Hydrogen production from beverage wastewater via dark fermentation and room-temperature methane reforming

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Hydrogen production from beverage wastewater via dark fermentation and room-temperature methane reforming Chiu-Yue Lin a,b,c,*, Hoang-Jyh Leu a,b,d, Keng-Huan Lee b a

Green Energy Development Center, Feng Chia University, Taiwan Master's Program of Green Energy Science and Technology, Feng Chia University, Taiwan c Department of Environment Science and Engineering, Feng Chia University, Taiwan d Center for General Education, Feng Chia University, Taiwan b

article info

abstract

Article history:

Direct and indirect bio-hydrogen generation via mesophilic (35
C) dark fermentation and

Received 21 April 2016

room-temperature (25
C) methane reforming, respectively, were studied to maximize bio-

Received in revised form

hydrogen production from a wastewater in an environment-friendly way. In lab-scale dark

5 July 2016

fermentation, beverage wastewater substrate was used for bio-hydrogen production and

Accepted 5 July 2016

its effluent was fed into a bio-methane producing reactor. In methane reforming, a lab-

Available online xxx

made plasma-assisted methane-reformer using a steel-silk catalyst was used. A peak hydrogen production rate (HPR) of 20 L H2/L-d was obtained at hydraulic retention time

Keywords:

(HRT) 8 h, pH 5.6 and substrate concentration 40 g total sugar/L. A peak methane pro-

Beverage wastewater

duction rate of 12 L CH4/L-d was obtained for the methane-fermenter fed on the hydrogen-

Bio-hydrogen

fermenter effluent with an influent beverage wastewater concentration of 30 g total sugar/

Bio-methane

L. The methane reforming had a methane conversion rate of 12.7% and an HPR of 132 L H2/

Dark fermentation

L-d. The hydrogen gas produced from both direct and indirect ways did not contain CO.

Plasma-assisted methane-reformer

Moreover, the present reforming method had an efficiency of 0.0048 L/min-W indicating it

Steel-silk catalyst

is comparable to other methods. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Bioenergy of bio-hydrogen and bio-methane produced from organic wastes/wastewaters via anaerobically biological conversion is attractive for their sustainable characteristics. Hydrogen fermentation is promising because this process is more environment-friendly comparing with chemical processes. H2 fermentation generates liquid metabolites such as alcohols, acetate, propionate and butyrate that are readily to convert anaerobically into methane. This simultaneous

hydrogen and methane production method is called as twophase process or two-stage process [9] or “HyMeTek” process with high hydrogen production rate [16]. To meet the hydrogen demand of “hydrogen society era”, stable and enough amount of hydrogen supply is important. To maximize the hydrogen production via dark fermentation, the methane produced in this two-stage fermentation could be reformed into hydrogen via reforming methods. There are many hydrogen-generation methane-reformers, such as (1) plasma reformer, (2) high-temperature catalytic

* Corresponding author. Department of Environment Science and Engineering, Feng Chia University, Taichung 40724, Taiwan. Fax: þ886 4 35127114. E-mail address: [email protected] (C.-Y. Lin). http://dx.doi.org/10.1016/j.ijhydene.2016.07.028 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Lin C-Y, et al., Hydrogen production from beverage wastewater via dark fermentation and roomtemperature methane reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.028

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steam reformer, (3) non-thermal plasma reformer, and (4) non-thermal plasma-assisted catalysis reformer [4,19]. Lowtemperature plasma-assisted reforming method operating at low pressure is an energy-saving process comparing with well-used hot/thermal plasmas or high-temperature catalytic methods (which having high-temperature requirements of 300e700
C). Moreover, plasmas-assisted catalytic methanereforming hydrogen generation systems have higher hydrogen yield, hydrocarbon conversion rate, selection rate and energy efficiency [5,6,8]. For the catalysts, though the activity is low but it is cost-effective to use some non-precious metals such as Fe, Co, Ni and Cu. Based on the consideration of utilizing waste materials as feedstock, low energy-consumption technology and cost effective methane-reforming method were used in increasing hydrogen production. Beverage wastewater, therefore, was used as the substrate to produce bio-hydrogen and biomethane; low-temperature plasma-assisted and steel silkcatalytic reforming method was also applied to the biomethane for increasing the hydrogen production.

