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Simultaneous coproduction of hydrogen and methane from sugary wastewater by an “ACSTRH–UASBMet” system
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Simultaneous coproduction of hydrogen and methane from sugary wastewater by an “ACSTRHeUASBMet” system Bing Wang a, Yongfeng Li a,*, Dexin Wang a, Ruina Liu a, Zhigang Wei a, Nanqi Ren b a b
School of Forestry, Northeast Forestry University, Harbin 150040, China School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150040, China
article info
abstract
Article history:
A two-phase “ACSTRHeUASBMet” system has been investigated at the stepwise decreased
Received 18 January 2013
HRT for the simultaneous production of hydrogen and methane in this study. Hydrogen
Received in revised form
could be continuously produced from the two-phase hydrogen fermentation of sugary
28 March 2013
wastewater in ACSTR and effluents from hydrogen fermentation were converted into
Accepted 13 April 2013
methane in UASB reactor. At optimum conditions (HRTH: 5 h, HRTMet: 15 h), the highest
Available online 11 May 2013
hydrogen production rate of 5.69 (0.06) mmol L1 h1 was obtained from sugary wastewater and methane was continuously produced from effluents of hydrogen fermentation
Keywords:
with a production rate of 3.74 (0.13) mmol L1 h1. The total bioenergy recovery by
Sugary wastewater
coproduction of hydrogen and methane from sugary wastewater reached 19.37 W and a
Two-phase anaerobic fermentation
total of 92.41% of substrate was converted to the biogas (hydrogen and methane) with two-
process
phase anaerobic fermentation.
Hydrogen production rate
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Methane production rate
reserved.
Bioenergy recovery
1.
Introduction
The current imperative global issues such as petroleum depletion and global warming are leading to new developments in the fuel markets all over the world. Bioenergy like hydrogen and methane have the potential to replace a part of our need for fossil fuels, especially in the transport sector because of their assumed many merits and perceived environmental benefits [1]. Among many bioenergy production methods, a great deal of attention has been focused on anaerobic production of hydrogen rather than methane in particular because it has many advantages including no
carbon dioxide emission and high energy density [2e6]. The exploration of substrate for hydrogen production is a vital and effective way of tapping clean energy from renewable sources in a sustainable approach. The main substrate for fermentative hydrogen production was synthetic wastewater containing carbohydrates substances. Numerous studies have been focused on glucose and sucrose and optimal operation parameters such as hydraulic retention time (HRT), organic loading rate (OLR), pH and temperature were fully investigated [7,8]. While in terms of the dual benefit of fermentative hydrogen production and waste degradation, carbohydraterich actual wastewaters were often utilized as substrates.
* Corresponding author. Tel.: þ86 13903614476; fax: þ86 45182192120. E-mail addresses: [email protected], [email protected] (Y. Li). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.065
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 7 7 7 4 e7 7 7 9
Cheese whey wastewater [9], rice winery [10], and apple processing wastewater [11] were proved to be feasible substrates showing the maximum hydrogen production rate of 2.79, 7.09, 4.02 mmol L1 h1, respectively. To improve energy recovery, the two-phase anaerobic fermentation process for coproduction of hydrogen and methane has attracted a great deal of attention. Until now this process has been operated with various types of actual wastes including food waste [12], olive pulp [13] and red canary grass [14]. However, to the best of our knowledge, there is no paper on the utilization of sugary wastewater as a sole carbon source for sequential coproduction of hydrogen and methane from a two-phase process. Hydrogen production by anaerobic fermentation process is highly dependent on the conditions of this process, such as pH, OLR, HRT and hydrogen partial pressure, which affect the microbial metabolic balance and subsequent fermentation metabolites. Among these conditions, as HRT is related to the amount of organics that can be handled per unit time [15], it has a direct impact on substrate uptake efficiency, microbial population and metabolic pathway. It is generally held that high HRT allows for growth, by not washing out slow growing methanogens and acetogens, including competitors for substrates. However, a low HRT may reduce substrate uptake efficiency, active biomass retention, and therefore, the overall process efficiency [16]. Based on above information, this study aimed to investigate the fermentative bioproduction of hydrogen from sugary wastewater in anaerobic continuous tank stirred reactor (ACSTR) at various HRT. The effluents of fermentative hydrogen production process were used for continuous methane production in an upflow anaerobic sludge bed (UASB) at various HRT, the total bioenergy recovery of hydrogen and methane coproduction from sugary wastewater was evaluated.
2.
Material and methods
2.1.
Substrate and inoculums
China). As shown in Table 1, the sugary wastewater contained a high concentration of carbohydrates which correspond to readily fermentative sugars. In addition, it contained enough nitrogen and phosphorus sources which are essential for cultivation of microorganisms. On one hand, diluted sugary wastewater with COD concentration of 6 g L1 was used as substrate for the first-phase acidogenic reactor. On the other hand, the effluent of the acidogenic reactor was used as substrate (pH was adjusted to about 7.0 with alkali addition) for the second-phase methanogenic reactor. The raw sludge was obtained from a local municipal wastewater treatment plant (Harbin, China) and screened by a sieve. To be used as inoculum for hydrogen production, the raw sludge was inoculated into the sequencing batch reactor (SBR) with sugary wastewater and enriched by aerating intermittently to inactivate hydrogen-consuming bacteria, especially methanogens. The system pH was controlled at 4.5 (0.2) using alkali solution. During the enrichment process, biological activity of hydrogen-producing bacteria was examined by analyses of glucose consumed. After enough enrichment over 30 days, the hydrogenic sludge with TSS 12.91 (0.43) g L1 and VSS 8.35 (0.12) g L1 was inoculated within the acidogenic reactor of the two-phase anaerobic fermentation process. In case of methane production, without any pretreatment, the raw sludge with TSS 21.54 (0.17) g L1 and VSS 14.22 (0.26) g L1 was inoculated into the methanogenic reactor of the two-phase anaerobic fermentation process. The system pH was controlled at 6.9 (0.2) which is susceptible for the effective function of methanogenic metabolic process. Operating temperature for the reactor with sugary wastewater was 35 C. Metabolism activities of methanogens were examined by detecting methane content in produced biogas from methanogenic reactor. After the methanogenic sludge got enough enrichment over three months, the effluent of the acidogenic reactor began to be used as the substrate for methanogenic reactor with addition of alkali solution to regulate pH.
