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Energy xxx (2014) 1e6

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Optimization of anaerobic co-digestion of Solidago canadensisL. biomass and cattle slurry

Yiqing Yao, Hongmei Sheng, Yang Luo, Mulan He, Xiangkai Li, Hua Zhang, Wenliang He,Lizhe An*

Ministry of Education, Key Laboratory of Cell Activities and Stress Adaptations, School of Life Sciences, Lanzhou University, Lanzhou 730000, China

a r t i c l e i n f o

Article history:Received 21 October 2013Received in revised form22 August 2014Accepted 5 September 2014Available online xxx

Keywords:Anaerobic digestionMethane productionSolidago canadensis L.Cattle slurry

* Corresponding author. Tel.: þ86 931 8912126; faxE-mail address: lizhean@lzu.edu.cn (L. An).

http://dx.doi.org/10.1016/j.energy.2014.09.0130360-5442/© 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Yao Y, et al(2014), http://dx.doi.org/10.1016/j.energy.20

a b s t r a c t

SC (Solidago canadensis L.) was digested with CS (cattle slurry). The process stability, methane productionby anaerobic digestion, and the efficiency of organic matter removal were measured. The maximummethane production of 143.7 L/kg volatile solids was obtained at a SC:CS ratio of 1:3 and a substrateconcentration of 6% (based on volatile solids); however, the difference between total methane produc-tion for SC:CS ratios of 1:1 and 1:3 was not significant (p > 0.05). Therefore, based on the SC treatmentcapacity, the optimum SC:CS ratio is 1:1 for this application. For a 6% substrate concentration, the totalmethane production (129.6 L/kg volatile solids) at a SC:CS ratio of 1:1 was 123.5% higher than that of acontrol. The pH was fairly constant (6.8e7.6). The removal efficiencies of total solids, volatile solids,cellulose, hemicellulose, and soluble chemical oxygen demand were 37.3, 41.6, 23.6, 34.8, and 38.8%,respectively, and the T80 was 30.0% shorter than that for maximum methane production. These resultsindicate that the process stability and methane production efficiency of SC can be improved by CSaddition.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Invasive plants are widely recognized as a serious environ-mental problem, which can impact or destroy ecosystem func-tioning and ecosystem biodiversity [1]. Invasive plants pose a greatthreat to many terrestrial and aquatic ecosystems, because they candisplace native plant species and alter geomorphological processesand nutrient cycles [2,3].

SC (Asteraceae) (Solidago canadensis L.), which originated inNorth America, has successfully invaded Europe, Asia, and Australia[4,5]. This species grows in densely monospecific stands, has theability to spread locally via rhizomes, and has a high growth rate[6]. SC was introduced to Eastern China in 1913 as an ornamentalplant, after which its seeds were dispersed from gardens to naturalenvironments by wind and other mechanisms [7]. Since then, SChas spread into croplands in Shanghai, Jiangsu, and ZhejiangProvinces. SC is currently found in various habitats, includingroadsides, orchards, gardens, abandoned farmland, and the greenspaces of some cities [8]. In China, the abundance and diversity of

: þ86 931 8625 576.

., Optimization of anaerobic c14.09.013

native plant communities has decreased greatly, because of thestrong allelopathic effects of SC on native plants, arbuscularmycorrhizal fungi, and soil-borne pathogens, and the detrimentaleffects of SC on soil nutrient cycling, microbial functional diversity,and the trophic structures of insect-associated communities[9e13].

The biomass yields of SC are high, and converting SC tomethaneby AD (anaerobic digestion) may be a good option for SC disposal.SC produces many seeds, which germinate easily in a wide range ofsoils and can be dispersed by the wind [6]. However, harvesting SCbefore flowering for methane production has the potential tocontrol its spread. Additionally, CS (cattle slurry) might be a goodcandidate for digestionwith SC for methane production, because ofthe presence of additional nutrients and the microorganism pop-ulations present in CS; these may improve the process balance andmethane production by AD [14,15].

The objectives of this study were to (1) determine whether theprocess stability of AD of SC can be enhanced by CS; (2) investigatethe effects of substrate concentration and CS proportion in themixture on daily and total methane production and determine theoptimum conditions; (3) analyze the AD process performance; and(4) assess the substrate degradation after AD.

o-digestion of Solidago canadensis L. biomass and cattle slurry, Energy

Table 2C/N ratios for mixtures.

