 L/kg
VS
209 L/kg
VS
Leaves
BMP
1.5
35
< 20%
90
2 g VS/L
Branches
BMP
1.5
35
< 20%
90
2 g VS/L
Blend of grass,
leaves and branches
BMP
1.5
35
< 20%
90
2 g VS/L
123 L/kg
VS
134 L/kg
VS
143 L/kg
VS
Owens and
Chynoweth, 1993
Owens and
Chynoweth, 1993
Owens and
Chynoweth, 1993
Willow
(stem and bark, 20
samples)
Particle size
(mm)
6
16
Reference
Koch et al., 2009
Owens and
Chynoweth, 1993
2.2. Pretreatment of Lignocellulosic Biomass for Anaerobic Digestion
Lignocellulosic biomass is mainly composed of cellulose, hemicellulose,
lignin, and extractives (Sjöström, 1993). Cellulose or β-1-4-glucan is a linear
polysaccharide polymer or glucose made of cellobiose units (Delmer and Amor,
1995; Morohoshi, 1991). Dominant sugars in hemicellulose are xylose in hardwoods
and agriculture residues (Persson et al., 2006; Lavarack et al., 2002; Emmel et al.,
2003) along with other sugars such as galactose, glucose and arabinose. Lignin is a
complicated molecule constructed of phenylpropane units linked with hemicellulose
and cellulose form a 3D structure. Lignin is difficult to degrade and is the most
recalcitrant component of the plant cell wall. A high proportion of lignin in
lignocellulosic biomass usually indicates high resistance to chemical and enzymatic
degradation (Taherzadeh and Karimi, 2008).
Lignin is poorly degradable in anaerobic conditions. The rate and extent of
lignocellulose utilization is severely limited by lignin due to the intense cross-linking
of cellulose with hemicellulose and lignin (Lehtomäki, 2006). Moreover, the
crystalline structure of cellulose prevents penetration by micro-organisms or
extracellular enzymes. Compared to cellulose, structure of hemicellulose is more
random, amorphous and thus it is less resistance to hydrolysis (Ademark et al., 1998;
Mod et al., 1981 and O’Dwyer, 1934). Although cellulose and hemicellulose are
relatively easily decomposed by microorganism, the biodegradability is lowered
when these compounds are embedded within the lignocellulose complex. The
surrounding lignin protects them from microbial attack (Lübken et al., 2010).
17
Figure 2.2. Effect of pretreatment on lignocellulosic biomass (adapted from Hsu et
al., 1980 Hsu et al., 1980 T.A. Hsu, M.R. Ladisch and G.T. Tsao, Alcohol from
cellulose, Chemical Technology 10 (1980) (5), pp. 315–319)
Due to the recalcitrant nature of most lignocellulosic biomass, pretreatment is
commonly employed to facilitate accessibility of hydrolytic enzyme to cellulose
(Lehtomäki, 2006). Effective pretreatment results in disruption of barriers that
prevent penetration of hydrolytic enzymes (Mosier et al., 2005) as illustrated in
Figure 2.2. An ideal pretreatment is represented by an increase in surface area and
reduction in lignin content and crystallinity of cellulose.
Pretreatments can be carried out either physically, chemically or biologically,
or as a combinations of these. Pretreatments have been quite intensively studied for
facilitating the enzymatic hydrolysis and consequent increase in ethanol production
from lignocellulosic substrates (Sun & Cheng 2002), but there is less information
available on the effects of pre-treating lignocellulosic biomass for methane
production. Pretreatment methods tested to be effective for improving methane yield
18
from lignocellulosic biomass include steam explosion, thermal hydrolysis, wet
oxidation, pre-incubation in water, and treatment with ultrasound or radiation
(Hashimoto 1986, Sharma et al. 1989, Sun & Cheng 2002, Fox & Noike 2003).
Acids, alkalis, solvents or oxidants are chemicals generally used for pretreatment of
lignocellulosic biomass (Sun & Cheng 2002). Biological method such as microbial
and enzymatic pretreatment is an attractive alternative as it does not command
intensive pretreatment condition (heat and pressure) or dangerous chemicals, but the
cost effectiveness is yet to be established (Frigon et al., 2011).
Almost all lignocellulosic biomass requires size reduction such as grinding
before biological conversion in order to increase the total surface area exposed to
microbial attack. Physical methods employed such steam explosion and heat
treatment showed an increase in methane yield of 7 to 20% (Bauer et al., 2009;
Gunaseelan, 1994).
Gunaseelan (1994) used HCl to pretreat dried Parthenium hysterophorus L.,
a terrestrial weed and obtained a 45% increase in methane yield. Fernandes et al.
(2009) used a weaker acid, maleic acid to pretreat hay, straw and bracken. They
found that the pretreatment inhibited hay and straw AD but lead to a 57% increase in
methane yield from the digestion of bracken. Fernandes et al. (2009) therefore
concluded that the effect of pretreatment was more profound in lignocellulosic
biomass that has higher lignin content (bracken). The disadvantage of acid
pretreatment prior to AD process is that neutralization is required to increase pH to
an optimal level during start-up.
19
Biological pretreatment methods that have been employed so far include
utilization of white rot fungi (Müller and Trösch, 1986) and directly use of lignindegrading enzymes on lignocellulosic biomass (Frigon et al., 2011). Müller and
Trösch (1986) showed that pretreating wheat straw with white rot fungi resulted in
an increase in methane yield of 28%. However, the treatment time was as long as 90
days. Frigon et al. (2011) also showed promising results when summer fresh
switchgrass was pretreated with peroxidases (lignin peroxidase and manganese
peroxidase) and pectinases (Pectate-lyase and Poly-galacturonase). The methane
yields were enhanced by 29% to 83%, respectively. Methane yield was further
increased to 90% when biological pretreatment was followed by alkaline
pretreatment (Frigon et al., 2011). Although biological pretreatment showed
promising results, the drawbacks of such a method are longer retention times.
Chemical methods such as alkaline treatment are known to break the bonds
between hemicellulose and lignin and to swell the fibres and increase the pore size,
therefore facilitating hydrolysis (Baccay & Hashimoto 1984, Pavlostathis & Gossett
1985a). Alkaline pretreatment is one of the most promising pretreatments leading to
methane yield enhancements of 4% to 69% (Table 2.3).
20
Table 2.3. Various pretreatment methods applied on lignocellulosic biomass in anaerobic digestion.
Feedstock
Pretreatment
Pretreatment condition
AD system
Enhancement
level
7% - 20%
Reference
Batch, 37.5C
and S/I ratio of 0.3
Methane
production
296 –
331 L/kg VS
Wheat straw
Steam explosion
20 bar steam,
160C - 200C for
10 to 20 min.
Wheat straw
Pretreated with P.florida
(white rot fungi)
Incubated for
30 , 60 and 90 days
Batch, 37C, 30 days
and S/I ratio of 0.5.
201 –
234 L/kg VS
9% - 28%
Müller and
Trösch, 1986
Dried
Parthenium
hysterophorus L.,
terrestrial weed
a) HCl
b) NaOH
c) Heat
a) 8% (w/w), 26C
for 24 hr
b) 3% (w/w), 26C
for 24 hr
c) 120C for 1 hr
Batch, 26C, 21 days
and S/I ratio of 0.7.
157 –
236 L/kg VS
12% - 69%
Gunaseelan,
1994
Hay
a) Ca(OH)2
b) (NH4)2CO3
c) Maleic acid
a) 10%, 85C and16 hr.
b) 4 g/L, 120C for 2 hr.
c) 5.8 g/L, 150C for 0.5 hr.
BMP, 35C and
40 days
230 -300 L/kg VS
Adverse effect
Fernandes et al.,
2009
Straw
a) Ca(OH)2
b) (NH4)2CO3
c) Maleic acid
a) 10%, 85C and16 hr.
b) 4 g/L, 120C for 2 hr.
c) 5.8 g/L, 150C for 0.5 hr.
BMP, 35C and
40 days
150 – 320 L/kg
VS
Adverse effect
to 28%
Fernandes et al.,
2009
Bracken
a) Ca(OH)2
b) (NH4)2CO3
c) Maleic acid
a) 10%, 85C and16 hr.
b) 4 g/L, 120C for 2 hr.
c) 5.8 g/L, 150C for 0.5 hr.
BMP, 35C and
40 days
90 - 170 L/kg VS
29% - 240%
Fernandes et al.,
2009
Switch grass
(fresh summer
harvest)
a) Lignin peroxidase (LiP)
b) Manganese peroxidase
(MnP)
c) MnP followed by NaOH
d) Pectate-lyase
e) Poly-galacturonase
a) 22C for 8 hr
b) 37C for 8 hr
c) (b) and 7 g/L ,55C for 3hr
d) Ambient temperature
for 24 hr
e) Ambient temperature
for 24 hr
BMP, 35C and
38 - 134 days
202 – 298 L/kg
VS
29% - 90%
Frigon et al.,
2011
21
Bauer et al.,
2009
Compared with acid or oxidative reagents, alkali treatment appears to be a more
effective method in breaking the ester bonds between lignin, hemicellulose and cellulose
(Gaspar, 2007). This has been demonstrated by Vaccarino et al. (1987) and Silverstein et
al. (2007).
