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Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion
international journal of hydrogen energy xxx (xxxx) xxx
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Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by twostage anaerobic digestion Kanathip Promnuan a, Takaya Higuchi b, Tsuyoshi Imai b, Prawit Kongjan c, Alissara Reungsang d,f, Sompong O-Thong a,e,* a
Biotechnology Program, Faculty of Science, Thaksin University, Phatthalung 93210, Thailand Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Yamaguchi 755-8611, Japan c Department of Science, Faculty of Science and Technology, Prince of Songkhla University, Pattani 94000, Thailand d Department of Biotechnology, Faculty of Technology, Khon Kaen University, Khon Kaen 40002, Thailand e Research Center in Energy and Environment, Faculty of Science, Thaksin University, Phatthalung 93210, Thailand f Research Group for Development of Microbial Hydrogen Production Process from Biomass, Khon Kaen University, Khon Kaen 40002, Thailand b
highlights The SRB consortium was effective for sulfate removal and hydrogen production. The sulfate removal and hydrogen yield from RSW was 87% and 101 mL H2$gVS1. The COD/SO2 4 ratio of 13 and pH 6 was suitable for hydrogen production from RSW. Trace element addition increased CH4 production rate for 2 times.
article info
abstract
Article history:
The sulfate-reducing bacteria (SRB) consortium (Desulfovibrio sp. Desulfitibacter sp. Dethio-
Received 5 July 2019
sulfatibacter sp. and Clostridium sp.) was investigated for sulfate removal and biohythane
Received in revised form
production from rubber sheet wastewater (RSW). The RSW at COD/SO4 2 ratio of 13 and pH
18 September 2019
6 was suitable for sulfate reduction and biohythane production. Hydrogen yield, sulfate
Accepted 26 October 2019
removal efficiency, and methane yield were 101 mL H2$gVS1, 87%, and 629 mL CH4$gVS1,
Available online xxx
respectively. The hydrogen and methane production rate was 14.53 mL H2$d1and 45 mL
Keywords:
drolysis rate (kh) from 0.19 d1 to 0.23 d1 and methane production rate from 31e48 to 42
COD/SO2 4 ratio
e97 mL CH4$d1. The COD removal of RSW in acidogenic stage and methane stage was
Biohythane production
11.3% and 72.4e76.2%, respectively. The sulfate removal of RSW in acidogenic stage and
Rubber sheet wastewater
methane stage was 87% and 9%, respectively. Two-stage anaerobic fermentation obtained
Trace element availability
simultaneous biohythane production and sulfate removal with efficiently for energy re-
Two-stage anaerobic digestion
covery from RSW.
CH4$d1, respectively. Hydrogen effluent with trace element supplemented improved hy-
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Biotechnology Program, Faculty of Science, Thaksin University, Phatthalung 93210, Thailand. E-mail address: [email protected] (S. O-Thong). https://doi.org/10.1016/j.ijhydene.2019.10.237 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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Introduction High sulfate content wastewater is frequently found in latex and rubber processing industries. Rubber processing wastewater has a pH of 3.7e5.5, chemical oxygen demand (COD) concentration of 3,500e14,000 mg$L1 and sulfate concentration of 500e2,000 mg$L1 [1]. The wastewater from rubber sheet production has a pH of 5.0e5.9, COD concentration of 1,928e15,069 mg$L1 and sulfate concentration of 136e472 mg$L1 [2]. The concentrated rubber latex wastewater has a pH of 3.6e4.7, COD concentration of 5,430 mg$L1, sulfate concentration of 1,819 mg$L1, and ammonia nitrogen concentration of 733.12 mg$L1 [3,4]. The skim latex wastewater has a pH of 4e5, COD concentration of 29,220e35,830 mg$L1 and sulfate concentration of 258e3,580 mg$L1 [5e7]. The rubber processing industries wastewaters are high sulfate concentration resulting in bad smell under the anaerobic condition from the reduction of sulfate to hydrogen sulfide (H2S). The conventional anaerobic wastewater treatment system was not suitable for treated rubber processing industries wastewaters, including rubber sheet wastewater (RSW). The high sulfate in rubber sheet wastewater has negatively effects methanogenic archaea resulting in low COD and sulfate removal efficiency. The two-stage anaerobic digestion process has the potential for separating the acidogenic bacteria and sulfatereducing bacteria from the methanogenesis archaea. The sulfate and hydrogen sulfide (H2S) could remove from the wastewater in the first stage before the methanogenesis in the second stage to avoiding the hydrogen sulfide inhibition and high hydrogen sulfide (H2S) contamination in biogas. Previously report shown that sulfate reduction by sulfatereducing bacteria (SRB) can obtain in the acidogenic stage [8e11]. Hwang et al. [12] demonstrated the synergies effect between sulfate-reducing bacteria and hydrogen-producing bacteria under high sulfate concentration of 20,000 mg$L1 with the H2 production rate of 2.95e9.40 L$d1 and hydrogen yields of 1.6e2.0 mol H2$mol1 glucose. High hydrogen production from sucrose under high sulfate concentration of 3000 mg SO4 2 L1 at low pH of 5.5 by a mixed microbial consortium was reported by Lin and Chen [13]. The SRB can utilize sulfate as an electron acceptor by competing with hydrogen-producing bacteria and methane-producing archaea for carbon sources [14]. The COD/SO4 2 ratio at lower than 0.67 has insufficient for the sulfate reduction [15]. The carbon sources to sulfate ratio (COD/SO4 2 ratio) is a key parameter for balancing the competition between SRB and the other bacteria in the anaerobic digestion process [16]. The high sulfate removal efficiencies of >95% were achieved from wastewater with COD/SO4 2 ratios of 4 and 1 [12]. The fermentative acidogenic bacteria acts on the degradation of organic compounds for conversion to hydrogen, ethanol, and volatile fatty acids (VFAs), which will be used by the SRB to reduce sulfate [17]. However, in the acidification process, the free sulfide has precipitated metal ions that act as functional groups of electron transporter systems and coenzymes required for the growth of microorganisms the methanogenesis stage [18]. However, still lacking knowledge on the effect of COD/SO4 2 ratio in RSW on biohythane production
and trace metals availability for methanogen in the second stage. The effect of COD/SO4 2 ratio in RSW on fermentative hydrogen production, sulfate reduction efficiency, methane production, and biohythane production via two-stage anaerobic digestion was investigated. Trace metals availability in first stage effluent and trace element supplementation into first stage effluent for improved methane production in the second stage was investigated.
Materials and methods Substrate and inoculum The rubber sheet wastewater (RSW) and anaerobic microbial sludge were collected from Yangkaw cooperative rubber sheet plant, Songkhla, Thailand. Rubber sheet wastewater was transported to the laboratory and analyzed for the chemical and physical composition (Table 1). The RSW was stored at 4 C until used. The SRB consortium was enriched from the anaerobic sludge with Postgate medium C [19] following composition of 6.0 g$L1 sodium lactate, 4.5 g$L1 Na2SO4, 1.0 g L1 NH4Cl, 1.0 g$L1 Yeast extract, 0.5 g L1 KH2PO4, 0.3 g$L1 Sodium citrate$2H2O, 0.06 g$L1 CaCl2$6H2O, 0.06 g$L1 MgSO4$7H2O, 0.004 g$L1 FeSO4$7H2O. Postgate medium C was adjusted pH to 7.0 ± 2. The enriched culture was incubated under a temperature of 30 C. The activity of the SRB was confirmed by sulfate removal efficiency, hydrogen production, and hydrogen sulfide production analysis. The microbial community of SRB consortium was analyzed by polymerase chain reaction-denaturating gradient gel electrophoresis (PCR-DGGE) [20]. The biomass concentration of SRB consortium of 40 g$L1 was used as inoculum in the first stage for sulfate reduction and hydrogen production. The anaerobic microbial sludge for methane inoculum was acclimatized with RSW by cultivated on 20% RSW at pH 7.5 (adjusting the pH with NaHCO3) as the substrate. The enriched
Table 1 e Chemical, physical and trace metals composition of rubber sheet wastewater. Parameters pH Chemical oxygen demand (COD) Total solids (TS) Volatile solids (VS) Protein Carbohydrate Lipid Alkalinity SO4 2 COD/SO4 2 ratio Mg Mn Fe Co Ni Cu Zn Mo
Unit (mg$L1) 5.0 6667 4619 2260 5300 1100 150 3000 514 13 89 0.16 0.23 <0.01 <0.01 0.41 7.7 <0.01
Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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methane-producing sludge was incubated for 5 days before used as inoculum to reduce the methane production from the remaining organic compound in sludge. The enriched methane-producing sludge with volatile suspended solid of 80 g$L1 was used as inoculum for methane production.