Concentration (%) Control 3:1 1:1 1:3

4% 27.8 27.3 24.2 19.86% 30.6 29.0 25.1 21.78% 32.3 31.0 26.1 22.3

Y. Yao et al. / Energy xxx (2014) 1e62

2. Methods

2.1. Feedstock and inoculum

SC was collected before flowering from an abandoned fieldlocated in the suburbs of Hangzhou, Zhejiang Province, China. TheSC was stored at 4 �C after shredding to a small size (7e12 mm). CSand inoculumwere collected from a biogas plant digesting manurein Linxia, Gansu Province, China. The characteristics of SC, CS, andinoculum are shown in Table 1.

2.2. Anaerobic digestion

The required amounts of SC and CS were loaded into digesters.The mixture ratios of SC:CS were 3:1, 1:1, and 1:3, based on VS(volatile solids). The volume of inoculum seeded into each digesterwas 250 mL/L. For SC (control), and SC:CS ratios of 3:1, 1:1, and 1:3,the substrate concentrations for ADwere 4, 6, and 8% (based on VS),respectively. The C/N ratios for the digesters are shown in Table 2.Digestion of SC alone was used as a control, and the VS content ofthe control was the same as for the three mixture ratios for aspecific substrate concentration. The tests were conducted in batchmode at laboratory scale. The volume of each digester was 2 L andthe working volume was 1 L. The digester headspaces were flushedwith N2 gas for about 5 min to obtain anaerobic conditions, afterwhich they were capped tightly with rubber stoppers and incu-bated at 35 �C without shaking. Digestion experiments were con-ducted in triplicate for each condition.

2.3. Analytical methods

2.3.1. Chemical analysesTS (Total solids), VS, sCOD (soluble chemical oxygen demand),

VFAs (volatile fatty acids), and pH were determined according tostandard methods [16]. An elemental analyzer (varioEL cube; Ele-mentar Analysensysteme GmbH)was used tomeasure total carbon,total nitrogen, and total hydrogen. The cellulose and hemicellulosecontents were determined using the procedure described by VanSoest et al. [17].

2.3.2. Biogas analysesBiogas production was measured every 2 d using the water-

displacement method; the total volume of biogas was calculatedafter the test. A GC (gas chromatograph) (7890A; Agilent Technol-ogies, Wilmington, DE, USA) equipped with a thermal conductivitydetector and a 25 m � 530 mm � 20 mm chromatographic columnwas used to analyze the methane content of the biogas. For GCanalysis, the carrier gas was hydrogen (35 mL/min), the injectorport and detector temperatures were 75 �C and 150 �C, respectively,and the composition of the standard gas (YQD-09; Qingdao HuaQing Co., Shandong, China) was 30.1% N2, 39.9% CH4, and 30.0% CO2.

Table 1Characteristics of different materials.

Parameter Solidago Cattle Inoculum

Canadensis Manure

Total solids (%) 35.3 ± 0.01 25.6 ± 0.00 15.2 ± 0.14Volatile solids (%) 90.5 ± 0.01 72.6 ± 0.04 56.3 ± 0.14Total carbon (%) 55.2 ± 0.20 36.4 ± 0.00 33.8 ± 0.12Total nitrogen (%) 1.4 ± 0.02 1.8 ± 0.13 2.0 ± 0.04pH value 5.6 ± 0.04 8.3 ± 0.02 7.6 ± 0.00

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2.4. Statistical analyses

ANOVA (Analysis of variance) was performed to determinewhether the observed differences between two or more groups ofexperimental results were significant. A p value less than 0.05 wasconsidered to indicate significance.

3. Results and discussion

3.1. Methane production

As shown in Fig. 1, daily methane production was significantlyaffected by the substrate concentration and the mixture ratio. Forthe control, daily methane production with a 4% substrate con-centration was continued until the end of AD. When the substrateconcentration was increased from 4 to 6%, the level of dailymethane production was very low during the first 20 d of AD, andthen increased gradually. This can be attributed to the high

Fig. 1. Effects of SC:CS ratio on daily methane production at different concentrations(digestion time: 30 d). A: 4% concentration; B: 6% concentration; and C: 8%concentration.

o-digestion of Solidago canadensis L. biomass and cattle slurry, Energy

Fig. 3. pH variation during AD for different SC:CS ratios (digestion time: 30 d). A: 4%concentration; B: 6% concentration; and C: 8% concentration.