The reasons for alkali consumption during alkali pretreatment of lignocellulosic
biomass include: 1) saponification of uronic and acetyl esters, 2) reaction with free
carboxyl groups and 3) neutralization of acidic compounds produced from degradation of
lignin and holocellulose (Pavlostathis and Gossett, 1985b). According to Pavlostathis and
Gossett (1985b), maximal NaOH consumption measured to be at 5.5g NaOH/ 100g TS
for a 30-day pretreatment period.
Table 2.4 presents various alkaline pretreatment studies on lignocellulosic
biomass in anaerobic digestion. NaOH is regarded as one of the most effective alkaline
agents to delignify lignocellulosic biomass as shown by Hashimoto (1986) in
pretreatment of wheat straw. In his study, the effect of pretreatment on methane yield
with NaOH was compared with ammonium hydroxide (NH4OH) and calcium hydroxide
(Ca(OH)2. The highest enhancement in methane yield (28%) was attained with NaOH.
Pretreatment duration with NaOH was also shorter compared to other alkaline agents. In
addition, Hashimoto (1986) also showed that increasing the alkaline concentration during
pretreatment resulted in an increase in methane yield.
Frigon et al. (2011) reported that increasing the pretreatment temperature from
ambient temperature to 55C increased methane yields of switch grass by 16%. The
increment in methane yield increased slightly to 25% when the temperature was further
22
increased from 55 to 121C. Hashimoto (1986) showed that increasing alkaline
pretreatment temperature above 150C (highest temperature at 225C) resulted in adverse
effect where methane yield decreased with increase in pretreatment temperature. With
such minimal increase in methane yield and adverse effect shown in these studies,
pretreatment at elevated temperature (above 55C) is not recommended.
Pang et al. (2008) studied the pretreatment of corn stover with NaOH at relatively
low moisture content. The pretreatment was conducted at 80% moisture content, ambient
temperature for 3 weeks. The highest methane yield was attained from corn stover
pretreated with 8% NaOH and at corn stover loading of 65 g TS/L. However, the most
significant enhancement in methane production (91%) compared to untreated corn stover
was observed at the highest corn stover loading of 80 g/L with 8% NaOH pretreatment.
In another study conducted by Zheng et al. (2009), pretreatment of corn stover
was conducted at higher moisture content (88%) with NaOH at ambient temperature for 3
days. The highest methane yield was attained from corn stover pretreated with 4% NaOH
and at corn stover loading of 35 g TS/L. The enhancement in methane yield at this
condition was 30% compared to untreated corn stover. However, the most significant
enhancement in methane yield (69%) compared to untreated corn stover was observed at
the highest corn stover loading of 65 g/L with 2% NaOH pretreatment. The enhancement
in methane yield of 69% was noted. Both studied conducted by Pang et al. (2008) and
Zheng et al.(2009) shows that the effect of NaOH pretreatment on methane yield
enhancement in anaerobic digestion was affected by the substrate loading level.
23
Zhu et al. (2010) conducted a study on pretreating corn stover at a low moisture
content of 52% at ambient temperature for 24 hours followed by SS-AD (TS content of
22% at 37C). The highest biogas yield of 372.4 L/kg VS (37% higher than untreated
corn stover ) was obtained with 5% NaOH pretreatment. Increases in lignin degradation
from 9.1% to 46.2% were observed when NaOH loading was increased from 1 to 7.5% in
the pretreatment step.
Pretreatment of feedstock prior to AD was demonstrated to enhance biogas
production and VS reduction. However, additional costs incurred due to additives or
additional energy requirement thus require a balance between the pretreatment and biogas
production. NaOH pretreatment has been approved to be one of the most promising
pretreatment methods for lignocellulosic biomass in anaerobic digestion due to lower
energy demand and potential adjustment of pH of AD process by the alkali residue.
24
Table 2.4. Alkaline pretreatment of lignocellulosic biomass in anaerobic digestion
Feedstock
Alkali loading
Temperature
(C)
AD system
Methane
production
Methane
enhancement
Reference
80
Pretreatment
duration
24 hr
Wheat straw
NH4OH
(17 –
34 g OH-/kg VS)
Batch, 55C and 112
days or until biogas
ceased.
343 - 362 L/kg VS
8% - 14%
Hashimoto, 1986
Wheat straw
NaOH
(17 –
34 g OH-/kg VS)
90
1 hr
Batch, 55C and 112
days or until biogas
ceased.
328 - 383 L/kg VS
9% - 28%
Hashimoto, 1986
Wheat straw
NaOH
(21 –
62 g OH-/kg VS)
200 (using Parr
reactor)
1 hr
Batch, 55C and 112
days or until biogas
ceased.
231 - 246 L/kg VS
40% - 49%
Hashimoto, 1986
Wheat straw
NaOH
(41 g OH-/kg VS)
150 - 225
1 hr
Batch, 55C and 112
days or until biogas
ceased.
221 – 287 L/kg VS
No effect
Hashimoto, 1986
Wheat straw
NaOH, NH4OH
or Ca(OH)2
(34 g OH-/kg VS)
95
1hr
Semi-continuous, 55C
and 25 days.
3.2 – 3.8 m3/m3.day
3% - 23%
Hashimoto, 1986
Corn stover
NaOH (2 - 6%)
Ambient
3 days
Batch, 35C and 75
days.
130 to 211 L/kg VS
0% - 69%
Zheng et al., 2009
Corn stover
NaOH (4 - 10%)
Ambient
3 weeks
Batch, 35C and 75
days.
390 – 470 L/kg VS
(biogas yield)
6% - 91%
(based on
biogas yield)
Pang et al., 2008
25
Table 2.4. Alkaline pretreatment of lignocellulosic biomass in anaerobic digestion (Cont.)
Feedstock
Alkali loading
Temperature
(C)
AD system
Methane
production
Methane
enhancement
Reference
Ambient
Pretreatment
duration
24 hr
Corn stover
NaOH (1-7.5%)
Batch, 37C, 40 days
and 22% TS
267-372 L/kg VS
Up to 37%
Zhu et al., 2010
Corn stover
NaOH (1-5 %)
37C
Same as AD
Simultaneous alkaline
treatment and AD.
Batch, 37C, 40 days
and 22% TS.
199 – 301 L/kg VS
(biogas yield)
Adverse effect
up to 9%
Zhu et al., 2010
Switch grass
(winter harvest)
NaOH (7 g/L)
55C - 121C
15 min.- 3 hr
BMP, 35C and 38 days
112 -140 L/kg VS
16% - 25%
Frigon et al., 2011
26
2.3. Operation of SS-AD
Although AD being a maturing technology, failure in operation of AD can be
avoided thorough understanding of critical operational and process design parameters.
TS contents up to 30% have minimal effects on conversion rates and efficiency, at
TS contents of 30 to 50% dramatic decreases in biogas production were observed (Jewell
et al., 1993). Increasing the TS content means a decrease in moisture content; this
proportionately affects concentrations of other variables such as alkalinity, free ammonia
(Jewell et al., 1993) and VFA. TS contents of 30 to 40% have been reported to cause
inhibition of AD due to build-up of VFA (Wujcik and Jewell, 1980). Most of the SS-AD
is operated with solid contents between 20% and 40% (Valorga, 2011).
As nitrogen could be bound in lignin structures (Deublin and Steinhauser, 2008),
C/N ratios are only recommended to be used as an indication in operation of AD process.
Various optimal C/N ratios (ranging from 15 to 30) have been reported in published
literature depending on the feedstock (Li et al., 2011b). Scharer and Moo-Young (1979)
reported an optimal C/N ratio of 25 to 35 in AD. A balance in C/N ratio is required as
high nitrogen contents can lead to increases in ammonia production (release of
ammonium compounds from degradation of nitrogenous compounds) which could be
inhibitory whereas low nitrogen contents could cause a negative impact on microbial
metabolism, as nitrogen is required for protein formation (Deublin and Steinhauser,
2008). In a study conducted by Shiralipour and Smith (1984), addition of nitrogenous
compound (in the form of urea) increased gas production in AD (batch in 250ml serum
bottle, digestion duration varied with sample) of root, shoot and plant material. The
39
nitrogen concentration ranged from 0 to 10 mg/L and the highest methane yield was
observed at 10 mg/L.
