Giant miscanthus (Miscanthus×giganteus) and Chinese fountaingrass (Pennisetum alopecuroides (L.) Spreng), cultivated for landscaping and soil conservation, are potential energy crops. The study investigated the effect of combined thermal and alkaline pretreatments on biogas production of these energy crops. The pretreatment included two types of alkali (6% CaO and 6% NaOH) at 22, 70 and 100 °C. The alkaline pretreatment resulted in a greater breakdown of the hemicellulose fraction, with CaO more effective than NaOH. Pretreatment of giant miscanthus with 6% CaO at 100 °C for 24 h produced a CH4 yield (313 mL g−1 volatile solids (VS)) that was 1.7 times that of the untreated sample (186 mL g−1 VS). However, pretreatment of Chinese fountaingrass with 6% CaO or 6% NaOH at 70 °C for 24 h resulted in similar CH4 yields (328 and 302 mL g−1 VS for CaO and NaOH pretreatments) as the untreated sample (311 mL g−1 VS). Chinese fountaingrass was more easily digestible but had a low overall CH4 yield per hectare (1,831 m3 ha−1 y−1) compared to giant miscanthus (6,868 m3 ha−1 y−1). This study demonstrates the potential of thermal/alkaline pretreatment and the use of giant miscanthus and Chinese fountaingrass for biogas production.

INTRODUCTION

Giant miscanthus (Miscanthus×giganteus) and Chinese fountaingrass (Pennisetum alopecuroides (L.) Spreng) are grown for landscaping and soil conservation along the roadsides in Taian, Shandong Province, People's Republic of China. The annual grassland biomass energy of the province was estimated at 79.1 PJ (Zhou et al. 2011). The harvested biomass is a potential biofuel feedstock due to these species' high growth rates, low nutrient demand, tolerance to stress and high biomass yield (Iqbal & Lewandowski 2014). Biogas can be potentially produced by anaerobic digestion of giant miscanthus and Chinese fountaingrass. In addition to biogas production, anaerobic digestion produces a residue (digestate) that is rich in ammonium-nitrogen and other nutrients that are readily available to crops, thus reducing the reliance on mineral fertilisers. However, the amount of biogas harnessed from these perennial grasses can be limited by the high lignocellulose content. Low CH4 potential of 0.084–0.19 L g−1 volatile solids (VS) has been reported from Miscanthus species (Menardo et al. 2013; Vasco-Correa & Li 2015). Hence, some form of pretreatment is needed to hydrolyse the fibres into digestible sugars.

Several technologies for the pretreatment of lignocellulose material have been developed for second-generation bioethanol and biogas production (Zheng et al. 2014). The choice of pretreatment method depends on the biomass type, bioprocess and desired objectives. Alkaline pretreatment has been reported to be efficient for lignocellulose agricultural materials, and treatment with NaOH leads to swelling, decreased degree of polymerisation and crystallinity of cellulose, increased surface area and lignin solubilisation (Alvira et al. 2010; Wan et al. 2011). Improved biogas potential and CH4 production rates have been reported when alkaline treatment with NaOH was applied to corn stover (Xiao et al. 2013) and bamboo waste (Shen et al. 2014). However, there are environmental and economic concerns with the use of NaOH. Ca(OH)2 can be used as an alternative base, which is safer and cheaper and easily recovered by precipitation with CO2 but is less efficient than NaOH (Xiao et al. 2013). To the best of the authors' knowledge, a comparison of the effect of low temperature and alkaline pretreatment with NaOH or CaO using giant miscanthus and Chinese fountaingrass has not yet been performed.

The objectives of our study were to investigate the effect of combined alkaline and thermal pretreatment of giant miscanthus and Chinese fountaingrass on (1) the chemical composition (cellulose, hemicellulose and lignin), (2) the soluble nutrients concentration and (3) CH4 production using laboratory-scale batch tests.

MATERIALS AND METHODS

Feedstock characteristics

Giant miscanthus and Chinese fountaingrass were grown at the Experimental Station, Shandong Agricultural University, Taian, Shandong Province, People's Republic of China. The biomass was harvested from three independent fields in September 2013. For giant miscanthus, the biomass yield was 23.1 t ha−1 yr−1, the total solids (TS) was 94.4% and the VS was 90.1% of TS. For Chinese fountaingrass, the biomass yield was 5.6 t ha−1 yr−1, TS was 94.0% and VS was 90.0% of TS. The two grass species were coarsely ground, air dried and stored at room temperature before use.