Y. Yao et al. / Energy xxx (2014) 1e6 3

substrate concentration (6%) and the single substrate (SC), whichcontributed to the high concentration of VFAs (Fig. 4b), and to thelow pH values (Fig. 3b; pH < 6.0), which were not maintainedwithin the neutral range [17]. In an AD reactor, fermentative bac-teria, acetogenic bacteria, andmethanogenic bacteria constitute themicrobial consortium. The grow rate of methanogenic bacteria islower than those of fermentative and acetogenic bacteria, thereforemethanogenic bacteria are sensitive to changes in the environ-mental conditions [18]. After a long period of adaptation by themethanogens, the VFAs were steadily consumed in the methanefermentation process, and the daily methane production increasedaccordingly [19]. When the substrate concentration was furtherincreased to 8%, the daily methane production decreased, becauseof the high substrate concentration and the high level of VFAs(Fig. 3c). When CS was added to the reactor to a SC:CS ratio of 3:1,the levels of daily methane production at 4 and 6% substrate con-centrations were higher than those of the controls. These resultsindicate that CS addition improved the buffering capacity of the AD.As the control, when the substrate concentration was increasedfrom 6 to 8%, daily methane production stopped. When the amountof CS in the mixture was increased to a SC:CS ratio of 1:1, the dailymethane production with a 4% substrate concentration increasedrapidly. The peak value (25.4 L/kg VS) was achieved on the fourthday, and was 135.2 and 22.1% higher than those for the control anda SC:CS ratio of 3:1, respectively. A temporary rapid decrease inproductionwas then observed, followed by a gradual decrease untilAD was complete. This is because of the readily biodegradableorganic matter, which can be used directly for methane production[20]. After most of the available substrate had been used, the dailymethane production declined [21]. The trend in the daily methaneproductionwith a 6% substrate concentrationwas similar to that fora 4% concentration. When the substrate concentration wasincreased to 8%, the methane production process was unstable andlarge fluctuations were observed, which were in accordance withthe pH (Fig. 3c). This may be because of the high ratio of SC:mi-croorganisms (CS and inoculum), which partially inhibitedmethanogen activity [14].When themixture ratiowas 1:3, the levelof daily methane production was higher than those in the otherexperiments for each substrate concentration; the methane pro-duction process was more stable than in the other experimentswith 4 and 6% substrate concentrations.

The above results show that the AD process stability wasenhanced by CS addition. Furthermore, the substrate concentrationshould be in the range 4e6%. When the substrate concentrationwas increased to 8%, increasing the amount of CS did not effectivelyimprove the process stability.

As shown in Fig. 2, the total methane production was signifi-cantly influenced by the substrate concentration. When the sub-strate concentration was increased, the total methane productiongenerally decreased. The reason may be acidification of the

Fig. 2. Effects of SC:CS ratio and concentration on total methane production (digestiontime: 30 d).

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digestion system caused by accumulation of VFAs (Fig. 4) at highsubstrate concentrations. This result is in agreement with those ofLiew et al. (2011) [22]. The addition of CS improved methane pro-duction, and the total methane production increased withincreasing CS proportion in the mixture for the three substrateconcentrations. The maximum methane production for each sub-strate concentrationwas obtained at a SC:CS ratio of 1:3; the valueswere 139.9 ± 3.37 L/kg VS, 143.7 ± 1.99 L/kg VS, and 139.1 ± 12.78 L/kg VS, respectively, for substrate concentrations of 4, 6, and 8%. Thisis because of the better nutrient balance and appropriate micro-organism activity in the feedstock [23]. The maximum methaneproduction was therefore obtained at a SC:CS ratio of 1:3 and a 6%substrate concentration; the methane production was 147.8%higher than that of the control, but only 10.9% higher than thatachieved at a SC:CS ratio of 1:1. ANOVA of the data indicated thatthe differences were not significant (p > 0.05). The increase in theCS proportion led to a decrease in the SC proportion at a certainworking volume. Considering the capacity of the SC treatment, aSC:CS ratio of 1:1 and a 6% substrate concentration are recom-mended for practical applications. For the control and a SC:CS ratioof 3:1, methane productions at a substrate concentration of 8%werefailure due to the rapid hydrolysis and the subsequent build-up ofVFAs (Figs. 3c and 4c) [23,24]. Additionally, the C/N ratio may be amajor factor; the impact of the C/N ratio on AD has been thoroughlyinvestigated, and the optimum C/N ratio is 20e25 [25,26]. If it is

o-digestion of Solidago canadensis L. biomass and cattle slurry, Energy

Fig. 4. VFA variations during AD for different SC:CS ratios (digestion time: 30 d). A: 4%concentration; B: 6% concentration; and C: 8% concentration.