Biohythane production The two-stage anaerobic digestion for biohythane production from RSW was carried out in a batch test according to Giordano et al. [21]. The RSW with four COD/SO4 2 ratio of 13, 3, 2, and 1 were tested for biohythane production. The COD/SO4 2 ratio of raw RSW was 13. The COD/SO4 2 ratios of 3, 2, and 1 were obtained by adding sodium sulfate (Na2SO4) of 2, 4, and 6 g$L1, respectively. The initial pH of the RSW was adjusted to 7.0, 6.0, and 5.0 by adding sodium bicarbonate (NaHCO3). The 20% v/v of SRB consortium inoculum was added to batch reactors and incubated at mesophilic temperature (35 C) for 7 days. All of the experiment was done in duplicate. The biogas in headspace bottles was collected every 24 h and analyzed for biogas composition by gas chromatography. Methane inoculum was added to bottles after 7 days of incubation at the substrate to inoculum ratio (S:I ratio) of 2:1 based on the VS basis and adjusted the initial pH to 7.5 by adding sodium bicarbonate (NaHCO3). The methane production from first stage effluent was continuously incubated for 14 days at a temperature of 35 C. The biogas in headspace bottles was collected every 24 h and analyzed for the composition. The effect of trace element supplementation into first stage effluent on methane production in the second stage was investigated. The effluent of the first stage was supplemented with 1 mL$L1 of trace element solution preparing as described previously by Angelidaki et al. [22]. The first stage effluent was mixed with methane inoculum at S:I ratio of 2:1 based on the VS basis and adjusted the initial pH to 7.5 by adding sodium bicarbonate (NaHCO3). The methane production was continuously incubated for 14 days at a temperature of 35 C. The biogas in headspace bottles was collected every 24 h and analyzed for the composition.
Analytical methods The COD, sulfate, sulfide, total solids (TS), and VS was determined according to the standard methods [23]. The pH of the wastewater was measured using a pH meter (Horiba D75 Portable). The H2S in biogas was analyzed by gas chromatography (GC-14A, Shimadzu) connected with a flame photometric detector (GC-FPD) and packed column (Rt®XLSulfur Micropacked, Restek). The concentration of H2, N2, CH4, and CO2 in biogas were measured by gas chromatography (GC-8A, Shimadzu) connected with thermal conductivity detector (GC-TCD) and packed column (Shin Carbon, Restek). The volatile fatty acid in the first stage and second stage effluent was measured by gas chromatography (GC17A, Shimadzu) connected with a flame ionization detector (GC-FID) and Stabilwax®-DA fused silica column (30 m 0.53 mm). Biogas volume was monitored daily by water displacement according to O-Thong et al. [24]. The metals composition was analyzed by ICP-AES (PerkinElmer, Optima 3300DV).
Data calculation The kinetic values of hydrogen and methane production from RSW by two-stage anaerobic digestion process were predicted from the Gompertz equation as following Eq. (1). ðl tÞ yt ¼ ym $exp exp Rmax $ e $ þ1 ym
(1)
where yt is the accumulation hydrogen or methane yield at the time t (mL$g1) and ym is the maximum hydrogen or methane yield (mL$g1). The Rmax is the maximum hydrogen or methane production rate (mL$g1$d1), l is the lag-phase time (d), and e is exponential constant 2.718 [25]. The hydrolysis constant (d1) was estimated by a first-order kinetic model as described by Trzcinski and Stuckey [26]., which was written as Eq. (2). Y ¼ Ymax ð1 expðkh tÞÞ
(2)
where Y is cumulative hydrogen or methane yields at time t, and Ymax is the ultimate hydrogen or methane yields. The value for kh in the equation was estimated by plotting ln (1-Y/ Ymax) versus time.
Results and discussion Hydrogen production and sulfate removal from RSW in the first stage The SRB consortium was composed of Desulfovibrio sp. Desulfitibacter sp. Dethiosulfatibacter sp. and Clostridium sp. (Fig. 1). The SRB consortium was high sulfate reduction and hydrogen production activity. The rubber sheet wastewater (RSW) at COD/SO4 2 ratio of 13 and 3 with initial pH 6e7 was the most effective for sulfate reduction with a sulfate removal efficiency of 86e94% (Table 2). While RSW at all COD/SO4 2 ratio with initial pH 6 and 5 was suitable for hydrogen production. The H2S production from RSW at COD/SO4 2 ratio of 13 with initial pH 6e7 were 1,800e2,000 and 1,900e2,100 ppm, respectively. The H2S production from RSW at COD/SO4 2 ratio of 3 with initial pH 6e7 were 2,800 and 2,600 ppm, respectively (Fig. 2). The H2S production rate (Rmax) at COD/ SO4 2 ratio 13 and 3 with an initial pH of 6e7 was 0.23e0.37 mL H2S$d1. Sulfate removal from RSW was depended on COD/SO4 2 ratio. The SRB consortium also has sulfate reduction activity at initial pH 5 with a sulfate removal efficiency of 52e77% in all COD/SO4 2 ratio. Typically, SRB has optimal growth at a pH between 6 and 8. The sulfate removal at initial pH 5 indicates the ability of acid-tolerant of this SRB consortium. The acid-tolerant SRB was inlined with Lopes et al. [27] found that the wastewater at COD/SO4 2 ratio of 4 and 1 with an initial pH of 5 has a high acid-tolerant SRB activity with a sulfate removal efficiency of 95%. Hydrogen production was depended on initial pH and high hydrogen production was achieved at all COD/SO4 2 ratio at pH 5e6. The hydrogen production from RSW in the first stage at different initial pH and COD/SO4 2 ratios were shown in Fig. 3. The maximal H2 yield of 139 mL H2$gVS1 was obtained at COD/SO4 2 ratio 2 and initial pH 5. The hydrogen yield of
Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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Fig. 1 e The composition of sulfate-reducing consortium enriched from rubber sheet wastewater with high hydrogen production and sulfate reduction ability.
COD/SO4 2 ratios 13, 3, 2, and 1 at an initial pH of 5e6 was 94e101, 105.5e105.8, 75e139, and 8e132 mL H2$gVS1, respectively. In addition, the maximum H2 production rate (Rmax) of 19.86 mL H2$gVS1$d1 was achieved at COD/SO4 2 ratios 2 and initial pH 5 with short lag phase (l) of 0.09 d1. The
RSW at COD/SO4 2 ratio of 2 and initial pH 5 still has the ability of the hydrogen-producing bacteria, indicating hydrogen production could be achieved at high sulfate concentration of 4,000 mg SO2-4$L1. Hwang et al. [28] also confirmed that the H2 production rate of 2.8 L$d1 was achieved at low pH (5.5) and high sulfate concentration (3,000 mg$L1). Hwang et al. [12] also reported that high sulfate concentrations up to 20,000 mg/l would not affect the hydrogen yields in a continuous reactor. The RSW at COD/SO4 2 of 13 and 3 with an initial pH of 5e6 was also a high hydrogen yield of 94e105 mL H2$gVS1, respectively. Possible simultaneous hydrogen production and sulfate reduction from RSW could obtain at COD/ SO4 2 of 13 and an initial pH of 6. The RSW at COD/SO4 2 a ratio of 13 and pH 6 was suitable for simultaneous sulfate reduction and hydrogen production. Hydrogen yield and sulfate removal efficiency of RSW at COD/SO4 2 a ratio of 13 and pH 6 were 101 mL H2$gVS1 and 87%, respectively with the hydrogen production rate of 14.53 mL H2$d1. The high hydrogen production from RSW was achieved at low initial pH (5e6) due to acidic condition reduce the competition between SRB and hydrogen-producing bacteria. The complex organic materials was a breakdown and provided a volatile fatty acid and hydrogen as carbon and energy sources for SRB [29]. Therefore, at low pH could decrease the activity of SRB and enhanced activity of hydrogen-producing bacteria. The hydrogen production from skim latex serum (SLS) by a twostage anaerobic fermentation was also obtained at pH of 5.0e6.0 with the maximum H2 production of 1.57 L H2$L1 and H2 yield of 41.3 mL H2$gVS1at pH of 5.0e6.0 [7]. The pH of the wastewater has been reported to be a very important factor in hydrogen production under high sulfate concentration [30]. Biological hydrogen production is efficient at pH 5.0e6.5 under moderate sulfate concentration up to 300 mg/L [31]. The COD/SO4 2 ratios were not effected on hydrolysis rate (kh) but effected on hydrogen yields. The hydrolysis rate of RSW in the first stage (hydrogen production and sulfate reduction) was high at initial pH 5 in all COD/SO4 2 ratios (Table 2). The kh of RSW at COD/SO4 2 ratios 13, 3, 2 and 1 with pH 5 was 0.35e0.42, 0.33e0.44, 0.35e0.45 and 0.34e0.41 d1, respectively. The substrate degradation in the first stage was
Table 2 e Hydrogen production, hydrogen sulfide production, sulfate removal efficiency, and COD removal efficiency from rubber sheet wastewater in first stage fermentation at difference COD/SO4 2 ratios by SRB consortium. COD/SO4 2 pH ratios
13
3
2
1
7 6 5 7 6 5 7 6 5 7 6 5
H2 production
H2S production 1
Rmax l (d) H2 yield Kh (d ) l (d) H2S yield Kh Rmax (mL H2,gVS1) (mL H2S,gVS1) (d1) (mL H2,d1) (mL H2S,d1) 3.20 14.53 13.48 0.10 15.12 15.01 2.72 10.78 19.86 0.10 1.26 18.98
0.47 0.90 0.24 0.45 0.70 0.35 0.49 0.66 0.09 0.43 0.59 0.23
22.43 101.72 94.38 0.67 105.81 105.08 19.06 75.47 139.03 0.69 8.79 132.86
0.35 0.36 0.42 0.33 0.40 0.44 0.35 0.30 0.45 0.41 0.31 0.34
0.33 0.16 0.34 0.37 0.23 0.32 0.70 0.15 0.82 0.49 0.35 0.11
0.76 0.83 0.43 0.36 0.40 0.40 0.34 0.12 0.26 0.18 2.16 0.50
0.65 0.60 0.85 0.95 0.96 0.71 1.63 0.78 1.82 1.32 0.96 0.27
1.69 1.21 1.28 1.68 1.27 1.67 1.77 1.10 1.77 1.71 1.19 1.39
SO4 2 COD removal removal efficiency (%) efficiency (%) 86 87 77 94 94 76 75 73 69 52 67 52
9 11 10 30 33 28 47 47 47 47 62 52
Note: Rmax ¼ maximum production rate; l ¼ lag phase; Kh ¼ hydrolysis constant.
Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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Fig. 2 e Daily hydrogen sulfide production at COD/SO4 2 ratio 13 (a), 3 (b), 2 (c), and 1 (d).
Fig. 3 e Cumulative hydrogen yields in the first stage from RSW at various COD/SO4 2 ratios of 13 (a), 3 (b), 2 (c), and 1 (d). Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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Fig. 4 e The VFAs distribution in the first stage effluent at various COD/SO4 2 ratios and various initial pH. depended on pH more than COD/SO4 2 ratio similar to hydrogen production. The result indicated that hydrogenproducing bacteria play an important rule of substrate degradation in the first stage. The sulfate concentration in RSW was not effected on kh but effected on hydrogen yield due to competition between SRB and hydrogen-producing bacteria for carbon sources. The kh values were a positive correlation with hydrogen yields. High hydrogen yield of 139 mL H2$gVS1 had high kh value of 0.45 d1. The reducing of COD/SO4 2 ratios from 13 to 1 increased COD consumption by SRB for sulfate reduction. The COD/SO4 2 ratio of 13 has sulfate removal efficacy of 77e87% with COD removal efficiency of 9e11%. While the COD/SO4 2 ratios of 3, 2 and 1 have sulfate removal efficacy of 76e94%, 69e75%, and 52e67%, respectively with COD removal efficiency of 28e30, 47%, and 47e62%, respectively. Results indicated that RSW at COD/SO4 2 ratio of 13 has lower COD consumption for sulfate reduction compared to the others. Most of COD in RSW at COD/SO4 2 ratio of 13 was consumed by hydrogen-producing bacteria for hydrogen production. Sharma and Melkania [32]. also confirm that high hydrogen production from the organic fraction of municipal
solid waste (OFMSW) using co-culture of Enterobacter aerogenes and E. coli was obtained at high COD/SO4 2 ratio of 12e20.
Soluble metabolites and trace metals availability in the first stage The RSW was mainly composed of protein and complex organic polymers were digested to monomers by hydrolytic bacteria/fermentative bacteria [6,7]. The hydrolytic bacteria in SRB consortium could hydrolyze complex organic polymers in RSW into propionic acid, acetic acid, and butyric acid (Fig. 4). Low COD/SO4 2 ratios of 3, 2, and 1 have low VFAs concentration. The VFAs was consumed by SRB for sulfate reduction in low COD/SO4 2 ratios of 1e3 more than high COD/SO4 2 ratios of 13. The SRB consortium can utilize VFAs as electron donor and sulfate as an electron acceptor in the sulfate reduction process. The sulfate reductions by propionate sulfate-reducing bacteria, acetate sulfate-reducing bacteria, and hydrogenotrophic sulfate-reducing bacteria were according Eqs. (3)e(5) [7].
Table 3 e Trace metals remaining in the first stage effluent at various COD/SO4 2 ratios. COD/SO4 2 ratios
13
3
2
1
Trace metals (mg,L1)
pH
7 6 5 7 6 5 7 6 5 7 6 5
Total trace metals (mg,L1)
Mg
Mn
Fe
Co
Ni
Cu
Zn
Mo
10.5 8.2 7 6 4.5 3.4 7.8 5 4.1 4.8 2.1 0.5
0.04 0.03 0.04 0.02 0.02 0.03 0.04 0.02 0.04 0.03 0.03 0.04
0.92 0.6 0.01 0.01 0.18 0.01 0.3 0.01 0.01 0.01 0.02 0.01
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
0.01 0.04 0.07 0.1 0.02 0.05 0.03 0.11 0.08 0.09 0.06 0.12
0.24 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
11.74 8.93 7.16 6.17 4.76 3.53 8.21 5.18 4.27 4.97 2.25 0.71
Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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11 9 15 5 5 10 13 11 15 39 23 41 85 85 85 66 56 70 31 43 34 37 28 40 0.22 0.22 0.23 0.22 0.23 0.22 0.22 0.24 0.20 0.23 0.21 0.21 725.8 664.9 723.2 605.9 645.9 557.5 705.9 840.1 455.4 691.2 566.5 735.8 0.5 1.0 0.4 0.7 0.3 0.5 0.1 0.6 0.9 0.3 0.4 1.1 74.9 70.4 79.0 64.2 71.4 58.0 75.1 97.9 42.3 76.8 57.9 75.1 13 10 22 2 5 9 18 21 19 22 31 38 1
2
3
Note: Rmax ¼ maximum production rate; l ¼ lag phase; Kh ¼ hydrolysis constant.
88 81 64 57 57 64 52 48 51 47 27 45 0.20 0.21 0.19 0.20 0.20 0.19 0.20 0.21 0.18 0.20 0.20 0.19 673.1 629.7 489.8 579.1 603.2 544.4 594.2 634.3 441.3 546.0 548.7 550.9 1.4 1.3 0.7 1.2 1.4 1.8 1.3 1.3 0.5 1.7 1.5 1.4 48.1 44.9 34.9 41.4 43.1 38.9 42.4 45.3 31.5 39.0 39.2 39.4
The RSW with high COD/SO4 2 ratios has higher methane production than low COD/SO4 2 ratios. Decreasing of COD/ SO4 2 ratios in RSW decreased methane production. High methane production also achieved at initial pH 6 and 7 (Fig. 5). The RSW at COD/SO4 2 ratio 13 with initial pH 6 was suitable for methane production with methane yield and methane production rate of 629 mL CH4$gVS1 and 45 mL CH4$d1, respectively. The COD/SO4 2 ratio of 13, 3 2, and 1 at pH 6e7 have methane yield of 629e673, 579e603, 594e603, 594e634, and 546e548 mL CH4$gVS1, respectively (Table 4). The COD/ SO4 2 ratio of 13, 3 2, and 1 at pH 5 have a methane yield of 489, 544, 441, 550 mL CH4$gVS1, respectively. Methane production from RSW of all COD/SO4 2 ratio at pH 6e7 was higher than pH 5. Cetecioglu et al. [34] report that the COD/SO4 2 ratios have a small effect on the methane production, although the presence of sulfate may affect the metabolic pathways of methanogens. The long lag phase of 0.70e1.39 d1 was achieved at all COD/SO4 2 ratios from first stage effluent with without trace element addition. The long lag phase was caused by inhibition and nutrient deficiency during the process and resulting in a negative impact on hydrolysis and acidification of organic matter [35]. The first stage effluent was deficient in trace metals for methane production. Trace element solution was supplemented to the first stage effluent for improving methane production. Trace element supplementation improved hydrolysis rate (kh ¼ 0.23 d1) compared with without trace element supplementation (kh ¼ 0.19 d1). In addition, the maximum methane production rate (Rmax) of 79.01 mL CH4$gVS1$d1 and shortest lag phase of 0.37 d1 was observed at the of COD/SO4 2 ratio of 13 and initial pH of 5 with trace element supplementation. The COD/SO4 2 ratio of 13 and
7 6 5 7 6 5 7 6 5 7 6 5
Methane production from RSW in the second stage
13
The increasing of sulfate concentration in RSW resulted in more COD consumption by SRB for sulfate reduction and decreasing VFAs concentration in first stage effluent. The sulfide and hydrogen sulfide production in the first stage could react with the metal in the fermentation broth and reduce its availability. The remaining trace elements in the first stage effluent were low when compared with raw RSW (Table 3). The highest trace metals of 11.74 mg$L1 was observed at COD/SO4 2 ratio 13 with initial pH 7. The COD/SO4 2 ratio of 1 has the lowest amount of trace elements of 4.97 mg$L1 at initial pH 7. Decreasing of COD/SO4 2 ratio resulted in a decreasing amount of trace metals in fermentation effluent. The H2S produced by SRB consortium could precipitate trace metals as metals sulfide [18]. Additionally, sulfide could combine with iron in cytochromes affecting bacteria functionality [33]. Therefore, the trace element could be added into the second stage reactor for better methane production.
