Bioresource Technology 171 (2014) 233–239

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Anaerobic digestion of giant reed for methane production Liangcheng Yang, 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, USA

h i g h l i g h t s  Anaerobic digestion (AD) of giant reed was at various total solids (TS) contents.  Increasing TS from 8% to 38% negatively affected methane yield.  Highest volumetric methane production was obtained at 20–23% TS.  Methane yield from solid state AD was 16% lower than that from liquid AD at F/E = 2.0.  Cellulose showed the highest contribution to methane production.

a r t i c l e

i n f o

Article history: Received 1 July 2014 Received in revised form 7 August 2014 Accepted 9 August 2014 Available online 18 August 2014 Keywords: Solid-state anaerobic digestion Giant reed Methane Cellulose Total solids

a b s t r a c t As a fast growing plant, giant reed has good potential to be used as a feedstock for methane production via anaerobic digestion (AD). The effect of total solids (TS) content, an AD operating parameter, was studied. Results showed that increasing TS from 8% to 38% decreased methane yield, due to the inhibition of volatile fatty acids (VFAs) and total ammonia nitrogen (TAN); while the maximum volumetric methane production was obtained at 20–23% TS. Comparison of solid-state AD (SS-AD) at 20% TS and liquid AD (L-AD) at 8% TS was conducted at feedstock to effluent (F/E) ratios of 2.0, 3.5, and 5.0. The best performance was achieved at an F/E of 2.0, with methane yields of 129.7 and 150.8 L-CH4/kg-VS for SS-AD and L-AD, respectively. Overall organic components were degraded by 17.7–28.5% and 24.0–26.6% in SS-AD and L-AD, respectively; among which cellulose showed the highest degradation rate and the highest contribution to methane production. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Lignocellulosic biomass is considered to be a promising feedstock for bioenergy production due to its abundance, renewability, and the fact that it does not compete with food or feed production. Therefore, in recent years, efforts have been made to identify new sources of lignocellulose for bioenergy production (Zheng et al., 2014), among which, giant reed (Arundo donax L.) is gaining increasing interest. Giant reed is a perennial weed grass plant, and is usually cultivated in subtropical and warm temperate regions. The attractive features of giant reed for bioenergy production are its high growth rate, suitability for harvesting more than once per year, and tolerance to dry environments (Pilu et al., 2013). A previous study reported that giant reed could have a growth rate of about 5 cm per day in favorable environments (Pilu et al., 2013). In Central Italy, a 10-year study showed a higher ⇑ Corresponding author. Tel.: +1 330 263 3855. E-mail address: [email protected] (Y. Li). http://dx.doi.org/10.1016/j.biortech.2014.08.051 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

dry mass production rate of giant reed (37.7 ton per year per hectare) than of miscanthus (28.7 ton per year per hectare) (Angelini et al., 2009). Due to its high growth rate, giant reed can be harvested twice per year (harvest–regrow–harvest), producing 20% more biomass than that obtained from a single harvest (Ragaglini et al., 2014a). Giant reed is considered to be a drought-tolerant species and can adapt to marginal or sub-marginal lands, thus not competing with food crops for land (Angelini et al., 2009). Currently, giant reed is widely planted in East Asia, Mediterranean regions, and both East and West coasts of the U.S., and is primarily used as a source of fiber for printing paper (Fiore et al., 2014), an alternative to wood to make chipboard panels (Flores et al., 2011), or a raw material for manufacturing activated carbon (Sun et al., 2012). Giant reed has also been examined as a feedstock for bioenergy production. A few studies have investigated fermenting giant reed for ethanol production (Scordia et al., 2012, 2013). Anaerobic digestion (AD) has been employed to produce biogas from giant reed. Ragaglini et al. (2014b) showed the highest biochemical methane potential of giant reed to be 392 L per kg

