Optimization of hydrogen production in a granule-based UASB reactor

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Optimization of hydrogen production in a granule-based UASB reactor Bai-Hang Zhaoa, Zheng-Bo Yuea, Quan-Bao Zhaoa, Yang Mua, Han-Qing Yua,, Hideki Haradab, Yu-You Lib a

Department of Chemistry, University of Science & Technology of China, Hefei 230026, China Department of Civil Engineering, Tohoku University, Sendai 980-8579, Japan

b

ar t ic l e i n f o

abs tra ct

Article history:

Hydrogen production from sucrose in a granule-based upflow anaerobic sludge blanket

Received 24 November 2007

(UASB) reactor was optimized through employing response surface methodology (RSM)

Received in revised form

with a central composite design in this study. The individual and interactive effects of

6 March 2008

influent sucrose concentration (Sin) and hydraulic retention time (HRT) on anaerobic

Accepted 7 March 2008

hydrogen production were elucidated. Experimental results show that a maximum hydrogen yield of 1.62 mol-H2/mol-hexose was obtained under the optimum conditions

Keywords: Granule Hydrogen Optimization Response surface methodology (RSM) Upflow anaerobic sludge blanket

of Sin 14.5 g/L and an HRT 16.4 h. The hydrogen yield was individually dependent on Sin and HRT, while their interactive effect on the hydrogen yield was not significant. Throughout the experiments the hydrogen content fluctuated between 25.9% and 50.0%, but free of methane. Ethanol, acetate and butyrate were the main aqueous products and their yields all correlated well with Sin and HRT, indicating a mixed-type fermentation in this UASB reactor. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

(UASB) reactor

1.

Introduction

Hydrogen has attracted increasing worldwide attention as a clean energy source due to the following advantages: clean, efficient with a high energy yield (122 kJ/g), renewable, and no generation of toxic byproducts [1]. Physical, chemical and biological processes have been used for hydrogen production. Among them, hydrogen production through anaerobic fermentation is particularly attractive because hydrogen can be produced from renewable organic matters [2]. Most of experiments on hydrogen production from various types of wastewater have been carried out with an anaerobic continuous-flow stirred tank reactor (CSTR) at laboratory [3–5]. However, the CSTR is unable to retain a high level of biomass for hydrogen production due to its intrinsic structure. This

reserved.

might lead to a decrease in the hydrogen production capability and system stability [6]. For example, the suspended-growth hydrogen-producing biomass in a CSTR is sensitive to the fluctuation of operating parameters such as retention time, pH and temperature [7]. A significant variation of environmental parameters often leads to a failure of a hydrogen-producing CSTR [1]. To overcome this problem, an upflow anaerobic sludge blanket (UASB) reactor with separate reaction and settlement regions is desirable. The main feature of the UASB reactor is the retention of a large amount of anaerobic granules [8]. Compared to sludge flocs, sludge granules have good settling ability and can resist to the fluctuation of operating parameters [9]. Anaerobic hydrogen production is a complex process and is effected by many factors [10], e.g., hydraulic retention time

Corresponding author. Fax: +86 551 3601592

E-mail address: [email protected] (H.-Q. Yu). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.03.008

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(HRT) [5], influent substrate concentration [3], reactor configuration [4], wastewater specificity [11], pH [12], temperature [13], nutritional requirements [14], and presence of fermented products [15]. Among them, influent substrate concentration and HRT are found to be crucial parameters, in addition to pH [16]. Studies have been conducted to evaluate the individual effects of influent substrate concentration and HRT on hydrogen production, but in an independent variable form [5,9]. The interactive effects of the two factors on hydrogen production in a granule-based UASB reactor have not been reported previously. Response surface methodology (RSM) is a collection of mathematical and statistical techniques useful for analyzing the effects of several independent variables on the response [12]. It can be used to evaluate individual and interactive effects of independent factors in numerous chemical and biochemical processes and determine optimum conditions for desirable responses [17–19]. Therefore, in the present study hydrogen production from a synthetic sucrose-laden wastewater in a granule-based UASB was optimized through employing RSM with a central composite design. In particular the individual and interactive effects of influent sucrose concentration (Sin) and HRT on hydrogen production were elucidated. Furthermore, the formation of aqueous products was also explored.