CH4 production with trace element supplementation
(5)
2
4H2 þ H2 SO4 /H2 S þ 4H2 O DG0 ¼ 194:61 kJ
(4)
1
DG0 ¼ 108:3 kJ
CH4 production without trace element supplementation
CH3 COOH þ H2 SO4 /2CO2 þ 2H2 O þ H2 S
(3)
COD/SO4 2 pH ratios
þ 0:75H2 S DG0 ¼ 74:3 kJ
Table 4 e Methane production, kinetics parameters, and reactor performance of first stage effluent with and without trace element supplementation in second stage anaerobic fermentation.
C2 H5 COOH þ 0:75H2 SO4 /CH3 COOH þ CO2 þ H2 O
Rmax l (d) CH4 yield Kh (d ) COD removal SO4 removal l (d) CH4 Yields Kh (d1) COD removal SO4 2 removal Rmax (mL CH4,gVS1) efficiency (%) efficiency (%) (mL CH4,d1) (mL CH4,gVS1) efficiency (%) efficiency (%) (mL CH4,d1)
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Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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Fig. 5 e Cumulative methane yields in the second stage of first stage effluent without trace element supplementation at COD/SO4 2 ratio 13 (a), 3 (b), 2 (c), and 1 (d). initial pH 5 has a methane yield of 723 mL CH4$gVS1. Trace element supplementation into first stage effluent was increased methane production rate and methane yield with reduction of lag phase at all COD/SO4 2 ratio and all initial pH tested. Especially methane production at initial pH 5 and COD/ SO4 2 ratio of 13 was the biggest improvement with trace element supplementation (Fig. 6). Enhancement of methane production of 16e48% was achieved from trace element supplementation into first stage effluent. The trace element has stimulated the activities of archaea population. Methanosarcina siciliae and Methanoculleus bourgensis were a positive response to nickel/molybdenum/boron (Ni/Mo/B) addition [36]. Linville et al. [37] reported that trace element supplementation could enhance methane yield up to 56% of sewage sludge in anaerobic digestion. Facchin et al. [38] report that
Table 5 e Performance of biohythane production from RSW at COD/SO4 2 ratio of 13 and pH 6 by two-stage anaerobic digestion. Parameters H2 production (L,L1) CH4 production (L,L1) Biogas production (L,L1) H2 yield (mL,gVS1) CH4 yield (mLg,VS1) SO4 2 removal efficiency (%) COD removal efficiency (%) COD conversion to H2S (%)
First stage
Second stage
Biohythane systems
0.57 e 2.23 101 e 87 11 8.2
e 3.3 6.6 e 629 9 85 2.4
0.57 3.3 8.83 101 629 96 96 16
food waste supplemented with a mixture of trace element (Co, Mo, Ni, Se, W) can increase the methane production of 45e65%. The rice straw with Co and Ni supplementation enhanced methane yield up to 11.6% [39]. The available trace metals could act as a co-enzyme in the anaerobic degradation process and enhance the growth rate of microorganisms [40,41]. The distribution of COD and sulfur from the RSW as a substrate for biohythane by two-stage anaerobic digestion was shown in Fig. 7. The RSW at COD/SO4 2 ratio at 13 and pH 6 was optimum for biohythane production and sulfate removal without trace element addition. The COD removal of RSW at COD/SO4 2 ratio of 13 and initial pH of 6 in the first stage, the second stage without trace element supplementation, and the second stage with trace element supplementation was 11.3%, 72.4, and 76.2% respectively. The sulfate removal of COD/SO4 2 ratio at 13 pH 6 in the first stage, the second stage without trace element supplementation, and the second stage with trace element supplementation was 87%, 10%, and 9% respectively. The acidogenic stage of this condition has 2.9% of COD converting to hydrogen and 8.2% of COD consuming for hydrogen sulfide production. The methanogenic stage of this condition has 70% of COD converting to methane and 2.4% of COD consuming for hydrogen sulfide production. The methanogenic stage of this condition with trace element supplementation has 74% of COD converting to methane and 2.2% of COD consuming for hydrogen sulfide production. Total COD consuming for hydrogen sulfide production of both stage was 10.4e10.6%. Sulfate converting to hydrogen sulfide in acidogenic stage and methanogenic stage
Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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Fig. 6 e Cumulative methane yields in the second stage of first stage effluent with trace element supplementation at COD/ SO4 2 ratio 13 (a), 3 (b), 2 (c), and 1 (d).
Fig. 7 e COD balance and sulfur balance of biohythane production from rubber sheet wastewater at COD/SO4 2 ratio of 13 and initial pH 5 and 6 by two-stage anaerobic digestion. Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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was 87% and 9e10%, respectively. The RSW at COD/SO4 2 ratio of 13 and initial pH 5 was required trace element supplementation for better methane production. COD removal in the second stage without and with trace element supplementation was 58% and 86%, respectively. Trace element supplementation could increase microbial biomass from 0.25 g$L1 to 0.37 g$L1. The COD removal of RSW at COD/SO4 2 ratio of 13 and initial pH 5 in the first stage, the second stage without trace element supplementation, and the second stage with trace element supplementation was 10.2%, 66.7, and 95.2%, respectively. The sulfate removal of COD/SO4 2 ratio of 13 and pH 5 in the first stage, the second stage without trace element supplementation, and the second stage with trace element supplementation was 77%, 22%, and 15%, respectively. The acidogenic stage of this condition has 2.7% of COD converting to hydrogen and 6.7% of COD consuming for hydrogen sulfide production. The methanogenic stage of this condition has 55% of COD converting to methane and 2.7% of COD consuming for hydrogen sulfide production. The methanogenic stage of this condition with trace element supplementation has 83% of COD converting to methane and 2.2% of COD consuming for hydrogen sulfide production. Total COD consuming for hydrogen sulfide production of both stage was 8.9e9.4%. Sulfate converting to hydrogen sulfide in the acidogenic stage and the methanogenic stage was 77% and 15e22%, respectively. The biohythane production was 8.83 L$L1 of RSW with H2 and CH4 production of 0.57 and 3.3 L$L1 of RSW, respectively, with the remaining as CO2 (Table 5.). The biohythane production was inlined with Kongjan et al. [5] results that 2.25 L$L1 skim latex serum (SLS) of hydrogen and 6.41 L$L1 SLS of methane was achieved via the two-stages anaerobic digestion process. The hydrogen and methane production from skim latex serum (SLS) by two-stage anaerobic digestion was 1.57 L$L1 SLS and 12.2 L$L1 SLS, respectively [7]. Microbial responsible for biohythane production from rubber processing wastewater in the first stage was dominated by Enterobacter sp., Caldicellulosiruptor sp., Clostridium sp. and Thermoanaerobacterium sp. [42], while in the second stage was dominated by Methanosarcina sp. and Methanoculleus sp. [43]. The two-stages anaerobic digestion could promote biohythane production and reduce hydrogen sulfide toxicity in the second stage. Table 5 showed SO4 2 removal efficiency in the first stage up to 87% with 8.2% of COD consuming for H2S conversion. The COD consumption for H2S production in the second stage was only 2.4% of total COD removal efficiency. Lu et al. [14] showed the high stability biogas production performance was achieved at COD/SO4 2 ratio of 2e10 with COD consuming for H2S production of 0.4e1.5%. Normally, the SRB groups is higher outcompete for nutrients than methanogenic archaea [42]. Therefore, separation of steps is suitable for preventing competition between SRB and methanogenic archaea.
Conclusions The SRB consortium composed of Desulfovibrio sp. Desulfitibacter sp. Dethiosulfatibacter sp. and Clostridium sp. was effective for sulfate removal and hydrogen production.
Simultaneous biohythane production and sulfate reduction from RSW was obtained at COD/SO4 2 of 13 and initial pH of 6 by the two-stage anaerobic digestion process. Hydrogen yield and sulfate removal efficiency in the first stage were 101 mL H2$gVS1 and 87%, respectively. Methane yield and methane production rate in the second stage were 629 mL CH4$gVS1 and 45 mL CH4$d1, respectively. The COD in RSW was converted to hydrogen and methane for 2.7e2.9% and 70e80%, respectively while 9.4e10.6% of COD was consumed for sulfate reduction to hydrogen sulfide. Two-stage anaerobic fermentation obtained simultaneous biohythane production and sulfate removal with efficiently for energy recovery from RSW.
Acknowledgments The authors would like to thank the Research and Development Institute Thaksin University, Thailand Science Reserach and Inovation, and Thailand Research Fund through the Research and Researchers for Industries (PHD59I0069), TRF Senior Research Scholar (Grant No. RTA6280001) and TRF Mid-Career Research Grant (Grant No. RSA6180048) for financial support.