2.2.
In this study, the sugar wastewater used as experimental substrate for sequential hydrogen and methane coproduction was collected from local sugar refining industry (Harbin,
Table 1 e The characteristics of sugar wastewater used in this study. Parameters
Valuesa
Total suspend solid (TSS) Volatile suspended solid (VSS) Total chemical oxygen demand (TCOD) Chemical oxygen demand (COD) Total organic carbon (TOC) Carbohydrates
3.3 g L1
Alkalinity
1.1 g L1
1.5 g L1
2.2 g L1
27.3 g L1
Total nitrogen (TN) SO2 4
1.3 g L1
26.7 g L1
PO3 4
0.41 g L1
11.6 g L1
pH
6.5
Parameters
9.8 g L1
a Values were averaged of 5 determinations.
Valuesa
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Integrated two-phase fermentation system
An integrated two-phase “ACSTRHeUASBMet” anaerobic fermentation system was designed for simultaneous hydrogen and methane production in this study. The system consisted of acidogenic reactor for hydrogen production and a methanogenic reactor for methane production. The acidogenic reactor was anaerobic continuous stirred tank reactor (ACSTR). It had a working volume of approximately 7 L with an internal diameter of 18.5 cm and a height of 26 cm, and was operated in a continuous flow mode and mixed completely by a variable speed gear shaft mixer. After the enriched hydrogenic sludge was inoculated into this acidogenic reactor, a rest of the volume in the reactor was filled with the sugary wastewater under complete anaerobic condition with the aid of peristaltic pump. The system temperature was constantly maintained at 35 C by an electric jacket. The acidogenic reactor was operated in a continuous mode by supplying the sugary wastewater continuously at decreased HRT in steps. Five levels of HRT were designed to 12, 8, 6, 5 and 4 h, respectively. The upflow anaerobic sludge blanket reactor (UASB) was used as the methanogenic reactor. It was also operated in a continuous mode at decreased HRT: 35, 27, 21, 15 and 10 h,
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RA ¼
RM ¼
WA $ai $Mi þ VH $b1 100 WA $CODA
VM $b2 100 WM $CODM
(a)
(b)
Where RA and RM represent the conversion rate of substrate in acidogenic and methanogenic reactor, respectively. WA and WM represent the feed rate in acidogenic and methanogenic reactor (L/d), respectively. ai, b1 and b2 represent the COD equivalent of VFA i, hydrogen and methane, respectively. Mi presents the concentration of VFA i in the effluent (g L1). VH and VM represent the hydrogen production (L). CODA and CODM represent the COD of sugary wastewater and the effluent of acidogenic reactor (g L1).
In previous study, the ACSTR has showed effective and stable hydrogen production with efficient utilization of carbon sources [18e21]. Thus this type of reactor was used for continuous production of hydrogen from sugary wastewater in the acidogenic reactor in this study. Through continuous operation by controlling operating parameters, the acidogenic reactor was stabilized and biogas composed of only hydrogen and carbon dioxide could be produced gradually. Fig. 1(a) shows an HRT-dependent profile of volumetric hydrogen production rate and hydrogen content in biogas of the acidogenic reactor. As expected, the HRT significantly affected the production rate and content of hydrogen from the acidogenic
10
5
9 4
8 7
3
6 5
60 58 56
52
1 10 20 30 40 50 60 70 80 90 100 110 120 130 140
0
62
50
Time (d)
(b) 3.0
Total VFAs Acetate Propionate
2.5
Ethanol Butyrate
70 60 50
2.0
40 1.5
30
1.0
20
0.5 0.0
10 0
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
Time (d)
(c) 9
80
8
70
7
60
6 50
5
40
4
pH of acidogenic reactor effluent Total conversion rate of substrate Conversion rate of substrate into VFAs
3 2
Main metabolites distribution (%)
3
64
54
2
HPR Hydrogen content
4
66
6
HRT
11
Hydrogen content (%)
(a)12
0
30
Conversion rate of substrate (%)
P
Hydrogen production from sugary wastewater
HPR (mmol L-1 hr-1)
COD, pH, alkalinity and chemical composition of sugary wastewater was analyzed according to standard methods [17]. Total nitrogen (TN) was measured by a TN analyzer (Model 2 TNM-1, Shimadzu, Japan), and PO3 4 and SO4 were measured by an ion chromatography (IC, Model ICS-900, Dionex). Carbohydrate content was determined by a phenolesulfuric acid method. Biogas generated from each reactor was collected using a wet gas meter (Model LML-1, Changchun Filter Co. Ltd., Changchun, China) and analyzed everyday. Effluent samples from each reactor were also collected for metabolites analyses during the overall period of reactor operation. Hydrogen and methane were analyzed using a gas chromatography (SC-7, Shandong Lunan Instrument Factory). The gas chromatography was equipped with a thermal conductivity detector (TCD) and a stainless steel column (2 m 5 mm) filled with Porapak Q (50e80 meshes). Nitrogen was used as the carrier gas at a flow rate of 40 mL/min. Detection of volatile fatty acids (VFAs) in the fermentation solution was analyzed by another gas chromatograph (GC 112, shanghai Anal. Inst. Co.) with a flame ionization detector (FID). A 2-m stainless steel column was packed with the supporter GDX-103 (60e80 meshes). The temperatures of the injection port, oven, and detector were 220 C, 190 C, and 220 C, respectively. Nitrogen was used as the carrier gas at a flow rate of 30 mL/min. The conversion rate of substrate in the fermentative hydrogen and methane production process can be calculated from detected VFAs and hydrogen/methane production according to Equation (a) and (b). The adoptive COD equivalents of four VFAs (ethanol, acetate, propionate and butyrate) were 2.09, 1.07, 1.51 and 1.82, respectively. The hydrogen and methane COD equivalents (g O2/L H2 and CH4) were 0.71 and 2.86, respectively.