Y. Yao et al. / Energy xxx (2014) 1e64

higher than the appropriate range, accumulation of VFAs mayoccur; if it is lower than the appropriate range, methanogenesismay be inhibited by the high ammonia concentration [18,27]. TheC/N ratios for the control and a SC:CS ratio of 3:1 were 32.2 and31.0, respectively (Table 2).

Table 3(a)TS and VS (g) after 30 d of AD. (b) Degradation (%) of TS and VS after 30 d of AD.

Composition Concentration (%) Control

ATotal solids 4% 38.0 ± 1.53

6% 50.9 ± 1.808% 74.2 ± 0.00

Volatile solids 4% 26.1 ± 0.656% 32.4 ± 4.748% 57.3 ± 6.03

BTotal solids 4% 34.7 ± 2.64

6% 40.1 ± 7.278% 27.5 ± 0.00

Volatile solids 4% 40.8 ± 1.486% 43.5 ± 7.398% 31.9 ± 7.17

Note: The total solids and volatile solids contents of the mixtures before and after anaerowas measured.

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As a result, the methane production was improved by CS addi-tion, but the substrate concentration should not be more than 6%.

3.2. Variations in pH and VFAs

The pH of digester was used to indicate the process stability(Fig. 3). As shown in Fig. 3b and c, the pH increased with increasingCS proportion (the nitrogen source). Specifically, the pH of reactorsduring the stable state was 6.8e7.6; this is in accordance with theresults of previous studies that showed that the pH was 6.6e7.8under the optimum conditions, when the substrate concentrationwas high (4e10%) [28]. As shown in Fig. 3b, the pH values werebelow 6.0 during the initial stage of AD for a SC:CS ratio of 3:1 andthe control, and then increased to 6.8e7.6. These results show thatpH is a key factor, and could also be an indicator of the AD processperformance.

The VFA concentration is a good indicator of the metabolicstatus of digestion [29]. As shown in Fig. 4, the VFA concentrationsincreased to high levels in the initial stage of AD; this is typical forthe start-up stage of AD, because of the imbalances among hy-drolytic, fermentative, acetogenic, and methanogenic functionsduring this period [22,30]. As shown in Fig. 4b and c, the VFAconcentration decreased with increasing CS proportion. These re-sults show that an appropriate SC:CS ratio can benefit stable ADoperation. Moreover, the C/N ratio is a key parameter for AD, and asuitable C/N ratio (20e25) is required for stable biological conver-sions during AD [25,26]. For the three substrate concentrations, theC/N ratios of control, and SC:CS ratios of 3:1, 1:1, and 1:3 are shownin Table 2. As shown in Fig. 4, if the C/N ratio is too high, accumu-lation of VFAs occurs, which leads to inhibition of AD. At a con-centration of 8%, the VFA concentrations for control and SC:CS ratioof 3:1 were high, which led to a highly acidic environment andcessation of methane production (Figs. 3c and 4c). The VFA con-centration for a SC:CS ratio of 1:1 decreased rapidly from day 10 today 12, because of the high ratio of SC-microorganisms, whichinhibited methanogenesis during the initial period of AD [14]. TheVFAs were then steadily consumed after a period of acclimation andbreeding of methanogenic species.

