Biogas Cleaning by Hydrogen Sulfide Scrubbing and Bio-oxidation of

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Biogas Cleaning by Hydrogen Sulfide Scrubbing and Bio-oxidation of Captured Sulfides Dana Pokorna,*,† Jose M. Carceller,‡ Ladislav Paclik,† and Jana Zabranska† †

Department of Water Technology and Environmental Engineering, University of Chemistry and Technology, Prague, Technicka 1905, CZ-166 28 Prague 6, Czech Republic ‡ University of Valencia, Avenida Blasco Ibáñez 13, E-46010 Valencia, Spain ABSTRACT: Hydrogen sulfide and partially carbon dioxide can be absorbed into alkaline washing liquid with nitrates for upgrading a quality of biogas. Sulfides captured into the washing liquid are consequently biologically oxidized in an anoxic bioreactor by autotrophic denitrifying bacteria. Nitrates in the washing liquid serve as electron acceptors for sulfide bio-oxidation. Washing of hydrogen sulfide from biogas was examined in a lab-scale countercurrent scrubber that was packed with plastic carriers and operated at different biogas and washing liquid flows. The hydrogen sulfide concentration in treated biogas was investigated in the range from 3 to 12.3 g m−3. The influence of the hydrogen sulfide volumetric loading rate on required scrubber wetting Sw was determined. The anoxic bioreactor packed with the same plastic carriers for immobilization of bacteria was operated long term at the sulfide loading rate in the range from 21 g m−3 day−1 during the start-up period to 400 g m−3 day−1 at the end of the operation to determine the maximum value for sufficient sulfide removal efficiency. On the basis of results, it was concluded that the sulfide loading rate of 160 g m−3 day−1 can ensure stable operation and sufficient desulfurization efficiency. The nitrate loading rate was set up according to molar ratio S/N, which must be lower than the stoichiometric value of 0.625 for stable and sufficient sulfide removal, with the maximum of about 0.55. Possibilities of the process application are suggested and discussed.

1. INTRODUCTION Hydrogen sulfide is widely known as the most undesirable component of biogas because of its toxicity and other serious problems, such as odor, the corrosion of concrete,1 steel structures,2 and pumps or pipes in biogas plants.3 Moreover, it also significantly shortens a life of gas motors of co-generation units by corrosive action and debasing of lubricating oil4 and, thus, increases operational costs. Flue gases from biogas burning can contain exceeding concentrations of SOx. From this point of view, the limit concentration of hydrogen sulfide in biogas starts from 0.1 to 0.5 g m−3 depending upon the cogeneration unit manufacturers,5 and in many cases, it is necessary to remove hydrogen sulfide from biogas before its utilization. Many agricultural and industrial wastes used as a substrate for biogas production may contain substances that are direct precursors of sulfide formation under anaerobic conditions. They are mainly organic wastes with a high proportion of proteins and amino acids having sulfur incorporated in the molecule, such as pig manure,6 or some industrial wastes, e.g., from breweries, distilleries, and pharmaceutical,7 textile,8 and paper9 industries as well as mine water.10 Sulfides are produced by anaerobic bacteria as a result of cleavage of these proteins and their subsequent degradation. Other sulfides may be formed by activity of sulfate-reducing bacteria (SRB) from sulfates present in some industrial wastes and wastewaters. Dependent upon the sulfide concentration and pH value, sulfide S passes into biogas as hydrogen sulfide.11−16 Moreover, sulfides in liquid phase can inhibit the activity of some groups of anaerobic bacteria, particularly syntrophic acetogenic and methanogenic bacteria.17 © XXXX American Chemical Society