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volatile solids (VS), which is equivalent to a methane yield of 11,585–12,981 m3 ha 1 based on the biomass yield per hectare. Girolamo et al. (2013) pretreated giant reed using hydrothermal methods with sulfuric acid (H2SO4) as the catalyst, and increased methane yield by 4–23%. According to these studies, AD is an effective method for extracting bioenergy from giant reed that is reliable and has low greenhouse gas emissions. While most previous studies have employed liquid AD (L-AD) to digest lignocellulosic biomass, the high solids content in lignocellulosic biomass has spurred researchers to consider solid-state AD (SS-AD) to digest giant reed. Compared to L-AD, which is usually operated with less than 15% total solids (TS) content, SS-AD can handle high solids containing feedstocks and operates with TS higher than 15%. Consequently, compared to L-AD, SS-AD usually has a higher organic loading rate, smaller reactor volume, lower energy demand for heating, higher volumetric methane productivity, and less wastewater generation (Li et al., 2011). To date, no SS-AD of giant reed has been carried out and research is needed to identify the optimal TS range for SS-AD of giant reed. It is also important to know the contribution of each organic component, e.g. cellulose, hemicellulose, and protein, to methane production during SS-AD. Therefore, the objectives of this study were to examine the effects of TS on methane production from AD of giant reed; compare the performance of SS-AD and L-AD in digesting giant reed; and estimate the contribution to methane production from each organic component in giant reed. 2. Methods 2.1. Feedstock and inoculum Giant reed was planted in April 2013 at a research farm near Columbus, OH, and was harvested in October 2013. Upon receipt, giant reed was ground through a 12 mm sieve with a grinder (Mighty Mac, Mackissic Inc., Parker Ford, PA, USA). To increase the total solids content, fresh giant reed was air dried in a 40 °C oven for 2 days. Both fresh and dried samples were used as feedstocks for the AD tests. To inoculate the feedstock, effluent taken from a mesophilic liquid anaerobic digester (operated by quasar energy group in Zanesville, OH, USA) was centrifuged and then well mixed with the feedstock. Centrifuge permeate was used to adjust the overall TS content. Characteristics of the feedstocks and effluent are presented in Table 1. Table 1 Properties of initial feedstocks and inoculum. Parameters

Fresh feedstock

Dried feedstock

Centrifuged inoculuma

TS, % VS, % TC, % TN, % C/N ratio pH Alkalinity, g-CaCO3/kg VFAs, g/kg TAN, g-N/kg Extractivesb, % Celluloseb, % Hemicelluloseb, % Ligninb, % Crude proteinb, %

37.64 ± 0.54 34.43 ± 0.59 18.30 ± 1.93 0.54 ± 0.07 34.31 ± 1.04 7.68 ± 0.07 2.39 ± 0.57 0.48 ± 0.02 1.29 ± 0.11 19.27 ± 0.32 20.62 ± 3.59 6.88 ± 1.57 33.85 ± 1.83 8.39 ± 0.27

96.63 ± 0.20 88.43 ± 0.27 42.75 ± 0.67 1.24 ± 0.03 34.47 ± 0.42 7.06 ± 0.03 1.99 ± 0.08 1.24 ± 0.04 1.67 ± 0.12 19.27 ± 0.32 20.62 ± 3.59 6.88 ± 1.57 33.85 ± 1.83 6.67 ± 0.14

22.19 ± 1.41 12.43 ± 0.68 6.27 ± 0.12 0.66 ± 0.02 9.47 ± 0.17 8.00 ± 0.01 10.76 ± 0.21 0.93 ± 0.06 5.52 ± 0.15 15.02 ± 0.27 1.45 ± 0.28 0.86 ± 0.02 N/A 8.70 ± 0.24

TS = total solids, VS = volatile solids, TC = total carbon, TN = total nitrogen, C/ N = ratio of carbon over nitrogen, VFAs = volatile fatty acids, TAN = total ammonia nitrogen. Average ± S.E., n = 2. a Centrifuged at 10 k rpm for 10 min. b Based on TS, others are based on total weight.

