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(6) Biological cleanup of gases is conducted in three types of reactors: biofilters, biotrickling filters, and bioscrubers. The first two of them have...
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Impact of Different Packing Materials on Hydrogen Sulfide Biooxidation in Biofilters Installed in the Industrial Environment Krzysztof Ziemiński* and Włodzimierz Jan Kopycki Faculty of Biotechnology and Food Sciences, Institute of Fermentation Technology and Microbiology, Lodz University of Technology, 171/173 Wolczanska Strasse, 90-254 Lodz, Poland ABSTRACT: Three pilot scale anoxic biotrickling filters containing different packing materials were built and tested over an extended period in the municipal wastewater treatment plants (WWTP). Performance of biological beds containing polyethylene elements, ceramic Raschig rings, and keramzite particles was investigated at hydrogen sulfide concentrations in biogas ranging from 369 to 1733 mg/m3. The dependence of the number of colony forming units on hydrogen sulfide concentration and location in the bed was determined. Microbial enumeration was performed for all three columns using cultivation methods. The best results of desulfurization were observed for polyethylene elements. For this material the optimum empty bed retention time was 12 min and the minimum concentration of nitrate ions in a nutrient solution was 50 mg/L. The hydrogen sulfide reduction over 99% was achieved at loading rates (LR) of 1.79−5.08 g/m3/h. The increase in LR to 8.45 g/m3/h reduced the removal efficiency to 96%. The results of kinetic analysis enabled characterization of biological systems immobilized in each biofilter.

1. INTRODUCTION Biogas derived by anaerobic fermentation is an important source of energy. In 2013, only in EU countries production of electricity from biogas reached 13.4 million tons of oil equivalent (Mtoe) and was by 10.2% higher than in 2012. Poland is the eighth biogas producer in Europe with biogasbased energy production of 251.2 ktoe in 2013. As much as 58 agricultural biogas plants and 195 biogas plants associated with wastewater treatment plants and landfill sites were established until June 2015 in Poland (www.stat.gov.pl). The presence of hydrogen sulfide in biogas significantly increases costs of energy production. Its concentrations vary between 0.1 and 2% and depend on feedstock used in the anaerobic fermentation and process conditions.1 The necessity of hydrogen sulfide removal is caused not only by corrosion of burners but also by its toxicity and low odor threshold.2 In most cases, also in Poland, biogas is used to produce energy and heat via cogeneration in combined heat and power (CHP) plants. One of serious problems of this highly efficient biogas utilization technology consists of the negative impact of hydrogen sulfide on lubricants used in installations. Therefore, efficient and cheap in exploitation biogas desulfurization systems are sought after. The technology of hydrogen sulfide removal is in each case chosen based on its initial concentration, target purity, and application of biogas as well as amounts of the latter. Numerous physicochemical methods of biogas desulfurization were described in the literature, including adsorption on activated carbon,3 absorption by iron oxides,4 and chemical treatment using alkaline reagents.5 These processes are expensive, and furthermore, the reagents used to capture hydrogen sulfide must be utilized. Therefore, a biological desulfurization method appears as more attractive and cleaner. It has been proved that biological desulfurization is not only more effective and cheaper but also safer than physicochemical treatments.6 Biological cleanup of gases is conducted in three types of reactors: biofilters, biotrickling filters, and bioscrubers. The first two of them have been widely applied for biodesulfurization.7 © XXXX American Chemical Society

Oxidation of hydrogen sulfide is usually mediated by species of chemolitotrophic bacteria, using either oxygen or nitrate ions as ultimate acceptors of electrons. Microbiological process of biogas desulfurization in aerobic conditions can be conducted in biotrickling filters.8 The principal drawback of this technology is around 4% oxygen concentration in the biogas to be purified, which increases the risk of explosion, because methane and oxygen mixtures, containing 5−15% of the latter are prone to explosion.9 Nevertheless, the recent research proved that 2% addition of oxygen with ON/OFF control can facilitate safe and effective hydrogen removal in biotricklig filter.10 The presence of oxygen can negatively affect the ultimate methane levels.1 Therefore, anoxic desulfurization processes seem more attractive. Oxidation of hydrogen sulfide using nitrate ions as electron acceptors was studied by several authors. These studies11−15 showed that under anoxic conditions the efficiency of hydrogen sulfide removal depended on both its concentration and metabolic activity of microorganisms that formed biofilms on the surface of packing materials. Biodiversity of microflora, living in the filter and mediating desulfurization, depends on the type and structure of packing material, which not only must be characterized by the large specific surface area but also favor adhesion and growth of microorganisms as well as reduce the risk of filter clogging to maintain the intensive gas exchange.16,17 Other desirable properties of this material are low specific weight, high mechanical durability, and lack of changes in the loading density due to thickening. Characteristics of the packing material determine the construction of the biofilter and its usefulness in industrial conditions. Also the shape of particles and porosity of packing material play important roles in effectiveness biogas desulfurization processes. Materials used in Received: June 6, 2016 Revised: September 20, 2016