Main objectives of this work were (i) to determine main parameters for hydrogen sulfide scrubbing from biogas for the design of a pilot scrubber and (ii) to determine main parameters for anoxic sulfide oxidation in an immersed biofilter for the design of a pilot bioreactor. 1.1. Biological Removal of Hydrogen Sulfide from Biogas. Biological desulfurization methods are currently promoted to abiotic methods because they are less expensive and do not produce undesirable materials, which must be disposed. The end products of biological sulfide oxidation, elemental sulfur or sulfate, are no longer hazardous; elemental sulfur can be removed from the system in a solid state, and limits for discharge of sulfates in surface water are not so strict as for nitrogen for example.2,18 1.1.1. Aerobic Biological Desulfurization Process. Biological removal of sulfides is based on the activity of sulfuroxidizing bacteria (SOB) that need some acceptor of electrons released during the sulfide oxidation. From the technological point of view, chemolithotrophic SOB (Thiobacillus, Sulfolobus, Thermothrix, Thiothrix, and Beggiatoa), also known as colorless sulfur bacteria, are the most frequent in technological cultures. This group of SOB oxidizes sulfides under aerobic conditions using the atmospheric oxygen as an electron acceptor.19 A limited amount of air can be supplied to either the gas space of an anaerobic reactor or the fermentation mixture. This process is known as microaeration.20 Another way of aerobic sulfide Special Issue: 2nd International Scientific Conference Biogas Science Received: December 15, 2014 Revised: March 30, 2015

A

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1.2.1. Anoxic Sulfide Bio-oxidation as Part of the Technological Line in a WWTP. In this case, sulfides come from scrubbing of hydrogen sulfide from biogas, which is produced in anaerobic digestion of sewage sludge in a WWTP. Wastewater after nitrification can be used as the washing liquid in the biogas scrubber, and captured sulfides are led to the denitrification tank, where heterotrophic and autotrophic denitrification proceeds (Figure 1). Part of the organic

oxidation takes place in an external bioreactor with SOB in suspended form21 or immobilized on a carrier from suitable material.22−25 1.1.2. Anoxic Biological Desulfurization Process. In addition to chemolithotrophic SOB, there are also autotrophic denitrifying bacteria (ADB) that use reduced forms of sulfur (S2−, S0, S2O32−, S4O62−, and SO32−) as an electron donor for the reduction of nitrate via nitrite to nitrogen gas under anoxic conditions. ADB occurring in technical cultures include mainly Thiobacillus denitrificans, Paracoccus denitrificans, and Thiomicrospira denitrificans.19,26−28 Contrary to heterotrophic denitrifying bacteria, they do not need organic compounds as electron donors and produce less biomass, which can decrease operational costs of wastewater treatment. The process of autotrophic denitrification has the advantage of simultaneous sulfide and nitrate removal.29−31 End products of sulfide oxidation depend upon the molar ratio of sulfide S and nitrate N and reduced forms of sulfur.32−34 The stoichiometric molar ratio of sulfide S/nitrate N is 0.625 according to eq 1, when all sulfides are oxidized to sulfates and nitrates are completely reduced to nitrogen gas. Molar ratio of sulfide S/nitrate N < 0.625 means excess of nitrates and results in incomplete denitrification (eq 2). At a molar ratio of sulfide S/nitrate N > 0.625, the lack of nitrates results in incomplete denitrification and sulfides are partially oxidized only to elemental sulfur (eq 3).19,33,35,36

Figure 1. Anoxic sulfide bio-oxidation as part of the technological line in a WWTP: (1) denitrification, (2) nitrification, (3) biogas scrubber, (4) anaerobic digester, (5) organic substrate, (6) digestate, (7) wastewater, (8) nitrified wastewater (washing liquid), (9) biogas with S2−, (10) washing liquid with captured S2−, and (11) cleaned biogas.