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Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by twostage anaerobic digestion Kanathip Promnuan a, Takaya Higuchi b, Tsuyoshi Imai b, Prawit Kongjan c, Alissara Reungsang d,f, Sompong O-Thong a,e,* a
Biotechnology Program, Faculty of Science, Thaksin University, Phatthalung 93210, Thailand Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Yamaguchi 755-8611, Japan c Department of Science, Faculty of Science and Technology, Prince of Songkhla University, Pattani 94000, Thailand d Department of Biotechnology, Faculty of Technology, Khon Kaen University, Khon Kaen 40002, Thailand e Research Center in Energy and Environment, Faculty of Science, Thaksin University, Phatthalung 93210, Thailand f Research Group for Development of Microbial Hydrogen Production Process from Biomass, Khon Kaen University, Khon Kaen 40002, Thailand b
highlights The SRB consortium was effective for sulfate removal and hydrogen production. The sulfate removal and hydrogen yield from RSW was 87% and 101 mL H2$gVS1. The COD/SO2 4 ratio of 13 and pH 6 was suitable for hydrogen production from RSW. Trace element addition increased CH4 production rate for 2 times.
article info
abstract
Article history:
The sulfate-reducing bacteria (SRB) consortium (Desulfovibrio sp. Desulfitibacter sp. Dethio-
Received 5 July 2019
sulfatibacter sp. and Clostridium sp.) was investigated for sulfate removal and biohythane
Received in revised form
production from rubber sheet wastewater (RSW). The RSW at COD/SO4 2 ratio of 13 and pH
18 September 2019
6 was suitable for sulfate reduction and biohythane production. Hydrogen yield, sulfate
Accepted 26 October 2019
removal efficiency, and methane yield were 101 mL H2$gVS1, 87%, and 629 mL CH4$gVS1,
Available online xxx
respectively. The hydrogen and methane production rate was 14.53 mL H2$d1and 45 mL
Keywords:
drolysis rate (kh) from 0.19 d1 to 0.23 d1 and methane production rate from 31e48 to 42
COD/SO2 4 ratio
e97 mL CH4$d1. The COD removal of RSW in acidogenic stage and methane stage was
Biohythane production
11.3% and 72.4e76.2%, respectively. The sulfate removal of RSW in acidogenic stage and
Rubber sheet wastewater
methane stage was 87% and 9%, respectively. Two-stage anaerobic fermentation obtained
Trace element availability
simultaneous biohythane production and sulfate removal with efficiently for energy re-
Two-stage anaerobic digestion
covery from RSW.
CH4$d1, respectively. Hydrogen effluent with trace element supplemented improved hy-
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Biotechnology Program, Faculty of Science, Thaksin University, Phatthalung 93210, Thailand. E-mail address: [email protected] (S. O-Thong). https://doi.org/10.1016/j.ijhydene.2019.10.237 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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Introduction High sulfate content wastewater is frequently found in latex and rubber processing industries. Rubber processing wastewater has a pH of 3.7e5.5, chemical oxygen demand (COD) concentration of 3,500e14,000 mg$L1 and sulfate concentration of 500e2,000 mg$L1 [1]. The wastewater from rubber sheet production has a pH of 5.0e5.9, COD concentration of 1,928e15,069 mg$L1 and sulfate concentration of 136e472 mg$L1 [2]. The concentrated rubber latex wastewater has a pH of 3.6e4.7, COD concentration of 5,430 mg$L1, sulfate concentration of 1,819 mg$L1, and ammonia nitrogen concentration of 733.12 mg$L1 [3,4]. The skim latex wastewater has a pH of 4e5, COD concentration of 29,220e35,830 mg$L1 and sulfate concentration of 258e3,580 mg$L1 [5e7]. The rubber processing industries wastewaters are high sulfate concentration resulting in bad smell under the anaerobic condition from the reduction of sulfate to hydrogen sulfide (H2S). The conventional anaerobic wastewater treatment system was not suitable for treated rubber processing industries wastewaters, including rubber sheet wastewater (RSW). The high sulfate in rubber sheet wastewater has negatively effects methanogenic archaea resulting in low COD and sulfate removal efficiency. The two-stage anaerobic digestion process has the potential for separating the acidogenic bacteria and sulfatereducing bacteria from the methanogenesis archaea. The sulfate and hydrogen sulfide (H2S) could remove from the wastewater in the first stage before the methanogenesis in the second stage to avoiding the hydrogen sulfide inhibition and high hydrogen sulfide (H2S) contamination in biogas. Previously report shown that sulfate reduction by sulfatereducing bacteria (SRB) can obtain in the acidogenic stage [8e11]. Hwang et al. [12] demonstrated the synergies effect between sulfate-reducing bacteria and hydrogen-producing bacteria under high sulfate concentration of 20,000 mg$L1 with the H2 production rate of 2.95e9.40 L$d1 and hydrogen yields of 1.6e2.0 mol H2$mol1 glucose. High hydrogen production from sucrose under high sulfate concentration of 3000 mg SO4 2 L1 at low pH of 5.5 by a mixed microbial consortium was reported by Lin and Chen [13]. The SRB can utilize sulfate as an electron acceptor by competing with hydrogen-producing bacteria and methane-producing archaea for carbon sources [14]. The COD/SO4 2 ratio at lower than 0.67 has insufficient for the sulfate reduction [15]. The carbon sources to sulfate ratio (COD/SO4 2 ratio) is a key parameter for balancing the competition between SRB and the other bacteria in the anaerobic digestion process [16]. The high sulfate removal efficiencies of >95% were achieved from wastewater with COD/SO4 2 ratios of 4 and 1 [12]. The fermentative acidogenic bacteria acts on the degradation of organic compounds for conversion to hydrogen, ethanol, and volatile fatty acids (VFAs), which will be used by the SRB to reduce sulfate [17]. However, in the acidification process, the free sulfide has precipitated metal ions that act as functional groups of electron transporter systems and coenzymes required for the growth of microorganisms the methanogenesis stage [18]. However, still lacking knowledge on the effect of COD/SO4 2 ratio in RSW on biohythane production
and trace metals availability for methanogen in the second stage. The effect of COD/SO4 2 ratio in RSW on fermentative hydrogen production, sulfate reduction efficiency, methane production, and biohythane production via two-stage anaerobic digestion was investigated. Trace metals availability in first stage effluent and trace element supplementation into first stage effluent for improved methane production in the second stage was investigated.
Materials and methods Substrate and inoculum The rubber sheet wastewater (RSW) and anaerobic microbial sludge were collected from Yangkaw cooperative rubber sheet plant, Songkhla, Thailand. Rubber sheet wastewater was transported to the laboratory and analyzed for the chemical and physical composition (Table 1). The RSW was stored at 4 C until used. The SRB consortium was enriched from the anaerobic sludge with Postgate medium C [19] following composition of 6.0 g$L1 sodium lactate, 4.5 g$L1 Na2SO4, 1.0 g L1 NH4Cl, 1.0 g$L1 Yeast extract, 0.5 g L1 KH2PO4, 0.3 g$L1 Sodium citrate$2H2O, 0.06 g$L1 CaCl2$6H2O, 0.06 g$L1 MgSO4$7H2O, 0.004 g$L1 FeSO4$7H2O. Postgate medium C was adjusted pH to 7.0 ± 2. The enriched culture was incubated under a temperature of 30 C. The activity of the SRB was confirmed by sulfate removal efficiency, hydrogen production, and hydrogen sulfide production analysis. The microbial community of SRB consortium was analyzed by polymerase chain reaction-denaturating gradient gel electrophoresis (PCR-DGGE) [20]. The biomass concentration of SRB consortium of 40 g$L1 was used as inoculum in the first stage for sulfate reduction and hydrogen production. The anaerobic microbial sludge for methane inoculum was acclimatized with RSW by cultivated on 20% RSW at pH 7.5 (adjusting the pH with NaHCO3) as the substrate. The enriched
Table 1 e Chemical, physical and trace metals composition of rubber sheet wastewater. Parameters pH Chemical oxygen demand (COD) Total solids (TS) Volatile solids (VS) Protein Carbohydrate Lipid Alkalinity SO4 2 COD/SO4 2 ratio Mg Mn Fe Co Ni Cu Zn Mo
Unit (mg$L1) 5.0 6667 4619 2260 5300 1100 150 3000 514 13 89 0.16 0.23 <0.01 <0.01 0.41 7.7 <0.01
Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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methane-producing sludge was incubated for 5 days before used as inoculum to reduce the methane production from the remaining organic compound in sludge. The enriched methane-producing sludge with volatile suspended solid of 80 g$L1 was used as inoculum for methane production.