Inoculum size is another important parameter in SS-AD. Inoculum in SS-AD is
normally obtained from active anaerobic digesters (e.g. sludge reactors, bed reactors,
UASB, liquid digester) (Angelidaki et al. 2009) or recycle of SS-AD digestate when
leachate percolation systems are in used. Although the effect of large inocula on reducing
the lag phase duration is well known (Chen and Hashimoto, 1996), studies on the effect
of inoculum size on methane production in SS-AD are scarce. For SS-AD that are located
without easy access to an inoculum source, inoculum proportion therefore becomes a
critical operating factor that could potentially affect process economics due to associated
higher logistical costs (in hauling of inoculum). For SS-AD that is being integrated with
an operating liquid digester, excess inoculum could result in an increasing need for
digestate dewatering which requires greater energy consumption (from centrifugation or
dewatering process). Hence, the need to optimize such factor arises.
The optimal pH for methanogens is 6.8 to 7.2, while for acid-forming (acidogenic
and acetogenic) bacteria it is around 6 (Moosbrugger et al., 1993; Zoetemeyer et al.,
1982). The growth rate of methanogenic microbes decreases sharply below pH 6.6
(Mosey and Fernandes, 1989). Therefore, recommended operational pH value range from
6.6 to 7.4 in AD.
An important parameter in AD systems is alkalinity, which is a measure of the
chemical buffering capacity of the aqueous solution. This is essential such that sufficient
buffering capacity is provided to neutralize any possible VFA accumulation in the reactor
40
to maintain optimal pH values (Callander and Barford, 1983). The extent of pH drop by
VFA depends on the alkalinity concentration, which is defined as the total protonaccepting capacity of the carbonate weak-acid subsystem combined with the protonaccepting capacity of the water system (Loewenthal et al., 1991). However, pH
measurement should not be used as the sole indication of imminent failure in AD as a
well-buffered digester may not show signs of pH drop despite high concentrations of
VFA (Lahav and Morgan, 2004). VFA concentration and composition have long been
recognized as important monitoring parameters for control of AD (Chynoweth and Mah,
1971; Fischer et al., 1984; Hill and Bolte, 1989). In a study conducted by Ahring et al.
(1995), VFA composition was found to be a good parameter to predict process stability.
Significant increases in both isobutyrate and butyrate concentration were noted as early
as 2 days after a perturbation was induced (increasing temperature from 55C to 59C).
This was also reported by others that isoforms of butyrate and valerate are better indicator
of process instability (Hill and Bolte, 1989; Hill and Holmberg, 1988). The changes in
ratio of propionate to acetate were however found to be slow and therefore considered to
be unsuitable (Ahring et al., 1995).
41
Chapter 3 Effect of Substrate to Inoculum Ratio on Methane Production in SolidState Anaerobic Digestion of Lignocellulosic Biomass
3.1. Introduction
Lignocellulosic biomass feedstocks, such as energy crops, agricultural forestry and
municipal wastes, have attracted much attention in bioenergy related studies.
Lignocellulosic biomass feedstocks typically contain 35-50% cellulose, 20-35%
hemicellulose and 15-25% lignin (Wyman, 1994). Lignocellulosic biomass feedstocks
generated worldwide from agricultural residue alone came to an average of 1091 million
tons annually (Sánchez, 2009).
Although many of the studies on conversion of lignocellulosic biomass to bioenergy
have been limited to ethanol production, studies have also shown that substantial
conversion of biomass to methane can be achieved in anaerobic digestion (AD) (Turick et
al., 1991; Jerger et al., 1982; Chynoweth et al., 1985).
Lignocellulosic biomass feedstocks have been considered to be suitable for solidstate anaerobic digestion (SS-AD) mainly due to the abundance. SS-AD is generally
operated at a total solid (TS) content of 20% or higher (Guendouz et al., 2010). The
major advantages of SS-AD compared to liquid anaerobic digestion (operates at lower TS
content of 15% of less) include reduction in reactor volume, minimal agitation thus less
moving parts and lower energy input for heating due to smaller operating volume (Li et
al., 2011).
42
Published literature regarding the effects of substrate to inoculum (S/I) ratio (VS of
substrate to VS of inoculum) on methane yield is scarce (Raposo et al., 2006).
Furthermore, most of these studies were limited to liquid AD (Hashimoto, 1989;
Gunaseelan, 1995; Raposo et al., 2006).
Another challenge for biological converstion of lignocellulosic biomass is the
recalcitrant nature that impedes biodegradability. The rate of hydrolysis has been
regarded as a limiting factor in anaerobic digestion of lignocellulosic biomass. This is due
to shielded biodegradable carbohydrates (cellulose and hemicelluloses) by lignin which
causes longer retention times (Adney et al., 1990). Factors that could affect the anaerobic
biodegradability of lignocellulosic materials include: lignin content, cellulose and
hemicellulose content, cellulose crystallinity, pore volume, particle size, total surface
availability for enzymatic reaction and the structural characteristic of lignin (Cowling,
1975; Chandler et al., 1980; Jerger et al., 1982; Kenny et al., 1990; Tong et al., 1990).
Lignin content however is believed to be one of the critical factors that affect the
biodegradability of lignocellulosic biomass in AD (Tong et al., 1990; Turick et al., 1991).
Most of these studies on how lignin content affects methane yield were however limited
to liquid AD (Chandler et al., 1980; Tong, 1990; Chynoweth, 1993).
Therefore, the objective of this study is to determine the effects of S/I ratio and lignin
content of lignocellulosic biomass on methane production from SS-AD of lignocellulosic
biomass.
43
3.2. Materials and Methods
3.2.1. Feedstock and inoculum
Four types of lignocellulosic biomass were selected in this study: corn stover,
wheat straw, leaves and yard waste which is a blend of grass clippings and bushes. Both
corn stover and wheat straw were collected in October 2009 from farms operated by Ohio
Agricultural Research and Development Center (OARDC) in Wooster, OH. Fallen tree
leaves were collected in October 2010 from OARDC campus in Wooster, OH. Fresh yard
waste was obtained in October 2010 from a local composting site in Wooster, OH. All
feedstocks were oven dried at 40°C for 48 h in a convection oven (Precision Thelco
Model 18, Waltham, MA) to attain a moisture content of less than 10% and then stored in
air tight containers for later use. The samples were ground to pass through a 9 mm screen
with a grinder (Mackissik, Parker Ford, PA).
Effluent from a mesophilic liquid anaerobic digester (operate in Akron, OH) fed
with municipal waste was used as inoculum in this research. Effluent was kept in air-tight
drums at 4C in a walk-in cooler. Prior to use, the inoculum was starved for 1 week by
incubation at 37C with constant mixing to reactivate microbiological activity in stored
effluent and remove the easily degradable VS present in inoculum.
3.2.2. Solid-state anaerobic digestion
Four S/I ratios of 2, 3, 4 and 5 (Table 3.1) were tested at a fixed initial TS content
of 22% for all reactors. The inoculum and lignocellulosic biomass were mixed by a handmixer (Black & Decker, 250-watt mixer, Towson, MD) and deionized water (DI) was
added to adjust the TS content to 22% when needed. Well-mixed material was tapped and
44
loaded into a 1 L glass reactor. Reactors were incubated in a walk-in incubation room for
up to 30 days at 37 ± 1 C. Duplicate reactors were run for each S/I ratio and feedstock
combinations. Inoculum without any feedstock addition was used as a control. Biogas
generated was collected by a 5 L gas bag (CEL Scientic Tedlar gas bag, Santa Fe Springs,
CA) attached to the outlet of the reactor. Biogas composition and volume were
determined every 2 days during the 30-day AD.
Table 3.1. Design of reactors using four different feedstock
S/I ratio
Corn stover
2
3
4
5
Wheat straw
2
3
4
5
Yard waste
2
3
4
5
Leaves
2
3
4
5
Substrate TS, g
Inoculum TS, g
Substrate VS, g
Inoculum VS, g
Substrate wet weight, g
Inoculum wet weight, g
1.16
1.55
1.91
2.12
1.99
2.99
4.01
4.99
0.15
0.22
0.30
0.37
1.11
1.49
1.74
2.15
2.01
2.99
3.99
4.99
0.15
0.22
0.30
0.37
0.84
0.97
1.09
1.23
1.99
3.01
4.00
4.99
0.15
0.22
0.29
0.36
0.51
0.62
0.70
0.77
2.00
2.99
3.99
5.00
0.15
0.23
0.31
0.39
45
3.2.3. Analytical methods
The extractive content of feedstock and material taken from the reactor at the
beginning and end of the AD process was measured according to the NREL Laboratory
Analytical Procedure (Sluiter et al., 2008). Extractive-free solid fractions were further
fractionated using a two-step acid hydrolysis method based on NREL Laboratory
Analytical Procedure (Sluiter et al., 2010). Monomeric sugars (glucose, xylose, galactose,
arabinosev and mannose) and cellobiose in the acid hydrolysate were measured by HPLC
(Shimadzu LC-20AB, MD, USA) equipped with a Biorad Aminex HPX-87P column and
a refractive index detector (RID). Deionized water at flow rate of 0.6 ml/min was used as
the mobile phase. The temperatures of the column and detector were maintained at 80C
and 55C, respectively.