2.

Materials and methods

2.1.

Synthetic wastewater, reactor and inoculum

A synthetic sucrose-laden wastewater was used as the substrate and its composition was as follows (unit in g/L): sucrose 10; K2HPO4 0.32; NH4HCO3 0.82; NaHCO3 0.90; CaCl2 0.05; MgCl2  6H2O 0.1; NaCl 0.01; FeCl2 0.025; CoCl2  6H2O 0.005; MnCl2  4H2O 0.005; AlCl3 0.0025; (NH4)6Mo7O24  4H2O 0.005; H3BO3 0.005; NiCl2  6H2O 0.0005; CuSO4  5H2O 0.005; ZnCl2 0.005. The influent pH value of the wastewater was adjusted to 7.0 by adding 3 M NaOH or 3 M HCl. The experiment was conducted with a UASB reactor. The reactor had a reaction portion of 4.0 L with an internal diameter of 10 cm and a gas–solids separator portion of 2.0 L. The seed sludge for the UASB reactor was taken from a full-scale UASB reactor treating citrate-producing wastewater. After operation of 8 months the mature and stable hydrogenproducing granules were formed. The granules had smooth surface and a compact structure (Fig. 1).

2.2.

Operation of UASB reactor

The reactor was operated at 3071 1C with heating bands. After the reactor start-up, experiments were conducted to investigate the effects of Sin and HRT on hydrogen production in the UASB reactor according to the experiment designs. Each run lasted over one month to ensure the reactor to reach pseudo-steady state before changing to the next conditions. Throughout the experiments, the pH of the mixed liquor in the reactor was kept around 4.0 by adjusting the influent alkalinity through dosing NaHCO3. Effluent and biogas

Fig. 1 – The morphology of the hydrogen-producing granules: (a) digital image and (b) optical microscopic image of a cross-section through the granule. compositions were monitored. Only results obtained at pseudo-steady state were reported.

2.3.

Experimental design and date analysis

Based on RSM with a central composite design [17,19], the range and level of independent input variables, Sin and HRT, are shown in Table 1. The yields of hydrogen, ethanol and volatile fatty acids (VFAs) were, respectively, selected as the dependent output variables. Regression analysis was performed with quadratic equations: Y ¼ b0 þ b1 X1 þ b2 X2 þ b11 X21 þ b22 X22 þ b12 X1 X2

(1)

where X1 and X2 are real values of Sin and HRT, respectively, Y is the predicated response value, bi , bii and bij are coefficients estimated by the model and indicate the linear, quadratic and cross-product effects of X1 and X2 on the response. Probability (p) values were used to check the significance of each of the coefficients, which, in turn, were necessary to understand the pattern of the mutual interactions between the test variables. A smaller magnitude of the p value means a more significant correlation coefficient. Parameters of the equations and corresponding analysis of variances were

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evaluated using MATLAB 6.5 (MathWorks Inc., USA) and Minitab 14 (Minitab Inc., USA). Additional two batches of experiment were conducted to verify the statistical models.

2.4.

Analytical methods

The amount of biogas produced was recorded daily using a gas meter and the biogas contents were analyzed using a gas chromatograph (Lunan, Model SP-6800A) equipped with a thermal conductivity detector and a 1.5 m stainless steel ˚ molecular sieve. The temperatures of column packed with 5 A injector, detector and column were kept at 100, 105 and 60 1C, respectively. Argon was used as carrier gas at a flow rate of 30 mL/min. The concentrations of ethanol and VFAs, including acetate, propionate, i-butyrate, butyrate, and i-valerate, in the effluent were determined daily using the second gas chromatograph (Agilent, Model 6890NT) equipped with a flame ionization detector and a 30 m  0.25 mm  0.25 mm fused-silica capillary column (DB-FFAP). The liquor samples were first centrifuged at 12,000 rpm for 10 min, and were then acidified by formic acid and filtrated through 0.2 mm membrane and finally measured for free acids. The temperatures of the injector and detector were 250 and 300 1C, respectively. The initial temperature of oven was 70 1C for 3 min followed with a ramp of 20 1C /min for 5.5 min and to final temperature of 180 1C for 3 min. Nitrogen was used as carrier gas with a flow rate of