Materials and methods Dark fermentation for hydrogen and methane productions Anaerobic granule sludge collected from a wastewater treatment plant (up-flow anaerobic sludge blanket, UASB) of a fructose industry was used as seed inoculums for hydrogen and methane fermentations. Table 1 summarizes the characteristics of the seed sludge with pH, total solids and volatile suspended solids (VSS) concentrations of 7.8, 42,180 mg/L and 38,540 mg/L, respectively.

the reactor for 3 days. The substrate wastewater was collected from a beverage factory (producing mainly juices) located at Taoyuan County (in north Taiwan). The inorganic nutrients were (mg/L, Endo et al., [31]): 5240 NH4HCO3, 125 K2HPO4, 100 MgCl2$6H2O, 15 MnSO4$6H2O, 25 FeSO4$7H2O, 5 CuSO4$5H2O, 0.125 CoCl2$5H2O, 6720 NaHCO3. The characteristics of the raw substrate beverage wastewater were pH 3.25, oxidationreduction potential (ORP) 267 mV, soluble chemical oxygen demand (COD) 730e760 g/L, soluble carbohydrate concentration 780e820 g/L and NH3eN 150 mg/L. Determination of optimal organic loading rate: To obtain an optimal organic loading rate for peak hydrogen production, the substrate wastewater was tested at total sugar concentrations of 10, 20, 30 and 40 g/L and the reactor was named as H1, H2, H3 and H4, respectively (Table 2). The wastewater substrate concentration was increased when the reactor reached a steady-state condition and the experimental data were collected. Biogas amount, hydrogen concentration (%), hydrogen gas amount, biomass concentration (VSS), total sugar concentration, soluble COD (SCOD) and liquid metabolite concentrations were determined. For each substrate concentration, a steady-state condition reached when the variations of indicator parameter values (hydrogen concentration, hydrogen gas amount, biomass concentration) were less than 10%. Moreover, during our experiments, the determination of hydrogen concentration was conducted in triplicate because it is the most important parameter indicating biohydrogen production. Connection of H2 and CH4 fermenters: The hydrogen fermenter effluent of each wastewater substrate concentration (H1, H2, H3 and H4) was introduced separately into a methane-fermenter for methane production.

Methane production Hydrogen production Bioreactor, seed and substrate: Continuous hydrogen production (Fig. 1(a)) was operated in a CSTR (completely-stirred tank reactor) bioreactor (2.5 L, with a working volume of 2 L) seeded with an anaerobic granular sludge collected from a food wastewater treatment plant. The collected anaerobic sludge was heated at 95
C for 1 h to dominate hydrogenproducing bacteria and inactivate methane-consuming microflora. The bioreactor was operated at hydraulic retention time (HRT) 8 h, pH 5.6 ± 0.1 and temperature 35 ± 1
C (Table 2). To start-up the reactor, seed sludge and substrate beverage wastewater (total sugar concentration 20 g/L) were mixed in a volumetric ratio of 1:3; then they were cultivated in

Table 1 e Characteristics of the seed sludge for dark fermentation. Items pH Total solids (TS, mg/L) Volatile suspended solids (VSS, mg/L) Total chemical oxygen demand (TCOD, mg/L) Soluble COD (SCOD, mg/L) Total carbohydrate (mg/L) Soluble carbohydrate (mg/L) NH3eN (mg/L)

