Hydrogen sulfide removal from biogas in biotrickling filter system inoculated with Paracoccus pantotrophus

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Hydrogen sulfide removal from biogas in biotrickling filter system inoculated with Paracoccus pantotrophus Jinjuta Juntranapaporn a,b, Nunthaphan Vikromvarasiri c,**, Cheema Soralump a, Nipon Pisutpaisal b,d,* a

Department of Environmental Engineering, Kasetsart University, Bangkok, 10900, Thailand Biosensor and Bioelectronics Technology Centre, King Mongkut's University of Technology North Bangkok, Bangkok, 10800, Thailand c Department of Transdisciplinary Science and Engineering Major in Global Engineering for Development, School of Environment and Society, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-Ku, Tokyo 152-8550, Japan d Department of Agro-Industrial, Food and Environment Technology, Faculty of Applied Science, King Mongkut's University of Technology North Bangkok, Bangkok, 10800, Thailand b

article info

abstract

Article history:

Biogas generated from anaerobic digestion can be employed as fuel in either industrial

Received 28 November 2018

boilers or electric generators. The presence of a trace amount of hydrogen sulfide (H2S) in

Received in revised form

the biogas can severely impact on the biogas applications. Biotrickling filter (BF) inoculated

1 March 2019

with a pure strain of sulfide oxidizing bacterium, Paracoccus pantotrophus NTV02 was used

Accepted 10 March 2019

to remove H2S from the biogas in the present work. P. pantotrophus was successfully

Available online xxx

immobilized on plastic packing media in BF after 228 h inoculation. H2S containing biogas and liquid media was subsequently fed into BF under countercurrent direction with fixed

Keywords:

inlet gas flowrate and liquid media recirculation rate at 0.5 LPM (120 s EBRT) and 3.6 L/h.

Paracoccus pantotrophus

H2S in the biogas was varied in the range of 100e2,000 ppmv. BF achieved H2S removal

Thiosulfate

efficiency at 98.3e99.7% at its inlet concentration ranges of 300e1,500 ppmv of H2S. The

Hydrogen sulfide

removal efficiency (99.5%) little declined as H2S inlet reached 2,000 ppmv. The maximum of

Biogas

elimination capacity was 82.98 g H2S/m3$h at the loading rate of 83.58 g H2S/m3$h.

Biotrickling filter

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Biogas produced by the anaerobic digestion (AD) of organic wastes with 50e75% CH4 composition, can be used as fuel in

power plants, electric generators, and vehicles. It is considered as a versatile renewable and sustainable energy source. One cubic meter of biogas normally provides up to 5000e5500 kcal for heating. The uses of biogas are limited by the presence of trace H2S, which is biologically generated from the anaerobic digestion

* Corresponding author. Department of Agro-Industrial, Food and Environment Technology, Faculty of Applied Science, King Mongkut's University of Technology North Bangkok, Bangkok, 10800, Thailand. ** Corresponding author. E-mail addresses: [email protected] (N. Vikromvarasiri), [email protected], [email protected] (N. Pisutpaisal). https://doi.org/10.1016/j.ijhydene.2019.03.069 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Juntranapaporn J et al., Hydrogen sulfide removal from biogas in biotrickling filter system inoculated with Paracoccus pantotrophus, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.069

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of proteins and other sulfur-containing feedstocks during the biogas production. Low H2S concentration can affect living organisms; cause corrosion in combustion engines and nuisance odor of rotten eggs; and can be a pollutant in the air [1,2]. There are many methods for eliminating H2S in biogas. Physical and chemical methods have high operating costs and produce secondary pollutants that also need to be eliminated before discharge to the environment. The chemical methods need the addition of large amounts of chemical reagents for biogas treatment [3]. For these reasons, biological treatment processes have more advantages because these methods have a lower operational cost, eco-friendly, and produce fewer by-products [4]. The main biological treatment processes for H2S elimination are biofilters, bioscrubbers, and biotrickling filters. These processes employ chemotrophic bacteria, sulfide-oxidizing bacteria (SOB), with ability to oxidize H2S. The oxidizing reactions can be described by the equations as follows [5,6]: H2S 4 Hþ þ HS (non-biological) DG0 ¼ 45.9 kJ

(1)

HS þ 0.5O2 / S0 þ OH (biological) DG0 ¼ 209.4 kJ

(2)

þ 0 HS þ 2O2 / SO2 4 þ H (biological) DG ¼ 732.0 kJ

(3)