2.2. Anaerobic digestion To examine the effect of TS on AD performance, dried feedstock was mixed with inoculum at a feedstock to effluent (F/E, based on VS) ratio of 2.0 with TS contents of 38%, 33%, 28%, and 23%; while fresh feedstock was inoculated at the same F/E ratio with TS contents of 28%, 23%, 18%, 13%, and 8%. To compare SS-AD and L-AD performance, fresh giant reed was inoculated to achieve F/E ratios of 2.0, 3.5, and 5.0 and the TS was controlled at 20% and 8% for SS-AD and L-AD, respectively. L-AD reactors were loaded in a platform shaker (Innova 2300, New Brunswick, CT, USA) with a speed of 140 rpm. One liter glass reactors were used for all tests, which were conducted in a 37 °C incubation room. For each reactor, a 5 L Tedlar gas bag (CEL Scientific, Santa FeSprings, CA, USA) was attached to collect biogas every 2–4 days during the 50 day experimental period. 2.3. Analytical methods The volume of biogas was measured with a drum-type gas meter (Ritter, Bochum, Germany) and its composition (CH4, CO2, N2, and O2) was analyzed with a gas chromatograph (Agilent, HP 6890, Wilmington, DE, USA) equipped with a 30 m  0.53 mm  10 lm RtÒAlumina Bond/KCl deactivation column and a thermal conductivity detector. Helium gas was used as a carrier gas at a flow rate of 5.2 mL/min. The temperature of the detector was maintained at 200 °C, while the initial temperature of the oven was 40 °C and then increased to 60 °C within 1 min. TS, VS, pH, and alkalinity of digesting materials were measured based on the Standard Methods Examination of Water and Wastewater (APHA, 2005). Specifically, a 5 g sample was diluted with 50 mL DI water, then the pH and alkalinity was measured using an auto-titrating pH meter (Mettler Toledo, DL22 Food & Beverage Analyzer, Columbus, OH, USA). Total carbon and total nitrogen contents were determined using an elemental analyzer (Elementar Vario Max CNS, Elementar Americas, Mt. Laurel, NJ, USA). Total ammonia nitrogen (TAN) was measured based on a modified distillation and titration method (ISO 5664, 1984) using 4% boric acid with a AutoKjeldahl Unit K-370 (Buchi Labortechnik AG, Switzerland). Total Kjeldahl nitrogen (TKN) was analyzed based on the Kjeldahl nitrogen method (ISO 5663, 1984). The difference between TAN and TKN was assumed to be organic nitrogen, based on which, the crude protein content was calculated by multiplying by a factor of 6.25 (Hattingh et al., 1967). Volatile fatty acids (VFAs), which include propionic, acetic, isovaleric, butyric, isobutyric, and valeric acids, were analyzed using a gas chromatograph (Shimadzu, 2010PLUS, Columbia, MD, USA) equipped with a 30 m  0.32 mm  0.5 lm StabilwaxÒ polar phase column and a flame ionization detector, according to a method described previously (Shi et al., 2013). Extractives in feedstocks were measured based on the NREL Laboratory Analytical Procedure (Sluiter et al., 2008) using a Dionex ASE 300 extraction system (Thermo Scientific, Sunnyvale, CA), while extractive-free samples were used to determine the structural carbohydrates. Monomeric sugars (glucose, xylose, galactose, arabinose, and mannose) were analyzed using a high-performance liquid chromatograph (Shimadzu, LC-20AB, Columbia, MD, USA) equipped with a Biorad Aminex HPX-87P column and a refractive index detector. HPLC grade water was used as the mobile phase at a flow rate of 0.3 mL/min, and temperatures of the column and detector were maintained at 60 °C and 55 °C, respectively. 2.4. Theoretical methane contribution of each organic component Previously reported theoretical methane potentials of organic components, which assumed methane potential of 415 and 496 L/kg-VS for carbohydrates (cellulose and hemicellulose) and protein, respectively, were used in this study (Angelidaki and

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2.5. Modeling and data analysis

160

CH4 yield, L/kg-VS

A

Measured methane production data were fitted into a modified Gompertz model to determine the cumulative methane yield, maximum daily methane yield, and lag phase of giant reed during the AD process. Details about this model have been provided in a previous study (Dechrugsa et al., 2013). Data was analyzed using R Studio software with a significance level of 0.95.

