RETRACTED: Feasibility of biogas production from anaerobic co-digestion of herbal-extraction residues with swine manure

Bioresource Technology 102 (2011) 6458–6463

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Feasibility of biogas production from anaerobic co-digestion of herbal-extraction residues with swine manure Yan Li a,b, Xi-Luan Yan a, Jie-Ping Fan a, Jian-Hang Zhu a,⇑, Wen-Bin Zhou a,⇑ a b

Centre for Low-Carbon Biotechnology, School of Environmental and Chemical Engineering, Nanchang University, Nanchang 330031, China State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang 330047, China

a r t i c l e

i n f o

Article history: Received 16 February 2011 Received in revised form 27 March 2011 Accepted 29 March 2011 Available online 2 April 2011 Keywords: Herbal-extraction residues Swine manure Anaerobic co-digestion Biogas

a b s t r a c t The objective of this work was to examine the feasibility of biogas production from the anaerobic co-digestion of herbal-extraction residues with swine manure. Batch and semi-continuous experiments were carried out under mesophilic anaerobic conditions. Batch experiments revealed that the highest specific biogas yield was 294 mL CH4 g1 volatile solids added, obtained at 50% of herbal-extraction residues and 3.50 g volatile solids g1 mixed liquor suspended solids. Specific methane yield from swine manure alone was 207 mL CH4 g1 volatile solid added d1 at 3.50 g volatile solids g1 mixed liquor suspended solids. Furthermore, specific methane yields were 162, 180 and 220 mL CH4 g1 volatile solids added d1 for the reactors co-digesting mixtures with 10%, 25% and 50% herbal-extraction residues, respectively. These results suggested that biogas production could be enhanced efficiently by the anaerobic co-digestion of herbal-extraction residues with swine manure. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Biomass is an abundant and renewable energy source which could be derived from all organic materials produced by human and natural activities, including industrial wastes, municipal solid wastes, forestry wastes as well as agricultural wastes (Berndes et al., 2003; Wang and Keshwani, 2009). Among them, herbalextraction residues (HER) is a specific kind of agricultural waste due to the major importance of the Chinese herbal medicine industry in China (Wang et al., 2010). With the rapid development of the Chinese herbal medicine industry, about 1.5 million tons of solid wastes, according to statistics, were produced annually after the extraction of medical active components from natural plants (Cheng and Liu, 2010). Because of its abundance in cellulose, hemicelluloses, lignin as well as proteins, HER could be employed as a renewable energy source. Its environmentally friendly exploitation becomes an important topic due to the increase in environmental pollution and the shortage of energy resource. In addition, there is little information in the literature about biogas digested from HER or co-digested from the mixture of HER with other biomasses. Anaerobic co-digestion has been defined as the treatment of a mixture of at least two different biomasses with the aim of improving process efficiencies (Álvarez et al., 2010). Nowadays, there is an increasing interest in using anaerobic co-digestion process treating ⇑ Corresponding authors. E-mail addresses: [email protected] (J.-H. Zhu), [email protected] (W.-B. Zhou). 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.03.093

industrial and agricultural organic wastes and swine manure for biogas production (Kaparaju and Rintala, 2005; Riaño et al., 2011; Wu et al., 2010). The advantages of anaerobic co-digestion process often lie in balancing the carbon/nitrogen (C/N) ratio in the mixture, macro and micronutrients, pH, inhibitors/toxic compounds as well as dry matter (Hartmann et al., 2003). Thus, the manure specific methane production potential could be improved by co-digestion with other agro-residues (Chae et al., 2008; Lansing et al., 2010; Riaño et al., 2011). Moreover, the anaerobic digestion of agro-residues is often associated with poor buffering capacity during anaerobic treatment and its inherent high C/N ratio. Anaerobic co-digestion of them with swine manure could overcome these problems by maintaining a stable pH within the methanogenesis range due to the inherent high buffering capacity of swine manure (Banks and Humphreys, 1998). Additionally, swine manure could present high ammonia content and a wide variety of nutrients needed by the methanogens during the anaerobic process (Cheng, 2009). From another point of view, the codigestion of agro-residues with swine manures would also aid in overcoming ammonia inhibition related to the digestion of pure swine manure. However, it is unclear whether some agro-residues, e.g., HER, might have adverse effects when added into a stable digester for swine manure. The objective of the present work was to investigate the feasibility of developing anaerobic co-digestion of HER with swine manure for efficient biogas production. Batch experiments were carried out based on a Central Composite Design. The influence of the percentage of HER in the substrate and the substrate/inocula