A

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density of 160 kg/m3, the smallest equivalent diameter of 7.09 mm, and the highest voidage (of 79.0%) among the three materials. The ceramic Raschig rings had the highest bulk density of 700 kg/ m3 while the keramzite beads were characterized by the largest equivalent diameter of 11.9 mm. According to the literature, both shape and size of packing material particles are determining factors of the flow character of liquids and gases in various sites of the column.24 The most important parameter, deciding on the character of the flow, is the ratio of the diameter of packing material particles to the reactor diameter. These ratios were 0.035, 0.046, and 0.059 for the HDPE elements, Raschig rings, and keramzite beads, respectively. These values may also affect the liquid holdup of the packing materials. 2.1.3. Nutrients Solution. The solution of nutrients contained KH2PO4 (0.8 g/L), MgCl2·6H2O (0.5 g/L), FeSO4·7H2O (0.1 g/L), and Ca(NO3)2·4H2O (from 0.8 to 2.4 g/L). The resulting concentration of nitrate ions was ranging from 420 to 1260 mg/L (N−NO3− 95−285 mg/L), dependending on the hydrogen sulfide concentration in the biogas. The initial pH value of this solution was 6.9 ± 0.1 (it was not adjusted during the process). According to the literature,6 the optimum pH for the growth of microorganisms mediating the anoxic hydrogen sulfide oxidation usually ranges from 6.8 to 7.4. 2.1.4. Biofilters. Biogas desulfurization processes were conducted in the pilot industrial scale, using three biological trickling filters, designated as BTF1, BTF2, and BTF3 and packed with HDPE elements, ceramic Raschig rings, and keramzite beads, respectively. The biofilters were produced from acid-proof stainless steel and had an internal diameter of 0.2 m and total height of 1.56 m. They consisted of three identical modules (each 0.52 m in length), equipped with the mesh supporting the packing materials. The working volume of each biofilter was 0.049 m3 while the volume occupied by the packing material was 0.041 m3. Each module had two sampling valves. The biofilters were fed with biogas taken directly from the biogas plant pipeline and collected in a 60 L equalizing and dehydration tank. This tank was equipped with temperature controller. The biogas flow rate was measured using a gas flow controller (type 5850S Brooks). The biogas flew through the biofilters in a counter-current mode to a solution of nutrients (mineral salts), which trickled at the constant velocity of 1.02 m/h (0.28 × 10−3 m/s). The trickling velocity was controlled using a pump (LT-10 Magdos). The nutrients solution was kept in 60 L containers and was recycled around 8 times per day. The desulfurization process was conducted at temperatures ranging from 23 to 27 °C dependent on the temperature of influent biogas. The temperatures of the nutrients solution and each biofilter were controlled separately using a suitable controller and an electric heater (see Figure 1 for the experimental setup). 2.2. Analytical Methods. 2.2.1. Biogas Composition and Hydrogen Sulfide Concentration. The concentration of hydrogen sulfide in crude biogas and in purified biogas from each biofilter was measured once a day throughout the whole period of the experiment using a GA-21 BIO plus biogas analyzer (Madur Electronics) equipped with an electrochemical gas sensor (precision of ±5 ppm). GC analyses were carried out using a 7890A gas chromatograph (Agilent Technologies) equipped with a thermoconductometric detector (TCD) and a gas valve with a loop (the volume of 1 mL) to apply biogas samples. The biogas components were separated using a series of three packed columns: column 1, Hayesep (3 ft × 1/8 in., 80/100 mesh); column 2, Porapak Q (6 ft × 1/8 in., 80/100 mesh), and column 3, Molecular Sieves 5A (6 ft × 1/8 in., 80/100 mesh). Helium was used as the carrier gas (flow rate of 30 mL/min). 2.2.2. Nutrients Solution Analysis. Concentrations of salts in the recycled nutrients solution were monitored by ion exchange chromatography using a 850 Professional IC chromatograph (Metrohm) equipped with a conductometric detector, an autosampler (858 Professional Sample Processor), and the MagIC Net 2.0 software that governed the function of the chromatograph as well as data acquisition and processing. The anions were separated at 45 °C on a Metrosep A Supp column 7 (250 mm × 4.0 mm) using 3.6 mM aqueous Na2CO3 solution as the eluent. The cations were separated on

processes of hydrogen sulfide removal include polypropylene rings,18 ceramic spheres,19 polyurethane foam,13 and metallic or plastic Pall rings.12 Apart from the type of packing material, desulfurization conditions, which strongly affect the diversity of the packed bed microflora, decide the hydrogen sulfide reduction.20 Taking into consideration factors that may have an influence on the desulfurization process, three different packing materials with significantly different physical characteristics were selected to be tested in the same conditions. The aim of this work was to investigate the effect of three different packing materials such as ceramic Raschig rings, keramzite beads, and high-density polyethylene (HDPE) elements on the efficiency of anoxic biodesulfurization conducted in a pilot installation, operating in a biogas plant. Currently, hydrogen sulfide is removed from biogas produced in this plant by adsorption on ferric oxide(III)-containing granulate, which is expensive. This prompted us to study the efficiency of biological desulfurization.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. 2.1.1. Biogas. Biogas used in this study was derived from a local biogas plant located in Lodz and associated with a municipal wastewater treatment plant. The biogas plant is one of the largest Polish biogas producers, converting each day around 72 Mg sludge dried mass in four chambers for anaerobic fermentation, 10 000 m3 of total volume each. This plant produces 18 600−20 210 m3/day of biogas containing 56−72% CH4, 28−44% CO2, and 100−1600 ppmv of hydrogen sulfide (139−2224 mg/m3). The average biogas productivity is 0.35 m3/ kg of dried organic mass. Fluctuation of hydrogen sulfide concentration in biogas is related to the chemical composition of sludge subjected to fermentation and to use of ferric coagulants for dephosphatation supporting. An acceptable threshold of hydrogen sulfide concentration in biogas combusted in a combined heat and power system (CHP) (three electricity generators 0.933 MW each) should not exceed 150 ppmv. 2.1.2. Packing Materials. Parameters of the three packing materials used in the study are presented in Table 1. The ceramic Raschig rings