3.1.
HRT (h)
Analytical methods
Results and discussion
Total VFAs concentration (g/L)
2.3.
3.
pH of acidogenic reactor effluent
respectively. The effluent of acidogenic reactor containing lots of volatile fatty acids (VFAs) was used as a carbon source for methanogens. The methanogenic reactor was built cuboidally with an internal diameter of 14 cm and a height of 1.3 m, providing a working volume of approximately 20 L. A three separator was installed upside the reactor to prevent biomass washout. The temperatures were maintained through an electric jacket to be 35 C. In this study, the quasi-steady state was defined as the condition that the biogas varied within 5% for 10 days.
20 10 20 30 40 50 60 70 80 90 100 110 120 130 140
Time (d)
Fig. 1 e HRT-dependent profile of the first-phase acidogenic reactor: (a) Hydrogen production rate and hydrogen content. (b) Total VFAs concentration and main metabolites distribution in the effluent. (c) pH of effluent and conversion rate of substrate.
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3.2. Methane production from the effluents of fermentative hydrogen production The UASB reactor employed as the methanogenic reactor has prevailed successfully for anaerobic treatment of various types of wastewater to produce methane, because of its high treatment efficiency and excellent process stability [26]. The COD concentration of influent was kept at a constant level, and the OLR of methanogenic reactor was increased from an initial 2.1 g COD L1 d1 by decreasing the HRT step by step. Volumetric methane production rate (MPR) varying from 1.19 (0.05) to 3.74 (0.13) mmol L1 h1 (Fig. 2(a)) was found at decreasing HRT from 35 h to 10 h with methane content in biogas ranging from 61.5 (0.5) to 73.1 (2.1) %. It should be noticed that at optimum HRT of 5 h for acidogenic reactor, the residual sugars and hydrogen-producing bacteria could flow into methanogenic reactor resulting in the detected a trace of hydrogen in biogas evolved from methanogenic reactor. In methane production point of view, it seemed that a short HRT
4 3
25 20
2
15
1
65 60 55
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
0
50
Time (d)
(b) 90
COD removal efficiency OLR
80
10 9 8 7
70
6 5
60
4 3
50
OLR (g COD/L-1 d-1)
COD removal efficiency (%)
70
MPR Methane content
10
2 40
(c)
75
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140
1
Time (d) 90
9
80 8 70 60
7
50 6
pH of methanogenic reactor effluent Conversion rate of substrate 5
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140
40
Conversion rate of substrate (%)
HRT (h)
30
Methane content (%)
80
HRT
35
MPR(mmol/L-1 h-1)
(a)
pH of methanogenic reactor effluent
reactor. As HRT was decreased from 12 h to 5 h, hydrogen production rate gradually increased from 4.26 (0.06) to 5.69 (0.06) mmol L1 h1 though the maximum hydrogen content of 64.7 (0.7) % was obtained at HRT 6 h. When the HRT was decreased further to 4 h, however, hydrogen production rate abruptly decreased up to 3.38 (0.09) mmol L1 h1 with hydrogen content of 53.8 (0.8)%. This can be explained that inlet flow velocity caused by short HRT led to insufficient time for hydrolysis of sugars present in sugary wastewater. Thus an HRT of 5 h was determined to be the optimum HRT condition for continuous production of hydrogen from sugary wastewater in the first-phase acidogenic reactor. During acidogenesis of sugars, various kinds of metabolites are known to be generated along with hydrogen [22,23]. Generally, VFAs production was associated with conversion of substrate to metabolism intermediates under anaerobic microenvironment. In this study, the main VFAs components of fermentative hydrogen production were ethanol, acetate, propionate and butyrate at various HRT (Fig. 1(b)). The total VFAs concentration varied between 1.46 (0.03) and 1.99 (0.09) g L1. At all HRTs above 5 h, ethanol concentration was the highest accounting for 48.8e54.1% of total VFAs concentration and the next is acetate accounting for 21.7e23.6%. This indicated that the main fermentation pathway in acidogenic reactor was ethanoleacetate pathway. The propionate concentration is the least only accounting for about 10% of total VFAs. The main metabolites distribution induced the system pH at the level of about 4.5 without manual interference, which was consistent with the studies reported by Ren et al. [24] and Guo et al. [25]. The observed pH was favorable for effective hydrogen production fermentation by inhibiting methanogenesis. This might be the reason for relatively low COD removal efficiency in acidogenic reactor. As shown in Fig. 1(c), the conversion rate of substrate into VFAs and hydrogen varied obviously with decreasing HRT during hydrogen fermentation. At HRT 5 h, 7.70% of the influent COD in acidogenic reactor was converted to the hydrogen and 58.96% was converted to the VFAs readily fermented by methanogens to generate methane. Therefore, a total of 66.66% of conversion rate of substrate was obtained in acidogenic reactor.