Biohythane production The two-stage anaerobic digestion for biohythane production from RSW was carried out in a batch test according to Giordano et al. [21]. The RSW with four COD/SO4 2 ratio of 13, 3, 2, and 1 were tested for biohythane production. The COD/SO4 2 ratio of raw RSW was 13. The COD/SO4 2 ratios of 3, 2, and 1 were obtained by adding sodium sulfate (Na2SO4) of 2, 4, and 6 g$L1, respectively. The initial pH of the RSW was adjusted to 7.0, 6.0, and 5.0 by adding sodium bicarbonate (NaHCO3). The 20% v/v of SRB consortium inoculum was added to batch reactors and incubated at mesophilic temperature (35 C) for 7 days. All of the experiment was done in duplicate. The biogas in headspace bottles was collected every 24 h and analyzed for biogas composition by gas chromatography. Methane inoculum was added to bottles after 7 days of incubation at the substrate to inoculum ratio (S:I ratio) of 2:1 based on the VS basis and adjusted the initial pH to 7.5 by adding sodium bicarbonate (NaHCO3). The methane production from first stage effluent was continuously incubated for 14 days at a temperature of 35 C. The biogas in headspace bottles was collected every 24 h and analyzed for the composition. The effect of trace element supplementation into first stage effluent on methane production in the second stage was investigated. The effluent of the first stage was supplemented with 1 mL$L1 of trace element solution preparing as described previously by Angelidaki et al. [22]. The first stage effluent was mixed with methane inoculum at S:I ratio of 2:1 based on the VS basis and adjusted the initial pH to 7.5 by adding sodium bicarbonate (NaHCO3). The methane production was continuously incubated for 14 days at a temperature of 35 C. The biogas in headspace bottles was collected every 24 h and analyzed for the composition.
Analytical methods The COD, sulfate, sulfide, total solids (TS), and VS was determined according to the standard methods [23]. The pH of the wastewater was measured using a pH meter (Horiba D75 Portable). The H2S in biogas was analyzed by gas chromatography (GC-14A, Shimadzu) connected with a flame photometric detector (GC-FPD) and packed column (Rt®XLSulfur Micropacked, Restek). The concentration of H2, N2, CH4, and CO2 in biogas were measured by gas chromatography (GC-8A, Shimadzu) connected with thermal conductivity detector (GC-TCD) and packed column (Shin Carbon, Restek). The volatile fatty acid in the first stage and second stage effluent was measured by gas chromatography (GC17A, Shimadzu) connected with a flame ionization detector (GC-FID) and Stabilwax®-DA fused silica column (30 m 0.53 mm). Biogas volume was monitored daily by water displacement according to O-Thong et al. [24]. The metals composition was analyzed by ICP-AES (PerkinElmer, Optima 3300DV).
Data calculation The kinetic values of hydrogen and methane production from RSW by two-stage anaerobic digestion process were predicted from the Gompertz equation as following Eq. (1). ðl tÞ yt ¼ ym $exp exp Rmax $ e $ þ1 ym
(1)
where yt is the accumulation hydrogen or methane yield at the time t (mL$g1) and ym is the maximum hydrogen or methane yield (mL$g1). The Rmax is the maximum hydrogen or methane production rate (mL$g1$d1), l is the lag-phase time (d), and e is exponential constant 2.718 [25]. The hydrolysis constant (d1) was estimated by a first-order kinetic model as described by Trzcinski and Stuckey [26]., which was written as Eq. (2). Y ¼ Ymax ð1 expðkh tÞÞ
(2)
where Y is cumulative hydrogen or methane yields at time t, and Ymax is the ultimate hydrogen or methane yields. The value for kh in the equation was estimated by plotting ln (1-Y/ Ymax) versus time.
Results and discussion Hydrogen production and sulfate removal from RSW in the first stage The SRB consortium was composed of Desulfovibrio sp. Desulfitibacter sp. Dethiosulfatibacter sp. and Clostridium sp. (Fig. 1). The SRB consortium was high sulfate reduction and hydrogen production activity. The rubber sheet wastewater (RSW) at COD/SO4 2 ratio of 13 and 3 with initial pH 6e7 was the most effective for sulfate reduction with a sulfate removal efficiency of 86e94% (Table 2). While RSW at all COD/SO4 2 ratio with initial pH 6 and 5 was suitable for hydrogen production. The H2S production from RSW at COD/SO4 2 ratio of 13 with initial pH 6e7 were 1,800e2,000 and 1,900e2,100 ppm, respectively. The H2S production from RSW at COD/SO4 2 ratio of 3 with initial pH 6e7 were 2,800 and 2,600 ppm, respectively (Fig. 2). The H2S production rate (Rmax) at COD/ SO4 2 ratio 13 and 3 with an initial pH of 6e7 was 0.23e0.37 mL H2S$d1. Sulfate removal from RSW was depended on COD/SO4 2 ratio. The SRB consortium also has sulfate reduction activity at initial pH 5 with a sulfate removal efficiency of 52e77% in all COD/SO4 2 ratio. Typically, SRB has optimal growth at a pH between 6 and 8. The sulfate removal at initial pH 5 indicates the ability of acid-tolerant of this SRB consortium. The acid-tolerant SRB was inlined with Lopes et al. [27] found that the wastewater at COD/SO4 2 ratio of 4 and 1 with an initial pH of 5 has a high acid-tolerant SRB activity with a sulfate removal efficiency of 95%. Hydrogen production was depended on initial pH and high hydrogen production was achieved at all COD/SO4 2 ratio at pH 5e6. The hydrogen production from RSW in the first stage at different initial pH and COD/SO4 2 ratios were shown in Fig. 3. The maximal H2 yield of 139 mL H2$gVS1 was obtained at COD/SO4 2 ratio 2 and initial pH 5. The hydrogen yield of
Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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Fig. 1 e The composition of sulfate-reducing consortium enriched from rubber sheet wastewater with high hydrogen production and sulfate reduction ability.
COD/SO4 2 ratios 13, 3, 2, and 1 at an initial pH of 5e6 was 94e101, 105.5e105.8, 75e139, and 8e132 mL H2$gVS1, respectively. In addition, the maximum H2 production rate (Rmax) of 19.86 mL H2$gVS1$d1 was achieved at COD/SO4 2 ratios 2 and initial pH 5 with short lag phase (l) of 0.09 d1. The
RSW at COD/SO4 2 ratio of 2 and initial pH 5 still has the ability of the hydrogen-producing bacteria, indicating hydrogen production could be achieved at high sulfate concentration of 4,000 mg SO2-4$L1. Hwang et al. [28] also confirmed that the H2 production rate of 2.8 L$d1 was achieved at low pH (5.5) and high sulfate concentration (3,000 mg$L1). Hwang et al. [12] also reported that high sulfate concentrations up to 20,000 mg/l would not affect the hydrogen yields in a continuous reactor. The RSW at COD/SO4 2 of 13 and 3 with an initial pH of 5e6 was also a high hydrogen yield of 94e105 mL H2$gVS1, respectively. Possible simultaneous hydrogen production and sulfate reduction from RSW could obtain at COD/ SO4 2 of 13 and an initial pH of 6. The RSW at COD/SO4 2 a ratio of 13 and pH 6 was suitable for simultaneous sulfate reduction and hydrogen production. Hydrogen yield and sulfate removal efficiency of RSW at COD/SO4 2 a ratio of 13 and pH 6 were 101 mL H2$gVS1 and 87%, respectively with the hydrogen production rate of 14.53 mL H2$d1. The high hydrogen production from RSW was achieved at low initial pH (5e6) due to acidic condition reduce the competition between SRB and hydrogen-producing bacteria. The complex organic materials was a breakdown and provided a volatile fatty acid and hydrogen as carbon and energy sources for SRB [29]. Therefore, at low pH could decrease the activity of SRB and enhanced activity of hydrogen-producing bacteria. The hydrogen production from skim latex serum (SLS) by a twostage anaerobic fermentation was also obtained at pH of 5.0e6.0 with the maximum H2 production of 1.57 L H2$L1 and H2 yield of 41.3 mL H2$gVS1at pH of 5.0e6.0 [7]. The pH of the wastewater has been reported to be a very important factor in hydrogen production under high sulfate concentration [30]. Biological hydrogen production is efficient at pH 5.0e6.5 under moderate sulfate concentration up to 300 mg/L [31]. The COD/SO4 2 ratios were not effected on hydrolysis rate (kh) but effected on hydrogen yields. The hydrolysis rate of RSW in the first stage (hydrogen production and sulfate reduction) was high at initial pH 5 in all COD/SO4 2 ratios (Table 2). The kh of RSW at COD/SO4 2 ratios 13, 3, 2 and 1 with pH 5 was 0.35e0.42, 0.33e0.44, 0.35e0.45 and 0.34e0.41 d1, respectively. The substrate degradation in the first stage was
Table 2 e Hydrogen production, hydrogen sulfide production, sulfate removal efficiency, and COD removal efficiency from rubber sheet wastewater in first stage fermentation at difference COD/SO4 2 ratios by SRB consortium. COD/SO4 2 pH ratios
13
3
2
1
7 6 5 7 6 5 7 6 5 7 6 5
H2 production
H2S production 1
Rmax l (d) H2 yield Kh (d ) l (d) H2S yield Kh Rmax (mL H2,gVS1) (mL H2S,gVS1) (d1) (mL H2,d1) (mL H2S,d1) 3.20 14.53 13.48 0.10 15.12 15.01 2.72 10.78 19.86 0.10 1.26 18.98
0.47 0.90 0.24 0.45 0.70 0.35 0.49 0.66 0.09 0.43 0.59 0.23
22.43 101.72 94.38 0.67 105.81 105.08 19.06 75.47 139.03 0.69 8.79 132.86
0.35 0.36 0.42 0.33 0.40 0.44 0.35 0.30 0.45 0.41 0.31 0.34
0.33 0.16 0.34 0.37 0.23 0.32 0.70 0.15 0.82 0.49 0.35 0.11
0.76 0.83 0.43 0.36 0.40 0.40 0.34 0.12 0.26 0.18 2.16 0.50
0.65 0.60 0.85 0.95 0.96 0.71 1.63 0.78 1.82 1.32 0.96 0.27
1.69 1.21 1.28 1.68 1.27 1.67 1.77 1.10 1.77 1.71 1.19 1.39
SO4 2 COD removal removal efficiency (%) efficiency (%) 86 87 77 94 94 76 75 73 69 52 67 52
9 11 10 30 33 28 47 47 47 47 62 52
Note: Rmax ¼ maximum production rate; l ¼ lag phase; Kh ¼ hydrolysis constant.