120 Fresh Giant Reed Dried Giant Reed

80 3. Results and discussion

Volunmetric CH4 production, L/L-digester

Y=143+1.38 X -0.263X2 2 R adj=0.97

3.1. Effects of TS

40

B

The methane yield from the anaerobic digestion of giant reed was negatively affected as TS increased from 8% to 38% (Fig. 1A). The highest methane yield of 150.8 L/kg-VS feedstock was achieved at a low TS content of 8%, while at 38% TS content, the methane yield dropped to 66.6 L/kg-VS. The methane content in biogas was stabilized at 62–70% after about 20 days, and was independent of TS. The decreasing trend of methane yield in relation to TS fits a quadratic equation, a commonly used function for methane yield (Motte et al., 2013), with an adjusted R2 value of 0.97. Very likely, the decrease was caused by the accumulation of VFAs and TAN (Fig. 2). As the TS increases, the mass transfer barrier for diffusion of VFAs also increases (Bollon et al., 2013), and thus may lead to accumulation of VFAs. At 38% TS, the concentrations of propionic acid, acetic acid, isovaleric acid, butyric acid, and isobutyric acid were 0.91, 0.36, 0.26, 0.12, 0.03, and 0.02 g/kg, respectively. The concentration of propionic acid was higher than the previously reported threshold of 0.9 g/kg (Wang et al., 2009). As TS decreased to 33%, the isovaleric acid content remained the same while all the others were much lower than those at 38% TS. At TS lower than 33%, only acetic acid was detectable. Therefore, high concentrations of VFAs, especially the propionic acid and isovaleric acid, may have contributed to the inhibition of methanogenic activity and the reduction of methane yield. Likewise, high TAN may also have imposed inhibitory effects. A previous study on SS-AD of corn stover showed that TAN higher than 4.3 g/kg

10

8

6 Y= -1.73+1.07 X -0.0235X R2adj= 0.93

2

4 5

10

15

20

25

30

35

40

TS, % Fig. 1. AD methane yield and volumetric production at various TS conditions. Error bar represents standard error.

Sanders, 2004). The extractives are mainly comprised of free sugars, oligomers, and organic acids (Liew et al., 2012), therefore, their estimated theoretical methane potential was assumed to be the same as glucose (373 L/kg-VS). The theoretical contribution of each component in giant reed to methane production was obtained by multiplying its theoretical methane potential by its measured degradation data.

2.0

A1

Fresh feedstock

A2

Dried feedstock

VFA, g/kg

1.5

1.0

0.5

0.0

B1

B2

TAN, g/kg

4.5

3.0

1.5

0.0 8

13

18

23

28

23

28

33

38

TS content, % Fig. 2. Final total VFAs and TAN levels in digestates at various TS conditions. Error bar represents standard error.

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CH4 yield, L/kg-VS

160

SS-AD

120

L-AD

F/E=2.0 F/E=3.5 F/E=5.0

80

40

0 0

10

20

30

40

50

10

20

30

40

50

Operation time, day Fig. 3. Comparison of cumulative methane yields in SS-AD and L-AD at three F/E ratios.

equation with an adjusted R2 of 0.93. The inhibitory effects reduced methane yield at high TS contents (33–38%), while the relatively low organic matter loading at low TS contents (8–13%) decreased volumetric methane production. Compared to fresh giant reed, dried giant reed showed slightly (8–13%, Fig. 1B) higher volumetric methane production at TS of 23% and 28%, although their methane yields were actually lower (Fig. 1A). This result was caused by the 15–20% larger specific volume (volume of digesting material per unit VS) of fresh giant reed compared to the dried giant reed at the end of the test. Very likely, intracellular water was reduced during the drying process and thus decreased the specific volume of dried giant reed, while free water (water in permeate, added to adjust TS) was partially adsorbed into the micropores on the surface of dried giant reed, and therefore lowered its overall specific volume.

Table 2 Modified Gompertz model predicted cumulative methane yield, maximum daily methane yield, and lag phase, per kg-VS. R2

AD type

F/E ratio

Cumulative CH4 yield L

Max. daily CH4 yield L/day

Lag phase

SS-AD

2.0 3.5 5.0

130.1 83.3 63.1

5.0 3.5 2.1

5.0 12.0 27.2

0.99 0.99 0.99

L-AD

2.0 3.5 5.0

150.7 132.8 137.5

7.0 5.8 5.5

10.3 12.2 16.5

1.00 1.00 0.99

day

retarded microbial activities for hydrolysis of cellulose and methanogenesis from acetate (Wang et al., 2013). In this study, the TAN concentration increased with TS content, and reached 4.33 and 4.92 g/kg at TS of 33% and 38%, respectively; therefore, it may have played a role in reducing methane yields at high TS contents. A bell-shaped curve of volumetric methane production in relation to TS was obtained, with the highest values shown at TS of 20–23% (Fig. 1B). This curve can also be fitted with a quadratic

3.2. Comparison of SS-AD and L-AD at various F/E conditions 3.2.1. Methane production For both SS-AD and L-AD, the highest methane yield was achieved at an F/E of 2.0. At this condition, the methane yield from SS-AD (129.7 L/kg-VS) was slightly lower (16%) but comparable

12

SS-AD

L-AD

A

D

8

F/E=2.0

Daily CH4 yield, L/kg-VS

4 0

B

E

8

F/E=3.5 4 0

C

F

8

F/E=5.0 4 0 0

10

20

30

40

50

10

20

30

Operation time, day Fig. 4. Comparison of daily methane yields in SS-AD and L-AD at three F/E ratios.