Table 1. Parameters of the Three Packing Materials Used in Biofilters

are commonly used for outlet gases cleanup in technical absorbers.21 Rounded pieces of keramzite (lightweight expanded clay aggregate) are made by thermal processing of clay at around 1150 °C. The obtained material is chemically inert and resistant to water and corrosion. Therefore, it finds increasing application in processes of sorption and air purification.22 The third packing material was high density polyethylene elements, type 2H BCN 009, produced by GEA 2H Water Technologies (U.K.). Because of the low bulk density and high specific surface area, these elements are increasingly used in biotechnological processes.23 The HDPE elements had the lowest bulk B

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Figure 1. Schematic presentation of experimental setup: (1) packed beds, (2) sprinklers, (3) circulating pumps, (4) containers with the nutrients solution, (5) temperature regulators, (6) sampling valves, (7) biogas flow controllers, (8) biogas equalizing tank, (9) drain valve, (10) crude biogas supply, and (11) thermal insulation and heating system.

Table 2. Dependence of Biomass Concentration in Biofilters on the Zone and the Hydrogen Sulfide Concentration biomass concn, CFU/g dry mass of carrier a

step Vb

step II zone in biofilter

BTF1

BTF2

BTF3

BTF1

BTF2

BTF3

top middle bottom

4.6 ± 0.21 × 108 5.7 ± 0.16 × 107 6.3 ± 0.16 × 108

2.7 ± 0.14 × 108 1.5 ± 0.16 × 107 2.2 ± 0.16 × 108

4.3 ± 0.28 × 108 3.5 ± 0.24 × 107 5.0 ± 0.32 × 108

3.1 ± 0.16 × 106 2.3 ± 0.19 × 105 4.5 ± 0.21 × 106

7.2 ± 0.16 × 104 1.6 ± 0.19 × 104 3.5 ± 0.17 × 103

4.4 ± 0.22 × 106 3.3 ± 0.28 × 105 2.7 ± 0.21 × 105

a

Operational conditions: H2S concentration, 525 mg/m3; LR, 2.56 m/m3/h; EBRT, 12 min. bOperational conditions: H2S concentration, 1620 mg/ m3; LR, 7.90 m/m3/h; EBRT, 12 min.

Figure 2. Water holdup value vs the trickling velocity for biofilters BTF1, BTF2, and BTF3. a Metrosep C4 column (150 mm × 4.0 mm) at 25 °C and eluted with 1.7 mM HNO3 and 0.6 mM dipicolinic acid in water. 2.2.3. Inoculum Preparation and Biofilm Development. Microbial consortium isolated from pipes through which the sludge is pumped into the anaerobic fermentation chambers and internal walls of the chemical desulfurization unit was used as inoculum for the biological desulfurization processes. This consortium was cultivated at 25 °C for 14 days in a liquid enrichment medium T2 for Thiobacillus species.25 The resulting mixed culture was used to inoculate 60 L portions of the nutrients solution (3 L per 60 L). The inoculated solution of nutrients trickled counter-currently to the direction of biogas flow through the packed beds to enable the growth of microbial biofilm. For the first 10

days the rate of biogas flow was 30 L/h. After 10 days the rate of biogas flow was increased to 60 L/h, and the beds were sprinkled with the third portion of nutrients solution, which was not inoculated. Formation and growth of the biofilm on the tested packing materials were conducted for 30 days. Frequency of the nutrition solution exchange was once per 5 days. During this period, hydrogen sulfide levels in the biogas ranged from 139 to 348 mg/m3. 2.2.4. Microbiological Analysis. Total counts of bacteria in the packed biofilters were determined based on the number of colony forming units (CFU) on the 98th day of step II of the process and on the 212th day of step V (Table 2). Four randomly collected particles from three different parts (top, middle, and bottom) of each of three C

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Figure 3. Relation between empty bed retention time (EBRT), removal efficiency (RE), and loading rate (LR) for biofilters BTF1, BTF2, and BTF3. biofilters (BTF1, BTF2, and BTF3) were suspended in physiological saline solution (100 mL) and incubated with shaking (150 rpm) for 30 min at 25 °C. The supernatants were pooled, and a series of decimal dilutions of each were prepared and spread on Petri dishes with nutrient agar supplemented with Ca(NO3)2·4H2O (0.5 g/L). The dishes were incubated under anaerobic conditions for 14 days. The number of CFU per 1 g of packing material was calculated according to the standard method (APHA, 1995).