30
Time (d)
Fig. 2 e HRT-dependent profile of the second-phase methanogenic reactor: (a) Methane production rate and methane content. (b) OLR and COD removal efficiency. (c) pH of effluent and conversion rate of substrate.
of 10 h was the optimum condition for the methanogenic reactor in this study. However, as we know that the main aim of methanogenic reactor was to reduce final discharge concentration of COD as more as possible. In Fig. 2(b) the methanogenic reactor showed good amount of COD removal efficiency. The COD removal efficiency reached the maximum 79.2 (1.2) % when HRT was decreased to 15 h. The COD removal efficiency showed a remarkable decrease by 19.1%e 64.1 (0.7) % at HRT 10 h due to the observed sludge flotation and washout from methanogenic reactor. Though highest methane production rate was obtained at HRT of 10 h, high production was achieved at the expense of a large waste of organic substrate. In conclusion, regarding with the performance of the maximum removal efficiency of COD and additional production of methane, an HRT of 15 h was determined to be the optimum HRT condition of methanogenic reactor, at
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which the production rate of methane was calculated to be 3.74 (0.13) mmol L1 h1. The VFAs present in effluent of hydrogen fermentation process were generally utilized by methanogens in the process of methane generation. The visible reduction in VFAs concentration was observed in methaogenic process as compared to acidogenic process, which might be indicative of effective function of methanogens. During methaogenic process, sample analysis reflected that ethanol was converted totally and the acetate, propionate and butyrate concentration in the effluent varied inconsistently in the range of 0.04e0.08, 0.06e0.09 and 0.03e0.07 g L1, respectively. The utilization of VFAs for methane production can be considered to be positive aspects of sustainable hydrogen production during acidogenic process [29]. As it is known that the optimum pH range for methanogens was between 6.0 and 8, with an optimum near 7.0 and a pH value outside the range could lead to imbalance [27,28]. Fig. 2(c) depicts effluent pH and conversion rate of substrate into methane during methane generation process. The persistent alkaline microenvironment kept the methanogenic reactor near neutral condition with effluent pH in the range of 6.7e7.1 which was suitable for the growth of methanogens. From Fig. 2(c), it can be seen that at HRT 15 h, about 84.71% of influent COD was converted to the methane in methanogenic reactor. This is also the reason for the maximum reduction in COD concentration because the COD was mainly removed in the form of methane during methanogenic process. With substrate conversion rate of 7.70% for hydrogen in acidogenic reactor and 84.71% for methane in methanogenic reactor, a total of 92.41% of substrate was converted to biogas (hydrogen and methane) in the two-phase anaerobic fermentation process. For comparison, a total of 87.96% of substrate was converted into biogas in integrated two-phase process by Wang and Zhao [12].
3.3.
40.2 L/d. The coproduction of hydrogen and methane from sugary wastewater markedly increased the bioenergy recovery from 2.73 W only in hydrogen production to 19.37 W (Table 2). Li et al. [30] previously obtained a bioenergy recovery of 9.43e19.02 W from rice, potato, lettuce, and kitchen as well as paper wastes by two-phase fermentation for the coproduction of hydrogen and methane. In comparison, a higher bioenergy recovery was achieved by combining the hydrogen fermentation of wastewater and methane fermentation of hydrogen fermentation effluents. According to simple mass balance at optimum condition (HRTH: 5 h, HRTMet: 15 h), 1 L of sugary wastewater (COD 26.7 g L1) could produce 4.9 L hydrogen-rich biogas with hydrogen content of about 60.2% and 8.0 L methane-rich biogas with methane content of about 68.9% by two-phase anaerobic fermentation process. In conclusion, sugary wastewater was a promising substrate for simultaneous hydrogen and methane coproduction using two-phase anaerobic fermentation system. The low bioenergy recovery from hydrogen fermentation is a major bottleneck for the cogeneration of hydrogen and methane that must be improved to commercial advancement, however, the selection of carbohydrate-rich actual wastewaters was a vital process for improved bioenergy recovery from the coproduction of hydrogen and methane with anaerobic fermentation. To date, there are still various kinds of wastewaters that remain unexplored for hydrogen and methane production process, such as oil industry wastewaters having relatively low pH [31]. The concept of combined wastewaters may lead to a new path for hydrogen and methane coproduction. For example, Huang [32] could practically achieve a higher biohydrogen yield by combining textile and food wastewaters. In addition, a combination of solid organic wastes and wastewater could also be a novel approach for hydrogen and methane production [1].
Bioenergy recovery and mass balance
4. At optimum conditions (HRTH: 5 h, HRTMet: 15 h), the hydrogen production of 21.6 L/d was obtained from the two-phase hydrogen fermentation of sugary wastewater in ACSTR and effluents from hydrogen fermentation were converted into methane in UASB reactor with methane production of
Table 2 e Bioenergy recovery from hydrogen and methane coproduction in the two-phase system. Hydrogen, methane production, energy recovery, conversion rate of substrate into biogas COD concentration (g L1) Optimum HRT (h) Hydrogen production (L/d)a Methane production (L/d)a Bioenergy recovery in hydrogen production (W)b Total bioenergy recovery (W)b Conversion rate of substrate (%)
Two-phase “ACSTRHeUASBMet” system 6/3.69 5/15 21.6 40.2 2.73 29.37 92.41
a Hydrogen and methane production were adjusted at standard pressure and temperature, i.e. 1 atm and 0 C. b Energy density: 242 kJ/mol of hydrogen, 801 kJ/mol of methane.
Conclusion
Hydrogen and methane were simultaneously generated from sugary wastewater by the two-phase system composed of anaerobic continuous tank stirred reactor (ACSTR) and upflow anaerobic sludge reactor (UASB) in this study. The effects of the stepwise decreased HRT on this two-phase system were investigated. Experimental data obtained at optimum condition (HRTH: 5 h, HRTMet: 15 h) illustrated that sugary wastewater was a promising carbohydrate-rich substrate for the coproduction of hydrogen and methane through anaerobic fermentation and also established the most important operation parameters guideline for future industrial application.
Acknowledgments Financial support was from public welfare research program (the early warning technology of environmental risk about the influence of pharmaceutical wastewater on environmental microbiology), Ministry of Environmental Protection, China (Grant No.200909043).