Values 7.8 42,180 38,540 78,500 80 3040 17 4670

Bioreactor, seed and substrate: Continuous methane production (Fig. 1(b)) was operated in an agitated granular sludge bed bioreactor (2 L, with a working volume of 1.5 L) having a thin iron plate (with a pore size of 1 mm) inside to keep biomass sludge. The seed sludge was an anaerobic granular sludge collected from a food wastewater treatment plant and did not have heat-pretreatment. The substrates used were the effluents from the hydrogen-fermenter operating at different total sugar concentrations of 10 (H1), 20 (H2), 30 (H3) and 40 g/L (H4) and the methane-fermenter was named as M1, M2, M3 and M4, respectively (Table 2). M1, M2, M3 and M4 were operated at pH 7.2 ± 0.1 and temperature 35 ± 1
C with varying HRT. 2 N NaOH solution was used for pH adjustment. No extra nutrients were added to the substrates. Determination of optimal operation conditions: To obtain an optimal operation condition for peak methane production, M1, M2, M3 and M4 were individually operated at the HRTs of 72, 36, 18 and 12 h in a gradual way (Table 2). The HRT of the methanefermenter was reduced when the reactor reached a steady-state condition and the experimental data were collected. For each operating HRT, a steady-state condition reached when the variations of indicator parameter values (methane concentration, methane gas amount, biomass concentration) were less than 10%. To start-up the methane-fermenter, a mixture of seed sludge (500 mL) and substrate (hydrogen-fermenter effluent, 1 L, 25 g COD/L) were used; then it was cultivated for 3 days. The

Please cite this article in press as: Lin C-Y, et al., Hydrogen production from beverage wastewater via dark fermentation and roomtemperature methane reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.028

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4 oC

Substrate In-flow pumb

pH ORP

2N NaOH

o

35 C

Gas/liqued Separator

Hydrogen Fermentation Water Reactor Circulation Pump

Heater

A

pH, Temp. Controller

pH

Mixing Out-flow Tank

Gas meter pH ORP

pH

Circular Holes Iron 35 oC

Hydrogen Fermentation Reactor

Mixing Tank

Water Circulation Pump

In-flow pumb Methane digestion reactor Out-flow

Heater

20 rpm

2N NaOH

pH, Temp., Rotation Speed Controller

Out-flow

Fig. 1 e Schematic diagrams of dark fermentation: (a) bio-hydrogen and (b) bio-methane production systems.

integration system of hydrogen- and methane-fermenters was operated in a continuous feeding mode. For each substrate concentration, a steady-state condition reached when the variations of indicator parameter values (methane concentration, methane gas amount, biomass concentration) were less than 10%. Moreover, during our experiments, the determination of methane concentration was conducted in triplicate because it is the most important parameter indicating bio-methane production.

Methane reforming for hydrogen production Lab-made reformer: A dielectric barrier discharge (DBD) based reactor was used in plasma-assisted methane reforming experiments (Fig. 2). This lab-made reformer reactor consisted of a silk steel-catalyst and a glass tube with a copper-foil serving as the inner and outer electrodes [13]. The inner and outer electrodes connected to a high voltage power generator and grounded, respectively. The discharge region length was

50 cm with a discharge gap of 2 mm. The DBD reactor had an AC power supply (2.5 W, lab-made) with a peak-to-peak voltage of 40 kV and an optimized frequency of 33 kHz. Catalyst was packed directly in a glass tube as a single-stage plasma-catalytic system. The catalyst packing gave a reasonable thermal conduction, allowing the outer electrode temperature to approximately equal to the ambient temperature. This thermal conduction was measured by a K-type thermocouple attached to the outer electrode. This plasma system was operated at room-temperature. Methane reforming: The catalyst used was a commercial silk steel containing 6.9% carbon (w/w), oxygen 3.0% and iron 90.1% in the form of 5  4 mm pellet. Before being packed this catalyst pellet was crushed into non-uniform particle (18.6 g, 0.85e5 mm). The bio-methane gases collected from the methane-fermenter were used directly at 50 mL/min and 1 bar without any pretreatments (Table 2). For comparing the reforming performance, a synthetic pure methane gas was used.

Please cite this article in press as: Lin C-Y, et al., Hydrogen production from beverage wastewater via dark fermentation and roomtemperature methane reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.028

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Table 2 e Experimental conditions for direct (dark fermentation) and indirect (bio-methane reforming) bio-hydrogen productions. Item

Influent/Inflow gas

Temperature (
C) pH HRT (h) Inflow rate (mL/h)

Dark fermentation

Methane reforming

H2-fermenter

CH4-fermenter

Beverage wastewater, Total sugar 10e40 g/L (H1, H2, H3, H4) 35 6.7 8 e

H2-fermenter effluents (M1, M2, M3, M4)

Bio-methane from CH4-fermenter and pure methane gas

35 7.2 72, 36, 18, 12 e

Ambient, 25 e e 45, 90, 180, 360, 540, 720, 900

Fig. 2 e Schematic diagram of room-temperature plasmaassisted methane-reformer.