3.3. TS, VS, cellulose, hemicellulose, and sCOD removals

TS and VS are important parameters, and they indicate the de-gree of substrate biodegradation. As shown in Table 3and Fig. 2,higher methane production was generally associated with greaterreductions in TS and VS, and higher degradation efficiencies were

3:1 1:1 1:3

36.3 ± 0.41 39.7 ± 5.86 37.9 ± 2.4549.1 ± 5.16 59.3 ± 0.58 49.3 ± 1.0672.5 ± 3.39 64.1 ± 1.39 59.2 ± 6.0125.8 ± 0.86 25.9 ± 4.21 23.2 ± 2.0233.7 ± 1.13 37.4 ± 2.32 33.8 ± 3.6847.1 ± 1.68 42.0 ± 3.18 37.6 ± 3.82

39.8 ± 0.68 35.1 ± 9.10 42.5 ± 3.7741.4 ± 6.17 37.3 ± 8.69 42.5 ± 3.3532.4 ± 3.16 42.7 ± 1.24 49.2 ± 5.1541.4 ± 1.96 41.2 ± 9.56 47.5 ± 4.5947.4 ± 1.77 41.6 ± 9.32 46.1 ± 5.0844.0 ± 2.00 50.0 ± 3.78 55.3 ± 4.54

bic digestion was determined, then degradation (%) of total solids and volatile solids

o-digestion of Solidago canadensis L. biomass and cattle slurry, Energy

Table 4(a)Cellulose and hemicellulose quantities (g) after 30 d of AD. (b) Degradation (%) of cellulose and hemicellulose after 30 d of AD.

Composition Concentration (%) Control 3:1 1:1 1:3

ACellulose 4% 13.1 ± 0.48 13.5 ± 0.91 14.4 ± 2.36 12.1 ± 0.41

6% 17.9 ± 0.48 19.1 ± 4.21 20.3 ± 1.03 20.9 ± 0.738% 25.9 ± 0.74 23.0 ± 3.09 23.0 ± 0.49 22.6 ± 2.40

Hemicellulose 4% 24.6 ± 1.25 24.3 ± 0.71 27.6 ± 4.15 22.8 ± 1.236% 33.2 ± 0.51 32.6 ± 3.93 33.8 ± 0.40 34.2 ± 0.538% 48.8 ± 0.01 46.8 ± 1.92 42.1 ± 0.25 38.0 ± 4.17

BCellulose 4% 23.9 ± 2.79 26.0 ± 4.98 25.8 ± 2.21 41.1 ± 2.01

6% 21.7 ± 1.93 22.9 ± 7.03 23.6 ± 3.90 24.9 ± 2.558% 9.2 ± 2.66 26.7 ± 5.92 32.0 ± 1.44 37.9 ± 6.60

Hemicellulose 4% 27.1 ± 3.71 31.8 ± 2.01 26.1 ± 1.11 41.7 ± 3.176% 27.6 ± 1.07 33.3 ± 2.05 34.8 ± 0.78 37.8 ± 0.938% 15.9 ± 0.01 24.9 ± 3.09 36.7 ± 0.37 46.1 ± 5.90

Note: The cellulose and hemicellulose contents of the mixtures before and after anaerobic digestion was determined, then degradation (%) of cellulose and hemicellulose wereobtained.

Y. Yao et al. / Energy xxx (2014) 1e6 5

obtained for mixtures of SC and CS compared to those of the con-trols. For the three substrate concentrations (4, 6, and 8%), the TSand VS reductions in the mixtures were in the range 32.4e49.2%and 41.2e55.3%, respectively; they increased by 17.8e78.9% and27.1e73.4%, compared with those of the controls. The TS and VSremoval efficiencies increased as the SC:CS ratio changed from 3:1to 1:3, indicating the positive effects of CS addition. At almost allconcentrations, the highest TS and VS reductions were obtained at aSC:CS ratio of 1:3. These findings indicate that the high TS and VSdegradation efficiencies were in line with better availability of theorganic substrate and an improved ratio of nutrients, which lead tofacilitation of their assimilation by anaerobic flora and an increaseddegree of degradation [31]. In other words, the C/N ratio isimportant for the process stability andmethane production. For the4, 6, and 8% substrate concentrations, the C/N ratios for a SC:CS ratioof 1:3 were 19.8, 21.7, and 22.3, respectively; these are in the op-timum range of 20e25 [26].

As shown in Table 4, cellulose and hemicellulose reductions ofdigesters with mixtures of SC and CS as the substrate were higherthan those of the controls. Similar to TS and VS, the cellulose andhemicellulose reductions increased as the CS proportion in themixture increased, with values 7.9e312.0% and 17.3e189.9% higherthan those of the controls, respectively, being observed. An increaseof the CS proportion in the mixture improved the balance of nu-trients in the reactors, which facilitated assimilation of the organicsubstrate by anaerobic flora, giving high substrate reductions [31].Additionally, the high-efficiency degradation of organic matterdepends on an adequate methanogenic population with highmetabolic activity, and the population of microorganisms in CS isrich [14,19]. In general, methane production was in line with thecellulose and hemicellulose reductions.