2.6 mL/min. Sucrose concentration was measured using enthrone–sulfuric acid method [20]. An Olympus SZ-PT microscope equipped with a digital camera (Olympus U-PMTVS) was used for the digital imaging of the hydrogen-producing granules.

3.

Results and discussion

3.1.

Substrate removal and hydrogen production

Substrate removal efficiencies and hydrogen production are summarized in Table 2. Sucrose removal efficiencies kept nearly 99% except for Trials 6 and 7, suggesting a high capacity of this granule-based UASB reactor to remove organic substrate. The low sucrose removal efficiency of Trial 6 was attributed to the over substrate loading, while the low sucrose removal efficiency of Trial 7 might be associated with the sludge wash-out. A statistical analysis indicates that the interactive effect of Sin and HRT on sucrose removal efficiency was not significant (p-value ¼ 0.96740.05). During the entire operation period, hydrogen content fluctuated between 25.9 and 50.0%. No methane was observed. The biogas production rate, hydrogen yield and hydrogen production rate were in a range of 0.22–6.51 L/L/h, 0.27–1.65 mol-H2/mol-hexose, and 0.004–0.122 L/L/h, respectively.

3.2. Table 1 – Central composite experimental design matrix Trial

1 2 3 4 5 6 7 8 9–13

Code values

Real values

x1

x2

X1 (g/L)

X2 (h)

1 1 1 1 1.414 1.414 0 0 0

1 1 1 1 0 0 1.414 1.414 0

5.0 15.0 5.0 15.0 2.9 17.1 10.0 10.0 10.0

12.0 12.0 20.0 20.0 16.0 16.0 10.3 21.7 16.0

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Effect of Sin and HRT on hydrogen yield

To evaluate the effect of Sin and HRT on hydrogen yield, the design matrix of experimental conditions with the corresponding hydrogen yield ðY H2 Þ in Table 2 were subjected to regression analysis, generating the following quadratic equation: Y H2 ¼  331:9 þ 20:58X1 þ 36:83X2  0:62X21  1:05X22  0:16X1 X2

(2)

The high value of regression coefficient (0.865) at a 95% confidence suggests that the regression model was an accurate representation of the experimental data. Furthermore, the optimum conditions for maximizing Y H2 , calculated by setting the partial derivatives of Eq. (2) to zero with respect to the corresponding variables, were Sin of 14.5 g/L and HRT of

Table 2 – Reactor performance in 13 trials Trial

Removal (%)

Biogas (L/L/h)

H2 (%)

H2 yield (mol-H2/mol-hexose)

H2 producing rate (L/L/h)

1 2 3 4 5 6 7 8 9–13

98.870.9 98.770.6 99.170.1 99.870.1 99.670.0 67.073.2 85.776.8 99.570.1 99.770.1

0.05370.005 0.29870.019 0.04470.001 1.97870.030 0.00970.001 0.16870.007 0.27170.046 0.08570.008 0.15770.014

36.371.0 41.070.8 50.072.1 48.071.2 44.370.8 47.971.1 25.972.5 31.172.0 43.471.1

0.6470.02 1.4170.02 1.0470.04 1.6570.04 0.2770.01 1.4570.03 0.8070.02 0.8770.06 1.4570.02

0.01970.5  104 0.12271.4  104 0.01770.2  104 0.09470.3  104 0.00470.1  104 0.05870.7  104 0.07070.9  104 0.02671.7  104 0.0670.7  104