Analysis Hydrogen, methane, nitrogen and carbon dioxide concentrations in the biogas were determined with a Shimadzu (Japan) GC-14A gas chromatograph that was equipped with a thermal conductivity detector (stainless column, 55
C; injection temperature, 90
C; carrier gas, Ar; packing, Porapak Q; integrator, Shimadzu C-R3A Chromatopac). Volatile fatty acids (VFAs) concentrations were determined with a flame ionization detector (glass column, 145
C; injection temperature, 175
C; carrier gas, N2; packing, FON; integrator, Shimadzu C-R6A Chromatopac). Sugar concentration was determined using Anthrone-sulfuric acid method. Other water quality parameters (pH, oxidation-reduction potential (ORP), COD and VSS) were measured according to the procedures described in Standard Methods [1]. For gas component analysis (hydrogen and methane concentrations in %), triplicate was conducted and their average values were shown. Moreover, the CSTR bioreactors were operated to reach steady-state conditions for periods giving less variations (lower than 10%) for H2 and CH4 concentrations, H2 and CH4 gas amount, VSS concentration, ORP and pH values.

Results and discussion Hydrogen fermentation Dark fermentation is a common method to produce biohydrogen and bio-methane. The present work showed that bio-hydrogen production progressed at 8 h HRT and influent beverage wastewater concentrations of 10e40 g total sugar/L (Fig. 3). From the trends of daily variations of pH, ORP,

hydrogen gas concentration, hydrogen production rate (HPR) and utilization rate of total sugar for each tested beverage wastewater concentration (Fig. 3), it is known that the variations of these parameter values were dependent on the substrate concentrations. In Fig. 3, the data of hydrogen concentration (%) were average values with coefficients of variations ranging from 0.2 to 3%. With increasing substrate concentrations, ORP, H2 concentration and HPR increased but total sugar utilization decreased. Table 3 summarizes the monitoring parameter values when the fermenter reached steady-state conditions at 8 h HRT. ORP values varied from 420 to 455 mV, which was in the ranges of favoring fermentative hydrogen production (Kumar et al., [32]). Hydrogen concentration increased with the increasing substrate concentrations when beverage wastewater concentrations were lower than 30 g total sugar/L, with values of 39.1, 44 and 48% at 10, 20 and 30 g total sugar/L, respectively. However, higher beverage wastewater concentration (40 g total sugar/L) did not enhance hydrogen concentration (it was around 48%). On the other hand, HPR increased linearly with increasing substrate and biomass concentrations (Table 3). This fact agreed with the results obtained from another beverage wastewater study [17]. Noted that peak HPR of 20 L H2/L-d was obtained at 40 g total sugar/L but peak hydrogen yield (HY) of 1.53 mol H2/mol hexose was observed at 20 g total sugar/L. This HPR value is higher than that of a CSTR system using low strength beverage wastewater (5 g COD/L, HPR 11.39 L H2/L-d; [18]) but lower than that of an immobilized-cell system (an organic loading of 320 g/L-d hexose equivalent, HPR 55 L H2/Ld; [23]). Moreover, in the present work the variation trend of HPR values agreed with the variation of VSS concentration, indicating that high HPR might result from high biomass concentration. In this hydrogen production operation, the biogas contained hydrogen 48% and carbon dioxide 52%. There was no carbon monoxide or methane in the biogas when the reactor reached steady-state conditions. Fermentative hydrogen production accompanies with producing liquid metabolites such as alcohols and organic acids (VFAs). Table 4 summarizes metabolite concentrations when the H2 fermenter reached various steady-state conditions. Except butanol, the concentrations of other liquid metabolite components obviously increased with increasing total sugar concentrations in the feedstock. Butanol concentrations were 34e104 mg COD/L for all the test runs. Table 4 indicates that butyrate was the major liquid metabolite component with concentrations high to 12.6e33 g COD/L.