which in turn depends on bed material properties (porosity, biofilm wetting, and particle size) and trickling velocity.26 It was shown the liquid holdup also depends on the ratio of the reactor diameter to the particle diameter and particle size and liquid Reynolds number.28 Wetting and liquid mass transfer may be a crucial parameter defining elimination capacity.29 From this point of view, a good liquid distribution in biofilter is required to maximize contact between gas and liquid.30 3.2. Impact of Empty Bed Retention Time on Desulfurization Outcomes. The empty bed retention time (EBRT) is one of the crucial parameters deciding of biogas desulfurization outcomes.31 The influence of EBRT values and corresponding values of loading rate (LR) on the removal efficiency (RE) of hydrogen sulfide is presented in Figure 3. This series of experiments was performed at variable biogas flow rates and almost constant hydrogen sulfide concentration ranging from 433 to 494 mg/m3. This was around an average hydrogen sulfide level in the biogas produced in the biogas plant involved in the study. The trickling velocity was constant (1.02 m/h) in this step of experiments. Under the tested conditions, values of LR ranged from 1.27 g/m3/h to 7.23 g/ m3/h for the flow rate of 0.12 m3/h and hydrogen sulfide concentration of 433 mg/m3 and for the flow rate of 0.6 m3/h and hydrogen sulfide level of 494 mg/m3, respectively. The effect of EBRT value on the hydrogen sulfide removal efficiency was investigated for 3 days for each value of biogas volumetric flow rate. The first day was considered the period of adaptation to the changed loading rate and the measurements were conducted for the next 2 days. At the highest biogas flow rate of 0.6 m3/h and the corresponding EBRT value of 4.1 min, the content of hydrogen sulfide was reduced by 90.1% using the biofilter BTF1, packed with HDPE elements. The result for the biofilter BTF3, filled with keramzite beads, was by 3.6% lower, while the biofilter BTF2, packed with Raschig rings, enabled a decrease in the level of hydrogen sulfide by only 78.2%. These results were achieved at an LR value of 7.23 g/m3/h. The increase in EBRT to 8.2 min caused LR to decrease to 3.32 g/ m3/h. Under these conditions, the biofilters BTF1, BTF2, and BTF3 reduced the content of hydrogen sulfide by 99.5, 88.5, and 97.4%, respectively. When EBRT was extended to 12 min, LR was reduced to 2.24 g/m3h and the biofilters BTF1 and BTF3 removed 99.7 and 99.3% of hydrogen sulfide, respectively, while the biofilter BTF2 reduced its level by 92.5%. Further extension of EBRT up to 20.5 min has not

3. RESULTS AND DISCUSSION 3.1. Liquid Holdup. The total liquid holdup is the hydrodynamic parameter of biofilters determining efficiency and outcomes of biodegradation of diverse compounds using packed columns. This parameter enables determination of mass and heat transfer coefficients in the packed column during absorption processes and estimation of wetting efficiency, pressure drop at different bed void fractions.26 It is defined as the total volume of the liquid phase in the bed volume at any time and was determined by the gravimetric method.27 The curves presented in Figure 2 demonstrate that the rise of trickling velocities from 0.18 to 7.56 m/h (0.05 to 2.10 × 10−3 m/s) caused an increase in water holdup for each of the packing materials. The lowest liquid holdup values were observed for the biofilter BTF2 packed with Raschig rings, while the water holdup for biofilter BTF1 with HDPE elements was 19% higher. In the same trickling velocity range for biofilter BTF 3 packed with keramzite beads it was 90% higher than for biofilter BTF1. At the trickling velocity of 1.02 m/h, which was used further in this study, the value of total liquid holdup for the biofilter BTF1 was 0.0834 m3/m3. At the same trickling velocity it was 14% lesser for biofilter BTF2 and 79% higher for biofilter BTF3. In our research, liquid holdup values were correlated with biofilters voidage. Interestingly, holdups for biofilters BTF1 and BTF2 having voidages 79% and 70% were almost the same over the range of trickling velocity investigated. The highest holdup value was determined for BTF3 having lowest voidage, i.e., 49%. Other authors26 using Pall rings as the bed material observed highest holdup value for voidage of 69%, while significantly lower holdup was determined for voidage 41%. The multiphase trickle bed reactor design and its scaling-up requires thorough knowledge on the mass and heat transfer processes, pressure drop, liquid retention time, and wetting efficiency. These factors are determined by liquid holdup, D

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Biogas 0.2 m3/h 1.79−8.45 g of hydrogen sulfide/m3/h 1.77−7.93 g of hydrogen sulfide/m3/h (RE: 99.3−93.8%)

12.3 min

10.3−72.0 min 11.25−12.5 g of hydrogen sulfide/m3/h (RE > 99%)

Ca(NO3)2 265−1245 ppmv

1000−4000 ppmv 39

present study

500−1500 ppmv 10

plastic elements ceramic Raschig rings keramzite

NaNO3

NaNO3

tetrahydrate

5.6−16 min 7.6 g of hydrogen sulfide/m3/h (RE > 95%)

1.79−8.15 g of hydrogen sulfide/m3/ h (RE: 99.3−96.4%) 1.74−5.37 g of hydrogen sulfide/m3/h (RE: 96.7−59.6%)