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Available online at www.sciencedirect.com
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Simultaneous coproduction of hydrogen and methane from sugary wastewater by an “ACSTRHeUASBMet” system Bing Wang a, Yongfeng Li a,*, Dexin Wang a, Ruina Liu a, Zhigang Wei a, Nanqi Ren b a b
School of Forestry, Northeast Forestry University, Harbin 150040, China School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150040, China
article info
abstract
Article history:
A two-phase “ACSTRHeUASBMet” system has been investigated at the stepwise decreased
Received 18 January 2013
HRT for the simultaneous production of hydrogen and methane in this study. Hydrogen
Received in revised form
could be continuously produced from the two-phase hydrogen fermentation of sugary
28 March 2013
wastewater in ACSTR and effluents from hydrogen fermentation were converted into
Accepted 13 April 2013
methane in UASB reactor. At optimum conditions (HRTH: 5 h, HRTMet: 15 h), the highest
Available online 11 May 2013
hydrogen production rate of 5.69 (0.06) mmol L1 h1 was obtained from sugary wastewater and methane was continuously produced from effluents of hydrogen fermentation
Keywords:
with a production rate of 3.74 (0.13) mmol L1 h1. The total bioenergy recovery by
Sugary wastewater
coproduction of hydrogen and methane from sugary wastewater reached 19.37 W and a
Two-phase anaerobic fermentation
total of 92.41% of substrate was converted to the biogas (hydrogen and methane) with two-
process
phase anaerobic fermentation.
Hydrogen production rate
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Methane production rate
reserved.
Bioenergy recovery
1.
Introduction
The current imperative global issues such as petroleum depletion and global warming are leading to new developments in the fuel markets all over the world. Bioenergy like hydrogen and methane have the potential to replace a part of our need for fossil fuels, especially in the transport sector because of their assumed many merits and perceived environmental benefits [1]. Among many bioenergy production methods, a great deal of attention has been focused on anaerobic production of hydrogen rather than methane in particular because it has many advantages including no
carbon dioxide emission and high energy density [2e6]. The exploration of substrate for hydrogen production is a vital and effective way of tapping clean energy from renewable sources in a sustainable approach. The main substrate for fermentative hydrogen production was synthetic wastewater containing carbohydrates substances. Numerous studies have been focused on glucose and sucrose and optimal operation parameters such as hydraulic retention time (HRT), organic loading rate (OLR), pH and temperature were fully investigated [7,8]. While in terms of the dual benefit of fermentative hydrogen production and waste degradation, carbohydraterich actual wastewaters were often utilized as substrates.
* Corresponding author. Tel.: þ86 13903614476; fax: þ86 45182192120. E-mail addresses: [email protected], [email protected] (Y. Li). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.065
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 7 7 7 4 e7 7 7 9
Cheese whey wastewater [9], rice winery [10], and apple processing wastewater [11] were proved to be feasible substrates showing the maximum hydrogen production rate of 2.79, 7.09, 4.02 mmol L1 h1, respectively. To improve energy recovery, the two-phase anaerobic fermentation process for coproduction of hydrogen and methane has attracted a great deal of attention. Until now this process has been operated with various types of actual wastes including food waste [12], olive pulp [13] and red canary grass [14]. However, to the best of our knowledge, there is no paper on the utilization of sugary wastewater as a sole carbon source for sequential coproduction of hydrogen and methane from a two-phase process. Hydrogen production by anaerobic fermentation process is highly dependent on the conditions of this process, such as pH, OLR, HRT and hydrogen partial pressure, which affect the microbial metabolic balance and subsequent fermentation metabolites. Among these conditions, as HRT is related to the amount of organics that can be handled per unit time [15], it has a direct impact on substrate uptake efficiency, microbial population and metabolic pathway. It is generally held that high HRT allows for growth, by not washing out slow growing methanogens and acetogens, including competitors for substrates. However, a low HRT may reduce substrate uptake efficiency, active biomass retention, and therefore, the overall process efficiency [16]. Based on above information, this study aimed to investigate the fermentative bioproduction of hydrogen from sugary wastewater in anaerobic continuous tank stirred reactor (ACSTR) at various HRT. The effluents of fermentative hydrogen production process were used for continuous methane production in an upflow anaerobic sludge bed (UASB) at various HRT, the total bioenergy recovery of hydrogen and methane coproduction from sugary wastewater was evaluated.
2.
Material and methods
2.1.
Substrate and inoculums
China). As shown in Table 1, the sugary wastewater contained a high concentration of carbohydrates which correspond to readily fermentative sugars. In addition, it contained enough nitrogen and phosphorus sources which are essential for cultivation of microorganisms. On one hand, diluted sugary wastewater with COD concentration of 6 g L1 was used as substrate for the first-phase acidogenic reactor. On the other hand, the effluent of the acidogenic reactor was used as substrate (pH was adjusted to about 7.0 with alkali addition) for the second-phase methanogenic reactor. The raw sludge was obtained from a local municipal wastewater treatment plant (Harbin, China) and screened by a sieve. To be used as inoculum for hydrogen production, the raw sludge was inoculated into the sequencing batch reactor (SBR) with sugary wastewater and enriched by aerating intermittently to inactivate hydrogen-consuming bacteria, especially methanogens. The system pH was controlled at 4.5 (0.2) using alkali solution. During the enrichment process, biological activity of hydrogen-producing bacteria was examined by analyses of glucose consumed. After enough enrichment over 30 days, the hydrogenic sludge with TSS 12.91 (0.43) g L1 and VSS 8.35 (0.12) g L1 was inoculated within the acidogenic reactor of the two-phase anaerobic fermentation process. In case of methane production, without any pretreatment, the raw sludge with TSS 21.54 (0.17) g L1 and VSS 14.22 (0.26) g L1 was inoculated into the methanogenic reactor of the two-phase anaerobic fermentation process. The system pH was controlled at 6.9 (0.2) which is susceptible for the effective function of methanogenic metabolic process. Operating temperature for the reactor with sugary wastewater was 35 C. Metabolism activities of methanogens were examined by detecting methane content in produced biogas from methanogenic reactor. After the methanogenic sludge got enough enrichment over three months, the effluent of the acidogenic reactor began to be used as the substrate for methanogenic reactor with addition of alkali solution to regulate pH.