Please cite this article in press as: Lin C-Y, et al., Hydrogen production from beverage wastewater via dark fermentation and roomtemperature methane reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.028

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20 g

10 g

5.8

40 g

30 g

pH

5.7

pH

5.6 5.5 5.4 ORP

ORP(-mv)

480 450 420 390

H2 Conc.(%)

40 30 20 24 21 18 15 12 9 6 3

HPR(L/L-d) Total Sugar Utilization(%)

H2 Conc.

50

HPR

100 95 Total Sugar Utilization

90 85 80 0

10

20

30

40

50

60

70

80

Time(day) Fig. 3 e Daily variations of fermentative bio-hydrogen production at various influent total sugar concentrations (g/L). Moreover, the concentration ratios of butyrate to acetate ranged from 7 to 8, indicating the hydrogen fermentation was of a butyrate-type. Both results indicate the prospective hydrogen production characteristic of this beverage

wastewater. Many reports have shown that high butyrate concentration fraction in liquid metabolite and high concentration ratio of butyrate to acetate favor fermentative hydrogen production [17].

Table 3 e Direct bio-hydrogen production via dark fermentation at HRT 8 h and various wastewater concentrations (under steady-state conditions). Total sugar (g/L) H1 H2 H3 H4 a

pH 10 20 30 40

7.23 7.23 7.22 7.18

± 0.02 ± 0.02 ± 0.02 ± 0.04

ORPa (-mv) 500 491 490 483

±3 ±4 ±3 ±2

HPR (L/L-d) 5.0 ± 0.5 12.3 ± 0.6 15.4 ± 0.5 20.0 ± 0.7

H2 conc. (%) 39.1 44.0 48.0 48.2

± 1.5 ± 1.4 ± 0.8 ± 0.6

HY (mol H2/mol hexose) 1.24 ± 1.53 ± 1.31 ± 1.34 ±

0.4 0.7 0.4 0.6

Total sugar utilization (%) 99.1 ± 98.3 ± 96.8 ± 91.7 ±

0.3 0.5 0.8 1.0

VSS (g/L) 2.2 4.9 7.3 9.4

± 0.3 ± 0.4 ± 0.2 ± 0.3

ORP, oxidation-reduction potential; HPR, hydrogen production rate; HY, hydrogen yield; VSS, volatile suspended solids.

Please cite this article in press as: Lin C-Y, et al., Hydrogen production from beverage wastewater via dark fermentation and roomtemperature methane reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.028

Table 4 e Liquid metabolites in direct bio-hydrogen production via dark fermentation at HRT 8 h and various wastewater concentrations (under steady-state conditions). Total sugar (g/L)

Ethanol

Butanol

HAc*

HPr*

HBu*

HVa*

mg COD/L H1 H2 H3 H4

10 20 30 40

399.6 ± 119.1 687.6 ± 158.1 971.8 ± 88.4 1656.8 ± 214.0

34.3 ± 8.9 103.6 ± 14.0 80.4 ± 17.6 58.6 ± 21.4

1667.5 2493.0 4645.4 4701.8

± 110.9 ± 405.7 ± 724.7 ± 906.6

577.3 ± 106.2 674.5 ± 174.8 994.0 ± 174.2 1657.4 ± 157.0

SMP* g COD/L

12,616.2 19,102.8 24,974.5 32,938.8

± 549.3 ± 2807.4 ± 1479.3 ± 2074.2

110.7 ± 20.0 332.6 ± 15.4 862.8 ± 154.0 1722.8 ± 51.6

15.8 25.6 35.1 43.7

± 1.2 ± 1.5 ± 2.2 ± 2.7

*

HAc, acetate; HPr, propionate; HBu, butyrate; HVa, valerate; SMP, soluble metabolite product.