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terms of grams of volatile solid (VS) (g VS)/g of MLSS (g MLSS) ratio, and the percentage of HER in the substrate (% HER), measured in terms of VS of HER in relation to the organic carbon of the feed. The selected range for factor one (SC) was 1.1–5.9 g VS g1 MLSS. The selected range for factor two (% HER) was 0–100%. Factorial design levels were codified from 1 to +1. The central point was replicated five times in order to estimate experimental error. Axial points ensure design rotatability and their distance to the central point (a) was calculated according to Eq. (1).

ratio were evaluated in terms of methane yield. Finally, the effect of the feed component ratio of HER to swine manure on process performance was investigated in a semi-continuously fed stirred tank reactor (CSTR) under mesophilic anaerobic conditions. 2. Methods 2.1. Origin of HER, swine manure and inocula The HER investigated in this study was obtained from Jiangzhong herbal medicine group (Jiangxi, China). Swine manure was obtained from a pig farm (Jiangxi Evergreen Biotechnology Co., Ltd.) located in Fengcheng (Jiangxi, China). The anaerobic sludge used as inocula was collected from the anaerobic biogas digester in Jiangxi Evergreen Biotechnology Co., Ltd., the mixed liquid suspended solids (MLSS) and pH value of which were 21 ± 1.3 g L1 and 7.19 ± 0.6, respectively. The substrates and inocula were individually homogenized and subsequently stored at 4 °C for further use. The characterization of each waste employed was shown in Table 1. All samples were collected in triplicate, and the averaged data of the measurements were presented.

a ¼ 2k=4

ð1Þ

The selected response for analysis was the methane yield (ml CH4 ml g1ml VS added), calculated from the methane content in biogas and the volume of biogas produced from per unit of VS added. The variables, Xi, were coded as xi according to Eq. (2), such that X0 corresponded to the central value:

xi ¼ ðX i  X i Þ=DX i ; where i ¼ 1; 2; 3; . . . ; k;

ð2Þ

where xi is the dimensionless coded value of an independent variable, Xi is the actual value of an independent variable for the ith test, X i is the actual value of an independent variable at the centre point and DXi is the step change (Chong et al., 2009; Riaño et al., 2011). All the evaluated levels were combined in 13 different treatments (T1–T13). Codified and real values for both factors are presented in Table 2. For predicting the optimal point, a second order polynomial function (linear regression) was performed Eq. (3):

2.2. Batch experiments Batch experiments were carried out at 35 ± 2 °C for 30 days based on a Central Composite Design, which is a second order factorial design used when the number of runs for a full factorial design is too large to be practical (Box and Wilson, 1951). This type of factorial design usually consists of a 2k factorial nucleus, five replicates of the central point and 2k axial points, where k is the number of factors evaluated. More specifically, in the present study the two factors selected were the solid concentration (SC), measured in

Y ¼ b0 þ b1 X 1 þ b2 X 2 þ b11 X 21 þ b22 X 22 þ b12 X 1 X 2 ;

ð3Þ

where Y represents the predicted response, b0, b1, b2, b11, b22, and b12, are the regression coefficients. X1 and X2 are the evaluated factors (SC and %HER). The coefficient of determination (R2) was calcu-

Table 1 Composition of the substrates in batch experiments and semi-continuous digestion: herbal-extraction residue (HER) and swine manure. Parameters