Both types of grass had similar TS contents and high neutral detergent fibre (cellulose, hemicellulose and lignin) and acid detergent fibre (cellulose and lignin) contents (Table 1). Allison et al. (2011) reported a cellulose content of 47.1% TS, hemicellulose content of 29.4% TS and acid detergent lignin (ADL) content of 11.8% TS from Miscanthus species. Our results were comparable to the cellulose and hemicellulose contents but the ADL was lower. High lignin content in giant miscanthus species has been reported at an advanced maturity (Allison et al. 2011). The carbon to nitrogen ratio was also high (42.8 to 48.2) when compared to the ratio of about 20 to 30 recommended for efficient biogas production (Yadvika et al. 2004). Co-digestion of these feedstocks with other low C/N ratio substrates, such as manure, is recommended.

Table 1

Characteristics of giant miscanthus and Chinese fountaingrass and the biomass energy content. Mean values of duplicate analyses

Characteristics Giant miscanthus Chinese fountaingrass 
TS (%) 94.4 ± 0.1 94.0 ± 0.0 
VS (% of TS) 90.1 ± 0.3 90.0 ± 0.0 
C (% of TS) 46.1 ± 0.1 46.08 ± 0.5 
N (% of TS) 0.96 ± 0.01 1.08 ± 0.02 
C/N 48.2 ± 0.6 42.8 ± 0.3 
NDF (% of TS) 78.5 ± 0.4 76.5 ± 0.1 
ADF (% of TS) 47.1 ± 0.0 49.9 ± 0.4 
Cellulose (% of TS) 42.6 ± 0.0 40.5 ± 0.3 
Hemicellulose (% of TS) 28.6 ± 0.4 29.4 ± 0.5 
ADL (% of TS) 7.3 ± 0.1 6.6 ± 0.7 
CP (% of TS) 5.98 ± 0.07 6.72 ± 0.10 
CF (% TS) 1.54 ± 0.22 1.08 ± 0.10 
Biomass yield (t TS ha−1y−123.1 ± 0.3 5.6 ± 0.1 
Characteristics Giant miscanthus Chinese fountaingrass 
TS (%) 94.4 ± 0.1 94.0 ± 0.0 
VS (% of TS) 90.1 ± 0.3 90.0 ± 0.0 
C (% of TS) 46.1 ± 0.1 46.08 ± 0.5 
N (% of TS) 0.96 ± 0.01 1.08 ± 0.02 
C/N 48.2 ± 0.6 42.8 ± 0.3 
NDF (% of TS) 78.5 ± 0.4 76.5 ± 0.1 
ADF (% of TS) 47.1 ± 0.0 49.9 ± 0.4 
Cellulose (% of TS) 42.6 ± 0.0 40.5 ± 0.3 
Hemicellulose (% of TS) 28.6 ± 0.4 29.4 ± 0.5 
ADL (% of TS) 7.3 ± 0.1 6.6 ± 0.7 
CP (% of TS) 5.98 ± 0.07 6.72 ± 0.10 
CF (% TS) 1.54 ± 0.22 1.08 ± 0.10 
Biomass yield (t TS ha−1y−123.1 ± 0.3 5.6 ± 0.1 

ADL: acid detergent lignin, CP: crude protein, CF: crude fat, NDF is the sum of cellulose, hemicellulose and ADL, ADF is the sum of cellulose and ADL.

Thermal and alkaline pretreatment

To achieve 80% water content levels, giant miscanthus and Chinese fountaingrass were pretreated by adding 8 mL of 15 g L−1 NaOH to 2 g VS substrate (6% NaOH g−1 VSadded) at three temperature conditions: (1) 22 °C for 3 days, (2) 70 °C for 24 h and (3) 100 °C for 24 h. Pretreatment at 22 °C for 3 days was investigated in order to limit the energy input when compared to the treatments at 70 and 100 °C. Controls were also included by adding 8 mL ultrapure H2O to 2 g VS of substrate under the same temperature conditions outlined for NaOH. For the pretreatment with CaO, 0.12 g CaO and 8 mL of H2O were added directly to 2 g VS (6% CaO g−1 VSadded) and incubated at 70 °C and 100 °C for 24 h. The direct addition of 0.12 g CaO and incubation at higher temperatures was due to the low solubility of CaO (1.19 g L−1 at 25 °C). The alkaline pretreatments were performed in 50 mL closed falcon tubes, with the tubes placed in a beaker before incubation in a convection oven at the pre-set temperatures. After the alkaline and thermal pretreatments, the contents were allowed to cool under running tap water. Five sets of samples were prepared and used for the CH4 potential batch tests while two were dried at 55 °C for 2 days and used for fibre analysis.