S2 − + 1.6NO3− + 1.6H+ → SO4 2 − + 0.8N2 + 0.8H 2O (1) 2−

S

+

4NO3−

→ SO4

2−

+ 4NO2

−

S2 − + NO3− + 2H+ → S0 + NO2− + H 2O

substrate required for denitrification is thus substituted by reducing equivalents of sulfides because 1 g of S2− is equal to 2 g of chemical oxygen demand (COD) according to electron balance. The process of sulfide oxidation takes up to sulfates, which can be easily discharged with treated wastewater. The advantage of this application is in using an existing denitrification tank as the bioreactor, and only the scrubber is needed to be installed. The scrubbing can be carried out directely with activated sludge from the nitrification tank.39 1.2.2. Anoxic Sulfide Bio-oxidation in a Separate Bioreactor Coupled with a Scrubber in the Desulfurization Unit in a WWTP. The installation of the separate anoxic bioreactor coupled with the scrubber in the desulfurization unit has the advantage of using of adapted ADB with a high activity in the bioreactor and, thus, very intensive denitrification with only sulfides. The bioreactor can be designed as the completely mixed reactor,40 upflow sludge bed reactor,41−45 or fixed-film reactors with different carriers.31 Nitrified wastewater from the technological line of the WWTP (Figure 2) or sludge liquor after nitrification of ammonia nitrogen can be used as washing liquid. The effluent from the bioreactor, which may contain residual nitrate or even residual non-oxidized sulfides probably in a very low concentration, can be led in either the denitrification tank or the nitrification tank, where residual sulfides are oxidized to sulfates.46 1.2.3. Anoxic Sulfide Bio-oxidation in a Biogas Plant. Biogas plants are the other possible application sites.31 In some cases, digestate is dewatered and the separated solid phase is further processed.47−50 The separated liquid phase with a high concentration of ammonia nitrogen causes problems if it is not possible to use it for fertilizing. One of the methods of ammonia removal is biological nitrification, followed by denitrification.51,52 The nitrified liquid phase can be used as the washing liquid, which, with sulfides captured in the scrubber, can be fed to the external anoxic bioreactor, as

(2) (3)

There is also a pH effect on the end product of denitrification: N2 is the end product at pH above 7.3; at the values below 7.3, the proportion of N2O increases. The process of denitrification takes place in a relatively wide range of pH 6−9, with optimum pH 7.0−8.5. For efficient capture of hydrogen sulfide from biogas into the liquid, an alkaline washing liquid with higher pH is favorable, which decreases during the scrubbing.37,38 1.2. Application of Coupled Denitrification and Biogas Desulfurization Technology. The technology of autotrophic denitrification that couples the process of denitrification and biological biogas desulfurization can be applied in biogas plants or wastewater treatment plants (WWTPs) where biogas with a higher concentration of hydrogen sulfide is produced, and it is necessary to reduce it to an acceptable value for biogas energy use. The reasons of application may be highly specific: character of substrate limiting the use of microaeration, high concentration of ammonia nitrogen, which inhibits SOB, or composition of anaerobic fermenter feed in terms of substrates containing hydrogen sulfide precursors. Other reasons can be, for example, insufficient efficiency of existing facilities because of inappropriate technology or economic reasons, high operational costs of the applied abiotic desulfurization method. Arrangement of technology of simultaneous hydrogen sulfide and nitrate removal depends upon local conditions. Technology is based on hydrogen sulfide scrubbing in the washing liquid in the gas scrubber, followed by bio-oxidation of captured sulfides in the anoxic bioreactor in the presence of oxidized forms of nitrogen. Anoxic sulfide bio-oxidation may be realized in different ways that are further discussed. B

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hydrogen electrode. The concentration of dissolved oxygen (DO) was determined using the probe LDO101 connected to the portable device HQ30D flexi from Hach Lange. Samples for analyzing NO3−, NO2−, S2−, SO42−, and pH were taken at regular intervals every other day. Analysis of CO2 in biogas was made by gas chromatography with a thermal conductivity detector (TCD) and 2 m column packed with Separon AE. 2.2. Gas Scrubbing. Scrubbing experiments were carried out in the lab-scale equipment consisting of a scrubbing column, a washing liquid pump, and device controlling flows and composition of gas. The scrubber was a plexiglass cylinder with the height of 1 m and total volume of 28.3 L [liquid volume Vw was in the range from 4.2 to 6.0 L, including the volume of the pump tubing, with the volume for gas (Vg) being 26.1 L]. The scrubber was filled to a height of 0.72 m with plastic elements (a specific surface of 320 m2 m−3, a packing density of 100 kg m−3, and a total surface area of packing of 6.53 m2). The gas was dosed to the scrubbing column bottom at the flow rate Qg (L min−1), and the washing liquid was sprayed countercurrent and recycled at the flow rate Qr (L min−1). The quality of the washing liquid will be discussed in section 3.1.1. The scheme of the scrubbing apparatus is shown in Figure 4. For easier handling with gases and better reliability of scrubbing experiments, the scale of the scrubber was much higher than that corresponding to the laboratory bioreactor.

