Hsiu-Wen Ou1, Ming-Shean Chou2, Hsiao-Yu Chang 1

Institute of Environmental Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
Livestock Research Institute, Council of Agriculture, Executive Yuan, Tainan 71246, Taiwan


Received: December 27, 2019
Revised: February 17, 2020
Accepted: February 17, 2020
Download Citation: ||https://doi.org/10.4209/aaqr.2019.12.0647 

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Cite this article:

Ou, H.W., Chou, M.S. and Chang, H.Y. (2020). Removal of Hydrogen Sulfide from Biogas Using a Bubbling Tank Fed with Aerated Wastewater. Aerosol Air Qual. Res. 20: 643-653. https://doi.org/10.4209/aaqr.2019.12.0647


HIGHLIGHTS

  • The study developed a scrubber for removing over 80% of H2S in a biogas.
  • Increasing scrubbing liquid pH to 8.0 improved the H2S removal efficiency to 99%.
  • CO2 in the biogas is responsible for the pH decrease of the scrubbing liquid.
 

ABSTRACT

The work develops a simple bubbling tank scrubber that is fed with aerating wastewater for the removal of hydrogen sulfide (H2S) in biogas. A 2,000 L plastic tank, in which fixed liquid levels 0.80 and 1.0 m and volumes of 1.1 and 1.4 m3 was kept, was used for the scrubbing tests. A polyvinyl chloride (PVC) pipe with holes of 1 mm diameter was used to sparge the biogas into the tank liquid. Results indicate that with an influent liquid of pH 7.5–7.7, a flow rate of 23–25 L min–1, influent biogas flow rates of 0.050–0.200 m3 min–1, and a H2S concentration of 907 ± 212 ppm, the pH of the effluent liquid stabilized at 6.6–6.9. With gas/liquid rate ratio of 2–8 m3 m–3 liquid and volumetric gassing intensities of 0.04–0.20 m3 m–3 liquid min–1, average H2S removals of 86–71% were obtained. Absorption of CO2 in the tested digester gas into the scrubbing liquid caused a decrease in the pH, thus decreasing the H2S removal efficiency. Increasing pH of the scrubbing liquid to 8.0 improved the H2S removal efficiency to as high as 99%.


Keywords: Hydrogen sulfide; Bubbling scrubbing; Digester gas; Air pollution control.


INTRODUCTION


Biogas contains H2S which is corrosive to power generators and other metal components. The presence of H2S is a concern to not only owners of anaerobic digesters but also workers in biogas plants. H2S reacts with oxygen in air to form H2SO4, which is corrosive to most metals in the presence of water. Sulfate in air is also a main component of secondary inorganic aerosols in the atmosphere (Jiang et al., 2018; Li et al., 2018a; Li et al., 2018b; Tseng et al., 2019)

Another concern is the conversion of H2S to sulfur dioxide (SO2) during the combustion of biogas in power generator (Potivichayanon et al., 2006; Beristain-Cardoso et al., 2008; Krischan et al., 2012; Smith and Ndegwa, 2012; Solcia et al., 2014). Thus, the presence of H2S has been identified as a technological barrier to the production and utilization of biogas.

Techniques for removing H2S from biogas or industrial waste gases comprise physical, chemical, and biological methods. Chemical methods, such as the oxidative absorption of H2S into an aqueous solution of ferric sulfate, involve expensive chemicals or require the management of the sulfur slurry that is formed from H2S (Krischan et al., 2010; Liu and Wang, 2017, 2019; Pei and Wang, 2019). Physical methods, such as water scrubbing or adsorption, require the replacement of saturated scrubbing water or activated carbon (Pipatmanomai et al., 2009; Lien et al., 2014; Huang et al., 2019; Mao et al., 2020). In terms of microbiological methods, the removal of H2S has been intensively studied. Most studies have focused on fixed-film processes such as the use of biofilters (BFs), or biotrickling filters (BTFs), and bioscrubbing towers (BSTs) (Gadre, 1989; Nitta and Hirura, 1993; Yang and Allen, 1994; Jensen and Webb, 1995; Nishimura and Motoyuki, 1997; Potivichayanon et al., 2006; Liang and Liang, 2013; Lin et al., 2013; Su et al., 2013; Solcia et al., 2014; Su et al., 2014).