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Fig. 2 e Daily hydrogen sulfide production at COD/SO4 2 ratio 13 (a), 3 (b), 2 (c), and 1 (d).
Fig. 3 e Cumulative hydrogen yields in the first stage from RSW at various COD/SO4 2 ratios of 13 (a), 3 (b), 2 (c), and 1 (d). Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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Fig. 4 e The VFAs distribution in the first stage effluent at various COD/SO4 2 ratios and various initial pH. depended on pH more than COD/SO4 2 ratio similar to hydrogen production. The result indicated that hydrogenproducing bacteria play an important rule of substrate degradation in the first stage. The sulfate concentration in RSW was not effected on kh but effected on hydrogen yield due to competition between SRB and hydrogen-producing bacteria for carbon sources. The kh values were a positive correlation with hydrogen yields. High hydrogen yield of 139 mL H2$gVS1 had high kh value of 0.45 d1. The reducing of COD/SO4 2 ratios from 13 to 1 increased COD consumption by SRB for sulfate reduction. The COD/SO4 2 ratio of 13 has sulfate removal efficacy of 77e87% with COD removal efficiency of 9e11%. While the COD/SO4 2 ratios of 3, 2 and 1 have sulfate removal efficacy of 76e94%, 69e75%, and 52e67%, respectively with COD removal efficiency of 28e30, 47%, and 47e62%, respectively. Results indicated that RSW at COD/SO4 2 ratio of 13 has lower COD consumption for sulfate reduction compared to the others. Most of COD in RSW at COD/SO4 2 ratio of 13 was consumed by hydrogen-producing bacteria for hydrogen production. Sharma and Melkania [32]. also confirm that high hydrogen production from the organic fraction of municipal
solid waste (OFMSW) using co-culture of Enterobacter aerogenes and E. coli was obtained at high COD/SO4 2 ratio of 12e20.
Soluble metabolites and trace metals availability in the first stage The RSW was mainly composed of protein and complex organic polymers were digested to monomers by hydrolytic bacteria/fermentative bacteria [6,7]. The hydrolytic bacteria in SRB consortium could hydrolyze complex organic polymers in RSW into propionic acid, acetic acid, and butyric acid (Fig. 4). Low COD/SO4 2 ratios of 3, 2, and 1 have low VFAs concentration. The VFAs was consumed by SRB for sulfate reduction in low COD/SO4 2 ratios of 1e3 more than high COD/SO4 2 ratios of 13. The SRB consortium can utilize VFAs as electron donor and sulfate as an electron acceptor in the sulfate reduction process. The sulfate reductions by propionate sulfate-reducing bacteria, acetate sulfate-reducing bacteria, and hydrogenotrophic sulfate-reducing bacteria were according Eqs. (3)e(5) [7].
Table 3 e Trace metals remaining in the first stage effluent at various COD/SO4 2 ratios. COD/SO4 2 ratios
13
3
2
1
Trace metals (mg,L1)
pH
7 6 5 7 6 5 7 6 5 7 6 5
Total trace metals (mg,L1)
Mg
Mn
Fe
Co
Ni
Cu
Zn
Mo
10.5 8.2 7 6 4.5 3.4 7.8 5 4.1 4.8 2.1 0.5
0.04 0.03 0.04 0.02 0.02 0.03 0.04 0.02 0.04 0.03 0.03 0.04
0.92 0.6 0.01 0.01 0.18 0.01 0.3 0.01 0.01 0.01 0.02 0.01
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
0.01 0.04 0.07 0.1 0.02 0.05 0.03 0.11 0.08 0.09 0.06 0.12
0.24 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
11.74 8.93 7.16 6.17 4.76 3.53 8.21 5.18 4.27 4.97 2.25 0.71
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11 9 15 5 5 10 13 11 15 39 23 41 85 85 85 66 56 70 31 43 34 37 28 40 0.22 0.22 0.23 0.22 0.23 0.22 0.22 0.24 0.20 0.23 0.21 0.21 725.8 664.9 723.2 605.9 645.9 557.5 705.9 840.1 455.4 691.2 566.5 735.8 0.5 1.0 0.4 0.7 0.3 0.5 0.1 0.6 0.9 0.3 0.4 1.1 74.9 70.4 79.0 64.2 71.4 58.0 75.1 97.9 42.3 76.8 57.9 75.1 13 10 22 2 5 9 18 21 19 22 31 38 1
2
3
Note: Rmax ¼ maximum production rate; l ¼ lag phase; Kh ¼ hydrolysis constant.
88 81 64 57 57 64 52 48 51 47 27 45 0.20 0.21 0.19 0.20 0.20 0.19 0.20 0.21 0.18 0.20 0.20 0.19 673.1 629.7 489.8 579.1 603.2 544.4 594.2 634.3 441.3 546.0 548.7 550.9 1.4 1.3 0.7 1.2 1.4 1.8 1.3 1.3 0.5 1.7 1.5 1.4 48.1 44.9 34.9 41.4 43.1 38.9 42.4 45.3 31.5 39.0 39.2 39.4
The RSW with high COD/SO4 2 ratios has higher methane production than low COD/SO4 2 ratios. Decreasing of COD/ SO4 2 ratios in RSW decreased methane production. High methane production also achieved at initial pH 6 and 7 (Fig. 5). The RSW at COD/SO4 2 ratio 13 with initial pH 6 was suitable for methane production with methane yield and methane production rate of 629 mL CH4$gVS1 and 45 mL CH4$d1, respectively. The COD/SO4 2 ratio of 13, 3 2, and 1 at pH 6e7 have methane yield of 629e673, 579e603, 594e603, 594e634, and 546e548 mL CH4$gVS1, respectively (Table 4). The COD/ SO4 2 ratio of 13, 3 2, and 1 at pH 5 have a methane yield of 489, 544, 441, 550 mL CH4$gVS1, respectively. Methane production from RSW of all COD/SO4 2 ratio at pH 6e7 was higher than pH 5. Cetecioglu et al. [34] report that the COD/SO4 2 ratios have a small effect on the methane production, although the presence of sulfate may affect the metabolic pathways of methanogens. The long lag phase of 0.70e1.39 d1 was achieved at all COD/SO4 2 ratios from first stage effluent with without trace element addition. The long lag phase was caused by inhibition and nutrient deficiency during the process and resulting in a negative impact on hydrolysis and acidification of organic matter [35]. The first stage effluent was deficient in trace metals for methane production. Trace element solution was supplemented to the first stage effluent for improving methane production. Trace element supplementation improved hydrolysis rate (kh ¼ 0.23 d1) compared with without trace element supplementation (kh ¼ 0.19 d1). In addition, the maximum methane production rate (Rmax) of 79.01 mL CH4$gVS1$d1 and shortest lag phase of 0.37 d1 was observed at the of COD/SO4 2 ratio of 13 and initial pH of 5 with trace element supplementation. The COD/SO4 2 ratio of 13 and
7 6 5 7 6 5 7 6 5 7 6 5
Methane production from RSW in the second stage
13
The increasing of sulfate concentration in RSW resulted in more COD consumption by SRB for sulfate reduction and decreasing VFAs concentration in first stage effluent. The sulfide and hydrogen sulfide production in the first stage could react with the metal in the fermentation broth and reduce its availability. The remaining trace elements in the first stage effluent were low when compared with raw RSW (Table 3). The highest trace metals of 11.74 mg$L1 was observed at COD/SO4 2 ratio 13 with initial pH 7. The COD/SO4 2 ratio of 1 has the lowest amount of trace elements of 4.97 mg$L1 at initial pH 7. Decreasing of COD/SO4 2 ratio resulted in a decreasing amount of trace metals in fermentation effluent. The H2S produced by SRB consortium could precipitate trace metals as metals sulfide [18]. Additionally, sulfide could combine with iron in cytochromes affecting bacteria functionality [33]. Therefore, the trace element could be added into the second stage reactor for better methane production.