Methane potential batch test

Batch tests were performed to assess the CH4 potential from treated and raw untreated giant miscanthus, and Chinese fountaingrass. The pretreatments used for the CH4 potential batch tests were 6% NaOH or 6% CaO per g VS at 70 °C for 24 h and 6% CaO per g VS at 100 °C for 24 hours. Avicel cellulose was included in the batch tests as a control to assess the cellulose degrading activity of the inoculum. Anaerobic seed sludge, used as the inoculum, was collected from a mesophilic (40 °C) industrial-scale biogas digester (Lethbridge Biogas LP, Canada), which co-digests dairy and hog slurry manure with other commercial food wastes. The inoculum was pre-incubated for 5 days before use in the batch tests in order to reduce the background CH4 production. The inoculum had a pH of 8.08, total alkalinity of 16.7 g L−1, TS of 2.2% and VS of 64.0% of TS (Nkemka et al. 2015). The tests were performed in triplicate in 500 mL E-flasks under mesophilic (40 °C) condition. The inoculum to substrate ratio (ISR) was fixed at 3:1 in the experiments in order to avoid possible NaOH inhibition from the pretreated samples (Fang et al. 2011). The average TS content in the batches was 12.2% and the E-flasks were manually swirled daily for the first 2 weeks and then every other day until completion of the experiment. The rest of the experimental set-up was as previously described (Nkemka et al. 2015).

In order to investigate the effect of the pretreatment on the soluble ion concentration in the anaerobic digestate of giant miscanthus, another batch test was performed by digesting untreated and pretreated (6% NaOH at 70 °C for 24 h and 6% CaO at 100 °C for 24 h) giant miscanthus at an ISR of 1:1. A selected low ISR (1:1) was used compared to the 3:1 ISR, since the inoculum contained a high concentration of soluble ions.

Analytical methods

The TS and VS were determined using Standard Methods (APHA 1998). The total carbon and total nitrogen were analysed using a CN analyser (Carlo Erba, Rodano, Italy). Corn stover was used as the internal standard and the procedure for C and N determination was as previously reported (Tabatabai & Keith 2003). The crude protein was calculated by multiplying a conversion factor (6.25) of nitrogen to protein. The energy content of the perennial grasses was analysed using a CAL2K combustion calorimeter (Randburg, South Africa). The internal standard was benzoic acid with an energy value of 26.453 J g−1 and the combustion was performed under an oxygen atmosphere at 1,500 kPa. Cellulose, hemicellulose and ADL were analysed using the fibre analysis method (Van Soest et al. 1991). The pH was measured before and after pretreatment using an Orion pH meter (Model 290 Beverly, Maine). The biogas volume was measured with a 100 mL gas-tight syringe (Cadence Science, Cranston, USA). Its composition (CH4 and CO2) was analysed using a Micro-GC (Varian 4900, Palo Alto, USA) equipped with a thermal conductivity detector. The analytical procedure and settings have been reported elsewhere (Nkemka et al. 2015). The CH4 collected was normalised to 273 K and 1 atm. The concentrations of soluble ion concentrations (Cl, NO3, NH4+, PO43−, K+, Na+, SO42−, Mg2+ and Ca2+) in the anaerobic digestate were determined on the supernatants of filtered (0.42 μm) samples using ion chromatography (ICS-1000 and DX-600, Dionex, Sunnyvale, USA). Water-soluble nitrogen (Water-N) and dissolved organic carbon (DOC) were analysed on filtered (0.42 μm) samples using a Shimadzu TOC-VCSH/TNM-1 analyzer.

CH4 production rate, Ks, was estimated using the equation 
formula
1
where Go is the ultimate CH4 potential, Gt is the CH4 produced at time t, t is the time in days and Ks is the hydrolysis constant in days, which is the same as the methanation rate since hydrolysis is rate limiting.

Statistical analysis was performed using SPSS 16.0, and the univariate test was performed using the Tukey test at a probability level of 0.05.

RESULTS AND DISCUSSION

Effect of thermal and alkaline pretreatment on lignocellulose fibre

The combined alkaline and thermal treatment resulted in the solubilisation of the hemicellulose fraction, while leaving the ADL lignin and cellulose fraction relatively unchanged. Table 2 presents the effect of the combined alkaline and thermal treatment on lignocellulose fibre and pH. CaO was more efficient in dissolving the hemicellulose fraction when compared to the use of NaOH. The highest hemicellulose solubilisation rates of 66.8% were obtained for giant miscanthus with 6% CaO at 100 °C for 24 h, and 63.9% was obtained for Chinese fountaingrass with 6% CaO at 70 °C for 24 h. Low hemicellulose solubilisation of 31.8 and 41.5% was obtained with 6% NaOH at 70 °C for 24 h for both giant miscanthus and Chinese fountaingrass, respectively. Approximately 9.4% and 12.5% solubilisation of hemicellulose was due to the thermal effect of the controls at 70 °C and 100 °C for 24 h for giant miscanthus (results not shown). About 40% hemicellulose, 25% cellulose and 10% lignin solubilisation have been reported when pretreating grass silage with 5% NaOH g−1 VS at 60 °C (Xie et al. 2011). A cellulose solubilisation of less than 7% was reported when pretreating corn stover with 6% NaOH at 20 °C for 3 days (Xiao et al. 2013). Despite the fact that cellulose fibre was relatively unaffected by the pretreatment in the present study, others have reported that pretreatment might have resulted in decreased cellulose crystallinity, increased pore size and surface area (Alvira et al. 2010; Xiao et al. 2013).