H2S in a gas stream can also be removed by bubbling the stream through gas spargers into an activated sludge tank. Water-soluble H2S is absorbed into the mixed liquor and subsequently degraded by the microorganisms in the liquor (Sublette et al., 1994; Chou et al., 2010). The bubbling method can be easily applied when the activated sludge system is located near the waste gas stream(s). The process can be used to remove H2S in gases that are emitted from anaerobic digesters, landfill fields, lift wells for sewage treatment, and paper and pulp plants (Syed et al., 2006; Zhang et al., 2018). Sublette et al. (1994) developed a process that uses flocculated Thiobacillus denitrificans to remove up to 1,500 ppm of H2S from sour gas. Using a 0.5-m3 pilot-scale bubble column, up to 97% of H2S was removed by complete oxidation to sulfate, which accumulated in the reactor medium. H2S removal was limited by mass transfer rather than the biodegradation rate of the system. Amano et al. (1999) used a suspension of T. thiooxidans with sodium citrate as a buffering agent to remove H2S from a gas stream. They obtained a removal rate of 0.96 g H2S m–3 d–1. Shimko et al. (1987) also proposed a patented process that involves bubbling a gas stream with 1,700–5,400 mg H2S m–3 and 1,800–5,100 mg CS2 m–3 into an activated sludge tank with a liquor depth of 3–4 m. Chou et al. (2010) used an activated sludge aeration tank (width × length × height = 0.4 m × 0.4 m × 3 m) with a 2 mm-orifice air sparger to treat gaseous H2S. They tested the operational stability and the relationships between the removal of H2S and the influent H2S concentration (50–900 ppm), aeration intensity (0.083–0.50 m3 m–3 min–1), liquid depth (0.5–3 m), and concentration of mixed-liquor-suspended solids (MLSS = 970–2,800 mg L–1). They experimentally obtained H2S removal efficiencies of 96% and < 98% at liquid depth values of 0.5 m and < 1 m, respectively, under their specified operational conditions. Their experimental results also revealed that no sludge bulking problem occurred at total sulfide loadings of 47–148 g S kg–1 MLSS d–1.

The bubbling approach requires a column or tank fed with a stream of activated sludge liquor that may be supplied from an existing aerating pond or fed with supplemental nutrients (such as carbon, nitrogen, and phosphorus) to sustain the microorganisms that oxidize the absorbed sulfide. This requirement limits the practical application of the approach because considerable attention must be paid to maintaining the health of the microorganisms. This study developed a simple bubbling tank scrubber that is fed with aerated wastewater for the removal of H2S from biogas that is vented from anaerobic digesters used to treat wastewater from dairy farms. The effects of the bubbling rate, liquid depth, and the pH of the scrubbing liquid on the degree and rate of H2S removal were investigated.


METHODS


The experimental setup comprised a full-scale bubbling tank and an influent gas supply system (Figs. 1 and 2). A 2,000-L plastic tank with an inner diameter of 1.33 m; fixed water levels of 0.80–1.0 m; and 1,100–1,400 L of scrubbing liquid was used for the scrubbing tests. A 2-inch polyvinyl chloride (PVC) perforated pipe was formed into a rectangle and used to sparge the biogas into the tank liquid. A total of 536 holes with a diameter of 1 mm and a constant pitch of 10 mm were present along the pipe, and four holes were present 90° apart along the perimeter of the pipe. Biogas from the digesters of a dairy wastewater plant was collected in a full-scale plastic bag. The biogas was filtered and then passed through a ring-type blower and sparged into the tank liquid. Aerating wastewater for the aerobic treatment of the effluent from anaerobic digesters was fed into the tank as a scrubbing liquid and flowed over the tank into the aerobic pond under the influence of gravity. Flow rates of both the gas and scrubbing liquid were regulated using rotameters. 

Fig. 1. Schematics of the experimental system.Fig. 1. Schematics of the experimental system. 