The TS and VS of feedstocks, inoculum, and digestate were analyzed at the
beginning and end of the AD process according to the Standard Methods for the
Examination of Water and Wastewater (APHA, 2005). Total carbon and nitrogen
contents were determined by an elemental analyzer (Elementar Vario Max CNS,
Elementar Americas, Mt. Laurel, NJ, USA). Total volatile fatty acids (TVFAs) and
alkalinity were measured using a 2-step titration method (McGhee, 1969; Hach Lange,
2010). Samples for pH, TVFA, and alkalinity measurement were prepared by diluting a
5-g sample with 50 ml of deionized water and subsequently filtering it using cheese cloth.
The filtrate was then analyzed using an auto-titrator (Mettler Toledo, DL22 Food &
Beverage Analyzer, Columbus, OH, USA). The TVFAs/alkalinity ratio was calculated to
determine the risk of acidification, a measure of the process stability (Lossie and Pütz,
46
2010). The volume of biogas collected in a Tedlar bag was measured with a drum-type
gas meter (Ritter, TG 5, Germany) and the composition of biogas (CO2, CH4, N2 and O2)
was analyzed using a GC (Agilent Technologies, HP 6890, DE, USA) equipped with a
10-ft stainless steel column 45/60 Molecular Sieve 13X and a Thermal Conductivity
Detector. Helium at a flow rate of 5.2 ml/min was used as a carrier gas. The temperature
of the detector was set at 200C. The temperature of the column oven was initially
programmed at 40C for 4 minutes, elevated to 60C at 20 C/minute and held for 5
minutes.
Methane yield expressed as L/kg VS was calculated by the volume of methane gas
produced per kg of VS loaded in reactor at start-up. Methane yield expressed in L/kg
VSfeedstock however refers to methane yield corrected by subtracting the methane volume
obtained from the control (Angelidaki et al, 2009; Chynoweth et al., 1981) for every1 kg
of feedstock VS loaded in reactor at start-up. Methane productivity of lignocellulosic
biomass is expressed in L/Lwork: volume of methane gas produced per unit working
volume of reactor.
3.2.4. Statistical analysis
Statistical significance was determined by analysis of variance (ANOVA) using SAS
software (Version 8.1, SAS Institute Inc., Cary, NC, USA) with a threshold p-value of
0.05.
47
3.3. Results and discussion
3.3.1. Composition analysis of inoculum and lignocellulosic biomass
Table 3.2 shows the TS, VS, bulk density, carbon and nitrogen content, extractives,
lignin, cellulose, hemicellulose, total VFAs, alkalinity, of inoculum and lignocellulosic
biomass tested in this study. The bulk density of the ground lignocellulosic biomass
varied substantially. Yard waste has the highest bulk density of 252.0 g/L, followed by
leaves (154.9 g/L), and corn stover (84.3 g/L); whereas wheat straw has the lowest bulk
density of 62.3 g/L.
Among the four types of feedstocks, cellulose contents of corn stover (33.7%) and
wheat straw (32.3%) were relatively higher; followed by yard waste (21.7%) and leaves
(12.2%). Similar results were obtained for hemicellulose content among the feedstocks
tested (corn stover - 19.1%, wheat straw - 17.9%, yard waste - 14.2% and leaves 10.6%). The highest lignin content was found in yard waste (26.02%) followed by leaves
(23.1%). Relatively lower lignin content was noted in corn stover (15.2%) and wheat
straw (17.4%).
Another component that is worth mentioning is the extractives, which includes
compounds such as free sugar, oligomers and organic acid (Chen et al., 2007; Chen et al.,
2010). These compounds are easily degradable and can potentially contribute to biogas
generation (Tong and Mccarty, 1991) however depending on the composition of
extractives. Leaves contained the highest amount of extractives (34.9%, water and
48
ethanol soluble combined) followed by yard waste (17.8%), wheat straw (13.4%) and
corn stover (9.9%).
Table 3.2. Charateristics of inoculum and lignocellulosic biomass.
Parameters
Inoculum
TS %
10.6 ± 0.00
VS %
6.9 ± 0.00
Bulk density, g/L
N/D
Carbon content , % a
4.6
Nitrogen content, % a
0.6
Extractives, % b
N/D
Lignin, % b
N/D
Cellulose, % b
N/D
Hemicellulose, % b
N/D
pH
8.5 ± 0.0
TVFA, g/L
7.5 ± 0.9
Alkalinity, g CaCO3/L
18.3 ± 0.3
a
As total weight of sample
b
As TS of sample
N/D = not determined
Standard deviation shown in parenthesis
Corn stover
97.0 ± 0.1
92.6 ± 0.0
84.3 ± 1.9
42.6
0.6
9.9 ± 0.7
15.2 ± 0.2
33.7 ± 0.2
19.1 ± 0.5
N/D
N/D
N/D
Wheat straw
97.8 ± 0.0
92.6 ± 0.0
62.3 ± 1.4
45.8
0.6
13.4 ± 0.0
17.4 ± 0.3
32.3 ± 0.2
17.9 ± 0.2
N/D
N/D
N/D
Yard waste
98.0 ± 0.4
94.6 ± 0.0
252.0 ± 4.0
49.0
0.7
17.8 ± 0.9
26.0 ± 0.0
21.7 ± 1.6
14.2 ± 0.1
N/D
N/D
N/D
Leaves
95.9 ± 0.1
89.2 ± 0.0
154.9 ± 2.6
48.8
1.1
34.86 ± 1.8
23.06 ± 0.3
12.23 ± 0.7
10.58 ± 0.3
N/D
N/D
N/D
3.3.2. Biogas production
The total methane yields in term of L/kg VS during 30 days of SS-AD are presented in
Figure 3.1(a). Overall, methane yield obtained from SS-AD of these feedstocks increases
with decreasing S/I ratio. Among the four S/I ratios tested, the highest total methane yield
was observed at S/I ratio of 2 for all the feedstocks tested( corn stover -81.2 L/kg VS,
wheat straw-66.9 L/kg VS, yard waste 40.8 L/kg VS and leaves -55.4 L/kg VS). The total
methane yields observed at S/I ratio of 3 for all feedstocks except leaves, were however
not significantly different (P>0.05) from methane yields obtained at S/I ratio 2. Lower
methane yield was observed for all feedstock except wheat straw as the S/I ratio
increased to 4. The methane yields were substantially decreased when S/I ratio was
increased to 5. Higher S/I ratio could contribute to organic overloading; which was
49
indicated by the presence of higher concentration of organic acids (Chynoweth, 1993).
Study conducted by Hashimoto (1989) on anaerobic digestion of wheat straw in batch
operation at 35C showed that significant reduction in ultimate methane yield was
observed at S/I ratio above 4. Gunaseelan (1995) observed increase in methane yield with
decreasing S/I ratio in liquid AD of Parthenium, a terrestrial weed. Hashimoto et at.
(1989) also observed increase in methane yield with decreasing S/I ratio in liquid AD of
wheat straw. Recommended S/I ratio for liquid AD that have been reported in literature
ranges from 0.5 to 2.3 (Turick et al., 1991; Chynoweth et al, 1993; Neves et al., 2006).
Based on this study, recommended S/I ratio for SS-AD of lignocellulosic biomass (corn
stover, wheat straw, yard waste and leaves) is around 2 which is in agreement with that
obtained in liquid anaerobic digestion studies.
Compared to the methane yield obtained in the liquid anaerobic digestion studies, the
methane yields obtained from SS-AD of lignocellulosic biomass in this study were lower.
For corn stover, Tong et al. (1990) observed a methane yield of 360 L/kg VS, which is
approximately 4.4-fold higher that methane yield obtained in this study. For wheat straw,
Tong et al. (1990) and Hashimoto (1989) observed a methane yield of 332 L/kg VS,
which is approximately 5-fold higher that methane yield obtained in this study. For yard
waste, Owens and Chynoweth (1993) observed a value of 143 L/kg VS, which is
approximately 3.5-fold higher that methane yield obtained in this study. For leaves,
Owens and Chynoweth (1993) observed a value of 123 L/kg VS, which is approximately
2.2-fold higher that methane yield obtained in this study.