Batch experiments

TOC (g kg1) VS (%) TKN (g kg1) C/N ratio* pH value Moisture content (%) *

Semi-continuous digestion

HER

SM

HER

SM

164.8 ± 5.2 32.2 ± 1.6 4.50 ± 0.17 36.6 5.69 ± 0.14 58.30 ± 1.20

81.29 ± 3.3 23.5 ± 1.2 5.64 ± 0.22 14.4 7.78 ± 0.21 70.81 ± 1.90

156.8 ± 7.2 31.9 ± 1.4 4.65 ± 0.23 33.7 5.90 ± 0.25 59.20 ± 2.10

71.24 ± 4.2 20.7 + 1.7 4.86 ± 0.19 14.7 7.59 ± 0.19 72.90 ± 1.70

C/N ratio was defined as the ratio of TOC to TKN.

Table 2 Codified, real values and responses for swine manure co-digestion with HER in batch experiments. Treatmentsa

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 a b

Codified values

Real values

SC (g VS g1 MLSS)

%HER

SC (g VS g1 MLSS)

%HER

0 0 1 1 0 1.4142 1 0 0 1 0 1.4142 0

1.4142 0 1 1 1.4142 0 1 0 0 1 0 0 0

3.50 3.50 5.20 5.20 3.50 5.90 1.80 3.50 3.50 1.80 3.50 1.10 3.50

0.00 50.00 14.64 85.36 100.00 50.00 85.36 50.00 50.00 14.46 50.00 50.00 50.00

Data are means of three replicates, except T2, T8, T9, T11 and T13 were performed once, respectively. The unit is ml CH4 g1 VS added.

Real responseb

Predicted responseb

207 ± 9 294 216 ± 5 254 ± 11 243 ± 10 279 ± 9 275 ± 6 301 297 235 ± 12 283 282 ± 10 298

198 294 229 260 244 283 270 294 294 237 294 270 294

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lated to achieve the proportion of data variability that is explained by the model, thus the quality of fit to the model. The p-values of the parameter estimation were used to validate the model and pvalues less than 0.05 indicate the significant model terms. Multiple regression analysis for the data sets collected was performed using Design-Expert 7.1.6 Trial software. The anaerobic assays were conducted in 29 bottles (500 mL) with an inocula volume of 150 mL and a total liquid volume of 375 mL, respectively. All treatments were performed in three replicates, except Treatment 2 (T2), T8, T9, T11 and T13 was performed in one bottle, respectively. Two blanks containing 150 mL of inocula and 225 mL of distilled water were also run to determine the endogenous biogas production of the anaerobic sludge. The bottles were closed with a septum and the headspace flushed with pure N2 to remove the oxygen. The volume of biogas produced was measured using water displacement method. 2.3. Semi-continuous digestion of different mixtures Semi-continuous co-digestion of HER with swine manure was carried out in two identical continuously stirred tank reactors (CSTRs) with a total volume of 10 L and a working volume of 7 L, namely R1 and R2. A water bath was used to maintain the temperature of the digesters at 35 ± 2 °C. Digesters were mounted separately on a mechanical stirrer, stirring continuously at 100 rpm. The outlets provided on the top of each digester were used for feeding influent, withdrawing effluent and for collecting biogas. Biogas was daily measured by displacement of water. Feeding VS was maintained constant during the whole experiment resulting in a VS loading rate of about 2.90 g VS L1 d1 under hydraulic retention time (HRT) of 30 days. Varied feeding VS ratios of HER and swine manure were evaluated. After inoculating the digester with 7 L of digested anaerobic sludge, R1 was used to co-digest HER with swine manure in a feeding VS ratio of 75% swine manure and 25% HER, whereas R2 was performed with swine manure alone. After 30 days, R1 was fed with swine manure and HER, in a feeding VS ratio of 50% swine manure and 50% HER, whereas the feed in R2 was made up 90% swine manure and 10% HER. The digesters were fed once a day every weekday. Prior to each feeding, a volume equal to the feeding volume was removed to maintain a constant digester volume. The characteristics of substrates are shown in Table 3. The semi-continuous digestion experiments on R1 and R2 were performed two batches, respectively. And the composition of influent and effluent were determined twice a week except the pH which was monitored daily. The results from the analysis of each mixture at steady state were used for evaluating the effect of codigestion on biogas production efficiency. 2.4. Analyses Volatile solid (VS), total organic carbon (TOC), total Kjeldahl nitrogen (TKN), and pH (Sartorius basic pH meter PB-10, Germany) were performed in accordance with APHA Standard Methods