Figure 2. WWTP with a separated anoxic bioreactor: (1) denitrification, (2) nitrification, (3) biogas scrubber, (4) anaerobic digester, (5) organic substrate, (6) digestate, (7) wastewater, (8) nitrified wastewater (washing liquid), (9) biogas with S2−, (10) washing liquid with captured S2−, (11) cleaned biogas, and (12) anoxic bioreactor.

shown in Figure 3. If all of the liquid phase is not processed to ammonia removal, it is possible to oxidize only a part of the

Figure 3. Anoxic sulfide bio-oxidation in a biogas plant: (1) denitrification, (2a) nitrification of the liquid phase of the digestate, (2b) nitrification tank in the WWTP, (3) anaerobic digester, (4) biogas scrubber, (5) anoxic bioreactor, (6) separation, (7) digestate, (8) liquid phase of the digestate (NH4+), (9) organic substrate, (10) biogas with S2−, (11) oxygen, (12) washing liquid, (13) washing liquid with captured sulfides, (14) cleaned biogas, (15) nitrogen, and (16) washing liquid with sulfates.

Figure 4. Scheme of the H2S scrubbing apparatus: (1) biogas influent, (2) cleaned biogas, (3) influent of fresh liquid, (4) effluent of washing liquid with sulfides, (5) washing liquid recirculation, (6) sampling, (7) mass flowmeters, (8) pump, (9) H2S detector, and (10) packing. Biogas was modeled as a result of limited laboratory conditions. Methane, because it is only slightly soluble, does not significantly affect absorption of acidic hydrogen sulfide and carbon dioxide and, also, in terms of safety, has been replaced by another inert gas, nitrogen. Modeled gas consisted of nitrogen gas, carbon dioxide, and hydrogen sulfide. The amount of gas passing through the scrubber was controlled by the mass flow meters (model GFM from Aalborg), having a measuring range up to 500 L min−1. The ASIN H2S analyzer with a measuring range from 0 to 6 g m−3 hydrogen sulfide (Aseko) was installed to check the concentration of hydrogen sulfide in the biogas outlet. The analyzer was purged with air after each measurement to avoid damaging because the sensor does not tolerate long-term exposure to hydrogen sulfide. 2.3. Lab-Scale Anoxic Bioreactor. A lab-scale bioreactor for autotrophic denitrification was designed similar to the gas scrubber for technical simplicity (Figure 5). The plexiglass cylinder of 1 m height with an inner diameter of 0.19 m was packed to the height of 0.72 m with the same type of plastic elements as the scrubber and was operated as a downflow immersed biofilter with recycle and total liquid volume of 20.0 L. The effluent from the bioreactor flew to a settler for removal of excess biomass, especially during the startup procedure. The bioreactor was inoculated with 10 L of activated sludge from the

ammonia nitrogen to provide a sufficient amount of the oxidized form of nitrogen.

2. MATERIALS AND METHODS 2.1. Analytical Methods. Concentrations of N NO2− and N NO3− were measured spectrophotometrically with amide sulfanilic acid and N-(1-naphthyl)ethylenediamine (NED) dihydrochloride and with 2,6-dimethylphenol, respectively. Sulfides were determined by iodometric titration with sodium thiosulfate and starch as an indicator.53 The acid-neutralizing capacity (ANC) was determined using acidimetric titration to pH 4.5. Sulfates were analyzed using a capillary isotachophoresis with two electrolytes: leading electrolyte LE, HCl (8 × 10−1 M), β-alanin (1.5 × 10−3 M), 1,3-bis(tris(hydroxymethyl)methylamino)propane (BTP) (3.10−1 M), and methylhydroxyethylcellulose (MHEC) at pH 3.7; terminating electrolyte TE, citric acid (10 × 10−3 M). For the measurement of pH and oxidation−reduction potential (ORP), pH meter WTW pH 730 Series inoLab was used. Probes for measurement of pH and ORP values were Hamilton Polilyte probe and WTW SensoLyt Pt argentochloride electrode, respectively. ORP values were not converted to a standard C

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The bioreactor was operated with the recycle of 25.6 L h−1 to a dilute sulfide concentration in the influent and to minimize dead zones in the reactor at low flows of sulfide liquid.