Fig. 2. The experimental bubbling tank and the gas sparger.
Fig. 2. The experimental bubbling tank and the gas sparger.

Initially, the tank was filled to a certain depth with the scrubbing liquid that was drawn from one of the aeration tanks; subsequently, the scrubbing liquid flow rate was adjusted to a preset value. A stream of biogas was then introduced into the tank and the flow rate was adjusted to a present value. H2S concentrations in the influent and effluent biogas and the pH of the effluent scrubbing liquid were measured at the end of 10-min fixed intervals until both the H2S concentration of the effluent biogas and the pH of the effluent scrubbing liquid reached quasi-steady values. Temperatures of the scrubbing liquid and gas were also measured during the operation. Concentrations of chemical oxygen demand (COD), suspended solids (SS), total solids (TS), dissolved oxygen (DO), and the basicity of the influent scrubbing liquid were measured during several runs.

Aqueous hydrogen sulfide has a pKa of 7.1 at 25°C in the following ionization reaction: H2S(aq) D HS + H+. Consistent with this pKa value, at pH 7, 55.7% of the total aqueous hydrogen sulfide ([H2S(aq)] + [HS]) is in an un-ionized state: α = [H2S(aq)]/([H2S(aq)] + [HS]) = 1/(1 + 107–7.1) = 0.557. At a higher pH, such as 8, α = 1/(1 + 108–7.1) = 0.112. Thus, a scrubbing liquid with a higher pH and acidic buffering capacity favors the chemical absorption of H2S. In the present study, for some cases, the scrubbing liquid was supplemented with sodium hydroxide solution (45%) by a dosing pump that was actuated by a pH controller to a preset pH range to test the effect of pH on H2S removal efficiency. Either calcium or magnesium hydroxide can also be used for pH control. However, CaCO3 or MgCO3 solids may form and precipitate in the scrubbing liquid and interfere with the operation. Sodium bicarbonate and carbonate are more expensive than sodium hydroxide.

H2S concentrations in the gas samples were measured using detection tubes (H2S detector tubes, Gastec Corporation, Japan) and a portable multiple gas analyzer (Dräger X-am 7000, Dräger Safety AG & Co. KGaA, Germany). The gas analyzer could also provide data on gaseous CH4, CO2, and O2. Data from 25 samples of the influent digester gas to the tank indicated CH4, CO2, and O2 contents of 64.5 ± 2.7%, 28.2 ± 1.5%, and 0.53 ± 0.22%, respectively, and H2S content was 1090 ± 300 ppm. The detected gases accounted for > 93% of all the components, with the remaining gases likely to be N2, H2O, NH3, and H2 (Seadi et al., 2008). Both detection tubes and the analyzer were calibrated using standard H2S gas at concentrations of 5, 10, 20, and 50 ppm. Liquid pH was detected using a pH meter (pH/mV Pocker Meter pH 330i, WTW, Germany), and COD, SS, TSS, DO, and basicity were measured according to the APHA (1989).


RESULTS AND DISCUSSION


Table 1 details the scrubbing liquid qualities and Table 2 details the properties of the raw and scrubbed biogases of the present study. 

Table 1. Scrubbing liquid qualities.

Table 2. Some operation data.

In general, the mechanistic model developed by Bielefeldt and Stensel (1999) describes the removal of VOCs from a contaminated gas stream sparged into a completely mixed activated sludge reactor as a function of the gas-liquid mass transfer and liquid VOC concentrations. The model can be modified and applied to the present system as follows:

where C and Co represent H2S concentrations in the exit and the influent biogas streams (mg m–3), respectively; m is the dimensionless Henry’s law coefficient; x is the molecular H2S concentration in the liquid phase (mg m–3); KGa is the overall volumetric mass transfer coefficient of H2S in biogas (min–1); Z is the depth of the liquid over the sparger (m); G is the influent gas flow rate (m3 min–1), and A is the cross-sectional area of the sparging tank (m2). In liquid phase, molecular hydrogen sulfide ionizes according to the following equations: 

Un-dissociated or molecular H2S in water (x or [H2S]) can then be related to the total sulfide ([H2S] + [HS] + [S2–]) in water by the following equation: 

Table 3 shows dependence of f on water pH. Water with a higher pH gives a lower x or [H2S] value which favors the absorption of gaseous H2S, according to Eq. (1). In addition, molecular or ionized H2S in the liquid may be chemically or biologically oxidized to elementary sulfur, sulfite, or sulfate by utilizing molecular oxygen in the liquid. The reactions help to gaseous H2S removal by enhancing its transfer to the liquid.