(1992). Samples from the beginning and the end of the experiment were analyzed. All measurements were conducted in triplicate and the averaged data were presented. The volume of biogas produced was measured using water displacement method. The fraction of CH4 was periodically analyzed by a gas chromatograph (SP6890) provided by Lunan Ruihong Company (Shandong, China) equipped with a thermal conductivity detector (Ruihong, Shandong, China) and a 2 m stainless steel column (Ruihong, Shandong, China) packed with 5 Å molecular sieves (its pore size is about 5 Å). High purity nitrogen gas (99.99%) was used as the carrier gas with a flow rate of 30 mL min1. The temperatures of the injection port, oven and detector were 120, 50 and 150 °C, respectively. Gas analyses were carried out two to three times a week during the first 2 weeks of the experiments. A 250 ll pressure-locked gas tight syringe (SP-6800A) (Ruihong, Shandong, China) was used for the gas sampling. All measurements were conducted in triplicate and the averaged data were presented. 3. Results and discussion 3.1. Chemical characteristics of swine manure and HER There were significant differences in the composition of the two biomass wastes (Table 1). HER had higher C/N ratios in the range of 33.7–36.6, as compared to swine manure, which presented C/N ratios in the range of 14.4–14.7. HER, as shown for pH values, was significantly acidic, whereas swine manure had a pH value of around 7.6. The characterization of the two wastes indicated that co-digestion of HER with swine manure could be a good solution to balance C/N ratio, overcoming the problems of digesting both substrates separately. 3.2. Batch experiments The experimental design data, real responses and predicted responses were presented in Table 2. Regression analyses were shown in Table 4 and resulted in the following second order polynomial Eq. (4):

Y CH4 ¼ 294:20  4:47  ðSCÞ þ 16:11  ð%HERÞ  8:79  ðSCÞ2  36:54  ð%HERÞ2  0:50  ðSCÞ  ð%HERÞ

ð4Þ

The regression showed that the model was significant because the Model F-value of 23.30 was greater than the calculated one (0.0003). The determined R2 coefficient obtained was 0.9433, meaning that the model explained 97.15% of the variability data. Although the ‘‘Pred R-squared’’ of 0.7061 is in reasonable agreement with the ‘‘Adj R-squared’’ of 0.9028, the ‘‘Lack of Fit F-value’’ of 2.51 implies the Lack of Fit is not significant relative to the pure error. There is a 19.79% chance that a ‘‘Lack of Fit F-value’’ this large could occur due to noise (Table 5). Moreover, the 0.7061 correlation coefficient indicated that the combination of both factors of SC and% HER had high importance in the yield of methane. p-values for the entire model terms were

Table 3 Characteristics of mixed feedstocks at varied different mixture ratio used in CSTR experiment. All parameters were measured in triplicate. Parameters

Total organic carbon (g L1) VS (%) Total nitrogen (g L1) C/N ratio pH value

HER (%) 0

10

25

50

30.20 ± 2.10 8.9 ± 0.4 2.10 ± 0.14 14.38 7.48 ± 0.18

31.91 ± 2.22 8.7 ± 0.5 1.93 ± 0.09 16.54 7.59 ± 0.21

33.89 ± 1.81 8.8 ± 0.4 2.84 ± 0.13 18.33 7.42 ± 0.13

37.33 ± 2.15 8.6 ± 0.6 1.56 ± 0.11 23.9 7.30 ± 0.24

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Y. Li et al. / Bioresource Technology 102 (2011) 6458–6463 Table 4 Regression of a full quadratic model for methane production. Factor