The biotrickling filter, a highly effective bioreactor, can cope with high H2S concentrations at higher flow rates, because the recirculating liquid in the system can improve the removal efficiency by controlling pH, nutrients, and sulfate concentration [7]. The biotrickling filter is operated by feeding air pollutants into the column before passing through the packing media containing microorganisms that can degrade the pollutants in the air [8]. From the previous study, Paracoccus pantotrophus was isolated and examined its sulfur-oxidizing activities. P. pantotrophus is chemolithoautotrophic and can grow in the medium containing sulfide and thiosulfate under aerobic conditions. The optimum temperature and pH for its growth are 37  C (range 15e42  C) and pH 8 (range 6.5e10.5), respectively [9]. This research examined the immobilization method of P. pantotrophus on the packing media using a thiosulfate medium as the recirculating liquid. The performance of P. pantotrophus for the H2S removal efficiency in biogas was evaluated in both short-term and long-term experiments.

Na2S2O3$5H2O [10]. pH was adjusted to 7 by 1 N NaOH and 1 N HCl. This medium was sterilized by autoclaving at 121  C, 15 psi for 15 min. Medium agar was prepared by adding bacto agar (16 g/L) to TMM broth. TMM broth was used in the immobilization process as recirculating liquid in the biotrickling filter.

Biotrickling filter preparation and abiotic experiments Biotrickling filter composes of glass column (4.75 cm inner diameter and 72 cm height) containing random packing media (GEA2H Water Technologies GmbH) with 1 L working volume. Previous to use, a glass column was sterilized by 95% ethanol for 5 min. Packing media are made from high-density polyethylene (HDPE; 1.2 cm diameter, surface 859 m2/m3), which can resist acid corrosion and high temperature. These media were weighed and sterilized by autoclaved at 15 psi and 121  C for 15 min and dried before packing into the column. An abiotic experiment was conducted to test thiosulfate and sulfide-oxidizing reaction of biotrickling filter system absent of P. pantotrophus activities. TMM medium without microbial inoculation was fed into the biotrickling filter system as recirculating liquid (60 mL/min of liquid flow rate), and 0.5 LPM of air was fed pass through 0.2 mm membrane filter to the system for the abiotic oxidizing reaction. This experiment was performed for 48 h. After that, sulfide oxidation in abiotic experiment was investigated by changing recirculating liquid to be TMM medium without thiosulfate and microbial inoculation, and fed a synthetic biogas (60% methane (CH4), 40% carbon dioxide (CO2), 5000 ppmv H2S balanced with nitrogen gas (N2) in order to adjust H2S concentration), which was mixed with air (2.5% v/v), into the system. H2S concentration was controlled at 100 ppmv. All gas tanks have valves, pressure gauges, and flow meters to control the gas flow rates. The flow rate of recirculatiing liquid and mixed gases were 3.6 L/h and 0.5 LPM, respectively. The empty bed residence times (EBRT) was 120 s. This experiment was conducted for 48 h. Inlet and outlet gases were monitored every 12 h. Liquid samples of each experiment were periodically collected and determined pH and sulfate concentrations.

Immobilization process of P. pantotrophus into biotrickling filter system

Materials and methods Paracoccus pantotrophus stain NTV02 P. pantotrophus stain NTV02 (KJ027465) was isolated and purified from the wastewater treatment plant of the leather industry (Samut Prakan province, Thailand) [10]. This pure culture stain was preserved in 15% glycerol at 20  C, and it was re-activated by culturing in the thiosulfate mineral medium (TMM) at 30  C, 180 rpm and 10% v/v of inoculum were used for subculture every 5e7 days.

Thiosulfate mineral medium Thiosulfate mineral medium (TMM) contains (g/L): 4.0 K2HPO4, 4.0 KH2PO4, 0.4 NH4Cl, 0.2 MgCl2$6H2O, 0.01 FeSO4$7H2O and 10

P. pantotrophus was reactivated from 20  C storage by inoculating in TMM medium containing 1 g/L glucose to enhance growth and sulfur oxidation activity [11]. After reactivation process, P. pantotrophus biomass grown at 4.58  107 CFU/mL was transferred to biotrickling filter system by adding into a fresh TMM medium, which was without glucose adding in order to avoid contamination from other organotrophs and heterotrophs during the operation of biotrickling filter. This medium was sprayed on the top of the packing media in the reactor and was continuously recirculated by the peristaltic pump at 3.6 L/h of flow rate, and airflow was 0.5 LPM. Fresh TMM medium was replaced when pH nearly decreased to 6.5, which was a suitable range of pH for P. pantotrophus. The reactor was operated in the room temperature (31.0 ± 1  C). The liquid samples were

Please cite this article as: Juntranapaporn J et al., Hydrogen sulfide removal from biogas in biotrickling filter system inoculated with Paracoccus pantotrophus, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.069

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Analytical techniques

periodically collected to monitored sulfate concentration, colony forming unit (CFU/mL), and pH in order to examine the performance of biotrickling filter.