According to Eq. (1), bubbling gas flow rate (G), influent gas H2S concentration (Co), liquid depth (Z), mass-transfer coefficient (KGa), molecular H2S concentration in the liquid phase (x, depends on pH), and m (depends mainly on liquid temperature) are among the affecting factors to the H2S removal. In the present study, effects of G, liquid pH, and Z on the H2S removal were tested and results discussed. Effect of Co on the removal was also discussed. 

Table 3. Dependence of f on water pH.


Effects of Bubbling Rate, Scrubbing Liquid pH, and Depth on H2S Removal

Fig. 3 details the variations in H2S removal and liquid pH over time during operation at a liquid depth (Z) of 0.8 m and gas injection rates (G) of 0.050, 0.100, and 0.200 m3 min–1. When G was 0.050 m3 min–1 at 30–32°C or when the volumetric gassing intensity (G/V) (gas injection rate per unit liquid volume) was 0.0455 m3 m–3 min–1, H2S removal was 86.4% ± 8.9% at pH 6.92 ± 0.12 for an influent H2S concentration of 918 ± 18 ppm and a scrubbing liquid influent flow rate of 25 ± 1 L min–1 in the quasi-steady state. As the gas injection rate increased from 0.050 to 0.200 m3 min–1, the proportion of H2S removed was reduced from 86.4% ± 8.9% to 70.6% ± 1.1%, and the pH of the scrubbing liquid decreased from 6.92 ± 0.12 to 6.75 ± 0.11. The influent H2S concentration did not affect its removal efficiency (Fig. 3). At the influent H2S concentrations of 620 and 1,410 ppm, the removal efficiency was approximately 70%, and the pH was 6.75 at the end of each operation (Fig. 3(c)). This could be because the efficiency was mass-transfer-controlled when the influent H2S concentration increased from 620 to 1410 ppm and the removal efficiency remained constant (at around 70%) (Chou et al., 2010). Increase in the biogas injection flow rate, which reduced the pH of the scrubbing liquid, affected the removal efficiency of H2S. 

Fig. 3. Time variations of H2S removal and liquid pH with operations of liquid depth Z = 0.8 m and gas injection rate G of (a) 0.050, (b) 0.100, and (c) 0.200 m3 min–1.Fig. 3. Time variations of H2S removal and liquid pH with operations of liquid depth Z = 0.8 m and gas injection rate G of (a) 0.050, (b) 0.100, and (c) 0.200 m3 min–1.

Fig. 4 details the time variations in the mean proportions of H2S removed and the liquid pH in the operations where G = 0.150 m3 min–1 and Z was 0.8, 0.9, and 1 m. Higher pH values of the influent and effluent liquid were associated with the removal of a higher amount of H2S. 

Fig. 4. Time variations of average values of H2S removal and liquid pH with operations of gas injection rate G = 0.150 m3 min–1 and liquid depths Z = of 0.8, 0.9, and 1.0 m.Fig. 4. Time variations of average values of H2S removal and liquid pH with operations of gas injection rate G = 0.150 m3 min–1 and liquid depths Z = of 0.8, 0.9, and 1.0 m.

Fig. 5 presents the effects of the influent biogas flow rate on the removal efficiency of H2S and the pH of the effluent liquid. When Z was 0.80 m, because the pH of the liquid decreased, the removal efficiency drastically decreased as the gas flow rate increased. When Z was 0.90 and 1.0 m, higher liquid pH values at all gas flow rates caused variations in efficiency; the gas flow rate and liquid level were lower than those when Z was .8 m.  

Fig. 5. Influences of influent biogas flow rate on the H2S removal and effluent liquid pH at quasi-steady states.Fig. 5. Influences of influent biogas flow rate on the H2S removal and effluent liquid pH at quasi-steady states.