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16.4 h. The corresponding maximum response value for hydrogen yield was estimated as 1.62 mol-H2/mol-hexose. The three-dimensional response surface and two-dimensional contour line, drawn based on the data in Table 2, are shown in Fig. 2. The hydrogen yield increased with the increasing Sin or HRT to its peak, but then decreased with a further increase in Sin or HRT. Moreover, the response surface of hydrogen yield showed a comparatively clear peak and the

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optimum conditions fell well in the design region. This indicates that both Sin and HRT had an individual significant influence on the hydrogen yield. As shown in Fig. 2b, the twodimensional contour line of the hydrogen yield with respect to Sin and HRT had a relatively rounded ridge, and the bij of 0.16 in Eq. (2) is small, implying that Sin and HRT were slightly interdependent, i.e., their interactive effect on hydrogen yield was not significant (p-value ¼ 0.68540.05).

3.3.

Effect of Sin and HRT on aqueous product formation

The yields of the aqueous products at various trials are summarized in Table 3. Butyrate, acetate and ethanol were the main aqueous products. Propionate was unstable and was easily influenced by environmental factors. An interesting phenomenon was observed, as isovalerate was produced only at Sin of 10.00 g/L and an HRT of 21.7 h. However, the reason is not clear yet. In a similar way, RSM with the central composite design was performed for evaluating the effect of Sin and HRT on the yield of ethanol and VFAs. Quadratic models at a 95% confidence were presented as following: Y et ¼ 154:0  1:73X1  15:65X2 þ 0:12X21 þ 0:60X22  0:11X1 X2

(3)

Y ac ¼ 35:47  8:17X1 þ 11:45X2  0:22X21  0:44X22 þ 0:48X1 X2

(4)

Y bu ¼ 655:9  23:9X1  61:87X2 þ 0:35X21 þ 1:66X22 þ 1:19X1 X2

(5)

Y TVFA ¼ 933:0  35:1X1  79:93X2 þ 0:07X21 þ 2:27X22 þ 1:78X1 X2

Fig. 2 – Effect of Sin and HRT on hydrogen yield: (a) threedimensional graph and (b) two-dimensional contour plot.

(6)

Y et , Y ac , Y bu and YTVFA represent the yield of ethanol, acetate, butyrate and total VFAs (TVFA), respectively. The high regression coefficients (0.927, 0.830, 0.942) suggest that the regression equations of ethanol, acetate, and TVFA yield were appropriate for modeling the experimental results. However, butyrate yield was not predicted well. The three-dimensional response surface and two-dimensional contour line for the yields of ethanol and VFAs obtained are, respectively, shown in Figs. 3 and 4. Both Sin

Table 3 – Yields (in unit of mg/g-hexose) of VFAs and ethanol in thirteen trials Trials

Ethanol

Acetate

Propionate

Butyrate

Isovalerate

1 2 3 4 5 6 7 8 9–13

4175.8 3472.0 61715.4 45710.8 5179.2 3475.3 3875.8 74711.3 3574.4

9873.3 2074.6 8871.6 4874.0 10175.6 5072.2 5273.8 9279.8 81712.2

3873.9 270.4 2971.3 270.2 3672.2 871.2 470.1 68710.5 1472.7

10075.9 105712.2 11079.9 211710.2 8479.7 6674.9 116715.0 10679.3 7773.0

/ / / / / / / 4575.1 /

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and HRT had a significant individual influence on their yields. The minimum ethanol yield was estimated from Eq. (4) to be 30.3 mg/g-hexose as Sin and HRT were 13.5 g/L and 13.8 h, respectively. The acetate yield decreased substantially with

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the increasing Sin, but it increased as the HRT increased. Figs. 3c and 4c illustrate that the butyrate yield decreased to a minimum level as both Sin and HRT increased, but then increased with a further increase in Sin and HRT. The

Fig. 3 – Effect of Sin and HRT on the yields of: (a) ethanol; (b) acetate; (c) butyrate; and (d) TVFA with three-dimensional graphs.

Fig. 4 – Effect of Sin and HRT on the yields of: (a) ethanol; (b) acetate; (c) butyrate; and (d) TVFA with two-dimensional contour graphs.