2.2.
In this study, the sugar wastewater used as experimental substrate for sequential hydrogen and methane coproduction was collected from local sugar refining industry (Harbin,
Table 1 e The characteristics of sugar wastewater used in this study. Parameters
Valuesa
Total suspend solid (TSS) Volatile suspended solid (VSS) Total chemical oxygen demand (TCOD) Chemical oxygen demand (COD) Total organic carbon (TOC) Carbohydrates
3.3 g L1
Alkalinity
1.1 g L1
1.5 g L1
2.2 g L1
27.3 g L1
Total nitrogen (TN) SO2 4
1.3 g L1
26.7 g L1
PO3 4
0.41 g L1
11.6 g L1
pH
6.5
Parameters
9.8 g L1
a Values were averaged of 5 determinations.
Valuesa
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Integrated two-phase fermentation system
An integrated two-phase “ACSTRHeUASBMet” anaerobic fermentation system was designed for simultaneous hydrogen and methane production in this study. The system consisted of acidogenic reactor for hydrogen production and a methanogenic reactor for methane production. The acidogenic reactor was anaerobic continuous stirred tank reactor (ACSTR). It had a working volume of approximately 7 L with an internal diameter of 18.5 cm and a height of 26 cm, and was operated in a continuous flow mode and mixed completely by a variable speed gear shaft mixer. After the enriched hydrogenic sludge was inoculated into this acidogenic reactor, a rest of the volume in the reactor was filled with the sugary wastewater under complete anaerobic condition with the aid of peristaltic pump. The system temperature was constantly maintained at 35 C by an electric jacket. The acidogenic reactor was operated in a continuous mode by supplying the sugary wastewater continuously at decreased HRT in steps. Five levels of HRT were designed to 12, 8, 6, 5 and 4 h, respectively. The upflow anaerobic sludge blanket reactor (UASB) was used as the methanogenic reactor. It was also operated in a continuous mode at decreased HRT: 35, 27, 21, 15 and 10 h,
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RA ¼
RM ¼
WA $ai $Mi þ VH $b1 100 WA $CODA
VM $b2 100 WM $CODM
(a)
(b)
Where RA and RM represent the conversion rate of substrate in acidogenic and methanogenic reactor, respectively. WA and WM represent the feed rate in acidogenic and methanogenic reactor (L/d), respectively. ai, b1 and b2 represent the COD equivalent of VFA i, hydrogen and methane, respectively. Mi presents the concentration of VFA i in the effluent (g L1). VH and VM represent the hydrogen production (L). CODA and CODM represent the COD of sugary wastewater and the effluent of acidogenic reactor (g L1).
In previous study, the ACSTR has showed effective and stable hydrogen production with efficient utilization of carbon sources [18e21]. Thus this type of reactor was used for continuous production of hydrogen from sugary wastewater in the acidogenic reactor in this study. Through continuous operation by controlling operating parameters, the acidogenic reactor was stabilized and biogas composed of only hydrogen and carbon dioxide could be produced gradually. Fig. 1(a) shows an HRT-dependent profile of volumetric hydrogen production rate and hydrogen content in biogas of the acidogenic reactor. As expected, the HRT significantly affected the production rate and content of hydrogen from the acidogenic
10
5
9 4
8 7
3
6 5
60 58 56
52
1 10 20 30 40 50 60 70 80 90 100 110 120 130 140
0
62
50
Time (d)
(b) 3.0
Total VFAs Acetate Propionate
2.5
Ethanol Butyrate
70 60 50
2.0
40 1.5
30
1.0
20
0.5 0.0
10 0
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
Time (d)
(c) 9
80
8
70
7
60
6 50
5
40
4
pH of acidogenic reactor effluent Total conversion rate of substrate Conversion rate of substrate into VFAs
3 2
Main metabolites distribution (%)
3
64
54
2
HPR Hydrogen content
4
66
6
HRT
11
Hydrogen content (%)
(a)12
0
30
Conversion rate of substrate (%)
P
Hydrogen production from sugary wastewater
HPR (mmol L-1 hr-1)
COD, pH, alkalinity and chemical composition of sugary wastewater was analyzed according to standard methods [17]. Total nitrogen (TN) was measured by a TN analyzer (Model 2 TNM-1, Shimadzu, Japan), and PO3 4 and SO4 were measured by an ion chromatography (IC, Model ICS-900, Dionex). Carbohydrate content was determined by a phenolesulfuric acid method. Biogas generated from each reactor was collected using a wet gas meter (Model LML-1, Changchun Filter Co. Ltd., Changchun, China) and analyzed everyday. Effluent samples from each reactor were also collected for metabolites analyses during the overall period of reactor operation. Hydrogen and methane were analyzed using a gas chromatography (SC-7, Shandong Lunan Instrument Factory). The gas chromatography was equipped with a thermal conductivity detector (TCD) and a stainless steel column (2 m 5 mm) filled with Porapak Q (50e80 meshes). Nitrogen was used as the carrier gas at a flow rate of 40 mL/min. Detection of volatile fatty acids (VFAs) in the fermentation solution was analyzed by another gas chromatograph (GC 112, shanghai Anal. Inst. Co.) with a flame ionization detector (FID). A 2-m stainless steel column was packed with the supporter GDX-103 (60e80 meshes). The temperatures of the injection port, oven, and detector were 220 C, 190 C, and 220 C, respectively. Nitrogen was used as the carrier gas at a flow rate of 30 mL/min. The conversion rate of substrate in the fermentative hydrogen and methane production process can be calculated from detected VFAs and hydrogen/methane production according to Equation (a) and (b). The adoptive COD equivalents of four VFAs (ethanol, acetate, propionate and butyrate) were 2.09, 1.07, 1.51 and 1.82, respectively. The hydrogen and methane COD equivalents (g O2/L H2 and CH4) were 0.71 and 2.86, respectively.