Microbial density in the recirculating medium was monitored by colony forming units by drop plate technique (CFU/mL) [12]. Sulfate concentration was measured by the turbidimetric method [13]. pH was observed by pH meter (Mettler Toledo FE20 Five Easy, Germany). Gas components were determined both inlet and outlet gas concentrations by the biogas analytic handheld instrument (GFM416, Gas Data, England).

H2S removal by P. pantotrophus This experiment was performed in order to primarily test the ability of P. pantotrophus for H2S removal from synthetic biogas in the biotrickling filter system. The synthetic biogas, which composed of CH4, CO2, N2, H2S, and air was used as an inlet gas to the biotrickling filter by introducing it upwards into a packed bed. The level of H2S concentrations was varied in a range of 50e300 ppmv, and the gas flow rate was 0.5 LPM. Thiosulfate in TMM medium in the recirculating liquid was removed after the immobilization process because this study preferred to use energy sources derived from only H2S oxidation for microorganism activities. The recirculating liquid was continuously pumped and sprayed to the upper packaging media, and then it was recirculated during the experiment with flow rates 3.6 L/h. The experiments were conducted at room temperature (32.0 ± 0.5  C) and operated for 490 min pH was not controlled in order to observe the activities of P. pantotrophus. The parameters, which were H2S inlet and outlet concentrations, pH, CFU/mL, and sulfate concentrations, were monitored for this experiment.

Results and discussion Abiotic experiments An abiotic reaction of thiosulfate oxidation can occur in the biotrickling filter as shown in Eq. (4) [14]. Therefore, TMM broth without P. pantotrophus inoculation was used for the investigation. þ 0 2 S2O2 3 þ H2O þ 2O2/ 2SO4 þ 2H DG ¼ 818.3 kJ

The abiotic experiment demonstrated that the main oxidation reaction of thiosulfate to sulfate occurred from the activities of P. pantotrophus. The results showed that a negligible level of sulfate was produced (4.16 mg/L) after 48 h of operation, and the pH value showed almost no change during the experiment (pH 7.98e8.00) (Fig. 1A). The average H2S removal efficiency was 5%, which placed the removal efficiency in the range of 0e9.5% (Fig. 1B). The sulfate concentration gradually increased from 0.0 to 5.2 mg/L, and the pH was slightly decreased from 8.00 to 7.98 after 48 h operation. These results showed that the oxidation of thiosulfate and sulfide from the abiotic reaction in the biotrickling filter was negligible.

Long-term experiment of H2S removal by P. pantotrophus This experiment was performed in order to explore the H2S removal efficiency of P. pantotrophus in the biotrickling filter. The immobilization process for this experiment operated the same as the procedure described in the preliminary investigation. Synthetic biogas was fed at 0.5 LPM of flow rate with the empty bed residence times (EBRT) of 120 s H2S concentrations were varied at the level of 100e2000 ppmv. TMM without thiosulfate was recirculated in the system at 3.6 L/h. The experiments were operated at room temperature (30.5 ± 1.5  C) for 120 h. Gas and liquid samples were collected and analyzed as previously experiment.

Immobilization process of P. pantotrophus on reactor media The immobilization process was successfully performed by inoculating P. pantotrophus on the packing media in the

12

A pH, [Sulfate] (mg/L)

8 6 4 2 0 0

12

24 Time (h)

36

48

120

B

10

100

8

80

6

60

4

40

2

20

0

[H2S inlet] (ppmv), H2S removal efficiency (%)

pH, [Sulfate] (mg/L)

12 10

(4)

0 0

12

24

36

48

Time (h)

Fig. 1 e The change of pH (-) and sulfate concentration (A ) in the abiotic experiment of thiosulfate oxidation (A). The change of inlet concentration of H2S (C), pH (-), sulfate production (A ), and H2S removal efficiency (D) in the abiotic experiment by using synthetic biogas (B). Please cite this article as: Juntranapaporn J et al., Hydrogen sulfide removal from biogas in biotrickling filter system inoculated with Paracoccus pantotrophus, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.069

4 5000

12

4000

10

3000

8

2000

6

1000

4

0

0

24

48

72

96

120

144

168

192

216

240

pH, LogCFU/mL

[Sulfate] (mg/L)