Table 2

Effect of combined thermal and alkaline pretreatment on the fibre composition of giant miscanthus and Chinese fountaingrass

  Cellulose (% of TS) Hemicellulose (% of TS) AD lignin (% of TS) pH before treatment pH after treatment 
Giant miscanthus 
 Controla at 22 °C for 3 d 43.6 ± 0.6 a 28.2 ± 0.2 a 8.0 ± 0.5 a 6.9 ± 0.0 5.6 ± 0.0 
 Control at 70 °C for 24 h 40.4 ± 0.1 b 25.9 ± 1.6 ab 8.2 ± 0.1 a 6.9 ± 0.0 5.2 ± 0.3 
 Control at 100 °C for 24 h 39.2 ± 0.2 b 24.8 ± 1.5 b 11.1 ± 0.5 b 6.9 ± 0.0 5.1 ± 0.2 
 6% NaOH at 22 °C for 3 d 41.0 ± 1.7 b 23.6 ± 0.0 b 9.2 ± 0.9 a 12.3 ± 0.0 9.4 ± 0.1 
 6% NaOH at 70 °C for 24 h 42.8 ± 0.5 ab 19.5 ± 2.7 c 8.4 ± 0.2 a 12.3 ± 0.0 9.1 ± 0.1 
 6% NaOH at 100 °C for 24 h 41.8 ± 0.1 ab 22.3 ± 0.6 bc 8.0 ± 0.2 a 12.3 ± 0.0 10.2 ± 0.0 
 6% CaO at 70 °C for 24 h 41.2 ± 0.2 b 17.5 ± 0.0 c 9.9 ± 0.4 a 11.1 ± 0.0 9.0 ± 0.1 
 6% CaO at 100 °C for 24 h 40.4 ± 0.4 b 9.3 ± 1.2 d 9.8 ± 0.2 a 11.1 ± 0.0 8.0 ± 0.1 
Chinese fountaingrass 
 Control: 22 °C, 3 d 39.7 ± 0.9 a 24.2 ± 0.2 a 6.2 ± 0.4 a 7.1 ± 0.0 6.0 ± 0.1 
 6% NaOH at 22 °C for 3 d 42.9 ± 0.5 a 22.2 ± 0.1 a 7.1 ± 0.9 a 12.3 ± 0.1 10.4 ± 0.2 
 6% NaOH at 70 °C for 24 h 39.7 ± 0.9 a 17.2 ± 6.1 a 8.3 ± 0.9 b 12.3 ± 0.1 9.4 ± 0.2 
 6% NaOH at 100 °C for 24 h 41.7 ± 5.9 a 22.6 ± 0.9 a 7.1 ± 0.3 a 12.3 ± 0.1 10.1 ± 0.6 
 6% CaO at 70 °C for 24 h 38.1 ± 1.1 a 10.6 ± 1.9 b 7.7 ± 0.4 a 12.0 ± 0.1 9.3 ± 0.1 
  Cellulose (% of TS) Hemicellulose (% of TS) AD lignin (% of TS) pH before treatment pH after treatment 
Giant miscanthus 
 Controla at 22 °C for 3 d 43.6 ± 0.6 a 28.2 ± 0.2 a 8.0 ± 0.5 a 6.9 ± 0.0 5.6 ± 0.0 
 Control at 70 °C for 24 h 40.4 ± 0.1 b 25.9 ± 1.6 ab 8.2 ± 0.1 a 6.9 ± 0.0 5.2 ± 0.3 
 Control at 100 °C for 24 h 39.2 ± 0.2 b 24.8 ± 1.5 b 11.1 ± 0.5 b 6.9 ± 0.0 5.1 ± 0.2 
 6% NaOH at 22 °C for 3 d 41.0 ± 1.7 b 23.6 ± 0.0 b 9.2 ± 0.9 a 12.3 ± 0.0 9.4 ± 0.1 
 6% NaOH at 70 °C for 24 h 42.8 ± 0.5 ab 19.5 ± 2.7 c 8.4 ± 0.2 a 12.3 ± 0.0 9.1 ± 0.1 
 6% NaOH at 100 °C for 24 h 41.8 ± 0.1 ab 22.3 ± 0.6 bc 8.0 ± 0.2 a 12.3 ± 0.0 10.2 ± 0.0 
 6% CaO at 70 °C for 24 h 41.2 ± 0.2 b 17.5 ± 0.0 c 9.9 ± 0.4 a 11.1 ± 0.0 9.0 ± 0.1 
 6% CaO at 100 °C for 24 h 40.4 ± 0.4 b 9.3 ± 1.2 d 9.8 ± 0.2 a 11.1 ± 0.0 8.0 ± 0.1 
Chinese fountaingrass 
 Control: 22 °C, 3 d 39.7 ± 0.9 a 24.2 ± 0.2 a 6.2 ± 0.4 a 7.1 ± 0.0 6.0 ± 0.1 
 6% NaOH at 22 °C for 3 d 42.9 ± 0.5 a 22.2 ± 0.1 a 7.1 ± 0.9 a 12.3 ± 0.1 10.4 ± 0.2 
 6% NaOH at 70 °C for 24 h 39.7 ± 0.9 a 17.2 ± 6.1 a 8.3 ± 0.9 b 12.3 ± 0.1 9.4 ± 0.2 
 6% NaOH at 100 °C for 24 h 41.7 ± 5.9 a 22.6 ± 0.9 a 7.1 ± 0.3 a 12.3 ± 0.1 10.1 ± 0.6 
 6% CaO at 70 °C for 24 h 38.1 ± 1.1 a 10.6 ± 1.9 b 7.7 ± 0.4 a 12.0 ± 0.1 9.3 ± 0.1 