50
Digester operating at higher TS content (2 to 3 times higher than in liquid AD) may
indicate a higher gas production per unit volume which therefore increases the cost
effectiveness of SS-AD (Jewell, 1993). Therefore, methane productivity (L/Lwork) of each
feedstock was evaluated in addition to the methane yield (L/kg VS). Methane
productivity observed in this study is shown in Figure 1(b). For all lignocellulosic
biomass feedstocks tested, the highest methane productivity was noted with S/I ratio of 2
and 3. The difference in bulk density of each feedstock (shown in table 1) implies that
more VS could be loaded per unit working volume with feedstock having higher bulk
density. At S/I ratio of 2 and 3, methane productivity from SS-AD of corn stover was the
highest (an average of 10.8 L/Lwork) among four feedstocks tested. This was followed by
SS-AD of leaves with an average methane productivity of 9.1 L/Lwork, which was 16%
less than that of corn stover. Methane productivity noted in SS-AD of both wheat straw
and yard waste was considerably lower. Methane productivity attained in SS-AD of
wheat straw was 7.8 L/Lwork, an average of 29% less than the methane productivity of
corn stover. The lowest methane productivity was observed with SS-AD of yard waste;
approximately 36% less compared to methane productivity attained with corn stover. A
methane productivity of 9 L/Lwork was obtained from liquid AD of corn stover in the
study of Zheng et al. (2009), which was 17% lower than that obtained in this study with
SS-AD.
51
(a)
Total methane yield (L/kg VS)
90
S/I = 2
S/I = 3
S/I = 4
S/I = 5
(n = 2)
80
70
60
50
40
30
20
10
0
Corn stover
Wheat straw
Yard waste
Leaves
Lignocellulosic biomass
(b)
Methane productivity (L/Lwork)
14
S/I = 2
S/I = 3
S/I = 4
S/I = 5
(n = 2)
12
10
8
6
4
2
0
Corn stover
Wheat straw
Yard waste
Leaves
Lignocellulosic biomass
Figure 3.1. Methane production obtained from SS-AD of lignocellulosic biomass in 30
days. (a) Methane yield expressed as L/kg VS (VS = total VS); (b) Methane productivity
expressed as L/Lwork.
52
Figure 3.2 shows that daily methane yield observed during the 30 days’ SS-AD
period. Daily methane yield for the initial 4 days was minimal (less than 1.5 L/kg VS) for
corn stover and wheat straw; higher daily methane yields up to 2.2 L/kg VS were
however observed for yard waste and leaves during the initial 4 days of SS-AD. This was
likely caused by the higher extractives contents (Table 3.1) in yard waste and leaves.
At S/I ratio of 4, peaks in daily methane yield in SS-AD of yard waste and leaves were
delayed to day 18 or later. This was however not found with SS-AD of corn stover and
wheat straw. This lag phase was likely attributed by the low inoculum proportion. When
the S/I ratio was increased, the volatile fatty acid generated from the extractives was also
increased. Due to the high extractives content of leaves and yard waste, the amount of
volatile fatty acids might inhibit the methanogens and thus delayed the methane peak.
a) Corn stover
Daily methane yield (L/kg VS)
9
8
S/I = 2
7
S/I = 3
S/I = 4
6
S/I = 5
5
4
3
2
1
0
0
2
4
6
8
10
12
14 16 18
Time (day)
53
20
22
24
26
28
30
b) Wheat straw
Daily methane yield (L/kg VS)
14
S/I = 2
12
S/I = 3
10
S/I = 4
8
S/I = 5
6
4
2
0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
Time (day)
c) Yard waste
Daily methane yield (L/kg VS)
5
S/I = 2
S/I = 3
4
S/I = 4
3
S/I = 5
2
1
0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
Time (day)
d) Leaves
54
Daily methane yield (L/kg VS)
7
S/I = 2
6
S/I = 3
5
S/I = 4
4
S/I = 5
3
2
1
0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
Time (day)
Figure 3.2. Daily methane yield of lignocellulosic biomass in 30-day SS-AD. (a) Corn
stover, (b) Wheat straw, (c) Yard waste and (d) Leaves
First-order kinetic model has been successfully used to characterize the methane
production of lignocellulosic biomass in liquid AD (Tong, 1990; Turick, 1991; Jewell,
1993; Chynoweth, 1993; Angelidaki et al., 2009). The first-order kinetic model shown in
equation (1) can be linearized as shown in equation (2).
(
)
Nomenclatures:
t = time in term of day
Mu = methane yield obtained in 30 days, L/kg VSfeedstock
55
Mt = methane yield obtained at time t, L/kg VSfeedstock
M = methane yield potential remained at time, M = Mu – Mt
As shown in Figure 3.3., there was a linear relationship between the logarithmic
methane production and reaction time in SS-AD of lignocellulosic biomass. Correlation
coefficients (r2) between 0.91 and 0.98 (Table 3.3.) were obtained at S/I ratio 2 to 4. This
indicates that the methane production in SS-AD of lignocellulosic biomass followed the
simple first-order kinetic model. At S/I ratio 4 and 5, the r2 values reduced to 0.70 and
0.67 for yard waste and leaves, respectively. This could be explained by the lag phase
noted in daily methane production at S/I ratios of 4 and 5 (Figure 2(c) and (d)) which was
not observed in corn stover and wheat straw.
3.0
Corn stover
2.5
ln. ( Mu / M )
Wheat straw
Yardwaste
2.0
Leaves
1.5
1.0
0.5
0.0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
Time (day)
Figure 3.3. Logarithmic plot for methane production from lignocellulosic biomass with
S/I ratio of 2
56
Table 3.3. Correlation coefficients of lignocellulosic biomass using first-order kinetic
model
Feedstock
S/I ratio 1
Correlation
coefficient, r2
Corn stover
2
0.9684
3
0.9559
4
0.9749
Wheat straw
2
0.9681
3
0.9594
4
0.9719
Yard waste
2
0.9759
3
0.9879
4
0.8560
5
0.7020
Leaves
2
0.9194
3
0.9652
4
0.6717
5
0.5734
1 Regression analysis was not performed for failed SS-AD process
3.3.3. Relationship between biogas production and lignin content
As shown in Figure 3.4, an inverse linear relationship was obtained between the
methane yield and lignin content of each feedstock. Strong linear correlations were
obtained at S/I ratio of 2 and 3 (r2 = 0.95 and 0.97, respectively), while the r2 value was
0.84 at S/I ratio 4. According to a study on liquid AD conducted by Tong (1990), a poor
linear correlation (r2 = 0.59-0.69) was observed between methane conversion efficiency
(expressed as percentage of methane yield over theoretical methane yield) and lignin
content of lignocellulosic biomass. Although it was inadequate for the prediction of
methane production rate based solely on lignin content, slightly higher r2 value (0.82)
between lignin content and rate constant, k (determined from simple first-order model)
was obtained by Tong et al. (1990). Chandler et al. (1980) however reported a high r2
value of 0.94 for the inverse linear relationship between lignin content and VS
57
destruction. Published literature on this topic is however limited to liquid AD of
lignocellulosic biomass. This study shows that methane yields of lignocellulosic biomass
in SS-AD could be predicted based upon the lignin content of feedstock.
Total methane yield (L/kg VSfeedstock)
120
S/I = 2
100
S/I = 3
S/I = 4
80
R² = 0.9469
60
R² = 0.8393
40
R² = 0.9717
20
0
10
15
20
25
Lignin content (as % of dry matter)
30
Figure 3.4. Correlation between total methane yield (expressed in L/kg VSfeedstock) and
lignin content of lignocellulosic biomass
3.3.4. Degradation of holocellulose and extractives
Degradation of holocellulose and extractives during 30-day SS-AD with S/I ratio of 2
was presented in Figures 3.5. Compositional analysis was performed on samples obtained
at the beginning and day 30 of the SS-AD process. Changes in the composition were
expressed as the weight of each compound measured based on initial loading level of 100
g TS. The highest cellulose degradation of 41% was observed during SS-AD of corn
stover. This was followed by wheat straw (36%), leaves (16%) and yard waste (6%). A
similar trend was noted in hemicellulose degradation. The highest hemicellulose removal
58
was noted with SS-AD of corn stover (34%) followed by wheat straw (35%), leaves
(21%) and yard waste (7%). The degradation of cellulose and hemicellulose was affected
by the lignin content of tested lignocelluosic biomass; higher degradation was associated
with lower lignin content.
A different trend was however observed in the degradation of extractives among the
four feedstocks. The highest degradation of 57% in extractives was observed in SS-AD of
leaves. This was followed by yard waste (56%), corn stover (14%) and wheat straw (0%).
The degree of extractives degradation among the feedstock tested was in agreement to the
daily methane yield trend discussed in section 3.3.2; high levels of degradation of
extractives in leaves and yard waste corresponded with high daily methane yield
observed during the early phase (initial 4 days) of SS-AD.
25
Initial
Cellulose, g
20
Final
(n = 2)
15
10
5
0
Corn stover
Wheat straw
Yard waste
Lignocellulosics biomass
59
Leaves
Hemicellulose, g
16
14
Initial
12
Final
(n = 2)
10
8
6
4
2
0
Corn stover
Wheat straw
Yard waste
Lignocellulosics biomass
Leaves
30
Initial
25
Etractives, g
Final
20
(n = 2)
15
10
5
0
Corn stover
Wheat straw
Yard waste
Lignocellulosics biomass
Leaves
Figure 3.5. Degradation of holocellulose and extractives during 30 days of SS-AD, based
on initial loading of 100g TS.