Fig. 4 e Daily variations of (a) bio-methane production rate and (b) COD removal rate in the CH4-fermenter feeding on H2fermenter effluents of various total sugar (TS) concentrations and HRTs. Please cite this article in press as: Lin C-Y, et al., Hydrogen production from beverage wastewater via dark fermentation and roomtemperature methane reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.028

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Methane fermentation The hydrogen-fermenter operating at influent wastewater concentrations of 10 (H1), 20 (H2), 30 (H3) and 40 (H4) g total sugar/L had effluents with COD concentrations of 15.8 ± 1.2, 25.6 ± 1.5, 35.1 ± 2.2 and 43.7 ± 2.7 g COD/L, respectively. These effluents were further used for the methane fermentations (M1, M2, M3 and M4) (Table 2). Fig. 4(a) depicts the daily variations of methane production rates (MPR) of the methanefermenter fed on the hydrogen-fermenter effluents discharged at various HRTs (12e72 h). The average values of COD removal rate and MPR for the fermenters reached steady-state conditions are summarized in Table 5. MPR was HRTdependent with short HRTs (12 and 18 h) having high MPR values. Short HRTs had high organic loading rates and then enhanced methane production. However, at HRT 12 h the MPR obtained at M4 (receiving 40 g total sugar/L hydrogenfermenter effluent) was lower than that of M1 (receiving 10 g total sugar/L hydrogen-fermenter effluent) (Fig. 4). This might result from the fact that for the methane-fermenter, the shortest HRT of 12 h gave too high organic loading rate and then reduced the COD removal efficiency (71% vs. 35%). Based on the above results of hydrogen and methane fermentations, for this beverage wastewater feedstock, 30 g total sugar/L was considered as the critical substrate concentration value giving high hydrogen and methane productions at 12 h HRT. Our previous studies also indicated that 20e30 g total sugar/L were optimal influent concentrations for giving high HPR [11]. In this methane production operation, the biogas contained methane 70% and carbon dioxide 30% with no carbon monoxide. For the purpose of efficiently reforming methane into hydrogen, the inflow methane concentration was targeted at >75% in the biogas with a substrate COD removal efficiency of >80% during methane fermentation. Fig. 4(b) indicates that for M3 (receiving H3 effluent) at HRT 18 h, the average methane gas

Table 5 e Performance of methane-fermenter under steady-state conditions. Methane fermenter M1 (H1, 10 g/L)a

M2 (H1, 20 g/L)

M3 (H1, 30 g/L)

M4 (H1, 40 g/L)

a

HRT (h)

MPR (L/L-d)

72 36 18 12 72 36 18 12 72 36 18 12 72 36 18 12

1.5 ± 0.2 2.6 ± 0.4 4.3 ± 0.7 6.2 ± 0.2 3.7 ± 0.6 5.6 ± 0.4 8.6 ± 0.4 10.2 ± 0.3 3.6 ± 0.5 6.2 ± 0.6 10.1 ± 0.5 11.5 ± 0.3 4.0 ± 0.8 5.5 ± 0.4 6.3 ± 0.6 3.8 ± 0.4

COD removal rate (%) 97.2 88.4 84.5 72.5 97.1 94.2 84.3 72.2 91.0 89.5 82.2 78.5 96.4 73.4 67.4 35.1

± 2.8 ± 8.1 ± 8.5 ± 13.1 ± 3.1 ± 12.3 ± 16.3 ± 20.6 ± 24.2 ± 6.2 ± 15.5 ± 18.6 ± 3.5 ± 19.4 ± 3.6 ± 7.0

Total sugar concentration in the influent for hydrogenfermenter.

7

concentration was 75.1% (with a COD removal rate of 82.8%). Therefore, this biogas was introduced into the plasma system for producing hydrogen (being discussed in next session).