3. RESULTS AND DISCUSSION 3.1. Scrubbing Experiments. Scrubbing experiments were carried out in batch mode because of technical possibilities and conditions in the laboratory at the ambient temperature of 21 ± 1 °C. The following parameters could be changed during experiments: the input hydrogen sulfide concentration CH2S in (g m−3), gas flow rate Qg (L min−1), liquid recycle flow rate Qr (L min−1), and liquid volume Vw (L). Hydrogen sulfide and CO2 in output gas, sulfide concentration, pH, and ANC of the washing liquid were monitored at adjusted washing liquid and gas flows and input concentration of hydrogen sulfide. Each set parameters can be characterized by the mass flow of H2S Qm, volumetric hydrogen sulfide loading rate Bg, hydrogen sulfide loading of liquid Bw, and hydrogen sulfide removal efficiency E (eqs 4−7).

Figure 5. Scheme of the anoxic bioreactor: (A) sulfide influent, (B) nitrate influent, (1) recirculation of liquid, (2) pumps, (3) packing with immobilized bacteria, (4) settler, and (5) outflow gas.

Q m = Q gC H2S in

Central Wastewater Treatment Plant Prague (CWWTP) with a total suspended solids concentration (TSS) of 4 g L−1 and 75% of volatile suspended solids (VSS). Influent to the bioreactor was modeled as a result of the different times of experiments and incompatible flows. The synthetic influent consisted of two stock solutions, which had been prepared separately to avoid chemical sulfide oxidation outside the bioreactor. Both a solution of sulfides (A) and a solution of nitrates and micronutrients (B) were buffered by hydrogen carbonate and carbonate, respectively (Table 1), and mixed just before entering the bioreactor. The

Bg = (Q gC H2S in)/Vg Bw = (Q gC H2S in)/Vw

(mg L−1)

solution B

(mg L−1)

Na2S·0.5H2O NaHCO3

250−2500 800−1600

KNO3 Na2HPO4·2 H2O K2HPO4 Na2CO3 micronutrient solution

730−3100 560 240 400 18 mL L−1

(4)

(g m−3 min−1)

(5)

(g m−3 min−1)

E = 100(C H2S in − C H2S out)/C H2S in

(%)

(6) (7)

Examples of the hydrogen sulfide concentration in the effluent gas at given Qg and Vw and at various initial concentrations of hydrogen sulfide are in Figure 6.

Table 1. Composition of Solutions A and B solution A

(g min−1)

Table 2. Composition of Micronutrient Solution compound

concentration (mg L−1)

NH4Cl CaCl2·6H2O MgCl2·6H2O FeCl2·6H2O MnCl2·4H2O CoCl2·6H2O NiO2·6H2O CuCl·6H2O H3BO3 (NH4)6Mo7O24·4H2O Se

100 400 500 120 5 20 5 0.4 0.5 0.5 0.5

Figure 6. Time course of the hydrogen sulfide concentration in effluent biogas at Qg = 40 L min−1, Vw = 5.4. L, and various initial concentrations of hydrogen sulfide.

The permitted limit of the hydrogen sulfide concentration in biogas is around 0.5 g m−3 for most producers of motor generators. From breaktrough curves at different parameters (Figure 6), time (ta) can be evaluated representing the time needed to reach limit concentration. The time ta and the volume of washing liquid Vw are used for calculation of the corresponding flow of liquid Qw in a continuous system (eq 8). The generalized parameter of required liquid flow Qw related to the volume of the scrubber Vg is hydraulic loading of the scrubber gas space, scrubber wetting Sw (eq 9). The dependence of the absorption rate ra on the scrubber wetting

composition of the micronutrient solution is shown in Table 2. The total influent flow rate was 0.24 L h−1 provided by a two-line peristaltic pump controlled by software LabView (National Instruments, Austin, TX). Gas spaces of storage vessels with solutions A and B as well as the reactor were filled with N2 gas through storage bags to maintain anoxic conditions. The temperature of experiments was 21 ± 1 °C. D

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Energy & Fuels Sw at given parameters (eq 10) is documented in Figure 7. A higher Sw also caused a higher absorption rate. Q w = Vw /ta

(L min−1)

(8)

Sw = Q w /Vg

(L L−1 min−1)

(9)

ra = (C H2S in − C H2S out)Q g /Vg

(g m−3 min−1)

(10)