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minimum butyrate yield was estimated to be 73.7 mg/ghexose at Sin of 6.4 g/L and HRT of 16.3 h. Similar to acetate, the TVFA yield decreased with the increasing Sin (Figs. 3d and 4d). At a higher Sin (415 g/L), the TVFA yield increased with the increasing HRT. In addition, p value analysis suggests that there was a relatively slight interaction between the variables for yields of ethanol, acetate and butyrate (p40.05), and that their interactive effects on TVFA yield was significant (p ¼ 0.001o0.05).

3.4.

H2 producing rate (L/L/h)

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0.12

0.08

y = 0.01418+0.01992*x

0.04

R = 0.8798

Fermentation type in the UASB reactor 0.00

Acetate, ethanol and butyrate have been found as major end products in continuously flow acidogenic reactors [1,10]. Key factor affecting fermentation type in acidogenesis is the ratio of NADH/NAD+ inside the microbial cells and the accumulated mass of fermentation products [21]. Substrate oxidation is normally accomplished by dehydrogenation, which produces a large number of NADH because of the lack of an electron transport chain. For the fermentation to proceed continuously and also to maintain a proper NADH/NAD+ ratio, acetate is always produced along with various amounts of propionate, butyrate, or ethanol [9,21]. In one-year operation of this UASB reactor, the fractions of VFAs remained in the following order: butyrate 4 acetate 4 ethanol b propionate (Fig. 5). This suggests that a mixed-type fermentation exited in the reactor, and that butyrate-type or butyrate–acetate-type was the main fermentation domain. Clostridium species that are responsible for the butyrate–acetate fermentation route might be the main microorganisms in our reactor [21]. Ren et al. [22] reported that butyrate-type fermentation lacks the stability for NADH/NAD+ accumulation, which can be readily turned into propionate-type fermentation if the operating conditions are slightly changed. The microbial regulation may take place in order to maintain a proper NADH/NAD+ ratio, and the evolution of hydrogen through NADH pathway may be driven by the necessity of re-oxidizing the NADH of metabolic reaction. Our experimental results indicate that the granule-based UASB reactor had a stability and capacity of prolonged hydrogen production from organic wastewaters.

100 Ethanol

Acetate

Propionate

Butyrate

Isovalerate

Percentage (%)

75

50

25

0 1

2

3

4

5 Trial

6

7

8

9

Fig. 5 – Distribution of VFAs and ethanol at various trials.

0

2

4

6

B/A ratio Fig. 6 – Relation between butyrate/acetate ratio and hydrogen production rate.

Butyrate to acetate ratios (B/A) was directly proportional to hydrogen production rate in this work (Fig. 6). This result is similar to that of Kim et al. [23], who reported that B/A was directly proportional to hydrogen yield and might be a quantitative indicator of substrate metabolism and hydrogen production in a hydrogen-producing reactor. The B/A ratios (in molar) were in a range of 0.57 to 3.65, which was also consistent with that reported by Kim et al. [23]. Both butyrate–acetate-type and ethanol–acetate-type fermentation have a good potential for hydrogen production [24]. Recently, Rodriguez et al. [25] established a model about fermentation product distribution and predicted that butyrate should be the major product at a high hydrogen partial pressure. Such a prediction is confirmed by the experimental results in our study (Tables 2 and 3).

3.5. Confirmation experiments and adequacy of the models To confirm the validity of the statistical experimental strategies and to better understand hydrogen production in the granule-based UASB reactor, two additional confirmation experiments were conducted. The experiment conditions and results are listed in Table 4. The experimental results reveal that the hydrogen yield, TVFA yield and sucrose removal efficiency measured were close to those estimated using RSM with the central composite design. This confirms that RSM with a central composite design analysis was a useful tool to optimize the hydrogen production in the UASB reactor. As illustrated in Fig. 7, the residual plots for the models and data set on hydrogen yield and total VFA both showed no trends. A check of the constant variance assumption could be addressed because a random plot of residuals meant homogeneous error variances across the observed values [17]. Better predictions of the maximum responses along with constant variance in residual plots indicate adequacy of the quadratic models, which could be used to evaluate the hydrogen production performance of a UASB reactor.