40

50

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L. Yang, Y. Li / Bioresource Technology 171 (2014) 233–239 Table 3 Changes in total VFAs, pH, and alkalinity. Conditions

SS-AD TS = 20%

L-AD TS = 8%

Total VFAs, g/kg

F/E = 2.0 F/E = 3.5 F/E = 5.0 F/E = 2.0 F/E = 3.5 F/E = 5.0

pH

Alkalinity, g-CaCO3/kg

Initial

Final

Initial

Final

Initial

Final

1.29 ± 0.14 1.69 ± 0.10 2.46 ± 0.06 1.01 ± 0.15 1.38 ± 0.11 1.42 ± 0.01

0.12 ± 0.00 0.13 ± 0.01 0.95 ± 0.09 0.12 ± 0.00 0.09 ± 0.01 0.17 ± 0.00

7.76 ± 0.00 7.69 ± 0.01 7.77 ± 0.01 7.84 ± 0.04 7.73 ± 0.01 7.67 ± 0.03

8.37 ± 0.03 8.41 ± 0.01 8.34 ± 0.03 8.10 ± 0.07 7.93 ± 0.00 7.96 ± 0.01

9.68 ± 0.03 9.04 ± 0.02 7.72 ± 0.02 10.43 ± 0.23 7.92 ± 0.01 7.44 ± 0.06

14.69 ± 0.22 12.69 ± 0.45 12.27 ± 0.17 11.57 ± 0.14 11.68 ± 0.06 11.61 ± 0.14

Average ± S.E., n = 4.

Table 4 Contents of organic components in the initial and final stages, g. Conditions

SS-AD TS = 20%

L-AD TS = 8%

Cellulose

F/E = 2.0 F/E = 3.5 F/E = 5.0 F/E = 2.0 F/E = 3.5 F/E = 5.0

Hemicellulose

Extractives

Initial

Final

Initial

Protein Final

Initial

Final

Initial

Final

%

VS removal

15.2 15.0 15.0 6.2 7.5 7.4

7.0 ± 0.5 7.9 ± 0.5 10.8 ± 0.3 2.6 ± 0.1 3.5 ± 0.4 4.1 ± 0.3

11.8 9.7 8.9 6.5 7.6 6.1

9.3 ± 0.7 8.5 ± 0.5 8.5 ± 0.2 4.5 ± 0.0 5.6 ± 0.1 5.1 ± 0.1

5.3 5.1 5.1 2.2 2.6 2.5

4.0 ± 0.2 4.2 ± 0.1 4.7 ± 0.2 1.4 ± 0.1 1.9 ± 0.1 2.0 ± 0.0

21.9 18.5 17.2 9.3 9.5 8.5

16.7 ± 0.1 14.4 ± 0.0 14.1 ± 0.1 6.6 ± 0.1 7.0 ± 0.1 6.7 ± 0.0

24.6 ± 0.1 20.7 ± 0.1 20.1 ± 0.1 26.9 ± 0.1 24.1 ± 0.1 24.0 ± 0.1

(p > 0.05) to that produced from L-AD (150.8 L/kg-VS, Fig. 3). Applying the modified Gompertz model, a similar conclusion was obtained, with a cumulative methane yield of 130.1 and 150.7 L/kg-VS for SS-AD and L-AD, respectively, at an F/E of 2.0 (Table 2). The measured methane yields were very close to those predicted by the modified Gompertz model, indicating that the digestion process had been completed. As discussed in the TS effect section, the higher methane yield in L-AD than in SS-AD was largely due to its faster mass transfer and less inhibition. Methane yields from both systems were 45–60% lower than previously reported values using giant reed for methane production, which could be because previous studies applied a much higher inoculation rate (F/E ratio = 0.5), or operated at thermophilic temperatures (55–60 °C), or pretreated with acids (Girolamo et al., 2013; Ragaglini et al., 2014b). At an F/E of 2.0, the volumetric methane production of the L-AD reactors was 5.5–5.7 L/L-digester and was significantly (p < 0.05) lower than the 9.7–10.7 L/L-digester obtained from the SS-AD reactors, indicating that SS-AD was more competitive in terms of space efficiency than L-AD. Although SS-AD has much higher volumetric methane production and only slightly lower methane yield, selection of AD technology is affected by many other factors, e.g. availability of different feedstocks, system complexity and cost, readiness of technology, and operation cost. Cumulative methane yield, maximum daily yield, and peaking time/lag phase in both systems were dramatically affected by the F/E ratio. Increasing the F/E ratio from 2.0 to 5.0 significantly (p < 0.05) reduced methane yield by 65% and 15% for SS-AD and L-AD, respectively (Fig. 3), while the maximum daily methane yield was decreased by 60% and 21% for SS-AD and L-AD, respectively (Table 2). As shown in Fig. 4D–F, the major peak (first peak) in L-AD was delayed from day 16 at an F/E of 2.0 to day 23 at an F/E of 3.5 and then to day 26 at an F/E of 5.0. A primary reason for this delay was the slow growth and metabolism rate of methanogenic microbes under anaerobic conditions (Batstone et al., 2002); therefore, higher F/E ratios required a longer time for microbes to grow and thrive. In SS-AD, the delay of the major peak was even more obvious as the transfer of nutrients and transport of methanogenic microbes at 20% TS content took even longer than at 8% TS (Fig. 4A–C). A previous study showed that

