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7/17/2019 Enhancing the Solid-state Anaerobic Digestion of Fallen Leaves Through Simultaneous Alkaline Treatment
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Enhancing the solid-state anaerobic digestion of fallen leaves through
simultaneous alkaline treatment
Lo Niee Liew, Jian Shi, Yebo Li ⇑
Department of Food, Agricultural, and Biological Engineering, The Ohio State University/Ohio Agricultural Research and Development Center, 1680 Madison
Ave., Wooster, OH 44691-4096, United States
a r t i c l e i n f o
Article history:
Received 2 May 2011
Received in revised form 28 June 2011
Accepted 6 July 2011
Available online 14 July 2011
Keywords:
Anaerobic digestion
Dry fermentation
Alkali pretreatment
Biogas
Leaves
a b s t r a c t
Previous studies have shown that alkali pretreatment prior to anaerobic digestion (AD) can increase the
digestibility of lignocellulosic biomass and methane yield. In order to simplify the process and reduce the
capital cost, simultaneous alkali treatment and anaerobic digestion was evaluated for methane produc-
tion from fallen leaves. The highest methane yield of 82 L/kg volatile solids (VS) was obtained at NaOH
loading of 3.5% and substrate-to-inoculum (S/I) ratio of 4.1. The greatest enhancement in methane yield
was achieved at S/I ratio of 6.2 with NaOH loading of 3.5% which was 24-fold higher than that of the con-
trol (without NaOH addition). Reactors at S/I ratio of 8.2 resulted in failure of the AD process. In addition,
increasing the total solid (TS) content from 20% to 26% reduced biogas yield by 35% at S/I ratio of 6.2 and
NaOH loading of 3.5%. Cellulose and hemicellulose degradation and methane yields are highly related.
2011 Elsevier Ltd. All rights reserved.
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 en-
ergy crops, agricultural residue and municipal solid waste, 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 anaer-
obic digestion (AD) of organic materials. However, due to the recal-
citrant structure and composition of lignocellulosic biomass, such
as lignin that interlinks cellulose and hemicellulose layers, the con-
version efficiency is limited (Noike et al., 1985). Hydrolysis of lig-
nocellulosic biomass is rate-limiting because of the low
cellulolytic activity and low specific growth rate of cellulolytic mi-
crobes in anaerobic digesters (Lu et al., 2007). Therefore, pretreat-ment is often required to overcome biomass recalcitrance in order
to facilitate the access of hydrolytic enzymes to degradable carbo-
hydrates 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 treatments (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 pretreatment greatly im-
proves 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, 1985). However, alkaline pretreatment performed at low
moisture and ambient temperature is particularly attractive. In a
study conducted by Pang et al. (2008), a 48.5% increase in biogas
was achieved from corn stover pretreated with 6% NaOH at 80%
moisture content for 3 weeks at ambient temperature. In a parallel
study, a 72.9% increase in total biogas yield was achieved with low-
er NaOH loading (2%) and shorter pretreatment time (3 days),
when pretreatment moisture content was increased to 88% (Zheng
et al., 2009). Recently, Zhu et al. (2010) reported that anaerobicdigestion 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. These studies indicate that alkaline pre-
treatment of lignocellulosic biomass is feasible with lower mois-
ture content. However, pretreatment effectiveness is greatly
affected by moisture content, NaOH loading, and pretreatment
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
0960-8524/$ - see front matter 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2011.07.005
⇑ Corresponding author. Tel.: +1 330 263 3855.
E-mail address: [email protected] (Y. Li).
Bioresource Technology 102 (2011) 8828–8834
Contents lists available at ScienceDirect
Bioresource Technology
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lignocellulosic biomass and problems encountered in liquid AD,
such as floating and stratification of solids, can be avoided in
SS-AD (Chanakya et al., 1993). Compared to liquid AD (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., 2011). However, it requires large
amounts of inoculum, longer retention time, and nitrogen supple-
mentation when lignocellulosic biomass is used ( Jewell et al.,1993; Li et al., 2011). Furthermore, pretreatment is generally
required for lignocellulosic biomass to improve the efficacy of
SS-AD (Li et al., 2011). 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 knowl-
edge, 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 diges-
tion. 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 in
pH during acidogenesis, which can create a more stable environ-
ment for the methanogenic bacteria (Pavlostathis and Gossett,
1985). 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 simulta-
neous 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 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., 2010), NaOH loading needs to be adjusted with
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 normallycharged for collection and hauling of such wastes from residential
or commercial areas. The objectives of this study were to deter-
mine the effect of NaOH loading and S/I ratio on daily and cumula-
tive methane production during SS-AD of leaves. Changes in total
volatile fatty acids (VFA), 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.