Biogas 0.01−0.07 m3/h

Biogas 0.025− 0.075 m3/h

Biogas 0.0084−0.06 m3/h

9.6−214 g of hydrogen sulfide/m3/h 8.8 g of hydrogen sulfide/m3/h 12.5−14.6 g of hydrogen sulfide/m3/h 2.4 min 127 g of hydrogen sulfide/m3/h (RE > 99%) NaNO3 1400−14600 ppmv 12

gas phase flow rate loading rate range

99%) NaNO3 KNO3 Ca(NO3)2 tetrahydrate

open-pore polyurethane foam polypropylene Pall rings polyester fibers lava rock polyester fibers 850−8500 ppmv

empty bed retention time maximum elimination capacity attained electrons acceptor packing materials studied hydrogen sulfide concentration ref

Table 3. Operation Conditions and Performance Parameters of Different Biofilters E

13

improved an efficiency of biofilters BTF1 and BTF3 significantly, while the biofilter BTF2 was able to remove 4.6% hydrogen sulfide from the biogas more. In studies of other authors who conducted biogas desulfurization under anoxic conditions, EBRT values ranged from a few to more than 10 min. In the study with polyester fibers and volcanic rock particles as packing materials,9 values of EBRT ranging from 5.6 to 16 min corresponded to 95−99% removal of hydrogen sulfide at LR = 8.8 g/m3/h, Fernandez13 observed 99.5% of desulfurization when EBRT varied from 2.4 to 17.0 min at LR < 138 g/m3/h (Table 3), while Deng31 reported 60−95% desulfurization rate for EBRT of 3.94−15.76 min at LR = 17.96−4.54 g/m3/h. 3.3. Long Time Removal Efficiency and Effect of Loading Rate on Desulfurization Outcomes. The effect of variable hydrogen sulfide concentrations in the biogas on values of RE and EC as well as changes in concentrations of nitrate and sulfate ions in the nutrient solution was determined. The continuous process of biogas desulfurization utilizing the microorganisms immobilized as described in section 2.2.3 was carried out at the constant EBRT of 12 min and constant biogas flow rate through each of the biofilters, of 0.2 m3/h. The removal efficiency (RE) of hydrogen sulfide by biofilters BTF1 and BTF2, operating for 225 days, is presented in Figure 4A,B. The differences in efficiency between these biofilters in terms of hydrogen sulfide removal were significant. The efficiency of biofilters BTF1 and BTF3 was similar and therefore the function of the latter biofilter was not presented. In each of the five steps of this experiment, hydrogen sulfide concentration in the biogas flowing through the biofilters was different (Figure 4C). The analysis of curves presented in Figure 4A,B leads to the conclusion that the reduction in hydrogen sulfide concentration depended on three important parameters, such as the chemical composition of the bed material, hydrogen sulfide concentration in biogas, and concentration of nitrate ions in the nutrient solution. During this study, hydrogen sulfide level in the biogas was increased from 369 mg/m3 (step I) to 1733 mg/m3 (step V) (Figure 4C). The efficiency of hydrogen sulfide removal by the biofilter BTF1 during the period between the 30th and 196th day (steps I−IV) ranged from 98.7 to 100%. In the last step (step V), hydrogen sulfide level in the biogas subjected to purification ranged between 1627 and 1733 mg/m3 and RE varied from 96.4% to 97.1%. The efficiency of biogas desulfurization by the biofilter BTF2 was less satisfying. In the first step of the study, at the hydrogen sulfide concentrations in biogas ranging from 369 mg/m3 to 444 mg/m3, RE values were similar as in case of the biofilter BTF1. However, in the next steps, when hydrogen sulfide concentrations were increased, RE values were lower. The interplay between values of LR (g/m3/h), RE (%), and EC (g of hydrogen sulfide/m3/h) for the three biofilters is shown in Figure 5, which presents only mean values of these parameters that were measured during the stable operation of these biofilters. The analysis of curves presented in Figure 5 leads to the conclusion that when mean values of LR were not higher than 3.25 g/m3/h, the value of EC for the biofilters BTF1 and BTF3 was close to the theoretical EC of 100%. The EC of the biofilter BTF2 was lower than the theoretical one (LR = EC) by around 6.0%. When hydrogen sulfide concentration in the biogas was increased in step IV, the biofilters operated at the loading rates ranging from 6.05 to 6.68 g/m3/h. Under these conditions, the elimination capacity ranged from 5.97 to 6.59 and from 5.82 to

Biogas 0.06 m3/h

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Figure 4. Variation of hydrogen sulfide removal efficiency (RE), NO3− and SO42− concentration as a function of time: (A) biofilter BTF1, (B) biofilter BTF2, and (C) evolution of hydrogen sulfide concentration in biogas and pH values of nutrient solutions.