Coefficient estimate

Degree of freedom

Standard error

95% CI low

95% CI high

VIF

Intercept A–A B–B AB A2 B2

294.20 4.47 16.11 0.50 8.79 36.54

1 1 1 1 1 1

4.47 3.53 3.53 5.00 3.79 3.79

283.63 12.83 7.76 12.32 17.75 45.50

304.77 3.89 24.47 11.32 0.17 27.58

1.00 1.00 1.00 1.02 1.02

Standard deviation = 10.00; R-squared = 0.9433; Adj R-squared = 0.9028; C.V.% = 3.75; Pred R-squared = 0.7061; Adeq precision = 14.118.

Table 5 Analysis of variance for the regression model of methane production. Source

Sum of squares

Degrees of freedom

Mean value

F prob > F

p-Value

Significance

Model A–A B–B AB A2 B2 Residual Lack of Fit Pure error Cor total

11639.45 159.82 2077.28 1.00 537.18 9286.88 699.32 456.52 242.80 12338.77

5 1 1 1 1 1 7 3 4

2327.89 159.82 2077.28 1.00 537.18 9286.88 99.90 152.17 60.7

23.30 1.60 20.79 0.01 5.38 92.96

0.0003 0.2464 0.0026 0.9231 0.0535 <0.0001

Significant

2.51

0.1979

lower than 0.0003, except for the quadratic term associated with SC (Table 5), showing %HER and (%HER)2 are more significant model terms, as compared to SC and SC2. As an overall, the second order polynomial model fitted the experimental results corresponding to methane yield quite well. Here, the methane yield could be calculated from the produced biogas volume and methane content in the biogas. The averaged methane yield of all treatments (29 bottles) was 261.2 mL CH4 g1 VS added. Methane content was above 58% (data not shown) for all treatments. The highest specific methane yield was 294 mL CH4 g1 VS added (the mean of five treatments), obtained at the value of 3.50 g VS g1 MLSS and 50% for factors SC and% HER, respectively. The specific methane yield for HER and swine manure alone were 243 (T5) and 207 (T1) mL CH4 g1 VS added, respectively, both of which were lower than those obtained from the mixture when HER content was 50%, showing the advantages of anaerobic co-digestion of mixed biomass wastes for biogas production. Fig. 1 illustrated the accumulated methane production throughout the co-digestion time for T1, T3, T4, T5, T6 and Tm (the

Not significant

accumulated methane production of Tm was the means of T2, T8, T9, T11 and T13), all of which SC were equal to or above 3.5 g VS g1 MLSS. The maximal methane production of 6.19 L could be obtained on day 12 for T1 with the 100% of swine manure as the digestion substrate, while the maximal methane production of 7.25 L was delayed to be observed on day 22 for T5 with 100% of HER as the digestion substrate. The delayed maximal methane production might be resulted from the higher C/N ratio of the substrate as well as the lower biodegradability of HER (Cheng and Liu, 2010), as compared to swine manure. As also shown in Fig. 1, two-stage methane production could be observed when varied percentages of HER were co-digested with swine manure. At the end of the tests, the stabilization of the methane production was not really achieved, so methane production might have been underestimated. Meanwhile, all pH values were maintained in the range of about 7.0, due to the buffer capacity of swine manure, as previously reported (González-Fernández et al., 2008). With regard to treatments with a constant value for SC of 3.50 g VS g1 MLSS (T1, T5 and Tm), Tm with 50% HER presented the average maximum methane yield of 8.25 L on day 30. As compared to the anaerobic digestion of HER or swine manure alone, codigestion of HER with swine manure improved methane production. As described above, the delayed methane production in Tm might be due to the low biodegradability, partly resulted from the high C/N ratio of HER, as compared to swine manure, leading to a two-stage production of methane in Tm. On the other hand, as shown in Table 2, with regard to treatments with a constant value for %HER of 50%, a similar methane production level, in the range of 279–294 ml CH4 g1 VS added, was observed from the three levels of factor SC, indicated that factor SC seemed not to influence methane production obviously. 3.3. Semi-continuous single-stage digestion of different mixtures

Fig. 1. Accumulated methane production for HER and swine manure by anaerobic co-digestion in batch tests. All data were measured in triplicate.