3.1.
HRT (h)
Analytical methods
Results and discussion
Total VFAs concentration (g/L)
2.3.
3.
pH of acidogenic reactor effluent
respectively. The effluent of acidogenic reactor containing lots of volatile fatty acids (VFAs) was used as a carbon source for methanogens. The methanogenic reactor was built cuboidally with an internal diameter of 14 cm and a height of 1.3 m, providing a working volume of approximately 20 L. A three separator was installed upside the reactor to prevent biomass washout. The temperatures were maintained through an electric jacket to be 35 C. In this study, the quasi-steady state was defined as the condition that the biogas varied within 5% for 10 days.
20 10 20 30 40 50 60 70 80 90 100 110 120 130 140
Time (d)
Fig. 1 e HRT-dependent profile of the first-phase acidogenic reactor: (a) Hydrogen production rate and hydrogen content. (b) Total VFAs concentration and main metabolites distribution in the effluent. (c) pH of effluent and conversion rate of substrate.
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3.2. Methane production from the effluents of fermentative hydrogen production The UASB reactor employed as the methanogenic reactor has prevailed successfully for anaerobic treatment of various types of wastewater to produce methane, because of its high treatment efficiency and excellent process stability [26]. The COD concentration of influent was kept at a constant level, and the OLR of methanogenic reactor was increased from an initial 2.1 g COD L1 d1 by decreasing the HRT step by step. Volumetric methane production rate (MPR) varying from 1.19 (0.05) to 3.74 (0.13) mmol L1 h1 (Fig. 2(a)) was found at decreasing HRT from 35 h to 10 h with methane content in biogas ranging from 61.5 (0.5) to 73.1 (2.1) %. It should be noticed that at optimum HRT of 5 h for acidogenic reactor, the residual sugars and hydrogen-producing bacteria could flow into methanogenic reactor resulting in the detected a trace of hydrogen in biogas evolved from methanogenic reactor. In methane production point of view, it seemed that a short HRT
4 3
25 20
2
15
1
65 60 55
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
0
50
Time (d)
(b) 90
COD removal efficiency OLR
80
10 9 8 7
70
6 5
60
4 3
50
OLR (g COD/L-1 d-1)
COD removal efficiency (%)
70
MPR Methane content
10
2 40
(c)
75
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140
1
Time (d) 90
9
80 8 70 60
7
50 6
pH of methanogenic reactor effluent Conversion rate of substrate 5
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140
40
Conversion rate of substrate (%)
HRT (h)
30
Methane content (%)
80
HRT
35
MPR(mmol/L-1 h-1)
(a)
pH of methanogenic reactor effluent
reactor. As HRT was decreased from 12 h to 5 h, hydrogen production rate gradually increased from 4.26 (0.06) to 5.69 (0.06) mmol L1 h1 though the maximum hydrogen content of 64.7 (0.7) % was obtained at HRT 6 h. When the HRT was decreased further to 4 h, however, hydrogen production rate abruptly decreased up to 3.38 (0.09) mmol L1 h1 with hydrogen content of 53.8 (0.8)%. This can be explained that inlet flow velocity caused by short HRT led to insufficient time for hydrolysis of sugars present in sugary wastewater. Thus an HRT of 5 h was determined to be the optimum HRT condition for continuous production of hydrogen from sugary wastewater in the first-phase acidogenic reactor. During acidogenesis of sugars, various kinds of metabolites are known to be generated along with hydrogen [22,23]. Generally, VFAs production was associated with conversion of substrate to metabolism intermediates under anaerobic microenvironment. In this study, the main VFAs components of fermentative hydrogen production were ethanol, acetate, propionate and butyrate at various HRT (Fig. 1(b)). The total VFAs concentration varied between 1.46 (0.03) and 1.99 (0.09) g L1. At all HRTs above 5 h, ethanol concentration was the highest accounting for 48.8e54.1% of total VFAs concentration and the next is acetate accounting for 21.7e23.6%. This indicated that the main fermentation pathway in acidogenic reactor was ethanoleacetate pathway. The propionate concentration is the least only accounting for about 10% of total VFAs. The main metabolites distribution induced the system pH at the level of about 4.5 without manual interference, which was consistent with the studies reported by Ren et al. [24] and Guo et al. [25]. The observed pH was favorable for effective hydrogen production fermentation by inhibiting methanogenesis. This might be the reason for relatively low COD removal efficiency in acidogenic reactor. As shown in Fig. 1(c), the conversion rate of substrate into VFAs and hydrogen varied obviously with decreasing HRT during hydrogen fermentation. At HRT 5 h, 7.70% of the influent COD in acidogenic reactor was converted to the hydrogen and 58.96% was converted to the VFAs readily fermented by methanogens to generate methane. Therefore, a total of 66.66% of conversion rate of substrate was obtained in acidogenic reactor.