6.43 g/m3/h, for the biofilters BTF1 and BTF3, respectively, while in case of the biofilter BTF2, EC ranged from 4.61 to 4.98 g/m3/h. Under these conditions, the maximum reduction in hydrogen sulfide level by the biofilters BTF1, BTF3 and BTF2 was 98.7%, 96.2%, and 76.2%, respectively. In step V, when the LR values ranged from 7.94 to 8.45 g/m3/h, the values of EC

were reduced with respect to the theoretical curve by 2.84%, 5.04%, and 43.20% for the biofilters BTF1, BTF3, and BTF2, respectively. In the last step of the experiment, the highest reduction in hydrogen sulfide level, ranging from 96.4 to 97.1%, was observed in case of the biofilter BTF1, while EC values for the biofilters BTF3 and BTF2 were by around 2.67% and F

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Figure 5. Removal efficiency (RE) and elimination capacity (EC) dependence on hydrogen sulfide loading rate (LR) for biofilters BTF1, BTF2, and BTF3.

addition carbonate and hydrocarbonate ions (CO32−, HCO3−) occurring in water could also act as a buffer system. When pH ranged from 6.8 to 7.0, RE values varied between 90.9 and 99.9%, but at a pH of 6.5, RE was reduced to 70%. The slightly acidic pH of the nutrient solution used in this study had no significant negative effect on the efficiency of hydrogen sulfide removal and utilization of nitrate ions by bacteria, although other authors13 observed that at pH below 7 reduction of nitrate ions was retarded. According to the literature,33 an autotrophic denitrifying bacteria grows optimally in the pH range between 6.5 and 7.5. In this study, nitrate ions were utilized with the same efficiency in the pH range from 6.9 to 6.2. These results suggest that the optimum pH in denitrification processes depends on the specific features of the process and the microflora of biofilters. 3.5. Nitrates Source. The source of nitrate ions in this study was calcium nitrate that was selected not only because it was cheaper than potassium nitrate and sodium nitrate, which are usually used in anoxic processes of hydrogen sulfide removal but also because of the potential role in biofilm formation. Calcium cations may stimulate cohesion between the cells that form biofilms and positively affect the growth of microorganisms taking part in denitrification processes.34 The data presented in Figure 4A demonstrate that the efficiency of hydrogen sulfide removal depended on the concentration of nitrate ions and the high values of RE, above 97%, were achieved only at sufficiently high nitrate levels in the nutrient solution. In the first three steps (steps I−III) of the experiment, the initial concentration of nitrate ions was 420 mg/L. When the concentration of hydrogen sulfide in biogas purified by the biofilters was increased, the nutrient solution had to be supplemented with calcium nitrate more frequently. Therefore, in steps IV and V, the concentrations of NO3− ions in the nutrient solution were increased to 840 mg/L and 1260 mg/L, respectively. In the case of the biofilter BTF1, the high RE values (>97%) were achieved when the nitrate level ranged from 0.29 to 0.33 mg N−NO3/mg of removed hydrogen sulfide. The efficiency of hydrogen sulfide removal from biogas was reduced to around 90% when the concentration of NO3−

58.13% lower, respectively. The results obtained in this study are consistent with the data obtained previously in laboratory conditions9 showing above 95% reduction of hydrogen sulfide concentration at EC of 7.6 g hydrogen sulfide/m3/h and LR of around 8.8 g of hydrogen sulfide/m3/h. In our study, in which LR was similar, of 8.45 g of hydrogen sulfide/m3/h, the efficiency of hydrogen sulfide removal by the biofilter BTF1 was above 96%. For comparison,12,13 for ∼10-fold higher values of EC (around 100 g of hydrogen sulfide/m3/h) the removal efficiency >99% was achieved at significantly higher concentration of hydrogen sulfide. However, flow rate of biogas through tested biofilters (resulting in higher EBRTs) were noticeable smaller (Table 3). 3.4. Effect of pH on Hydrogen Sulfide Removal. The data presented in Figure 4C provide evidence that changes in the pH of the nutrient solution during the biogas desulfurization processes were only slightly dependent on the hydrogen sulfide concentration in the biogas. The slow decrease in pH from 6.9 to 6.4 was observed in steps I and II of the study, in the case of all the applied biofilters. The significant increase in the hydrogen sulfide concentration in biogas in the third step of the experiment caused a decrease in pH to 6.2−6.4 in all the biofilters. In step V of the study, when the mean hydrogen sulfide concentration in biogas was around 1673 mg/m3, the RE values for the biofilters BTF1 and BTF3 were above 96% and 93%, respectively, while the mean pH of the nutrient solution in these biofilters was 6.3. In case of the biofilter BTF2, the mean pH was only slightly lower, of 6.2, but the value of RE was significantly lower (below 64%), compared to the biofilters BTF1 and BTF3. Interestingly, the pH of the nutrient solution in the biofilters was maintained at the stable level over the long period of time (around 20 days) although it was not replaced with a fresh portion (it was only supplemented with nitrate ions). The small changes in the pH of nutrient solution might be caused by the buffering effect of KH2PO4 contained in this medium. The buffering effect of phosphate ions coming from the material of the bed was also observed.32 Also, correlation between the pH in a biofilter and efficiency of hydrogen sulfide removal was reported.13 In G

