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7/17/2019 Enhancing the Solid-state Anaerobic Digestion of Fallen Leaves Through Simultaneous Alkaline Treatment http://slidepdf.com/reader/full/enhancing-the-solid-state-anaerobic-digestion-of-fallen-leaves-through-simultaneous 1/7 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 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. 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 journal homepage: www.elsevier.com/locate/biortech

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7/17/2019 Enhancing the Solid-state Anaerobic Digestion of Fallen Leaves Through Simultaneous Alkaline Treatment

http://slidepdf.com/reader/full/enhancing-the-solid-state-anaerobic-digestion-of-fallen-leaves-through-simultaneous 1/7

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

j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c o m / l o c a t e / b i o r t e c h

7/17/2019 Enhancing the Solid-state Anaerobic Digestion of Fallen Leaves Through Simultaneous Alkaline Treatment

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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.

L.N. Liew et al. / Bioresource Technology 102 (2011) 8828–8834   8829

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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%).

8830   L.N. Liew et al. / Bioresource Technology 102 (2011) 8828–8834

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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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