Hydrogen production from methane reforming The performance of the DBD-based plasma-assisted methane reformer was enhanced via catalyst addition in the reactor. Fig. 5 depicts the variations of conversion rate (converting methane into hydrogen), hydrogen concentration (%) in the reformed product stream and HPR (production flow rate/ reactor volume, Q/V) at various methane gas inflow rates (45e900 mL/h) in reforming pure methane gas and the biomethane gas collected from the methane-fermenter M3 operating at HRT 18 h. In Fig. 5, the data of hydrogen and methane concentrations (%) were average values with coefficients of variations ranging from 0.4 to 4%. The experimental data indicate that the bio-methane gas could be efficiently reformed to produce hydrogen in a way like that of pure methane gas. Fig. 6 shows that in a continuous flow operation of reforming, when the bio-methane gas was introduced into the reformer, conversion rate, hydrogen concentration and HPR increased markedly during 80 min and then reached stable values. For the reformed product hydrogen, its HPR increased with the increasing methane inflow rate and had a peak value of 132 L H2/L-d being obtained at methane inflow rate 720 mL CH4/h. A higher inflow rate of 900 mL CH4/h did not enhance HPR, which might result from the fact that high inflow rate gave a too short reaction time and then resulting in an incomplete conversion. Moreover, an increasing methane inflow rate resulted in decreasing values of hydrogen concentration and methane conversion rate [a methane concentration ratio, ¼ (inflow concentration e outflow concentration)/(inflow concentration)]; a peak conversion rate of 32.9% occurred at the flow rate of 45 mL CH4/h. In the effluent gas, the methane concentration reached 68% and CO concentration was undetectable. This effluent gas could be applied for reforming again by another reactor to increase the values of total conversion rate and hydrogen concentration. In Fig. 6, the data of hydrogen and methane concentrations (%) were average values with coefficients of variations ranging from 0.2 to 3%. Comparing with conventional steam reforming of high temperature method, plasmabased dry reforming technologies are one of the most popular non-conventional room-temperature approaches [10]. In the present work, room-temperature methane reforming gave peak conversion rates of around 25e30% with very less energy consumption. Table 6 summarizes the dry reforming reactions reported in literature using different types of plasmas with reforming efficiencies (ratios of flow rate to power) ranging 0.0005e0.0234 L/min-W. The present work had a reforming efficiency of 0.0048 L/min-W indicating that the present room-temperature plasma-assisted reforming method using a steel-silk catalyst is quite comparable to other methods (see Table 6).

Significances of the experimental data In the present work, a methane reactor was successfully operated with receiving effluents from a hydrogen reactor

Please cite this article in press as: Lin C-Y, et al., Hydrogen production from beverage wastewater via dark fermentation and roomtemperature methane reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.028

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6 Pure CH4

HPR(mL/mL-hr)

5

Bio CH4

4 3 2 1 0

Hydrogen Conc.(%)

25 20 15 10 5 0

Methane Conc.(%)

80

60

40

20

Conversion Rate(%)

40 0

30

20

10

0 0

180

360

540

720

900

Inflow Rate(mL/hr)

Fig. 5 e Relationships between methane conversion rate, methane concentration, hydrogen concentration, HPR and methane inflow rates in methane reforming using pure and bio-methane.

feeding on a real sugar-contained beverage wastewater at various operating HRTs. Both the hydrogen- and methanefermenters had been operated at their optimal conditions separately to give peak hydrogen or methane production rates. This indicates that dark fermentation of sugarcontaining wastewater using a two-stage (producing H2 and CH4) anaerobic process to produce bio-hydrogen and biomethane is feasible. Recently, this two-stage anaerobic process has been reported to produce more energy (8e43%) than that of a conventional one-stage process (producing CH4) [21].

Moreover, from the view point of mass balance and practical practices it is known that hydrogen and methane fermentation enhances COD removal efficiency and sludge reduction in wastewater treatment. Since a beverage factory needs energy, therefore, it could use its sugar-containing wastewater to produce more energy via a two-stage anaerobic process (producing H2 and CH4) for its usage and simultaneously lower the cost of aerobic wastewater treatment. A mixture of H2 and CH4 is a hythane gas that having high combustion efficiency [28]. This gas had been shown

Please cite this article in press as: Lin C-Y, et al., Hydrogen production from beverage wastewater via dark fermentation and roomtemperature methane reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.028

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9

6 45 mL/hr 90 mL/hr 180 mL/hr 360 mL/hr 540 mL/hr 720 mL/hr 900 mL/hr

HPR(mL/mL-hr)

5 4 3 2 1 0

Hydrogen Conc.(%)

25 20 15 10 5

Bio-methane(mL/hr)

0

600

400

200

Conversion Rate(%)

0

30

20

10

0 0

20

40

60

80

100

120

Reaction Time(min) Fig. 6 e Time variations of methane conversion rate, hydrogen concentration and HPR during methane reforming at various methane inflow rates.