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Energy & Fuels

nutrient solution in biofilter BTF2 was the result of decreased consumption of nitrates and, in consequence, worsening of removal efficiency (RE). This suggested that the reaction of hydrogen sulfide oxidation was different than that described by reaction 1 and/or that nitrate ions contained in the nutrient solution were reduced to a lesser extent. Under anoxic conditions, sulfate ions (SO42−) may compete with nitrate ions as electron acceptors and at sufficiently high concentrations they may reduce the efficiency of denitrification. However, in the present study, the relatively high concentration of sulfate ions, around 2000 mg/L, had no negative impact on the desulfurization efficiency. The application of calcium nitrate as a source of nitrate ions also enabled one to remove an excess of sulfate ions from the nutrient solution in the form of calcium sulfate, which is insoluble at concentrations above 2.04 g/L. The usage of either sodium or potassium nitrate instead of calcium nitrate would increase the ionic strength of the nutrient solution that in turn would limit its supplementation with nitrate ions. The occurrence of phosphate ions in the nutrient solution may cause precipitation of insoluble calcium phosphate.13 In our study, the pH of freshly prepared nutrient solution was around 6.9 ± 0.1. At this pH, H2PO4− ions dominate in solution, which form with calcium ions the relatively well soluble (18 g/L) calcium dihydrophosphate Ca(H2PO4)2. Therefore, the supplementation of the nutrient solution with calcium nitrate did not cause precipitation of calcium phosphate. Although certain amounts of sediments appeared at the bottom of the tank containing the nutrient solution, the biofilters were not plugged with any sediments over the period of 255 days. The nitrogen content in Ca(NO3)2·4H2O was lowest as compared to potassium and sodium nitrate, i.e., 11.9% vs 13.8% and 16.5%, respectively. Nevertheless an operational cost of removal of 1 kg of hydrogen sulfide using calcium nitrate was 0.48 € and was comparable to that for sodium nitrate.13 3.6. Microbiology. The numbers of colony forming units (CFU) per gram of dry carrier at three different levels of each of three biofilters are presented in Table 2. Average concentrations of hydrogen sulfide in the biogas flowing through the biofilters were 525 mg/m3 in the first step and 1620 mg/m3 in the last step. At the lower hydrogen sulfide content, CFU levels were almost the same in each of three biofilters. The highest number of live microbial cells was observed in the biofilter BTF1 (packed with HDPE elements), particularly in its bottom part (6.3 ± 0.16 × 108). CFU values for the middle and top parts were lower, of 5.7 ± 0.16 × 107 and 4.6 ± 0.21 × 108, respectively (Table 2). In each of the three beds, the lowest CFU values were observed in the middle part, which was ascribed to less favorable conditions than in the bottom and top parts. The number of live microbial cells and their biodiversity is usually higher in the inlet part of biofilters used for biogas purification.36 This phenomenon is ascribed to the greater variety of available nutrients.37 The increase in hydrogen sulfide concentration in the biogas, in step V of the experiment, significantly reduced CFU values in each of the three biofilters. In the biofilter BFT1, CFU values for the top and bottom parts were 4.5 × 105 and 3.1 × 106, respectively. The similar CFU value was observed in the top part of biofilter BTF3. It is to note that reduction in hydrogen sulfide content was above 90% at these concentrations of microbial cells in these two biofilters. The lowest CFU levels, of 3.5 ± 0.17 × 103 at the bottom and 7.2 ± 0.16 × 104 at the top, were observed in the biofilter BTF2.

ions was decreased to 50 mg/L (N−NO3 of 11.3 mg/L). At this concentration of nitrate ions, microbial cells usually began to assimilate nitrite ions, which were contained in the nutrient solution. The levels of the latter ions in successive steps of the process were not high and for the biofilter BTF1 they ranged from 120 mg/L in the step I to 68.0 mg/L in step V. Desulfurization was much less efficient when the concentration of NO3− ions was reduced to 20 mg/L (N−NO3− was decreased to 4.52 mg/L). Other authors11,13 reported that the lowest concentration of nitrate ions, which did not terminate hydrogen sulfide removal, was 20 mg N−NO3−/L, and the biofilters used by these researchers began to work 1 day after supplementation of the nutrient solution with the sufficient dose of nitrate ions. The function of the biofilter BTF2 was different than the function of the biofilters BTF1 and BTF3. In step I, when LR ranged from 1.8 to 2.26 mg/m3/h, the demand for nitrate ions was like in the case of the biofilter BTF1. From step II, the efficiency of hydrogen sulfide removal was decreasing despite the concentration of nitrate ions in the nutrient solution above 100 mg NO3/L. Further supplementation with calcium nitrate increased the level of nitrates in the nutrient solution, but RE did not grow. The reaction of complete oxidation of hydrogen sulfide oxidation to sulfates occurs according to the following equation:11,35 5H 2S + 8NO3− → 5SO4 2 − + 4N2 + 4H 2O + 2H+

(1)