aWater was used in the control treatments.

Data in a column followed by the same lower case letter do not differ significantly at 0.05 probability level when using the post hoc multiple comparisons for observed means.

Pretreatment with NaOH and CaO decreases pH (Table 2). The pH dropped from 12.3 to 9.1 for giant miscanthus in the pretreatment with 6% NaOH at 70 °C for 24 h. A decrease in the pH from 11.1 to 8.0 was also recorded for the pretreatment with 6% CaO at 100 °C for 24 h. A decrease in pH during pretreatment might be beneficial in avoiding pH adjustments to neutral pH before anaerobic digestion. The hemicellulose backbone in the biomass cell wall is acetylated and the hydrolysis of hemicellulose can result in the liberation of acetic acid, which can decrease pH (Zhao et al. 2012).

Effect of thermal and alkaline pretreatment on the CH4 potential from giant miscanthus and Chinese fountaingrass

A higher CH4 yield was obtained after 27 days of digestion of the pretreated giant miscanthus when compared with the untreated at an ISR of 3:1 (Figure 1(a)). At an ISR of 3:1, CH4 yields from pretreatment of giant miscanthus with 6% CaO at 100 °C for 24 h (313 ± 29 mL g−1 VS) and with 6% NaOH at 70 °C for 24 h (297 ± 14 mL g−1 VS) were similar and both were significantly higher than the untreated giant miscanthus, which had a CH4 yield of 186 ± 72 mL g−1 VS (P < 0.015). The CH4 content from pretreated (57%) was also higher than the untreated giant miscanthus (46%). Compared to the CH4 production rate of untreated giant miscanthus (0.026 d−1), CH4 production rate increased to 0.069 d−1 (2.7 times) with 6% CaO at 70 °C for 24 h pretreatment, 0.076 d−1 (2.9 times) with 6% NaOH at 70 °C for 24 h pretreatment and 0.094 d−1 (3.6 times) with 6% CaO at 100 °C for 24 h pretreatment (Table 3).
Table 3

Methane yield and production rate in the anaerobic digestion of giant miscanthus and Chinese fountaingrass

Pretreatment CH4 yield after 27 days (mL g−1 VS) ISR Ks (d−1CH4 yield after day 14 (% of day 27) 
Untreated giant miscanthus 186 ± 72 3:1 0.026 ± 0.011 29 
 6% NaOH at 70 °C for 24 h 297 ± 14 3:1 0.076 ± 0.010 70 
 6% CaO at 70 °C for 24 h 258 ± 2 3:1 0.069 ± 0.015 84 
 6% CaO at 100 °C for 24 h 313 ± 29 3:1 0.094 ± 0.032 87 
Untreated giant miscanthus 163 ± 7b 1:1 0.014 ± 0.001 17a 
 6% NaOH at 70 °C for 24 h 195 ± 10b 1:1 0.024 ± 0.001 34a 
 6% CaO at 100 °C for 24 h 179 ± 34b 1:1 0.020 ± 0.000 30a 
Chinese fountaingrass 
 Untreated 311 ± 26 3:1 0.074 ± 0.010 34 
 6% NaOH at 70 °C for 24 h 302 ± 23 3:1 0.078 ± 0.018 69 
 6% CaO at 100 °C for 24 h 328 ± 73 3:1 0.095 ± 0.021 85 
Pretreatment CH4 yield after 27 days (mL g−1 VS) ISR Ks (d−1CH4 yield after day 14 (% of day 27) 
Untreated giant miscanthus 186 ± 72 3:1 0.026 ± 0.011 29 
 6% NaOH at 70 °C for 24 h 297 ± 14 3:1 0.076 ± 0.010 70 
 6% CaO at 70 °C for 24 h 258 ± 2 3:1 0.069 ± 0.015 84 
 6% CaO at 100 °C for 24 h 313 ± 29 3:1 0.094 ± 0.032 87 
Untreated giant miscanthus 163 ± 7b 1:1 0.014 ± 0.001 17a 
 6% NaOH at 70 °C for 24 h 195 ± 10b 1:1 0.024 ± 0.001 34a 
 6% CaO at 100 °C for 24 h 179 ± 34b 1:1 0.020 ± 0.000 30a 
Chinese fountaingrass 
 Untreated 311 ± 26 3:1 0.074 ± 0.010 34 
 6% NaOH at 70 °C for 24 h 302 ± 23 3:1 0.078 ± 0.018 69 
 6% CaO at 100 °C for 24 h 328 ± 73 3:1 0.095 ± 0.021 85 