to have the most combustion efficiency in a mixture ratio of H2:CH4:CO2 ¼ 16.5%:44.8%:38.7% [20]. However, most of the commercialized hythane gas is generated from natural gas and is not a green one. Bio-hythane obtained from mixing H2 and CH4 that produced via dark fermentation is a green biofuel [3,12]. The present work gave biogas productions of H2 48% þ CO2 52% and CH4 70% þ CO2 30% in H2 and CH4 fermentation, individually, at their peak production rates. Therefore, these biogases could be easily

mixed together to generate a green bio-hythane in any content ratios. In the room-temperature methane reforming for hydrogen production, an HPR of 132 L H2/L-d (5.5 mL H2/mL-h) was obtained at a methane inflow rate of 720 mL CH4/h. Since peak HPR 20 L H2/L-d was obtained from the hydrogen-fermenter, an increment of energy recovery could be evaluated for this two-stage bioenergy production system treating sugarcontaining wastewater. An 15% (20 L H2/L-d ÷ 132 L H2/L-d)

Please cite this article in press as: Lin C-Y, et al., Hydrogen production from beverage wastewater via dark fermentation and roomtemperature methane reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.028

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Table 6 e Comparison of reforming efficiency of plasma-based technologies. Plasma/Catalyst DBD/Ni/g-Al2O3 DBD/Ni/Al2O3 DBD/Ni/Al2O3 DC thermal plasma/Ni/Al2O3 DBD Gliding arc discharge Kilohertz spark-discharge Glow discharge plasma DC-pulsed plasma Arc-jet plasma DBD/steel silk catalyst

Flow rate (L/min)

Power (W)

Conversion (%)

Efficiency L/min W

References

0.025 0.05 0.25 36.7 0.5 12.7 0.15 1 0.135 4 0.012

60 38.4 65 9600 500 544 503 69 135 1000 2.5

50 60 28 88 35 40 75 95 55 50 32.9

0.0005 0.0013 0.0039 0.0039 0.0010 0.0234 0.0003 0.0145 0.0010 0.0040 0.0048

[25] [26] [27] [24] [29] [2] [30] [14] [22] [7] This work

increment in hydrogen production was obtained, which agrees with the observation in energy increment (8e43%) comparing to only methane production [21]. Carbon monoxide (CO) is the most poisonous gaseous material for proton exchange membrane fuel cells (PEMFC) and should be removed from the influent gas during electricity generation via hydrogen-based fuel cell application [15]. In the present work, the biogas produced in the hydrogen-fermenter giving peak HPR contained H2 48% and CO2 52% (without CO); the biogas produced in the methane-fermenter giving peak MPR contained CH4 70% and CO2 30% (also without CO). Moreover, the room-temperature plasma-assisted methane reforming system also did not produce CO. In other words, these biogas and reformed gas products did not contain CO. These results indicate the applicable characteristics of the biohydrogen gas produced from sugar-containing beverage wastewater via direct dark fermentation or indirect methane reforming for PEMFC electricity generation. On the other hand, this work used waste, low cost materials and low energy-consumption process. They included (1) beverage wastewater as substrate in bio-hydrogen production following bio-methane production without nutrient addition, (2) room-temperature and atmosphere pressure operation in plasma-assisted methane reforming, and (3) non-noble metal steel-silk containing 90% iron as a catalyst material in methane-reforming. These facts make the bio-hydrogen production processes more cost-effective and environmentally friendly.

Conclusions A continuous-feeding lab-scale system of dark bio-hydrogenfermentor connecting to a bio-methane fermentor feeding on the hydrogen-fermenter effluent was mesophilically operated to treat a sugar-containing beverage wastewater for maximizing bio-hydrogen production. In the fermentative biohydrogen production, peak HPR of 20 L H2/L-d was obtained at HRT 8 h, pH 5.6 and substrate concentration of 40 g total sugar/L. Hydrogen production from the bio-methane via plasma-assisted methane-reformer using a steel-silk catalyst at room temperature had a methane conversion rate of 12.7% and an HPR of 132 L H2/L-d. Moreover, the present reforming method was comparable to other methods from having a reforming efficiency of 0.0048 L/min-W.

Acknowledgments The authors gratefully acknowledge the financial support provided by Taiwan's Ministry of Science and Technology (MOST 102-2221-E-035-002-MY3 and MOST 103-2923-E-035001-MY3).

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