30
Time (d)
Fig. 2 e HRT-dependent profile of the second-phase methanogenic reactor: (a) Methane production rate and methane content. (b) OLR and COD removal efficiency. (c) pH of effluent and conversion rate of substrate.
of 10 h was the optimum condition for the methanogenic reactor in this study. However, as we know that the main aim of methanogenic reactor was to reduce final discharge concentration of COD as more as possible. In Fig. 2(b) the methanogenic reactor showed good amount of COD removal efficiency. The COD removal efficiency reached the maximum 79.2 (1.2) % when HRT was decreased to 15 h. The COD removal efficiency showed a remarkable decrease by 19.1%e 64.1 (0.7) % at HRT 10 h due to the observed sludge flotation and washout from methanogenic reactor. Though highest methane production rate was obtained at HRT of 10 h, high production was achieved at the expense of a large waste of organic substrate. In conclusion, regarding with the performance of the maximum removal efficiency of COD and additional production of methane, an HRT of 15 h was determined to be the optimum HRT condition of methanogenic reactor, at
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which the production rate of methane was calculated to be 3.74 (0.13) mmol L1 h1. The VFAs present in effluent of hydrogen fermentation process were generally utilized by methanogens in the process of methane generation. The visible reduction in VFAs concentration was observed in methaogenic process as compared to acidogenic process, which might be indicative of effective function of methanogens. During methaogenic process, sample analysis reflected that ethanol was converted totally and the acetate, propionate and butyrate concentration in the effluent varied inconsistently in the range of 0.04e0.08, 0.06e0.09 and 0.03e0.07 g L1, respectively. The utilization of VFAs for methane production can be considered to be positive aspects of sustainable hydrogen production during acidogenic process [29]. As it is known that the optimum pH range for methanogens was between 6.0 and 8, with an optimum near 7.0 and a pH value outside the range could lead to imbalance [27,28]. Fig. 2(c) depicts effluent pH and conversion rate of substrate into methane during methane generation process. The persistent alkaline microenvironment kept the methanogenic reactor near neutral condition with effluent pH in the range of 6.7e7.1 which was suitable for the growth of methanogens. From Fig. 2(c), it can be seen that at HRT 15 h, about 84.71% of influent COD was converted to the methane in methanogenic reactor. This is also the reason for the maximum reduction in COD concentration because the COD was mainly removed in the form of methane during methanogenic process. With substrate conversion rate of 7.70% for hydrogen in acidogenic reactor and 84.71% for methane in methanogenic reactor, a total of 92.41% of substrate was converted to biogas (hydrogen and methane) in the two-phase anaerobic fermentation process. For comparison, a total of 87.96% of substrate was converted into biogas in integrated two-phase process by Wang and Zhao [12].
3.3.
40.2 L/d. The coproduction of hydrogen and methane from sugary wastewater markedly increased the bioenergy recovery from 2.73 W only in hydrogen production to 19.37 W (Table 2). Li et al. [30] previously obtained a bioenergy recovery of 9.43e19.02 W from rice, potato, lettuce, and kitchen as well as paper wastes by two-phase fermentation for the coproduction of hydrogen and methane. In comparison, a higher bioenergy recovery was achieved by combining the hydrogen fermentation of wastewater and methane fermentation of hydrogen fermentation effluents. According to simple mass balance at optimum condition (HRTH: 5 h, HRTMet: 15 h), 1 L of sugary wastewater (COD 26.7 g L1) could produce 4.9 L hydrogen-rich biogas with hydrogen content of about 60.2% and 8.0 L methane-rich biogas with methane content of about 68.9% by two-phase anaerobic fermentation process. In conclusion, sugary wastewater was a promising substrate for simultaneous hydrogen and methane coproduction using two-phase anaerobic fermentation system. The low bioenergy recovery from hydrogen fermentation is a major bottleneck for the cogeneration of hydrogen and methane that must be improved to commercial advancement, however, the selection of carbohydrate-rich actual wastewaters was a vital process for improved bioenergy recovery from the coproduction of hydrogen and methane with anaerobic fermentation. To date, there are still various kinds of wastewaters that remain unexplored for hydrogen and methane production process, such as oil industry wastewaters having relatively low pH [31]. The concept of combined wastewaters may lead to a new path for hydrogen and methane coproduction. For example, Huang [32] could practically achieve a higher biohydrogen yield by combining textile and food wastewaters. In addition, a combination of solid organic wastes and wastewater could also be a novel approach for hydrogen and methane production [1].
Bioenergy recovery and mass balance
4. At optimum conditions (HRTH: 5 h, HRTMet: 15 h), the hydrogen production of 21.6 L/d was obtained from the two-phase hydrogen fermentation of sugary wastewater in ACSTR and effluents from hydrogen fermentation were converted into methane in UASB reactor with methane production of
Table 2 e Bioenergy recovery from hydrogen and methane coproduction in the two-phase system. Hydrogen, methane production, energy recovery, conversion rate of substrate into biogas COD concentration (g L1) Optimum HRT (h) Hydrogen production (L/d)a Methane production (L/d)a Bioenergy recovery in hydrogen production (W)b Total bioenergy recovery (W)b Conversion rate of substrate (%)
Two-phase “ACSTRHeUASBMet” system 6/3.69 5/15 21.6 40.2 2.73 29.37 92.41
a Hydrogen and methane production were adjusted at standard pressure and temperature, i.e. 1 atm and 0 C. b Energy density: 242 kJ/mol of hydrogen, 801 kJ/mol of methane.
Conclusion
Hydrogen and methane were simultaneously generated from sugary wastewater by the two-phase system composed of anaerobic continuous tank stirred reactor (ACSTR) and upflow anaerobic sludge reactor (UASB) in this study. The effects of the stepwise decreased HRT on this two-phase system were investigated. Experimental data obtained at optimum condition (HRTH: 5 h, HRTMet: 15 h) illustrated that sugary wastewater was a promising carbohydrate-rich substrate for the coproduction of hydrogen and methane through anaerobic fermentation and also established the most important operation parameters guideline for future industrial application.
Acknowledgments Financial support was from public welfare research program (the early warning technology of environmental risk about the influence of pharmaceutical wastewater on environmental microbiology), Ministry of Environmental Protection, China (Grant No.200909043).
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