aAmount of CH4 produced in 11 days (%).

bCH4 yield after 37 days of digestion.

Figure 1

Cumulative CH4 production for (a) giant miscanthus and (b) Chinese fountaingrass.

Figure 1

Cumulative CH4 production for (a) giant miscanthus and (b) Chinese fountaingrass.

Additionally, 87% of the CH4 was collected after 14 days from the pretreatment with 6% CaO at 100 °C for 24 h while only 29% was collected from the untreated sample, another indicator showing an improvement in the CH4 production rate. The CH4 yield of the cellulose control in our study was 407.2 ± 7.4 m L g−1 VS, close to the reported theoretical CH4 yield of 415 mL g−1 VS. This suggests that the inoculum used in our study was active during the CH4 potential batch test. No inhibition was observed for the two energy crops in the present study at the high ISR of 3:1 in the digestion of the thermal pretreated samples with 6% NaOH.

A lower CH4 yield was obtained at an ISR of 1:1 (163 mL g−1 VS) when compared to the ratio of 3:1 (186 mL g−1 VS). This was as expected for the untreated giant miscanthus sample. However, a lower CH4 yield was also obtained from the pretreated samples with 6% NaOH at 70 °C for 24 h (195 mL g−1 VS) and at an ISR of 1:1 when compared to the ISR of 3:1 (297 mL g−1 VS). Sodium ion inhibition of CH4 production has been reported for thermal pretreated grass silage using 7.5% NaOH at an ISR of 1:1 (Xie et al. 2011) and might account for the low yield.

There was no significant difference in the CH4 yield obtained after 27 days at an ISR of 3:1 from the digestion of the untreated and pretreated Chinese fountaingrass (P > 0.05) (Figure 1(b)). The CH4 yield of the untreated sample was 311 ± 26 mL g−1 VS, suggesting Chinese fountaingrass was easier to digest than untreated giant miscanthus, with a CH4 yield of only 187 ± 72 mL g−1 VS. The CH4 content ranged from 47% for the untreated to 57% for the pretreated Chinese fountaingrass.

Pretreatment with 6% NaOH and CaO at 70 °C for 24 h led to higher CH4 production rates (1.1 and 1.3 times) than the untreated Chinese fountaingrass, with 85% CH4 produced when pretreated with 6% CaO at 100 °C for 24 h, while 34% was produced from the untreated sample during the first 14 days. Thermal and alkaline treatment at 20 °C for 3 days with combined 4% NaOH and 2% Ca(OH)2 resulted in a 55% improvement in the CH4 yield from 185 mL g−1 VS untreated to 286 mL g−1 VS treated corn stover (Xiao et al. 2013). Improvement in the CH4 yield from 84 mL g−1 VS to 374 mL g−1 VS has been reported for steam pretreatment of Miscanthus at 220 °C for 10 minutes (Menardo et al. 2013). A lower giant miscanthus CH4 yield of 297 mL g−1 VS was obtained in our study, which could be due to the lower temperature (70–100 °C) used. Improvement of CH4 production rate from 0.07 d−1 to 0.15 d−1 with corresponding CH4 yields of 273 ± 6.8 and 283 ± 4.0 mL g−1 VS for the untreated and hydrothermal pretreatment of giant reed (Arundo L.) at 180 °C for 20 minutes has been reported (Di Girolamo et al. 2013). In a recent study, the high solubility of Ca(OH)2 at low temperature was explored and pretreatment of grass with 7.5% Ca(OH)2 at 10 °C for 19.2 h improved the methane yield by 37.3% (Khor et al. 2015). Based on the results of our studies, optimum pretreatment conditions vary among feedstocks, and optimisation of pretreatment conditions (time, temperature and alkaline concentration) is recommended for each substrate.