Theoretical methane contribution, L/kg-VS

Average ± S.E., n = 4. Initial values were calculated based on the composition of feedstock and inoculum.

120

SS-AD

L-AD

90

60

30

0 2.0

3.5

2.0

5.0

3.5

5.0

F/E ratio Cellulose

Extractives

Protein

Hemicellulose

Fig. 5. Theoretical contribution of organic components to methane production.

the diffusion coefficient of digesting material decreased by 3.7 times when TS was increased from 8% to 25% (Bollon et al., 2011). Therefore, high TS would prolong the startup phase. At an F/E of 5.0, the SS-AD reactors almost failed, with a very low methane yield and a long startup phase of 27.2 days (Table 2). 3.2.2. Changes in VFAs, TAN, pH, and alkalinity VFA concentrations dropped dramatically in both systems and only a small amount of VFAs remained at the end of the tests except for the SS-AD reactors with an F/E ratio of 5.0 (Table 3). At an F/E ratio of 5.0, the SS-AD digestate contained 0.70 g/kg propionic acid, 0.16 g/kg acetic acid, and 0.07 g/kg isovaleric acid. The high total VFA concentration, especially the propionic acid, in the final digestate indicated a retarded methanogenesis, which supported the observation of low methane yield at that condition. The pH values and alkalinity in both systems increased after the test, and their final values were close to each other. Final TAN concentrations in both systems were in the range of 2.4–3.2 g/kg, which were higher than their initial values but were lower than the inhibitory threshold of 4.3 g/kg (Wang et al., 2013). The

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Table 5 Comparison of energy crops for methane production via anaerobic digestion.

a b c

Feedstock

CH4 yield L/kg-VS

Biomass yield Mg/ha yr

CH4 yield Nm3/ha yr

Energy yield GJ/ha yr

Electricity yield MW h/ha yr

Reference

Giant reed Miscanthus Switchgrass

130–150 152b 212c

23.0a 16.5 4.3

2990–3450 2508 910

113–130 95 34

31–36 26 10

This study Borkowska and Molas (2013) Masse et al. (2011)

Cited from reference (Ragaglini et al., 2014a). Measured in our group, has not been published yet. Based on TS.