Figure 9. Influence of ANC on the amount of hydrogen sulfide absorbed in washing liquid.

hydrogen sulfide at the same other parameters (Qm, Qw, Bg, and Bw) increases with increasing washing liquid ANC. Wastewater ANC ranges in a municipal WWTP from approximately 5.5 to 6.9 mmol L−1, and ANC of the digestate liquid phase tends to have more than 300 mmol L−1. These data were obtained from several municipal WWTPs and biogas plants and were verified by our analysis. The ANC of washing wastewater is possible to increase by either the addition of NaHCO3 or the enrichment with liquid of high ANC, for example, with liquid phase of digestate. Mixing of wastewater with digestate in a ratio of 9:1, we obtained the washing liquid with ANC of about 35 mmol L−1, which can reduce the necessary hydraulic loading rate of the scrubber, wetting of the column by 34% in comparison to only wastewater. The decrease of the CO2 concentration in treated gas is also dependent upon ANC of washing liquid. The content of CO2 in the gas effluent decreased from 35 to 32−30% (v/v) at ANC of about 35 mmol L−1. 3.2. Bioreactor Operation. During the startup procedure, after washout of heterotrophic bacteria, part of bacteria was immobilized on the carrier surface and another part remained in the inner space of the carrier and, after a longer time of operation, formed a fine fibrous reticular structure. In a fixedfilm reactor, it is impossible to determine the total amount of biomass in the reactor and the main operational parameters were the volumetric loading rates of sulfides BS (mg L−1 day−1) and nitrates BN (mg L−1 day−1). In Figure 10, the course of the sulfide volumetric loading rate, removal efficiency, and concentration of DO is presented. The concentration of DO was maintained at a very low level,

Figure 7. Influence of the absorption rate ra on the scrubber wetting Sw at the following parameters: Qg = 40 L min−1, CH2S in = 3 g m−3, and Vg = 26.1 L.

Main basic parameters of the scrubber are the volumetric hydrogen sulfide loading rate Bg and scrubber wetting Sw. The influence of Bg on required Sw was examined in the wide range from 0.62 to 3.8 g m−3 min−1, and trends of their relation at different ratios of Qg/Qw are in Figure 8.

Figure 8. Minimum required scrubber wetting Sw related to the hydrogen sulfide loading rate Bg.

3.1.1. Quality of Washing Liquid. Nitrified water from the WWTP or nitrified liquid phase of digestate from biogas plants can be used as the washing liquid for hydrogen sulfide scrubbing. The quality of the washing liquid in terms of ANC (mmol L−1) is very important because hydrogen sulfide is an acidic component of biogas. Experiments shown in Figure 9 were performed to determine an effect of ANC on the hydrogen sulfide absorption in the washing liquid. It is apparent that a captured amount of

Figure 10. Efficiency of sulfide removal E, sulfide loading rate Bs, and DO during bioreactor operation. E

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Energy & Fuels around 0.6 mg L−1. This value does not cause any problems. Its increase to 1.6 mg L−1 did not result in a decrease in the efficiency of the sulfide removal, because oxygen was probably used by some heterotrophic bacteria. However, a larger increase in the DO concentration could result in the increase of ORP and, consequently, in the inhibition of autotrophic denitrification. The bioreactor was operated with the volumetric sulfide loading rate of 21 mg L−1 day−1 during the startup period. From the 75th day of bioreactor operation, Bs was increased to 140 mg L−1 day−1 with still stable operation. BN corresponded to BS according to the S/N molar ratio that was in the range from 0.35 to 0.55. From the 176th to the 200th day of operation, BS was extremely increased to 400 mg L−1 day−1 to find out the response of the system and its ability of regeneration. This overloading resulted in the change of the S/N molar ratio to 0.83. The quick negative response resulted in the rapid decrease of the sulfide removal efficiency to 45%, corresponding to decreases in the nitrate consumption and production of nitrites as a product of nitrate reduction. During this 25 day period, the concentration of ammonia nitrogen and COD increase from 3 to 100 mg L−1 and from 30 to 768 mg L−1, respectively. An increase of the VSS concentration in the effluent from the bioreactor was also observed. The same signals of destabilization after sulfide overloading are presented in the literature.30,42,54 A further consequence of sulfide overloading is the decrease in the sulfate production and accumulation of elemental sulfur in the reactor, which was also observed by other authors.33,45,54 The S/N ratio influences the oxidized form of sulfur; the limit for sulfate formation is indicated to be 0.65.41 The sulfide shock overloading also caused an increase in pH above 10, similar to results by Jing et al.41 and Mahmood et al.54 After returning of BS to 120 mg L−1 day−1, the process was relatively quickly restored, and during 10 days, the efficiency of sulfide removal was back at high values; NH4+ and COD decrease to the level of 3 and 30 mg L−1, respectively. Only production of sulfates was increased slowly, being restrained by oxidation of stored elemental sulfur, and a full recovery of the sulfate concentration in the effluent took 28 days. After 50 days of stable operation at BS = 120 mg L−1 day−1, this parameter was increased to 160 mg L−1 day−1 and the bioreactor was operated without problems for the next 50 days. The nitrate loading rate was set up according to molar ratio S/N, which was in the range of 0.37−0.8. In Figure 11, the relation of the molar ratio S/N to the efficiencies of sulfide and