Both giant miscanthus and Chinese fountaingrass have a high gross energy potential of 22.3 and 21.7 MJ kg−1 TS, respectively. Anaerobic digestion of the pretreated samples of these two grasses resulted in energy outputs of 14.0 and 15.5 MJ kg−1 TS, respectively, which were 68% and 72% of the gross energy potential (Figure 2). The energy output from the untreated samples was 39% and 63% for giant miscanthus and Chinese fountaingrass, respectively.
Figure 2

Bio-CH4 energy yield of giant miscanthus and Chinese fountaingrass. The pretreatment conditions used for the calculations were 6% CaO at 100 °C for 24 h for giant miscanthus and 6% CaO at 70 °C for 24 h for Chinese fountaingrass.

Figure 2

Bio-CH4 energy yield of giant miscanthus and Chinese fountaingrass. The pretreatment conditions used for the calculations were 6% CaO at 100 °C for 24 h for giant miscanthus and 6% CaO at 70 °C for 24 h for Chinese fountaingrass.

Combining the biomass yield with the CH4 yield, the gross CH4 yield per hectare of land for the pretreated giant miscanthus was 6,868 m3 ha−1 y−1, 3.8 times the value for Chinese fountaingrass (1,831 m3 ha−1 y−1). In a recent study, steam explosion of Miscanthus at 210 °C for 10 minutes resulted in a similar gross CH4 yield of 7,057 m3 ha−1 y−1 at a medium biomass yield of 21 t TS ha−1 y−1 (Theuretzbacher et al. 2014). The selection of a high-yield energy crop together with easy digestibility can therefore be important in reaching high biogas energy output.

Effect of thermal and alkaline pretreatment on soluble nutrients in anaerobic digestate of giant miscanthus

The inoculum contained a high concentration of soluble ions when compared to the untreated and pretreated samples of giant miscanthus (Table 4). The high soluble ion concentration in the inoculum reflected the high concentration in the digested samples. The water-soluble Ca2+, K+ and Mg2+ ion concentrations were not affected in the batch anaerobic digestion of pretreated and untreated giant miscanthus. The pH of the inoculum was 8.03 and the inoculum also had a high buffer capacity or bicarbonate alkalinity of 16.7 g L−1 (Nkemka et al. 2015). The solubility of Ca2+, K+ and Mg2+ ions has been reported to increase at low pH (Gilroyed et al. 2013). Increase in Na+ ions, 1,213.7 ± 140.5 mg L−1 in the batch test of samples from the pretreatment with 6% NaOH at 70 °C for 24 h, was due to the addition of NaOH in the treatment. This increase in Na+ ion concentration would not have a negative effect on the biogas process as Na+ ions are required by mesophilic methanogens in concentrations of 100 to 230 mg L−1, and higher concentrations of 3,500 to 5,500 mg L−1 are moderately inhibitory. In addition, the inhibitory level of Na+ ions also depends on sludge adaptation (Chen et al. 2008).

Table 4

Soluble nutrient concentrations in the inoculum used and in the giant miscanthus prior to 24 h pretreatment, after 24 h pretreatment, and after 27-day anaerobic digestion

  Inoculum Giant miscanthus before pretreatment Giant miscanthus after pretreatment
 
Giant miscanthus after anaerobic digestion
 
Concentration (mg L−1(Nkemka et al. 20156% NaOH at 70 °C 6% CaO at 100 °C Untreated 6% NaOH at 70 °C 6% CaO at 100 °C 
Soluble ions 
 Cl 999 ± 32 41.7 ± 4.1 47.8 ± 0.9 45.4 ± 0.5 709 ± 1 1,008 ± 24 957 ± 19 
 NO3-N 5 ± 1 1.4 ± 0.1 1.7 ± 0.1 1.6 ± 0.1 6 ± 1 5 ± 0 6 ± 2 
 SO4-S 32 ± 2 9.6 ± 0.2 13.0 ± 0.7 10.6 ± 2.0 29 ± 5 34 ± 5 35 ± 6 
 PO4-P 24 ± 6 3.8 ± 0.4 2.6 ± 0.1 0.8 ± 0.1 13 ± 0 20 ± 2 18 ± 1 
 Na 1,019 ± 34 6.1 ± 0.3 136.3 ± 6.6 7.4 ± 1.7 1,096 ± 2 1,214 ± 141 969 ± 15 
 NH4+-N 2,926 ± 160 0.2 ± 0.4 0.5 ± 0.1 – 3,456 ± 393 2,942 ± 105 2,884 ± 43 
 Mg 80 ± 2 8.1 ± 1.3 1.2 ± 0.2 8.3 ± 5.0 71 ± 7 74 ± 1 72 ± 0 
 K 2,561 ± 37 37.7 ± 3.2 40.3 ± 1.6 46.6 ± 2.4 2,745 ± 2 2,563 ± 91 2,177 ± 145 
 Ca 91 ± 1 8.6 ± 1.0 12.0 ± 3.5 82.4 ± 6.8 81 ± 13 87 ± 1 83 ± 1 
 DOC 1,626 ± 156 2,506.8 ± 41.6 7,594.0 ± 113.1 7,674.0 ± 80.6 1,264 ± 48 1,207 ± 97 964 ± 30 
 Water-N 2,705 ± 108 141.3 ± 20.6 284.0 ± 2.4 191.7 ± 16.3 2,849 ± 49 2,700 ± 38 2,468 ± 167 
  Inoculum Giant miscanthus before pretreatment Giant miscanthus after pretreatment
 