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2

Time (h)

Fig. 2 e The changes of pH (-), suspended cells (:), and sulfate production (A ) in the immobilization process of P. pantotrophus in the biotrickling filter.

biotrickling filter. P. pantotrophus grown at the logarithmic phase was inoculated and continuously recirculated during this process. TMM medium in the liquid recirculation was replaced when the pH level dropped lower than is suitable for P. pantotrophus growth (pH 6.5) (Fig. 2). The results showed that the change of pH corresponded with sulfate production. The pH level was controlled in the range of 6.43e8.02 throughout the experiment. The number of suspended cells reached 1.92  1010 CFU/mL after 120 h operation, and it remained stable after 144 h until the end of the experiment. The average number of suspended cells was 7.42  108 CFU/mL. The number of cells suspended in the liquid medium dropped after the first replacement of new medium, the drop of suspended cell in the liquid medium was not significant in the subsequent replacements. The immobilization process was continued until the sulfate production rate approached a steady state condition after 180 h operation, after which the sulfate production rates were not significantly different after TMM medium replacement. The average sulfate production rate was 64.02 ± 4.60 mg/L/h, and the sulfate concentrations before changing the TMM medium in liquid recirculation were similar with the average of 3,222.22 ± 230 mg/L.

H2S removal by P. pantotrophus P. pantotrophus is a sulfur-oxidizing bacterium that can oxidize the reduced sulfur compounds such as sulfide, elemental sulfur, thiosulfate, or organic sulfur by using oxygen as an electron acceptor in order to gain energy and support for their growth [15]. Free-thiosulfate TMM medium was replaced in the recirculating liquid with 3.6 L/h recirculation rate after the immobilization process in order to provide H2S as the sole energy source for P. pantotrophus. The synthetic biogas was mixed with air and fed into the biotrickling filter. The oxygen level in the system was 2.5e3.0% v/v, which was excessive for the H2S oxidation activity throughout this experiment. Inlet gas flow rate was at 0.5 LPM (120 s of EBRT). The H2S removal efficiency was 90e95% in the earliest stage when the inlet H2S concentrations were varied from 50 to 300 ppmv, then it was increased to 97.5e98.3% after the inlet H2S concentration reached 200e300 ppmv (Fig. 3). The pH value was slightly decreased, which correlated with the amount of total sulfate production (109.38 mg/L). Oxidation of H2S in the abiotic experiment was minimal; thus the main activities of H2S removal in the system were derived from the oxidation activities of P. pantotrophus.

10

250

8

200

6

150

pH

[Inlet H2S] (ppmv), [Sulfate] (mg/L), H2S removal effiency (%)

300

4

100

2

50 0

0

100

200

300

400

500

0

Time (min)

Fig. 3 e The change of inlet concentration of H2S (C), pH (-), sulfate production (A ), and H2S removal efficiency (D) in the preliminary investigation by using biotrickling filter inoculated with P. pantotrophus. Please cite this article as: Juntranapaporn J et al., Hydrogen sulfide removal from biogas in biotrickling filter system inoculated with Paracoccus pantotrophus, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.069

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100

2500

80

2000

70 60

1500

50 40

1000

30 20

500

pH, H2S removal efficiency (%)

Inlet [H2S] (ppmv), [Sulfate] (mg/L)

90

10 0

0 0

24

48

72

96

120

Time (h)

Fig. 4 e The inlet concentration of H2S (C), pH (-), sulfate production (A ), and H2S removal efficiency (D) in the long-term experiment by using biotrickling filter inoculated with P. pantotrophus.

Long-term experiment of H2S removal by P. pantotrophus The results of the preliminary experiment showed that P. pantotrophus was able to oxidize H2S in the biotrickling filter system. In this experiment, the immobilization process and gas running procedures were operated in the same manner as in the preliminary investigation, but the inlet H2S concentrations were varied at a higher level (100e2,000 ppmv). Fig. 4 showed that P. pantotrophus could remove 90% of 100 ppmv of H2S at the first stage, then the removal efficiency was increased in the range of 98.3e99.7% when the inlet H2S concentration increased from 300 to 1500 ppmv after 84 h of operation. The results showed that the H2S removal efficiency tended to increase with increased operation times. At 2,000 ppmv inlet concentration, the removal efficiency was 99.5% after 96 h of operation. Unexpectedly, the removal efficiency