increases of TAN and pH values indicated that organic components such as protein were degraded into ammonia during the AD process. 3.2.3. Degradation of organic components The degradation of organic components, e.g., cellulose, hemicellulose, protein, and extractives, are critical for methane production. Among the four organic components, cellulose and extractives were dominant in the initial samples. After digestion, the contents of the four organic compounds were all decreased in both systems, and the overall decreases of the four compounds were 17.5–31.7% for SS-AD and 26.6–36.7% for L-AD, with higher decreases obtained at lower F/E conditions (Table 4). The decreases of VS were 20.1–24.6% for SS-AD and 24.0–26.9% for L-AD. Paired t-tests showed that there was no significant difference (p > 0.05) in the overall decrease of the four organic components between the SS-AD and L-AD processes across the three F/E ratios. Furthermore, the overall decreases of the four compounds and VS were also close to the ratios of generated biogas weight (including CH4, CO2, N2, and O2) over the initial VS loading weight, which were 11.1–28.5% for SS-AD and 24.0–26.6% for L-AD, indicating that degraded organic components were effectively converted to biogas; while the differences in the decrease may have been caused by the conversion of VS to other compounds such as water vapor. Compared to L-AD, SS-AD showed significantly lower (p < 0.05) degradation rates in hemicellulose and protein, but comparable (paired t-test: p > 0.05) degradation rates in cellulose across the three F/E conditions (Table 4), indicating that L-AD was slightly more efficient in degrading organic components in giant reed than SS-AD. Among the three organic components, cellulose showed the highest degradation rates, which ranged from 28.4% to 54.0% for SS-AD and from 44.4% to 57.5% for L-AD, while degradation rates of protein and hemicellulose ranged from 4.4% to 29.9%. Degradation of extractives is complex as other components such as cellulose may be degraded into small extractable compounds, resulting in dynamic changes in concentration and composition of extractives. Given that extractives are usually easily digestible, the degradation rate of extractives should have been high (Zhong et al., 2011), or at least higher than the decrease (17–29%) of extractives during the AD process. Lignin is generally not degradable during digestion (Zhang et al., 2007). 3.2.4. Contribution of organic components to methane production By applying the theoretical methane potential, the contribution of each organic component to methane production can be calculated. Based on this calculation, cellulose contributed the highest amount of methane, followed by extractives, protein, and hemicellulose (Fig. 5). In L-AD, the total theoretical yield was 23–30% less than the measured methane yield; while in SS-AD, the theoretical yield was 44% and 24% lower than measured values at F/E ratios of 2.0 and 3.5, respectively, but was 29% higher at an F/E of 5.0. These differences could have several causes, such as the change in the composition of extractives, degradation of organic components to intermediate products other than methane, and methane contribution

from other organic components. Compared to L-AD, SS-AD had significantly lower (p < 0.05) theoretical methane yields from extractives, proteins, and hemicellulose, but no significant difference in cellulose (p > 0.05) across the three F/E conditions, which was in line with the degradation results. 3.3. Perspectives in using giant reed for methane production via AD Based on the measured methane yield of giant reed in this study and its biomass production from published studies, the potential of using giant reed for methane production via anaerobic digestion can be calculated. Without considering multiple harvesting, giant reed has a higher methane yield potential per hectare than miscanthus or switchgrass, all of which can grow on marginal lands with minimal water and nutrient requirements (Table 5). Theoretically, by applying SS-AD, giant reed harvested from one hectare of land can be converted to 2990 Nm3 methane, which is equivalent to 113 GJ energy or 31 MW h electricity. Note that an even higher giant reed dry biomass yield of 37.7 Mg per year per hectare was reported in Central Italy (Angelini et al., 2009), and a higher methane yield of 392 L/kg-VS was obtained with a low F/E ratio of 0.5 (Ragaglini et al., 2014b), showing there is potential for improving methane production from giant reed. Cutting giant reed in summer and harvesting it again in winter, as previously reported, could increase biomass yield by 20% (Ragaglini et al., 2014a). In addition, high percentages of cellulose, hemicellulose, and protein were not degraded in the AD process as shown in this study, therefore, pretreatment of giant reed would be a promising method for improving the degradation of organic components to increase methane yield (Di Girolamo et al., 2013; Zheng et al., 2014). Digesting giant reed at thermophilic temperatures (55–60 °C) instead of mesophilic temperatures (35–40 °C) may also improve its methane yield (Bolzonella et al., 2011). 4. Conclusion Increasing TS from 8% to 38% decreased methane yield from giant reed, while the highest volumetric methane production was obtained at 20–23% TS. At the most favorable F/E ratio of 2.0, the methane yield from SS-AD was 16% lower than that obtained from L-AD, and both decreased when the F/E ratio was increased to 3.5 and 5.0. Compared to L-AD, SS-AD showed significantly higher volumetric methane production. Among organic components, cellulose contributed the most methane production in both systems. This study also showed that giant reed has a higher methane yield potential per hectare than miscanthus or switchgrass. Acknowledgements This project was funded by USDA NIFA Biomass Research and Development Initiative Program (Award No. 2012-10008-20302). The authors wish to thank Mrs. Mary Wicks (Department of Food, Agricultural and Biological Engineering, OSU) for critical review.

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Anaerobic digestion of giant reed for methane production.

As a fast growing plant, giant reed has good potential to be used as a feedstock for methane production via anaerobic digestion (AD). The effect of to...
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