3.4. Conclusion
Among the four lignocellulosic biomass feedstocks studied, the methane yields of
crop residue (corn stover and wheat straw) were much higher than that obtained from
yard waste and leaves during SS-AD at TS of 22%. The S/I ratio gave the highest
60
methane yield was 2 for all feedstocks. However, due to the difference in bulk density
among the tested feedstocks, the methane productivity of leaves was higher than those of
wheat straw and yard waste, but lower than that of corn stover. The methane production
during SS-AD of lignocellulosic biomass followed the simple first-order kinetics model
with correlation coefficients of r2 ranging from 0.91 to 0.98. An inverse linear
relationship was obtained between the methane yield and lignin content of lignocellulosic
biomass. Methane production during SS-AD of corn stover and wheat straw was mainly
contributed by the degradation of cellulose and hemicellulose. This was shown by
substantial degradation of cellulose and hemicellulose (34% to 41%) for corn stover and
wheat straw compared to yard waste and leaves (6% to 21%) during 30 days of SS-AD.
Methane production during SS-AD of yard waste and leaves was however likely to be
contributed by degradation of extractives. This was shown by relatively higher
degradation of extractives (56% to 57%) during 30 days of SS-AD for yard waste and
leaves compared to corn stover and wheat straw (0% to 14%).
61
Chapter 4 Enhancing the Solid-state Anaerobic Digestion of Fallen Leaves through
Simultaneous Alkaline Treatment
4.1. Introduction
Due to concerns about the sustainability of petroleum supplies, the research
community is evaluating alternative resources for fuels and energy production.
Lignocellulosic biomass, such as energy crops, agricultural and municipal solid wastes, is
a promising renewable resource because it is widely available and can be converted to
various forms of fuel and energy. Biogas, which contains about 60-70% methane, can be
obtained from the anaerobic digestion (AD) of organic materials. However, due to the
recalcitrant structure and composition of lignocellulosic biomass, such as lignin that
interlinks cellulose and hemicellulose layers, the conversion efficiency is limited (Noike
et al., 1985). Hydrolysis of native lignocellulosic biomass is rate-limiting because of the
low cellulolytic activity and low specific growth rate of cellulolytic microbes in
anaerobic digesters (Lu et al., 2007). Therefore, pretreatment is often required to
overcome biomass recalcitrance in order to facilitate the access of hydrolytic enzymes to
degradable carbohydrates to improve sugar release and biogas production.
AD efficiency of lignocellulosic biomass can be improved by applying several
pretreatment methods including steam, acid, alkaline and biological treatment (Penaud et
al., 1999; Frigon et al., 2011). Alkaline pretreatment is often favored for anaerobic
digestion and sodium hydroxide (NaOH) was found to be one of the most effective
alkalis for improving biogas production (Taherzadeh and Karimi, 2008). Alkaline
62
pretreatment greatly improves the digestibility of lignocellulosic biomass through lignin
solubilization, removal of hemicellulose, disruption of interlinking ester bonds, and
neutralization of structural carboxylic acids (Mosier et al., 2005). In addition, alkalis help
to prevent a drop of pH during the subsequent acidogenesis process and increase the
efficiency of methanogenesis (Hashimoto, 1986; Pavlostathis and Gossett, 1985a).
Alkaline pretreatment performed at low moisture and ambient temperature is particularly
attractive. Zhu et al. (2010) reported that anaerobic digestion of alkaline pretreated corn
stover produced 37% more biogas compared with untreated corn stover. The pretreatment
was carried out with 5% NaOH at 53% moisture content for 1 day at ambient
temperature. In a study conducted by Pang et al. (2008), a 48.5% increase in biogas was
achieved with higher NaOH loading (6%) and moisture content (80%) for longer
retention time (3 weeks). In a follow-up study, a 72.9% increase in total biogas yield was
achieved with lower NaOH loading (2%) and shorter pretreatment time (3 days), when
pretreatment moisture content was increased to 88% (Zheng et al., 2009). These studies
indicate that alkaline pretreatment of lignocellulosic biomass is feasible with lower
moisture content but may require relatively higher NaOH loading or longer retention
time.
Solid-state anaerobic digestion (SS-AD) refers to an AD process operated at total
solids (TS) content of 20-55%. It has been used to digest the organic fraction of
municipal solid waste in Europe (Bolzonella et al., 2003). SS-AD is well suited to handle
lignocellulosic biomass and address problems encountered in liquid anaerobic digestion,
such as floating and stratification of solids can be avoided in SS-AD (Chanakya et al.,
63
1993). Compared to liquid anaerobic digestion (TS less than 15%), SS-AD has
advantages such as less energy needed for heating, finished materials with higher TS
content (20%), and no moving parts in the digester (Li et al., 2011b). However, it requires
large amounts of inoculum, longer retention time, and nitrogen supplementation when
lignocellulosic biomass is used (Jewell et al., 1993; Li et al., 2011b). Furthermore,
pretreatment is generally required for lignocellulosic biomass to improve the efficacy of
SS-AD (Li et al., 2011b). Pretreatment methods, such as alkaline treatment prior to the
AD process, have previously been established to increase the digestibility of
lignocellulosic biomass and methane yield in SS-AD systems (Zhu et al., 2010).
However, to our knowledge, no successful results have been reported on the simultaneous
alkaline treatment and SS-AD of lignocellulosic biomass.
Simultaneous alkaline treatment and digestion offers several benefits compared
with alkaline pretreatment followed by digestion. It can simplify the operation by
eliminating a separate reactor required for alkaline pretreatment and reducing material
handling. Additionally, the increase in alkalinity may help prevent a drop of pH during
acidogenesis, which can create a more stable environment for the methanogenic bacteria
(Pavlostathis and Gossett, 1985a). However, excessive NaOH loading may inhibit
anaerobic digestion either due to high pH or sodium ion toxicity (Rinzema et al., 1988).
A recent study by Zhu et al., (2010) tested simultaneous NaOH treatment and SS-AD of
corn stover at a C/N ratio of 18 and NaOH loading of 5%. However, no significant
improvement in biogas production was observed compared with untreated corn stover.
Appropriate NaOH loading needs to be established such that it is sufficient for
64
delignification while not inhibiting the AD process. Furthermore, as the amount and
activity of inoculum greatly affect methane yield and retention time for SS-AD (Raposo
et al., 2006; Li et al., 2011a), NaOH loading needs to match substrate-to-inoculum (S/I)
ratio during simultaneous alkaline treatment and SS-AD. Fallen leaves (leaf litter) are
potentially a low cost feedstock for SS-AD because a tipping fee is normally charged for
collection and hauling of such wastes from residential or commercial areas. The
objectives of this study were to determine the effect of NaOH loading and S/I ratio on
daily and cumulative methane production during SS-AD of leaves. Changes in total
volatile fatty acids (TVFAs), alkalinity, and pH were measured and correlated to methane
yield. In addition, degradation of cellulose and hemicellulose during SS-AD was
investigated and compared to methane yield to verify the effect of NaOH treatment.
4.2. Materials and methods
4.2.1. Feedstock and inoculum
Fallen leaves were collected from the campus of the Ohio Agricultural Research
and Development Center (OARDC) in Wooster, OH, USA (4048’33’’N, 8156’14’’W)
in October 2009. Leaves were dried at 40 °C for 72 h in a convection oven (Shel Lab
FX28-2, Sheldon Manufacturing, Cornelius, OR, USA) to achieve a moisture content of
less than 10% before storing in an air tight container. Prior to use, oven-dried leaves were
ground through a 9 mm sieve with a grinder (Mighty Mac, MacKissic Inc., Parker Ford,
PA, USA). Effluent from a mesophilic liquid anaerobic digester, which was fed food
processing waste and operated by quasar energy group (Wooster, OH, USA), was used
as the inoculum for SS-AD. Due to the low TS content, the effluent was dewatered by
65
centrifugation. TS content increased from 3.9% to 6.1% after dewatering. Characteristics
of leaves and inoculum are shown in Table 4.1. Structural carbohydrate and lignin
contents of leaves are based on dry matter, whereas the rest of the values are based on
total weight.
Table 4.2 Characteristics of leaves and inoculum.
Parameter
Leaves
Inoculum
Total solids (%)
91.6 ± 0.0
6.2 ± 0.0
Volatile solids (%)
85.1 ± 0.0
4.0 ± 0.0
Total carbon (%)
45.4 ± 0.2
2.7 ± 0.0
Total nitrogen (%)
0.9 ± 0.0
0.5 ± 0.0
Carbon to nitrogen (C/N) ratio
51.9 ± 1.8
5.5 ± 0.2
pH
6.8 ± 0.1
8.0 ± 0.0
Alkalinity (g CaCO3/kg)
3.5 ± 0.0
8.9 ± 0.1
Total volatile fatty acid (g/kg)
1.5 ± 0.1
3.3 ± 0.1
Water soluble extractives (%)
25.7 ± 0.4
N/D
Ethanol soluble extractives (%)
7.3 ± 0.3
N/D
Cellulose (%)
11.1 ± 0.4
N/D
Hemicellulose (%)
11.5 ± 0.1
N/D
Lignin (%)
22.7 ± 0.6
N/D
ND: not determined.