CH4 production with trace element supplementation
(5)
2
4H2 þ H2 SO4 /H2 S þ 4H2 O DG0 ¼ 194:61 kJ
(4)
1
DG0 ¼ 108:3 kJ
CH4 production without trace element supplementation
CH3 COOH þ H2 SO4 /2CO2 þ 2H2 O þ H2 S
(3)
COD/SO4 2 pH ratios
þ 0:75H2 S DG0 ¼ 74:3 kJ
Table 4 e Methane production, kinetics parameters, and reactor performance of first stage effluent with and without trace element supplementation in second stage anaerobic fermentation.
C2 H5 COOH þ 0:75H2 SO4 /CH3 COOH þ CO2 þ H2 O
Rmax l (d) CH4 yield Kh (d ) COD removal SO4 removal l (d) CH4 Yields Kh (d1) COD removal SO4 2 removal Rmax (mL CH4,gVS1) efficiency (%) efficiency (%) (mL CH4,d1) (mL CH4,gVS1) efficiency (%) efficiency (%) (mL CH4,d1)
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Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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Fig. 5 e Cumulative methane yields in the second stage of first stage effluent without trace element supplementation at COD/SO4 2 ratio 13 (a), 3 (b), 2 (c), and 1 (d). initial pH 5 has a methane yield of 723 mL CH4$gVS1. Trace element supplementation into first stage effluent was increased methane production rate and methane yield with reduction of lag phase at all COD/SO4 2 ratio and all initial pH tested. Especially methane production at initial pH 5 and COD/ SO4 2 ratio of 13 was the biggest improvement with trace element supplementation (Fig. 6). Enhancement of methane production of 16e48% was achieved from trace element supplementation into first stage effluent. The trace element has stimulated the activities of archaea population. Methanosarcina siciliae and Methanoculleus bourgensis were a positive response to nickel/molybdenum/boron (Ni/Mo/B) addition [36]. Linville et al. [37] reported that trace element supplementation could enhance methane yield up to 56% of sewage sludge in anaerobic digestion. Facchin et al. [38] report that
Table 5 e Performance of biohythane production from RSW at COD/SO4 2 ratio of 13 and pH 6 by two-stage anaerobic digestion. Parameters H2 production (L,L1) CH4 production (L,L1) Biogas production (L,L1) H2 yield (mL,gVS1) CH4 yield (mLg,VS1) SO4 2 removal efficiency (%) COD removal efficiency (%) COD conversion to H2S (%)
First stage
Second stage
Biohythane systems
0.57 e 2.23 101 e 87 11 8.2
e 3.3 6.6 e 629 9 85 2.4
0.57 3.3 8.83 101 629 96 96 16
food waste supplemented with a mixture of trace element (Co, Mo, Ni, Se, W) can increase the methane production of 45e65%. The rice straw with Co and Ni supplementation enhanced methane yield up to 11.6% [39]. The available trace metals could act as a co-enzyme in the anaerobic degradation process and enhance the growth rate of microorganisms [40,41]. The distribution of COD and sulfur from the RSW as a substrate for biohythane by two-stage anaerobic digestion was shown in Fig. 7. The RSW at COD/SO4 2 ratio at 13 and pH 6 was optimum for biohythane production and sulfate removal without trace element addition. The COD removal of RSW at COD/SO4 2 ratio of 13 and initial pH of 6 in the first stage, the second stage without trace element supplementation, and the second stage with trace element supplementation was 11.3%, 72.4, and 76.2% respectively. The sulfate removal of COD/SO4 2 ratio at 13 pH 6 in the first stage, the second stage without trace element supplementation, and the second stage with trace element supplementation was 87%, 10%, and 9% respectively. The acidogenic stage of this condition has 2.9% of COD converting to hydrogen and 8.2% of COD consuming for hydrogen sulfide production. The methanogenic stage of this condition has 70% of COD converting to methane and 2.4% of COD consuming for hydrogen sulfide production. The methanogenic stage of this condition with trace element supplementation has 74% of COD converting to methane and 2.2% of COD consuming for hydrogen sulfide production. Total COD consuming for hydrogen sulfide production of both stage was 10.4e10.6%. Sulfate converting to hydrogen sulfide in acidogenic stage and methanogenic stage
Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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Fig. 6 e Cumulative methane yields in the second stage of first stage effluent with trace element supplementation at COD/ SO4 2 ratio 13 (a), 3 (b), 2 (c), and 1 (d).
Fig. 7 e COD balance and sulfur balance of biohythane production from rubber sheet wastewater at COD/SO4 2 ratio of 13 and initial pH 5 and 6 by two-stage anaerobic digestion. Please cite this article as: Promnuan K et al., Simultaneous biohythane production and sulfate removal from rubber sheet wastewater by two-stage anaerobic digestion, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.237
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was 87% and 9e10%, respectively. The RSW at COD/SO4 2 ratio of 13 and initial pH 5 was required trace element supplementation for better methane production. COD removal in the second stage without and with trace element supplementation was 58% and 86%, respectively. Trace element supplementation could increase microbial biomass from 0.25 g$L1 to 0.37 g$L1. The COD removal of RSW at COD/SO4 2 ratio of 13 and initial pH 5 in the first stage, the second stage without trace element supplementation, and the second stage with trace element supplementation was 10.2%, 66.7, and 95.2%, respectively. The sulfate removal of COD/SO4 2 ratio of 13 and pH 5 in the first stage, the second stage without trace element supplementation, and the second stage with trace element supplementation was 77%, 22%, and 15%, respectively. The acidogenic stage of this condition has 2.7% of COD converting to hydrogen and 6.7% of COD consuming for hydrogen sulfide production. The methanogenic stage of this condition has 55% of COD converting to methane and 2.7% of COD consuming for hydrogen sulfide production. The methanogenic stage of this condition with trace element supplementation has 83% of COD converting to methane and 2.2% of COD consuming for hydrogen sulfide production. Total COD consuming for hydrogen sulfide production of both stage was 8.9e9.4%. Sulfate converting to hydrogen sulfide in the acidogenic stage and the methanogenic stage was 77% and 15e22%, respectively. The biohythane production was 8.83 L$L1 of RSW with H2 and CH4 production of 0.57 and 3.3 L$L1 of RSW, respectively, with the remaining as CO2 (Table 5.). The biohythane production was inlined with Kongjan et al. [5] results that 2.25 L$L1 skim latex serum (SLS) of hydrogen and 6.41 L$L1 SLS of methane was achieved via the two-stages anaerobic digestion process. The hydrogen and methane production from skim latex serum (SLS) by two-stage anaerobic digestion was 1.57 L$L1 SLS and 12.2 L$L1 SLS, respectively [7]. Microbial responsible for biohythane production from rubber processing wastewater in the first stage was dominated by Enterobacter sp., Caldicellulosiruptor sp., Clostridium sp. and Thermoanaerobacterium sp. [42], while in the second stage was dominated by Methanosarcina sp. and Methanoculleus sp. [43]. The two-stages anaerobic digestion could promote biohythane production and reduce hydrogen sulfide toxicity in the second stage. Table 5 showed SO4 2 removal efficiency in the first stage up to 87% with 8.2% of COD consuming for H2S conversion. The COD consumption for H2S production in the second stage was only 2.4% of total COD removal efficiency. Lu et al. [14] showed the high stability biogas production performance was achieved at COD/SO4 2 ratio of 2e10 with COD consuming for H2S production of 0.4e1.5%. Normally, the SRB groups is higher outcompete for nutrients than methanogenic archaea [42]. Therefore, separation of steps is suitable for preventing competition between SRB and methanogenic archaea.
Conclusions The SRB consortium composed of Desulfovibrio sp. Desulfitibacter sp. Dethiosulfatibacter sp. and Clostridium sp. was effective for sulfate removal and hydrogen production.
Simultaneous biohythane production and sulfate reduction from RSW was obtained at COD/SO4 2 of 13 and initial pH of 6 by the two-stage anaerobic digestion process. Hydrogen yield and sulfate removal efficiency in the first stage were 101 mL H2$gVS1 and 87%, respectively. Methane yield and methane production rate in the second stage were 629 mL CH4$gVS1 and 45 mL CH4$d1, respectively. The COD in RSW was converted to hydrogen and methane for 2.7e2.9% and 70e80%, respectively while 9.4e10.6% of COD was consumed for sulfate reduction to hydrogen sulfide. Two-stage anaerobic fermentation obtained simultaneous biohythane production and sulfate removal with efficiently for energy recovery from RSW.
Acknowledgments The authors would like to thank the Research and Development Institute Thaksin University, Thailand Science Reserach and Inovation, and Thailand Research Fund through the Research and Researchers for Industries (PHD59I0069), TRF Senior Research Scholar (Grant No. RTA6280001) and TRF Mid-Career Research Grant (Grant No. RSA6180048) for financial support.
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