35

The calculated theoretical N/S ratio for the above reaction is 1.60 mol/mol (0.699 mg/mg). The N/S ratios calculated for steps I−V of the long time removal efficiency experiment for biofilters BTF1 and BTF3 were ranging from 1.58 to 1.49 mol/ mol (0.692−0.651 mg/mg) and from 1.54 to 1.42 mol/mol (0.673−0.622 mg/mg), respectively. It may be concluded that the predominant oxidation mechanism was reaction 1, which was proved by almost theoretical N/S ratios for the above biofilters. For biofilter BTF2, the N/S ratios were in the range 1.44−0.86 mol/mol (0.629−0.375 mg/mg). Starting from step III, the stepwise worsening and significantly lower results of N/ S ratios were observed for this biofilter. It suggests that a different hydrogen sulfide oxidation reaction took place in this biofilter. The levels of nitrite ions were monitored over a period of each step. Interestingly, a slow increase in nitrite ions was observed during each step up to around 80−120 mg per of liter of nutrient solution. When concentration of nitrates dropped down below 25−50 mg/L, biofilters BTF1 and BTF3 still kept their desulfurization efficiency at a high level, most probably due to presence of relatively high concentrations of nitrites. After nitrates were exhausted completely, hydrogen removal was decreased quickly. As it was shown in Figure 4A,B, the concentration of sulfate ions in the nutrient solution gradually increased in successive steps of the process. In step I, after the first 24 days, it amounted to 877 and 894 mg/L for the biofilters BTF1 and BTF2, respectively. In the last three steps, steps III−V, the level of sulfate ions in the nutrient solution, which was used to sprinkle the biofilter BTF1, reached 2000 mg/L. Starting from step III, the concentration of sulfate ions in the nutrient solution trickling through the biofilter BTF2 was lower, compared to the biofilters BTF1 and BTF3. On the basis of the stoichiometry of reaction 1, the amount of 1.0 mg of sulfates is produced by reduction of 0.97 mg of nitrates. A significantly slower increase of sulfates concentration in H

DOI: 10.1021/acs.energyfuels.6b01374 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 6. Linear relationship between 1/R and 1/Cln for biofilters BTF1, BTF2, and BTF3.

used as a packing material,38 the Vmax and Km values were 60.48 g/m3/h and 411.6 mg/m3, respectively. The results of the kinetic analysis can be used as valuable data for selection of the most efficient packing materials in designing of industrial scale biofilters and controlling performance parameters.

Concentrations of microbial cells in biofilters operating under anoxic conditions depended primarily on the packing material and process conditions. The microbial counts ranged from 1.4 ± 0.14 × 109 to 1.23 ± 0.21 × 1010 cells were reported per gram of dry carrier,12 while others who used biofilter packed with plastic fibers found 2.4 × 105 cells per mL of this packing material.9 3.7. Kinetic Analysis. The results of kinetic analysis, which was performed for biofilters BTF1, BTF2, and BTF3, enabled one to compare the characteristics and performance of biological systems immobilized on the packing materials under steady state conditions. The assumption of sufficient oxygen availability in each biofilter and simple enzymatic onesubstrate reaction of hydrogen sulfide oxidation was adopted in calculations. At steady state conditions, the microbiological equilibrium in each biofilter was determined by rates of balanced growth and decay of microorganisms, thus kinetic constants were remaining constant within a considered period of time.3 The steady state experimental data for each biofilter were used to calculate the apparent kinetic parameters, according to the following equation:32

4. CONCLUSIONS The highest efficiency of hydrogen sulfide removal was achieved using the biofilter packed with HDPE elements. At the EBRT of 12 min, the maximum EC reached 7.97 g of hydrogen sulfide m3/h, which corresponded to RE > 96%. Furthermore, this packaging material ensured the best conditions for the immobilization of microorganisms. The lowest efficiency of hydrogen sulfide removal from biogas was observed in the biofilter packed with Raschig rings. The phosphates present in nutrient solution can act as a buffer. In regard to that, pH adjustment during the desulfurization process was not necessary. Calcium nitrate was found to be a suitable electron acceptor in biogas desulfurization processes.

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AUTHOR INFORMATION

Corresponding Author

1/R = K m/Vmax × 1/C ln + 1/Vm

*Phone: + (48) (42) 631-34-85. E-mail: [email protected].

where Cln is the log mean concentration = [(Cin − Cout)/ln(Cin/ Cout)] (mg of H2S m−3); R is the apparent removal rate = [(Vbiofilter/Qin)/(Cin − Cout)] (g of H2S m−3 h−1); Vmax is the maximum removal rate (g of H2S m−3 h−1); Km is the saturation constant for the gas phase (mg m−3); Qin is the biogas flow rate (m3 h−1); Vbiofilter is the biofilter volume (m3). The calculated values of 1/R plotted against 1/Cln using the least-square method are shown in Figure 6. The maximum removal rate Vmax and saturation constant Ks were obtained from the intercept and slope of the regression line equation for each biofilter. The highest value of the maximum removal rate, of 16.0 g/m3/h, was observed for biofilter BTF1. It reached 12.6 g/m3/h for biofilter BTF3, whereas for biofilter BTF2 it was 6.0 g/m3/h, i.e., more than 2.5 times lower than for BTF1. The saturation constant values for biofiters BTF1, BTF2, and BTF3 were 709.9, 294.3, and 582.3 mg/m3, respectively. For comparison, for biofilters containing pine bark, pozzolan with UP20, and UP20 alone,32 the following Vmax values, 14, 15, and 35 g/m3/h, respectively, and corresponding saturations constants in a gas phase Km of 25, 28, and 96 mg/m3 were obtained. For polyurethane foam

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present research was supported by the European Regional Development Fund and Polish Ministry of Science and Higher Education through the Operating Program for Innovative Economy for the period 2007−2013 Project No. UDAPOIG.01.03.01-196/09-00. The authors would also like to thank the Lodz Agglomeration Group Waste Treatment Plant, Poland, for providing the technical support in installation of pilot scale biofilters.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.6b01374 Energy Fuels XXXX, XXX, XXX−XXX