2. Methods
2.1. Feedstock and inoculum
Fallen leaves were collected from the campus of the Ohio Agri-cultural Research and Development Center (OARDC) in Wooster,
OH, USA (404803300N, 815601400W) in October 2009. Leaves were
dried at 40 C for 72 h in a convection oven (Shel Lab FX28-2, Shel-
don Manufacturing, Cornelius, OR, USA) to achieve a moisture con-
tent 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 centrifugation. TS content
increased from 3.9% to 6.1% after dewatering. Characteristics of
leaves and inoculum are shown in Table 1. Structural carbohydrate
and lignin contents of leaves are based on dry matter, whereas therest of the values are based on total weight.
2.2. Solid-state anaerobic digestion with simultaneous NaOH
treatment
Oven-dried and ground leaves were mixed thoroughly with an
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 condi-
tions). 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, when necessary, to obtain a TS content of 20%. 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 mea-
sured daily for the first 15 days and every 2 days afterwards. Dupli-cate reactors were run at each condition.
2.3. Analytical methods
The extractive content of leaves and materials 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 fraction-
ated using a two-step acid hydrolysis method based on NREL Lab-
oratory Analytical Procedure (Sluiter et al., 2010). Monomeric
sugars (cellobiose, glucose, xylose, galactose, arabinose, and man-
nose) in the acid hydrolysate were measured by HPLC (Shimadzu
LC-20AB, Columbia, MD, USA) equipped with a Biorad Aminex
HPX-87P column and a refractive index detector (RID). Deionizedwater at a 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 contents of leaves, inoculum, and digestate were
measured at the beginning and end of the AD process according to
the Standard Methods for the Examination of Water and Wastewa-
ter (APHA, 2005). Total carbon and nitrogen contents were deter-
mined by an elemental analyzer (Elementar Vario Max CNS,
Elementar Americas, Mt. Laurel, NJ, USA). Total volatile fatty acids
(VFA) and alkalinity were measured using a 2-step titration meth-
od (McGhee, 1968). Samples for pH, total VFA, and alkalinity mea-
surement 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 (MettlerToledo, DL22 Food & Beverage Analyzer, Columbus, OH, USA).
Table 1
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 ± 0.0Alkalinity (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.
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The total VFA/alkalinity ratio was calculated to determine the risk
of acidification, a measure of the process stability (Lossie and Pütz,
2010). The volume of biogas collected in a Tedlar bag was mea-
sured with a drum-type gas meter (Ritter, TG 5, Bochum, Germany)
and the composition of biogas (CO2, CH4, N2 and O2) was analyzed
using a GC (Agilent Technologies, HP 6890, Wilmington, DE, USA)
equipped with a 10-ft stainless steel column 45/60 Molecular Sieve
13X and a Thermal Conductivity Detector. Helium gas at a flow rateof 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 min, elevated to 60 C at
20 C/min, and held for 5 min. The sodium ion (Na+) concentration
in the digestate was analyzed by Inductively Coupled Plasma-Mass
Spectrometry (ICP-MS) (Agilent 7500, Agilent Technologies, Wil-
mington, DE, USA). Samples for ICP-MS analysis were prepared
by digestion using a microwave digester (MARSXpress™, CEM Cor-
poration, Matthews, NC, USA) programmed with a 15-min ramp-
up time to 200 C and was then maintained at 200 C for 15 min.
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.
3. Results and discussion
3.1. Biogas production
Fig. 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 gen-
eral, SS-AD requires higher inoculum levels than liquid AD. Reduc-
ing the amount of inoculum allows better reactor efficiency;
however, it may result in an increase in the accumulation of VFA
and lead to reactor upset (Li et al., 2011). For SS-AD without NaOH
addition, only S/I ratio of 4.1 gave good total methane yields amongthe 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 the 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%, respec-
tively. 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 5.0%
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 pro-
duced. Addition of NaOH at loading rates of 2.0%, 3.5%, and 5.0%
significantly improved biogas yield and the total methane yieldwas increased by 9-, 24-, and 21-fold, respectively, compared to
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 signifi-
cantly 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 load-
ing, 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 a signif-
icant increase in methane yield with NaOH addition at higher corn
stover loading.
Daily methane yields during SS-AD of leaves at S/I ratios of 4.1
and 6.2 are shown in Fig. 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 microor-
ganisms, 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 al., 1988). The Na+ concentration in this
study ranged from 1.0 g/kg at 1% NaOH loading to about 6.7 g/kgat 5.0% NaOH loading. Although the inhibitory Na+ level for
SS-AD has not been reported previously, inhibition due to high so-
dium ions concentration was likely to occur for reactors operated
at 5.0% NaOH. The lag phase of methane production with 5.0%
NaOH addition is probably caused by the high initial pH and high
concentration of sodium salt of fatty acid due to the fast conversion
of carbohydrates.