4.2.2 Solid-state anaerobic digestion with simultaneous NaOH treatment
Oven-dried and ground leaves were mixed thoroughly with appropriate amount of
inoculum effluent and NaOH pellets (pre-dissolved in effluent) to achieve three S/I ratios
(on VS basis) at 4.1, 6.2, and 8.2, with NaOH concentrations of 2%, 3.5%, and 5% (on
basis of dried leaves ) for each S/I ratio (a total of 9 conditions). Reactors without any
NaOH addition were run in parallel at each S/I ratio as controls. The C/N ratios were 18,
22 and 25, at S/I ratios of 4.1, 6.2 and 8.2, respectively. Deionized water was then added
66
to obtain a TS content of 20% when necessary. Mixed materials were loaded into 1-L
glass reactors. Reactors were sealed with a rubber stopper, and placed in a walk-in
incubator for 30 days at a constant temperature of 37C and without agitation. Biogas
generated was collected using a 5-L gas bag attached to the outlet of the reactor (CEL
Scientific Tedlar gas bag, Santa Fe Springs, CA, USA) and biogas composition and
volume were measured daily for the first 15 days and every two days afterwards.
Duplicate reactors were run at each condition.
4.2.3. Analytical methods
The analytical methods employed in as stated in section 3.2.3. Total volatile fatty
acids (TVFAs) and alkalinity were measured using a 2-step titration method (McGhee,
1969; Hach Lange, 2010). Samples for pH, TVFA, and alkalinity measurement were
prepared by diluting a 5-g sample with 50 ml of deionized water and subsequently
filtering it using cheese cloth. The filtrate was then analyzed using an auto-titrator
(Mettler Toledo, DL22 Food & Beverage Analyzer, Columbus, OH, USA). The
TVFAs/alkalinity ratio was calculated to determine the risk of acidification, a measure of
the process stability (Lossie and Pütz, 2010). The sodium ion (Na+) concentration in the
digestate was analyzed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)
(Agilent 7500, Agilent Technologies, Wilmington, DE, USA). Samples for ICP-MS
analysis were prepared by digestion using a microwave digester (MARSXpress™, CEM
Corporation, Matthews, NC) programmed with a 15-minute ramp-up time to 200C and
was then maintained at 200C for 15 minutes.
67
4.2.4. Statistical analysis
The analytical methods employed in as stated in section 3.2.4.
4.3. Results and discussion
4.3.1. Biogas production
Figure 4.1 demonstrates the effect of simultaneous NaOH treatment on total
methane yield during 30-day SS-AD at different S/I ratios. The S/I ratio is a critical factor
affecting SS-AD performance. In general, SS-AD requires higher inoculum levels than
liquid AD. Reducing the amount of inoculum allows better reactor efficiency; however, it
may result in an increase in the accumulation of VFAs and lead to reactor upset (Li et al.,
2010b). For SS-AD without NaOH addition, only S/I ratio of 4.1 gave good total methane
yields among the three tested S/I ratios, while at S/I ratios of 6.1 and 8.1, total methane
yields were very low, indicating failure of AD process. In general, simultaneous NaOH
treatment improved total methane yield for anaerobic digestion of leaves at all S/I ratios.
At S/I ratio of 4.1, NaOH additions of 2.0%, 3.5% and 5.0%, caused an increase
in methane yield of 11.7%, 21.5%, and 4.0%, respectively. The highest methane yield of
81.8 L/kg VS was attained with 3.5% NaOH loading. Increasing the NaOH loading from
3.5% to 5% resulted in decreased methane yield. The most significant improvement in
total methane yield was at S/I ratio of 6.2 with an increase in NaOH loading from 2.0% to
3.5%. When no NaOH was added, increasing the S/I ratio from 4.1 to 6.2 resulted in
upset of the AD process with about 3.0 L/kg VS of total methane produced. Addition of
NaOH at loading rates of 2.0%, 3.5% and 5.0% significantly improved biogas yield and
the total methane yield was increased by 9-, 24- and 21-fold, respectively, compared to
68
the control. The maximal enhancement (24-fold) in biogas yield was achieved with 3.5%
NaOH loading at which a total methane yield of 72.6 L/kg VS was obtained. However, it
was not significantly different (P>0.05) from that with 5.0% NaOH addition (63.6 L/kg
VS). When the S/I ratio was increased to 8.2, SS-AD failed to produce methane at 0%,
2.0%, and 3.5% NaOH loading. Although a methane yield of 37.3 L/kg VS was obtained
at 5.0% NaOH loading, it was much lower than that at S/I ratios of 4.1 and 6.2. This
indicates that simultaneous NaOH treatment not only improves methane yield but also
helps to mitigate the risk of process failure caused by high S/I ratios. Zheng et al. (2009)
also observed significant increase in methane yield with NaOH addition at higher corn
stover loading.
(n = 2)
Figure 4.1 Effect of S/I ratio and NaOH loading on total methane yield (digestion time:
30 days, TS: 20%).
69
Daily methane yields during SS-AD of leaves at S/I ratios of 4.1 and 6.2 are
shown in Figure 4.2. At S/I ratio of 4.1, the control showed no clear peaks and daily
methane yields remained at a level of 3-5 L/kg VS during the initial 20 days. However,
daily methane production for reactors subjected to 2.0% and 3.5% NaOH loading
demonstrated obvious maximal peaks of 9.5 L/kg VS at day 3 and 8.4 L/kg VS at day 6,
respectively. However, at 5.0% NaOH loading, peak methane yield of 9.4 L/kg VS did
not occur until day 15. Although, high NaOH loading helped to enhance the digestibility
of leaves, it is speculated that high levels of sodium ions and lignin degradation
compounds may inhibit metabolic activity of microorganisms, especially methanogens
(Rinzema et al., 1988). For liquid AD, the inhibitory Na+ concentration was reported in a
range of 3 to 6.5 g Na+/L (Rinzema et a., 1988). The Na+ concentration in this study
ranged from 1.0 g/kg at 1% NaOH loading to about 6.7 g/kg at 5% NaOH loadings.
Although the inhibitory Na+ level for SS-AD has not been reported previously, inhibition
due to high sodium ions concentration was likely to occur for reactors operated at 5.0%
NaOH. Furthermore, the high initial pH of 9.1 at 5.0% NaOH loading could also
contribute to the lag phase of methane production.
As presented in Figure 4.2b, daily methane yield at S/I ratio of 6.2, shows
different patterns compared with S/I ratio of 4.1 (Figure 4.2a). Overall, much lower daily
yields were achieved for reactors with S/I ratio of 6.2. The control reactor without NaOH
addition had very low methane yields during the 30-day period indicating reactor failure.
Simultaneous alkaline treatment at different NaOH loadings improved methane
production, although a long lag phase was observed for all reactors especially for those
70
with 3.5% and 5.0% NaOH loadings. The peak for daily methane production was delayed
to between day 11 to 15 for 3.5% and 5.0% NaOH loading, respectively, while the
methane yield for reactors with 2.0% NaOH loading only peaked on day 27. We
speculated that the accumulation of VFAs in reactors with S/I ratio of 6.2 created a highly
acidic environment (pH <6, Figure 4.4a) inhibiting the methanogenic bacteria and
causing failure of the reactor (Penaud et al., 1999). The delay in methane production at
NaOH loading of 3.5% and 5.0% could also be a result of the dynamic transition of
methanogen populations to adapt to acidic conditions (Delbès et al., 2001; Hori et al.,
2006).
(a)
71
(b)
Figure 4.2. Effect of NaOH loading on daily methane yield at S/I ratios of a) 4.1 and b)
6.2 (digestion time: 30 days, TS: 20%).
The effect of total solid (TS) loading on biogas production was also investigated.
Figure 4.3. shows that increasing TS from 20% to 26% significantly decreased biogas
production especially at low NaOH loadings. Biogas production in control reactors was
less than 3.0 L/kg VS for both 20% and 26% TS. Figure 3 also shows that simultaneous
NaOH treatment in SS-AD improved biogas production; however, at 26% TS, the biogas
production was much lower than that at 20% TS. A highly acidic environment (pH 5.56.6, data not shown) caused by VFA accumulation may have inhibited the methanogenic
bacteria at high TS content. Similar results were reported indicating that methane yield
decreased about 17% when TS increased from 20% to 30% during anaerobic digestion of
the organic fraction of municipal solid waste (OFMSW) (Fernandez et al., 2008).
72
(n = 2)
Figure 4.3. Effect of NaOH loading and TS content on total methane yield (digestion
time: 30 days, S/I ratio: 6.2).