decreased to 90% after 108 h operation and continuously declined to 75.0% after 120 h operation. H2S outlet concentration was 200e500 ppmv during the declination period. Therefore, H2S concentration at the level of 2,000 ppmv was considered to limit the biotrickling filter system in this study. Regarding the sulfate production, the sulfate concentration was gradually increased to 371.53 mg/L after 96 h operation. However, the sulfate production discontinued when the H2S concentration reached 2,000 ppmv. The result corresponded to the H2S removal efficiency. The pH was slightly dropped from 7.94 to 6.61 at the end of the operation, which was in the optimum pH range for P. pantotrophus (pH range 6.5e10.5) [9]. The previous study reported that P. pantotrophus was able to tolerate sulfate concentration above 3,000 mg/L [10]. pH and sulfate concentration in the recirculating medium can be controlled to prevent the threshold toxic to P. pantotrophus.

Table 1 e Comparison of the maximum H2S elimination capacity of current study with previous researches. Inlet gas

[H2S] (ppmv)

EBRT (s)

Microorganisms

Activated sludge Acinetobacter sp. and Alcaligenes faecalis Thiobacillus thioparus Acidithiobacillus thiooxidan Desulfurization bacteria Thiobacillus denitrificans

H2SþAir

0e190

14e84

H2SþN2

10e100

13e32

H2SþAir

10e90

9e60

H2SþAir

66

12e15

Biogas

500e600

84

H2SþAir

20e157

16

H2SþN2

100e2,000

69

Synthetic Biogas Synthetic Biogas

0e2,040

120

100e2,000

120

Activated sludge Halothiobacillus neapolitanus Paracoccus pantotrophus

Packaging materials

H2S elimination capacity (g H2S/m3$h)

References

Polypropylene pall ring Polypropylene pall ring

24.0

[20]

19.2

[21]

Lava rock

32.5

[22]

polyurethane foam BioSulfidEx

28.0

[23]

32.3

[24]

Open-pore polyurethane foam Glass ring

22.0

[25]

23.7

[19]

Random packing media (HDPE) Random packing media (HDPE)

79.0

[26]

83.90

This study

Please cite this article as: Juntranapaporn J et al., Hydrogen sulfide removal from biogas in biotrickling filter system inoculated with Paracoccus pantotrophus, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.069

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Firstly, pH can be controlled by daily adjustment to the optimum pH, and secondly replacement of the new recirculating medium to remove sulfate, and proton from the system. Pressure drop was not observed during the operation, and greater 2.0% oxygen was detected in the outlet gas. The breakdown of the system did not arise from pH, the clogging in the system, and oxygen supply, but it might instead have resulted from the excess H2S concentration. Previous studies reported that a toxic effect of higher concentrations of H2S could negatively contribute the removal efficiency as a shock loading effect [16]. In addition, Barton et al. (2014) reported that the high concentration of sulfide could affect to decrease the transcription of ribosomal protein-encoding genes of Desulfovibrio vulgaris strain Hildenborough, which corresponded to its growth rate [17]. Additionally, the limit of biogas utilization concerns the amount of H2S contained in the biogas. Some applications such as heating or boilers can resist 1,000 ppmv H2S, whereas some of the internal combustion engines required lower than 100e500 ppmv H2S depending on the type of engine [18,19]. Therefore, the biotrickling filter inoculated with P. pantotrophus in this study may have the potential to be used for some applications in which biogas contains H2S concentrations in the range of 1,500e2,000 ppmv, and the requirement for H2S concentration is lower than 5e500 ppmv. The maximum elimination capacity of 82.98 g H2S/m3/h at the loading rate of 83.58 g H2S/m3/h in this study. exerts the greater performance of the biotrickling filter compared to the previous studies (Table 1).

Conclusions The immobilization process of P. pantotrophus in a biotrickling filter system reached the steady state condition after 180 h operation. The removal of H2S from biogas by P. pantotrophus in the biotrickling filter was succeeded at the maximum H2S inlet concentration up to 2,000 ppmv. The maximum elimination capacity was 82.98 g H2S/m3$h at the loading rate of 83.58 g H2S/m3$h, which overcame the performances in the previous studies.

Acknowledgments The authors gratefully acknowledged the financial support from King Mongkut's University of Technology North Bangkok (KMUTNB-61-KNOW-043), Thailand Research Fund (DBG6280001), the Office of Higher Education Commission, the Royal Society (NAFyR2y180513), and the Royal Academy of Engineering (IAPP1617y9).

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Please cite this article as: Juntranapaporn J et al., Hydrogen sulfide removal from biogas in biotrickling filter system inoculated with Paracoccus pantotrophus, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.069