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Table 4 – Results of the confirmation experiments Sin (g/L)

15.0 16.3

HRT (h)

17.8 14.5

H2 yield (mol-H2/mol-hexose)

TVFA (mg/g-hexose)

Sucrose removal (%)

Measured

Calculated

Measured

Calculated

Measured

Calculated

2.370.09 2.070.01

1.60 1.62

166725 80715

199 119

99.970.5 95.373.4

89.4 82.6

30

Table 5 – Comparison of hydrogen yields and production rates for various studies

15

Reactor

Residuals

Y max H2 (mol-

0

-15

-30 0

40 80 120 Observed YH2 (mol-H2/mol-hexose)

160

UASB Sequencing batch reactor CSTR CSTR CSTR Biofilm reactor Membrane bioreactor Biofilm reactor

Rmax H2

Substrate

H2/molhexose)

Sucrose

1.62

0.123

This study

Glucose

0.56

0.231

[12]

Sucrose Glucose Glucose

1.15 2.06 1.95

0.021 0.033 0.254

[3] [26] [5]

Sucrose

0.64

1.32

[27]

Glucose

1.0

0.118

[4]

Fructose

0.58

0.007

[28]

(L/L/h)

References

20

Residuals

10

0

-10

-20 70

140 210 280 Observed TVFA (mg/g-hexose)

350

Fig. 7 – Residual plots of quadratic models: (a) hydrogen yield and (b) TVFA yield.

3.6.

Evaluation the granule-based UASB reactor

Table 5 summarizes the maximum values of hydrogen yield ðY max H2 Þ and hydrogen production rate ðRmax H2 Þ of various types of anaerobic hydrogen-producing systems with different substrates for comparison. The theoretical Ymax H2 of 4 mol/mol-hexose achieved when acetate was the sole fermentation end product. In fact, experimental hydrogen yields were not comparable with the theoretical maximum yield and most of hydrogen production rates were much lower than 2 L/L/h (Table 5). This might be contributed to the

bacterial metabolic flexibility [1] and the generation of aqueous products. The Y max H2 and Rmax H2 , 1.62 mol-H2/molhexose and 0.123 L/L/h, respectively, in this study, were comparable with those of the other studies. Tables 2 and 5 also show that a high hydrogen yield did not match with a high hydrogen production rate. It implies that the single factor of either Y max H2 or Rmax H2 could not completely represent the hydrogen-producing capacity of reactors. Moreover, there should be a difference with Sin and HRT for the optimum conditions between Y max H2 and Rmax H2 . For a hydrogen-producing reactor, both Y max H2 and Rmax H2 are main objectives to be maximized. Since the optimum conditions for them were not same, one of them should be selected as the output variable, and thus the corresponding operational conditions are chosen.

4.

Conclusions

In the present study, RSM with a central composite design was successfully used to evaluate both individual and interactive effects of Sin and HRT on anaerobic hydrogen production from sucrose in a granule-based UASB reactor. A maximum hydrogen yield of 1.62 mol-H2/mol-hexose was obtained under optimum conditions of Sin 14.5 g/L and HRT 16.3 h. The hydrogen yield was strongly affected by either Sin or HRT, while no statistical significant interactive effect was found. Ethanol, acetate and butyrate were the main aqueous

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products and their yields correlated well with Sin and HRT, implying that a mixed-type fermentation existed in this UASB reactor. The granule-based hydrogen-producing UASB was stable and had a capacity for prolonged hydrogen production.

Acknowledgments The authors wish to thank the NSFC-JST Joint Project (20610002), and National Basic Research Program of China (2004CB719602), Chinese Academy of Sciences (KSCX2-YW-G001), National Hi-Technology Development 863 Program of China (2006AA06Z340), for the support of this study. R E F E R E N C E S

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