Mechanisms of H2S Removal Other than Water Absorption

As detailed in Table 2, the oxygen concentrations in the influent biogas added to and the effluent biogas derived from the absorbing liquid were 0.53% ± 0.22% and 0.50% ± 0.22%, respectively. On average, only 0.03%, or 300 ppm, of O2 in the biogas was absorbed by the absorbing liquid, and the oxygen could be used to oxidize either 150 ppm of H2S that was transferred from the biogas (according to HS+ 2O2 → HSO4) or 600 ppm of H2S (according to 2HS + O2 → 2S0 + 2OH) (González-Sánchez and Revah, 2007). This amount of H2S (150–600 ppm) was approximately 14%–55% of 1,090 ± 300 ppm of H2S in the influent biogas. DO in the influent absorbing liquid at an average flow rate of 25 L min–1 was approximately 1 mg L–1, and the total DO input to the absorbing tank was 25 mg min–1. This amount of oxygen could oxidize 13 and 53 mg H2S min–1 (according to the two aforementioned equations), respectively. The DO could oxidize 95–380 ppm of H2S that was absorbed from the biogas at an influent flow rate of 100 L min−1, at, for example, approximately 30°C. Therefore, at most 245–980 ppm of H2S that was absorbed from the biogas with 1,090 ppm H2S could possibly be oxidized to elementary sulfur or various oxidation compounds of sulfur (such as S2O32–, SO32–, and SO42–) (Cline and Richards, 1969; Jorgensen, 1990; Zhang et al., 2008). Therefore, the H2S removal mechanisms in this process might include liquid absorption and chemical or biological oxidation.

There existed biological activity in the influent scrubbing liquid drawn from one of the aeration ponds for treating the dairy wastewater, and the activities responded for the microbial oxidation of the absorbed H2S. The aeration ponds were operated 24 hours a day and biological activity was kept at a quasi-steady state. The biological activity in the scrubbing liquid were assumed to be constant throughout the 100-minute operation time.


Adjustment of Liquid pH to Improve H2S Removal Efficiency

Fig. 6 details the effect of liquid pH on the removal efficiency of H2S when Z is 0.8 m. The pH was adjusted when the system reached a quasi-steady state. Data in Fig. 6 indicate that the condition where G is 0.100 m3 min–1 and the effluent or absorbing liquid was of pH 8 resulted in 99% removal of 760 ppm H2S in the influent gas. At pH 8, the H2S removal efficiency decreased as the gas injection rate increased, possibly because of a decrease in the mass transfer rate of the gaseous H2S to the scrubbing liquid at the higher gas injection rate, as Eq. (1) shows. The additional increase in the H2S removal rate when the liquid pH was adjusted to 8.5 was less than that when the liquid pH was adjusted to 8. Supplementation with an additional caustic solution to increase the pH to 8.5 was unnecessary. 

Fig. 6. Time variations of H2S removal and liquid pH with operations of liquid depth Z = 0.8 m and gas injection rate G = 0.20 m3 min–1 and liquid pH adjusted to (a) 8.0 after time of 70–80 minutes and (b) 8.5 after time of 110 minutes.Fig. 6. Time variations of H2S removal and liquid pH with operations of liquid depth Z = 0.8 m and gas injection rate G = 0.20 m3 min–1 and liquid pH adjusted to (a) 8.0 after time of 70–80 minutes and (b) 8.5 after time of 110 minutes.


Limitation of H2S Transfer Rate

According to data in Fig. 6, the H2S removal rates were > 94% when G was ≤ 0.150 m3 min–1 and the pH of the absorbing water was adjusted to 8.0 or 8.5. The mass transfer rate, R, of gaseous H2S to the scrubbing liquid can be described as R = KGa(C-mx) as shown in Eq. (1). Table 3 shows dependence of f on water pH. Water with a higher pH gives a lower [H2S] or x value which favors the absorption of gaseous H2S. When the scrubbing liquid pH is > 8.0, mx << C, and R equals approximately KGa C. This explains why when pH is > 8, the H2S removal efficiency could reach a high value at the fixed operating conditions of liquid depth, liquid temperature, liquid flow rate, and biogas flow rate. In addition, sparging holes with 2.0 mm (rather than 1.0 mm in the present study) in diameter had been used by authors and much lower H2S removal efficiencies got due to their lower gas-liquid interfacial area (a) available for the transfer.