4.3.2 Variation of pH, total volatile fatty acids (TVFAs) and alkalinity
Imbalances of hydrolytic, fermentative, acetogenic, and methanogenic functions during
anaerobic digestion can lead to reactor failure and low methane yield. For example,
accumulation of VFAs could result in a dramatic drop in pH, subsequently inhibiting
methanogenic bacteria and disrupting the performance of anaerobic digestion. Thus, pH
and total VFAs are common stress indicators used for monitoring AD operation (Ahring,
1995; Lahav and Morgan, 2004). Figure 4.4a shows the initial and final pH of batch
mode reactors during 30-day SS-AD. The initial pH values of all reactors, which ranged
from 7.5 to 9.1, were above the operational pH of 7.4 recommended by Lahav and
Morgan (2004). Figure 4.4a also shows that pH was maintained above 7.4 during SS-AD
at S/I ratio of 4.1 which indicates a “healthy” AD system. However, at S/I ratio of 6.2, the
73
final pH dropped to 5.3 and 6.6 (below 7.4) for reactors at no or 2% NaOH loadings,
respectively. This observation indicated failure in AD process, which is in agreement
with data shown in Figure 1 that the biogas production was low at no or 2% NaOH
loadings. It is also noted that at S/I ratio of 6.2, the final pH of the digestate at 5% NaOH
loading was lower than that of the digestate at 3.5% NaOH loading, probably due to the
higher total VFAs in digestate. Not surprisingly, the final pH for all reactors operated at
S/I ratio of 8.2 was below 7.4, which was associated with SS-AD process failure as
indicated by no or low biogas production as shown in Figure 3.1.
74
(a)
10
9
pH
8
7
6
Initial
Final
Control
Control
2.0% NaOH
2.0% NaOH
3.5% NaOH
3.5% NaOH
5.0% NaOH
5.0% NaOH
5
3.0
(b)
5.0
6.0
S/I Ratio
7.0
6.0
7.0
8.0
9.0
10
Initial
Control
2.0% NaOH
3.5% NaOH
5.0% NaOH
8
TVFA/Alkalinity
4.0
6
Final
Control
2.0% NaOH
3.5% NaOH
5.0% NaOH
4
2
0
3.0
4.0
5.0
8.0
9.0
S/I Ratio
Figure 4.4. Initial and final a) pH and b) TVFA/alkalinity ratios for reactors with different
NaOH loading and S/I ratio (digestion time: 30 days, TS: 20%).
75
In addition to pH measurement, both total VFAs and alkalinity were determined
since pH is not a sole indicator of AD failure (Ahring et al., 1995; Lahav and Morgan,
2004). The stability criterion for anaerobic digestion is often expressed by the ratio of
total VFAs to the buffering capacity measured as alkalinity - total VFAs/alkalinity ratio
(Koch et al., 2010). Although the optimal total VFAs/alkalinity ratio of each AD reactor
is unique, a ratio of 0.3 to 0.4 is generally regarded as optimal for liquid AD and a ratio
exceeding 0.6 is regarded as indicative of overfeeding (Lossie and Pütz, 2010). As shown
in Figure 4.4b, the initial total VFA/alkalinity ratio during start-up of all reactors was
approximately 0.5. The initial total VFA/alkalinity ratio of reactors without alkali
addition was found to be higher (ranging from 0.7 to 1.0) compared to reactors with
NaOH addition. Final TVFA/alkalinity ratios of all healthy reactors (with total methane
yield above 60.0 L/kg VS) were however at or below 1.6 in this study which was higher
than the limit of 0.6 for liquid AD. At S/I ratio of 8.2, TVFA/alkalinity ratios measured at
the end of 30-day SS-AD were substantially higher than the recommended ratio, which
indicates failure of SS-AD. The failure was likely caused by accumulation of organic
acids due to overfeeding. Simultaneous NaOH treatment at 3.5%-5.0% NaOH not only
helped to improve the digestibility of leaves but also increased the buffering capacity of
the digester to maintain suitable pH and VFA/alkalinity ratio, thus leading to higher
biogas production compared with no or low NaOH loadings.
4.3.3. Degradation of cellulose and hemicellulose
Table 4.1. shows the composition of leaves. Compared with other lignocellulosic
biomass, such as corn stover, leaves have extractive content of about 33.0% of the TS.
76
Lignin content (22.7%) of the leaves was comparable to other typical biomass; however,
the cellulose (11.1%) and hemicellulose (11.5%) contents were lower compared to corn
stover as reported elsewhere (Zhu et al., 2010). Due to the relatively low cellulose and
hemicellulose contents, the methane potential (81.8 L/kg VS) of leaves obtained in this
study was be lower than corn stover (Zhu et al., 2010).
(a)
Cellulose ( g/100g TSintial )
12
Initial
Final
10
(n = 2)
8
6
4
2
0
0.0
2.0
3.5
NaOH loading (as % of leaves TS)
5.0
(b)
Hemicellulose ( g/100g TSintial )
12
Initial
Final
10
(n = 2)
8
6
4
2
0
0.0
2.0
3.5
NaOH loading (as % of leaves TS)
5.0
Figure 4.5. Effect of NaOH loading on reduction of a) cellulose and b) hemicellulose (digestion
time: 30 days, TS: 20%, S/I ratio: 6.2).
77
Figure 4.5. illustrates cellulose and hemicellulose degradation during 30-day SSAD at S/I ratio of 6.1, comparing the initial and final compositions. In general, higher
methane yield was obtained in reactors having higher cellulose and hemicellulose
degradation. Cellulose and hemicellulose reduction was negligible for the control, which
was in line with the very low methane yield observed at this condition. Substantial
cellulose and hemicellulose degradation were observed for reactors with 2.0%, 3.5% and
5.0% NaOH loadings. However, not much difference was observed between 3.5% and
5.0% NaOH loading. The highest cellulose degradation of 36.0% and hemicellulose
degradation of 34.9% were observed at 3.5% NaOH loading, and were in agreement with
the highest methane production at this condition. Lignin degradation in leaves was not
significant (P > 0.05) for the control and 2.0% NaOH reactors but was significant in
reactors with 3.5% and 5.0% NaOH loading (data not shown). The higher delignification
of lignocellulosic biomass at these conditions was correlated with the higher methane
yield and degradation of cellulose and hemicellulose.
4.4. Conclusions
NaOH addition not only contributes to the delignification of lignocellulosic
biomass but also improves the buffering capacity of SS-AD by increasing the alkalinity.
The highest methane yield of 81.8 L/kg VS was obtained at S/I ratio of 4.1 with 3.5%
NaOH loading. At S/I ratio of 4.1, methane yield was not significantly (P>0.05)
improved by the NaOH addition. Enhancement of methane yield of 9 to 24-fold was
78
observed with NaOH addition at S/I ratio of 6.2. The increased methane yield was
however not higher than methane yield obtained at S/I ratio of 4.1.
79
Chapter 5 Conclusions and Suggestions for Future Research
Lignocellulosic biomass was shown to be a feasible source of feedstocks for
biogas production in SS-AD. The recommended S/I ratio in SS-AD for lignocellulosic
biomass is 2 as the highest methane yield was attained for each feedstock tested at this S/I
ratio. The composition of lignocellulosic biomass was shown to correlate with methane
yield. The highest methane yield was attained with SS-AD of corn stover (which had the
lowest lignin content and the highest holocellulose content), followed by wheat straw,
leaves and yard waste. An inverse linear relationship with correlation coefficients of 0.95
to 0.97 was obtained between methane yield observed and lignin content of
lignocellulosic biomass. In addition to holocellulose, extractives (water and ethanol
soluble) found in lignocellulosic biomass also contributed to the biogas production in SSAD. Substantial degradation of extractives was observed during SS-AD of yard waste
and leaves.
NaOH addition not only contributes to the delignification of lignocellulosic
biomass but also improves the buffering capacity of SS-AD by increasing the alkalinity.
The highest methane yield of 81.8 L/kg VS was obtained at S/I ratio of 4.1 with 3.5%
NaOH loading, which was in agreement with the highest cellulose (36.0%) and
hemicellulose (34.9%) degradation observed at this condition. At S/I ratio of 4.1,
methane yield was not significantly (P>0.05) improved by the NaOH addition. However,
enhancement of methane yield of 9 to 24-fold was observed with NaOH addition at S/I
ratio of 6.2.
80
In order to further improve the economics of SS-AD operation, operational costs
associated feedstock handling and preprocessing need to be further optimized. One area
that is of great concern is the heating of feedstocks during cold weather. Heating of low
moisture content material at the commercial digesters scale is likely to be less efficient
and more difficult compared to liquid phase digesters. Hence, further research that could
efficiently heat up the material in commercial scale digester will be of great interest. As
the availability of lignocellulosic biomass is likely to be seasonal, other feedstock
alternatives that could co-digest with lignocellulosic biomass should be further explored.
81
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