Fig. 1. Effect of S/I ratio and NaOH loading on total methane yield (digestion time: 30 days, TS: 20%).
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As presented in Fig. 2b, daily methane yield at S/I ratio of 6.2,
shows different patterns compared with S/I ratio of 4.1 (Fig. 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 reactorfailure. Simultaneous alkaline treatment at different NaOH load-
ings improved methane production, although a long lag phase
was observed for all reactors especially for those 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 accumula-
tion of VFA in reactors with S/I ratio of 6.2 created a highly acidic
environment (pH < 6, Fig. 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).
We also investigated the effect of total solid (TS) loading onmethane yield. Fig. 3 shows that increasing TS from 20% to 26%
significantly decreased methane yield especially at low NaOH load-
ings. Methane yield in control reactors was less than 3.0 L/kg VS for
both 20% and 26% TS. Fig. 3 also shows that simultaneous NaOH
treatment in SS-AD improved methane yield; however, at 26% TS,
the methane yield was much lower than that at 20% TS. A highlyacidic environment (pH 5.5–6.6, data not shown) caused by VFA
accumulation may have inhibited the methanogenic bacteria at
high TS contents. 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 muni-
cipal solid waste (OFMSW) (Fernandez et al., 2008).
3.2. Variation of pH, total volatile fatty acids (VFA) and alkalinity
Imbalances of hydrolytic, fermentative, acetogenic, and metha-
nogenic functions during anaerobic digestion can lead to reactor
failure and low methane yield. For example, accumulation of VFA
could result in a dramatic drop in pH, subsequently inhibiting
methanogenic bacteria and disrupting the performance of anaero-bic digestion. Thus, pH and total VFA are common stress indicators
Fig. 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%).
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used for monitoring AD operation (Ahring et al., 1995; Lahav and
Morgan, 2004). Fig. 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). Fig. 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
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 the AD process, which is in agreement with data shown in Fig. 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 VFA in the digestate.
Not surprisingly, the final pH for all reactors operated at S/I ratioof 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
Fig. 1.
In addition to pH measurement, both total VFA and alkalinity
were determined since pH is not a sole indicator of AD failure (Ahr-
ing et al., 1995; Lahav and Morgan, 2004). The stability criterion for
anaerobic digestion is often expressed by the ratio of total VFA to
the buffering capacity measured as alkalinity – total VFA/alkalinity
ratio (Koch et al., 2010). Although the optimal total VFA/alkalinity
ratio of each AD reactor is unique, a ratio of 0.3–0.4 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 Fig. 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 total VFA/alkalinity ratios of all healthy reac-
tors (with total methane yield above 60.0 L/kg VS) were however at
Fig. 3. Effect of NaOH loading and TS content on total methane yield (digestion
time: 30 days, S/I ratio: 6.2).
(a)
(b)
Fig. 4. Initial and final (a) pH and (b) total VFA/alkalinity ratios for reactors with different NaOH loading and S/I ratio (digestion time: 30 days, TS: 20%).
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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, total VFA/alkalinity ratios measured, at
the end of 30-day SS-AD were substantially higher than the recom-
mended 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% and 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
total VFA/alkalinity ratio, thus leading to higher biogas production
compared with no or low NaOH loadings.
3.3. Degradation of cellulose and hemicellulose
Table 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. Lignin content (22.7%) of the
leaves was comparable to other typical biomass; however, the cel-
lulose (11.1%) and hemicellulose (11.5%) contents were lower com-
pared 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 lower than corn stover (Zhu et al., 2010).
Fig. 5 illustrates cellulose and hemicellulose degradation dur-
ing 30-day SS-AD at S/I ratio of 6.1, comparing the initial and final
compositions. In general, higher methane yield was obtained inreactors having higher cellulose and hemicellulose degradation.
Cellulose and hemicellulose reduction was negligible for the con-
trol, which was in line with the very low methane yield observed
at this condition. Substantial cellulose and hemicellulose degrada-
tion 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% and5.0% 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. Conclusions
NaOH addition not only contributes to the delignification of lig-
nocellulosic 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 load-
ing, 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) im-proved by the NaOH addition. However, enhancement of methane
(a)
(b)
Fig. 5. Effect of NaOH loading on reduction of (a) cellulose and (b) hemicellulose (digestion time: 30 days, TS: 20%, S/I ratio: 6.2).
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yield of 9- to 24-fold was observed with NaOH addition at S/I ratio
of 6.2.
Acknowledgements
This project was supported by Ohio Agricultural Research and
Development Center (OARDC) Seeds Program (2008-043) and The
Ohio Third Frontier Program (10-059). The authors thank Mrs.Mary Wicks (Department of Food, Agricultural and Biological Engi-
neering, OSU) for reading through the manuscript and providing
useful suggestions.
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