The concentrations and removal efficiencies of sCOD are pre-sented in Table 5. In general, the sCOD removal efficiency increased

Table 5sCOD concentrations of substrate before and after 30 d of anaerobic co-digestion, and sC

sCOD concentration of 4% 8225.0 ± 28.28substrate before anaerobic 6% 12007.5 ± 166.17co-digestion (mg/L) 8% 17830.0 ± 238.41sCOD concentration of 4% 6330.0 ± 71.75substrate after anaerobic 6% 10127.5 ± 159.10co-digestion (mg/L) 8% e

sCOD removal efficiency 4% 23.0 ± 0.08(%) 6% 15.7 ± 0.00

8% e

Values for failing digesters were not determined.

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as the CS proportion increased for 4, 6, and 8% substrate concen-trations. When SC was fed into the digester alone, the lowest sCODremoval efficiency was reached at 6% substrate concentration. Forthe three substrate concentrations, the sCOD removal efficiencies ata SC:CS ratio of 1:3 were the highest, and were 77.4e259.9% higherthan those of the controls. These findings clearly demonstrate thatthe sCOD removal efficiency was in line with the TS and VS re-ductions. However, the concentrations of sCOD were high after AD,therefore the digested mixtures should be further treated prior todischarge.

3.4. Technical digestion time

The technical digestion time (T80) can be used to indicate thebiodegradability of the substrate [32,33]. T80 is the time required toreach 80% of total gas production [32]. The AD in this study lastedfor 30 d. For a 6% substrate concentration, the T80 values for control,and SC:CS ratios of 3:1, 1:1 and 1:3 were 28, 18, 14, and 20 d,respectively. T80 for the optimum conditions (SC:CS ratio of 1:1ratio and 6% substrate concentration) was 50.0, 22.2, and 30.0%shorter than those of the control, and the SC:CS ratios of 3:1 and1:3, respectively. These results indicate that the mixture wasdigested in a shorter time under the optimum conditions. Theeconomical benefit was significant for the increase of methaneproduction efficiency or treatment capacity.

4. Conclusions

SC can be used for methane production by digesting with CS.The process stability andmethane productionwere enhanced by CSaddition. The maximum methane production of 143.7 ± 1.99 L/kgVS was obtained at a SC:CS ratio of 1:3 and a 6% substrate con-centration. However, the maximum methane production was only

OD removal efficiencies during 30 d of AD.

5662.5 ± 64.58 7435.0 ± 261.63 8462.5 ± 117.7111282.5 ± 414.27 8865.0 ± 60.19 9850.0 ± 86.2218035.0 ± 461.67 16267.5 ± 337.92 16997.5 ± 61.154645.3 ± 26.52 5292.5 ± 244.95 3703.8 ± 128.387405.0 ± 60.25 5410.0 ± 146.02 5837.5 ± 191.00e 11685.0 ± 293.04 8162.5 ± 96.8817.5 ± 0.08 28.8 ± 0.01 56.5 ± 0.0635.8 ± 0.13 38.8 ± 0.06 40.8 ± 0.01e 28.2 ± 0.01 51.9 ± 0.03

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Y. Yao et al. / Energy xxx (2014) 1e66

10.9% higher than that achieved with a SC:CS ratio of 1:1 at thesame substrate concentration (p > 0.05). Considering the capacityof treating SC at one batch, the conditions SC:CS ratio 1:1 and 6%substrate concentration are optimum. The T80 under these condi-tions was 30.0% shorter than that for maximum methane produc-tion. These findings provide significant information for renewableenergy recovery and SC treatment in practical applications.

Acknowledgments

This study was supported by the National Key Basic ResearchProgram of China (973) (2013CB429904), the International Scienceand Technology Cooperation Projects (2009DFA61060), the StateKey Program of the National Natural Science Foundation of China(31230014), the Transformation Fund Plan for Agricultural Scienceand Technology Achievements of Gansu Province (0910XCNA066)and the Fundamental Research Funds for the Central Universities(lzujbky-2012-106 and lzujbky-2011-35).

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o-digestion of Solidago canadensis L. biomass and cattle slurry, Energy