Table 6 showed the performance data of CSTR treating the mixture of HER with swine manure, indicated that the highest biogas production could be achieved when 50% HER was added to the mixed feedstock. As also can be seen from Table 6, the biogas production rate and the specific methane yield of SM alone were 762 mL CH4 d1 and 151 mL CH4 g1 VS added d1, respectively,

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Table 6 Performance data of CSTR treating the mixture of HER with swine manure. HER%

Biogas production rate (mL d1) Methane content (%) Specific methane yield (mL g1 VS added d1) TOC reduction (%)

0

10

25

50

762 ± 14 57.3 ± 0.9 151 ± 7 73.2 ± 2.2

783 ± 12 60.2 ± 1.1 162 ± 6 62.7 ± 1.9

853 ± 16 61.3 ± 1.2 180 ± 8 66.4 ± 2.8

1002 ± 17 63.8 ± 1.4 220 ± 6 65.7 ± 2.1

and these values increased up to 1002 mL CH4 d1 and 220 mL CH4 g1 VS added d1 with 50% HER addition. The methane content increased from 57.3% with SM alone to 63.8% with a 50% HER addition. When the %HER was within the range of 0–50%, the reactor fed with higher concentrations of HER showed higher biogas production rates, higher compositions in methane as well as high specific methane yield. The specific methane yield increased 7.3%, 19.2% and 45.7% as compared to that obtained from the digestion of swine manure alone when 10%, 25% and 50% of HER was added, respectively. These results were in accordance with those found in literature (Amon et al., 2006; Liu et al., 2009; Cavinato et al., 2010; Gelegenis et al., 2007) that indicated that anaerobic co-digestion could increase CH4 production of manure digesters, depending on the operating conditions and the co-substrates used. The higher methane yields achieved in co-digestion of HER with SM, as compared with those achieved with SM alone at the same loading rate in the present study, were apparently due to the higher methane potential of HER, as demonstrated by batch experiments. This high methane potential achieved by the co-digestion of HER was probably due to the high anaerobic biodegradability of the cellulose and sugars, the main components of HER. On the other hand, the main components in typical pig manures are carbohydrates, hemi-cellulose and cellulose followed by proteins, fats and lipids and a small amount of lignin (4.4%) and starch (1.6%) (Iannotti et al., 1979). The biodegradability of pig manure has been reported to be dependent upon the lignin content, which is not only considered as refractory to anaerobic degradation, but also reduces the availability of other components, especially cellulose (Gelegenis et al., 2007; González-Fernández et al., 2008; Liu et al., 2009). Finally, NHþ 4 -N concentrations in the present study were far to reach reported toxics levels of >4 g L1 which would cause ammonia inhibition (De Baere et al., 1984; Cheng, 2009). The high content of ammonia in swine manure makes it possible to degrade HER biologically without the addition of external alkalinity and without addition of external nitrogen source (Table 1). On the other hand, as reported by previous study, significant increases in volumetric biogas production can be achieved by adding carbon rich agricultural residues to the co-digestion process with swine manure (Wu et al., 2010). These authors found that the C:N ratio of 20:1 was the best in terms of biogas productivity in the anaerobic co-digestion of swine manure with three crop residues as an external carbon source. HER addition to swine manure widened TOC:TKN ratio from 14.38 to 23.9 in 0% HER and 50% HER, respectively (Table 6). As mentioned, in this work, the swine buffer capacity contributed to the stability of the process. Co-digestion in the present study should be considered as a process for the simultaneous treatment of two different biomass wastes and as a solution for the problems of ammonia inhibition generally encountered during anaerobic digestion of pig manure. However, the process stability and efficiency have not been investigated. Our following works should be focused on the process

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