Effects of Biogas Absorption on the Basicity of the Scrubbing Liquid

Table 1 reveals that COD, SS, and TS did not differ significantly between the influent and effluent scrubbing liquids for H2S absorption. The decrease in the pH of the effluent liquid that was caused by the absorption of H2S and CO2 from the injected biogas reduced the H2S removal efficiency. Temperature may affect the H2S removal rate. The influent gas temperatures were between 33–47°C from April to September and 20–35°C from October to March. The effluent gas temperatures were 2–5°C lower than the influent ones due to a chilling effect from the gas bubbling through the scrubbing liquid. The scrubbing liquid temperatures were 30–34°C from April to September and 24–30°C from October to March. No temperature control for both the biogas and scrubbing liquid was conducted during the tests. Similar studies have not considered biogas and scrubbing water temperatures (Hagen et al., 2001; Lien et al., 2014). Tilahun et al. (2017, 2018) used membrane contactors to separate H2S from biogas; however, the mechanism of H2S removal is its diffusion through a porous membrane rather than a gas film (as in this study). The effects of biogas and scrubbing liquid temperatures on H2S removal will be investigated in a further study.

According to Fig. 3(c), when G was 0.200 m3·min–1, the mean effluent gas temperature was 38°C, the average influent H2S concentration was 1,410 ppm, and the influent H2S mass flow rate was 11.1 mmol min–1 (1,410 ppm × 0.200 Am3 min–1 × 10–6 ppm–1 × 273 Nm3/[38 + 273] Am3/22.4 Nm3 kmol–1 = 11.1 mmol min–1). Total absorption of the injected H2S into the influent liquid at a flow rate of 25 L min–1 may have reduced the water basicity by 0.336 meq L–1 from that of the influent water (11.1 mmol min–1/(25 L min–1) = 0.444 meq L–1 according to H2S(aq) ® HS + H+). The decrease in basicity was only 7.25% of that of 6.13 ± 0.3 meq L–1 in the influent liquid when titrated to pH 7 (Table 1).

The major drop in the basicity of the scrubbing liquid is caused by the absorption of CO2 from the biogas. Fig. 7 presents the time variations in the concentrations of CO2 in the influent and effluent gases and the pH of the effluent liquid when Z, G, and L were 0.8 m, 0.16 m3 min–1, and 25 L min–1, respectively. In the quasi-steady state, an average of approximately 2.33% of the CO2 in the influent biogas was absorbed into the liquid. The absorbed CO2 mass flow rate was 146 mmol min–1 (2.33% × 0.160 Am3 min–1 × 10–2 %–1 × 273 Nm3/[(38 + 273)Am3/(22.4 Nm3 kmol–1)] = 146 mmol min–1). The absorption of the CO2 into the influent liquid at a flow rate of 25 L min–1 reduced the basicity by 5.84 meq L–1 from that of the influent liquid (146 mmol min–1/(25 L min–1) = 5.84 mmol L–1 = 5.84 meq L–1, according to CO2(aq) + H2O ® HCO3 + H+). This basicity of 5.84 meq L–1 was approximately 95% of 6.13 ± 0.33 meq L–1 in the influent liquid (Table 1). The decrease in pH is attributable to CO2 absorption into the scrubbing liquid, which reduced the H2S removal efficiency. Therefore, with an increase in G/L (gas/liquid rate ratio), the scrubbing liquid absorbs a much more CO2 from the biogas and this results in the decrease in liquid pH and H2S removal efficiency. Fig. 5 displays that at a constant L of 25 L min–1, H2S removal efficiencies decreased with increasing gas flow rate (G) or G/L, and decreasing pH. With G extra plotting to 0 at Z = 1.00 m, H2S might approach to 100% as shown in Fig. 3(a). The attribution of CO2 in the biogas to the lowering liquid pH also supports the results that the H2S removal efficiencies were nearly independent with the influent H2S concentrations of < 1,500 ppm.