Giant miscanthus after anaerobic digestion
 
Concentration (mg L−1(Nkemka et al. 20156% NaOH at 70 °C 6% CaO at 100 °C Untreated 6% NaOH at 70 °C 6% CaO at 100 °C 
Soluble ions 
 Cl 999 ± 32 41.7 ± 4.1 47.8 ± 0.9 45.4 ± 0.5 709 ± 1 1,008 ± 24 957 ± 19 
 NO3-N 5 ± 1 1.4 ± 0.1 1.7 ± 0.1 1.6 ± 0.1 6 ± 1 5 ± 0 6 ± 2 
 SO4-S 32 ± 2 9.6 ± 0.2 13.0 ± 0.7 10.6 ± 2.0 29 ± 5 34 ± 5 35 ± 6 
 PO4-P 24 ± 6 3.8 ± 0.4 2.6 ± 0.1 0.8 ± 0.1 13 ± 0 20 ± 2 18 ± 1 
 Na 1,019 ± 34 6.1 ± 0.3 136.3 ± 6.6 7.4 ± 1.7 1,096 ± 2 1,214 ± 141 969 ± 15 
 NH4+-N 2,926 ± 160 0.2 ± 0.4 0.5 ± 0.1 – 3,456 ± 393 2,942 ± 105 2,884 ± 43 
 Mg 80 ± 2 8.1 ± 1.3 1.2 ± 0.2 8.3 ± 5.0 71 ± 7 74 ± 1 72 ± 0 
 K 2,561 ± 37 37.7 ± 3.2 40.3 ± 1.6 46.6 ± 2.4 2,745 ± 2 2,563 ± 91 2,177 ± 145 
 Ca 91 ± 1 8.6 ± 1.0 12.0 ± 3.5 82.4 ± 6.8 81 ± 13 87 ± 1 83 ± 1 
 DOC 1,626 ± 156 2,506.8 ± 41.6 7,594.0 ± 113.1 7,674.0 ± 80.6 1,264 ± 48 1,207 ± 97 964 ± 30 
 Water-N 2,705 ± 108 141.3 ± 20.6 284.0 ± 2.4 191.7 ± 16.3 2,849 ± 49 2,700 ± 38 2,468 ± 167 

Pretreatment with either 6% NaOH at 70 °C for 24 h or 6% CaO at 100 °C for 24 h resulted in an increase in the available nitrogen, which was measured as water-N. However, improvement in the nitrogen availability was not observed in the digested samples due to the high concentration of available nitrogen already present in the inoculum. An increase of 20–26% NH4+-N has been previously reported (Pabón-Pereira et al. 2014). A similar high concentration of NH4+-N of 3.5 g L−1 (with a pH of 8.3) was reported from a full-scale anaerobic digester treating manure and agro-industrial food wastes (Nges & Björnsson 2012).

The concentration of DOC after anaerobic digestion was lower for the pretreated samples with 6% NaOH at 70 °C for 24 h (1,207 g L−1) and 6% CaO at 100 °C for 24 h (964 g L−1) than for the untreated samples (1,626 g L−1). The decrease in DOC was due to the increased solubilisation during pretreatment and conversion of the DOC into biogas.

CONCLUSIONS

The effect of low temperature and alkaline pretreatment with NaOH or CaO using giant miscanthus and Chinese fountaingrass was evaluated. Combined alkaline and thermal pretreatments of giant miscanthus and Chinese fountaingrass with 6% NaOH or 6% CaO at 70 and 100 °C were effective in increasing the solubilisation of hemicellulose. Significantly higher CH4 yield was obtained from the pretreatment of giant miscanthus, while comparable yields were obtained from Chinese fountaingrass in comparison with the untreated samples. In addition, the CH4 production rate from pretreatments was 2.7 to 3.6 times higher than the untreated for giant miscanthus and 1.1 to 1.3 times higher than the untreated for Chinese fountaingrass. Reduction in the digestion time is therefore one of the merits of this pretreatment. This study also demonstrates that the selection of energy crops in terms of their biomass yield and easy digestibility can play a vital role in harnessing the bound energy in the form of biogas.

ACKNOWLEDGEMENTS

This work was supported by Natural Resources Canada under the Program of Energy Research and Development (PERD) and by Agriculture and Agri-Food Canada under the Growing Forward 2 Program.

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