nitrate removals is shown. It is evident that the efficiency of nitrate removal is on average 20% lower than the efficiency of sulfide removal. The main aim of this study is to propose operational parameters of the fixed-film bioreactor for dissolved sulfide removal, with sulfates as a preferential form of oxidized sulfur. Elemental sulfur can clog the carrier and, after a time, decrease an active surface of biofilm. Therefore, nitrates were dosed in excess to achieve maximal efficiency of the sulfide removal, with mainly sulfates as the end product. Some denitrifying bacteria are in fact able to reduce nitrates using both sulfides and organic substrate coming from the bacteria cultivation, which is confirmed by the formation of some COD during the process, and it is preferably used.34 On the basis of results of long-term fixed-film bioreactor operation, it was concluded that the sulfide loading rate of 160 g m−3 day−1 at an ambient temperature ensured stable operation and sufficient sulfide removal efficiency. For stable and sufficient sulfide removal, nitrates need to be in excess and S/N needs to be lower than the stoichiometric value, with a maximum of about 0.55.

4. CONCLUSION The quality of biogas containing hydrogen sulfide can be cleaned by scrubbing and subsequent bio-oxidation of captured sulfides. The aim of this study was to find out the appropriate conditions of scrubbing in the packed countercurrent column and the optimal conditions for efficient biological sulfide oxidation with nitrates in the anoxic fixed-film bioreactor. Washing of hydrogen sulfide from biogas was examined in the lab-scale scrubber packed with a plastic carrier and operated at different biogas and washing liquid flows. The hydrogen sulfide concentration in biogas was examined in the range from 3 to 12.3 g m−3. The influence of the volumetric hydrogen sulfide loading rate on required scrubber wetting was examined in the wide range from 0.62 to 3.8 g m−3 min−1, and trends of their relation at different ratios of gas and water flows were determined. Any available water containing nitrates can be used as the washing liquid for hydrogen sulfide scrubbing: nitrified water from the WWTP or nitrified liquid phase of the digestate from biogas plants. The effect of ANC on the hydrogen sulfide absorption in the washing liquid was studied, and the captured amount of hydrogen sulfide at the same parameters increases with the washing liquid ANC. The anoxic bioreactor was operated as the immersed biofilter with a plastic carrier for ADB immobilization. On the basis of results of the long-term operation, it was concluded that the sulfide loading rate of 160 g m−3 day−1 can ensure the stable operation and sufficient desulfurization efficiency. The nitrate loading rate was set up according to molar ratio S/N, which, for the stable and sufficient sulfide removal, must be lower than the stoichiometric value, with the maximum S/N of about 0.55. The possibilities and advantages of an application of this technology for simultaneous removal of hydrogen sulfide and nitrates depend upon local conditions and the different ways of realization of anoxic sulfide bio-oxidation were discussed.

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AUTHOR INFORMATION

Corresponding Author

Figure 11. Influence of the molar ratio S/N on the efficiency of sulfide and nitrate removal.

*E-mail: [email protected]. F

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Technological Agency of the Czech Republic (Project TA01020798) and Project MSM6046137308.

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DOI: 10.1021/ef502804j Energy Fuels XXXX, XXX, XXX−XXX