Fig. 7. Time variations of CO2 concentrations in the influent and effluent gases and effluent liquid pH (Liquid depth Z = 0.8 m, gas injection rate G = 0.16 m3 min–1, and water flow rate L = 25 L min–1).Fig. 7. Time variations of CO2 concentrations in the influent and effluent gases and effluent liquid pH (Liquid depth Z = 0.8 m, gas injection rate G = 0.16 m3 min–1, and water flow rate L = 25 L min–1).


Comparisons of Biogas Desulfurization Methods

Table compares biogas desulfurization methods. Tilahun et al. (2018) used a polydimethylsiloxane membrane contactor for selective H2S removal from the biogas. The H2S and CH4 in the biogas diffused through the membrane to an absorbing liquid. The results revealed that at the lowest loading rate (91 mg H2S m–2 h–1), absorption efficiencies of more than 98% for H2S and 59% for CO2 were achieved. Increasing the absorbing liquid pH (from 7 to 10) and the loading rate (from 91 to 355 mg H2S m–2 h–1) increased the H2S absorption capacity. Tilahun et al. (2017) also used a hybrid membrane gas absorption and bio-oxidation process for the removal of hydrogen sulfide from biogas. Membrane processes have promising real scale applications if they are cost-effective.

Unlike the water scrubbing method in which the saturated scrubbing water must be replaced (Lien et al., 2014), an activated sludge mixed liquor for polishing the effluent wastewater from the anaerobic digester(s) was used in the current approach. No additional fresh water is required for this proposed process. The effluent liquid from the simple absorption tank can be further treated to oxidize the absorbed H2S to sulfate, which is considerably less harmful to aqueous environments than sulfide ions.

Compared with biological approaches such as those involving BFs or BTFs, which require organic or inert packing materials, the proposed method neither requires these materials nor the need to control the microbial activities of the biofilms that are either inside or attached to the packing materials. In addition, the H2S removal efficiency from BFs may be unstable because of fluctuations in biological activity. The H2S removal efficiency of the proposed method can be stabilized by maintaining the scrubbing liquid depth, liquid pH, and the ratio of gas flow rates to liquid flow rates. However, the proposed method requires at least one activated aeration tank, a low-head liquid pump to deliver the scrubbing liquid, and a medium-pressure (approximately 0.1 bar) or a 1,000 mm water head blower to sparge the digester gas into the scrubbing liquid. The operating electrical energy cost of the proposed method is also higher than that of the method that uses biofilters.

Compared with the activated sludge tank bubbling processes (Shimko et al., 1987; Sublette et al., 1994; Amano et al., 1999; Chou et al., 2010), the proposed method requires no nutrients to sustain microbial growth and activity. In the proposed method, no attention is required to be paid to the health of the microorganisms to maintain H2S removal efficiency.

Biological processes such as those cited in Muñoz et al. (2015) may have H2S removal efficiencies higher than 90%. Compared with biological methods, the main limitation of the proposed process is its increased cost from additional chemicals (caustic soda) that are usually required to adjust the absorbing liquid to a pH of > 8 to obtain a H2S removal efficiency of more than 90%. However, this cost can be reduced to as low as zero by increasing the ratio of scrubbing liquid flow rate to gas flow rate, allowing the removal of over 90% of H2S in the influent biogas. The scrubbing liquid can be drawn from existing biological aeration tank(s) and drained back to the tank(s) to biologically oxidize the absorbed H2S to sulfate ions. The pumping cost should be taken into account.

Khoshnevisan et al. (2017) demonstrated that the sufficient residence time (RT) for biogas in biotrickling filters is a key factor for successful microaerobic desulfurization. A RT of greater than 5 h yields removal efficiencies (REs) of up to 90%. REs of 88% and 72% were obtained when an RT of approximately 2.5 h was tested. A study demonstrated a successful RE of 96% under variable RTs (from 59 to 97 min). However, a long RT, for example, of 1 h, implies that a big reactor is required to treat a sufficiently large flow of biogas. In the present study, a biogas rate of 200 L min–1 requires an absorbing water volume of only 1,100 L to attain an RE of more than 80%. Data are equivalent to an RT of approximately 5 min.

Another disadvantage of BTFs is the formation of elemental sulfur in the reactors. The sulfur gradually increases the pressure required for driving the gas through the filters and eventually clogs the filters. One approach to solving the clogging problem is to withdraw the accumulated solids from the packing materials after shutting down the filtration systems. The present system has no problems with clogging because most of the absorbed H2S is in H2S and HSaqueous states. Moreover, the suspended elemental sulfur, if any, flows out with the effluent absorbing liquid.

Pokorná et al. (2015) described microaeration as a straightforward, highly efficient, and economically viable technique for H2S removal from biogas. In the method, sulfide is oxidized to elemental sulfur by the action of sulfide oxidizing bacteria. However, the limitations of microaeration, such as partial oxidation of the soluble substrate, clogging in the walls and pipes by elemental sulfur, and toxicity to methanogens, were noted. The proposed method has no such problems.


Feasibility of the Process and Further Studies

A tank with a reasonable size and liquid depth is required for the practical application of the proposed method. Recebli et al. (2015) discovered that an average of 25 kg of manure per day (8.44% dry solids) produced from one bovine animal can generate 0.90 m3 of digester gas per day with 62% of CH4 at 25–40°C. Thus, a ranch that has 200 bovine animals can produce 180 m3 of digester gas per day (average: 0.125 m3 min–1) if all the manure is anaerobically fermented. A system size similar to the one tested in the present study can be used to remove 80% of H2S from the biogas when the scrubbing liquid flow is 25 L min–1. The liquid can be introduced from either an aerobic pond or the effluent pond of the wastewater treatment plant for the ranch and discharged back to the aerobic pond to oxidize the absorbed sulfides. The installation cost for the system is around 3,000 USD. The operation consumes electricity for the 1 HP ring blower and 1 HP water pump, both operated at around 50% capacity. Daily electricity cost is estimated to be around 1.8 USD for 18 kWh electricity. The cost is equivalent to USD 10 for 1,000 m3 of the biogas. The cost is around 3.3% of USD 300 for 1,000 m3 biogas cost.

This study presents only experimental data and discusses some influencing factors on the H2S removal from the biogas. Based on the data, theoretical analyses should be done in further studies to show quantitatively the influence of the key affecting factors on the H2S removal. In addition, H2S in the biogas from digesters for swine wastes were reported to be as high as 5,000 ppm (Su et al., 2014). Further studies should account for the high H2S concentrations in some biogases.


CONCLUSIONS


The results revealed that H2S in an anaerobic digester gas can be effectively removed by directly sparging it into a tank that is filled with liquid to a depth of 0.8–1 m. The liquid was obtained from an aerobic pond to treat the effluent from the digester of a dairy wastewater treatment plant. With an influent liquid of pH 7.5–7.7, a flow rate of 23–25 L min–1, influent biogas flow rates of 0.050–0.200 m3·min−1, and a H2S concentration of 907 ± 212 ppm, the pH of the effluent liquid stabilized at 6.6–6.9. The operation conditions were as follows: gas/liquid ratio of 2–8 m3 m–3 liquid), volumetric gassing intensities of 0.04–0.20 m3 m–3 liquid min–1), and an average tank liquid volume of 1.25 m3. Average H2S REs of 86.4%, 82.5%, 74.4%, and 70.6% were obtained at influent biogas rates of 0.050, 0.100, 0.150, and 0.200 m3 min–1, respectively. Data indicate that increasing the pH of the scrubbing liquid improved the H2S removal efficiency from biogas. CO2 absorption into the scrubbing liquid caused a decrease in the pH, thus decreasing the H2S removal efficiency.


ACKNOWLEDGMENTS


The authors would like to thank the Livestock Research Institute, Council of Agriculture, Executive Yuan of Taiwan, R.O.C. for financially supporting this research under Contract No 107AS-2.4.1-LI-L2.


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