Process Biochemistry 41 (2006) 708–715 www.elsevier.com/locate/procbio
Hydrogen sulfide removal by a novel fixed-film bioscrubber system S. Potivichayanon, P. Pokethitiyook *, M. Kruatrachue Department of Biology, Faculty of Science, Mahidol University, Bangkok 10400, Thailand Received 24 May 2005; received in revised form 14 September 2005; accepted 16 September 2005
Abstract The fixed-film bioscrubber was developed for hydrogen sulfide removal. Acinetobacter sp. MU1_03 and Alcaligenes faecalis MU2_03 are two new strains of microorganisms from the fixed-film bioscrubber systems found. Under certain conditions, they exhibited more than 91% of hydrogen sulfide removal efficiency while a mixture of the two strains was capable of 98% hydrogen sulfide removal. Removal efficiency increased with decreasing inlet gas flow rates, increasing the height of packing and empty bed retention time. During the operation, the pH decreased but did not fall below 6.4. Sulfate production increased when the removal efficiency increased due to the oxidation of hydrogen sulfide to sulfate. In addition, dissolved oxygen decreased during the same reaction. # 2005 Elsevier Ltd. All rights reserved. Keywords: Hydrogen sulfide; Fixed-film bioscrubber; Acinetobacter sp. MU1_03; Alcaligenes faecalis MU2_03; Mixed culture; Removal efficiency
1. Introduction Hydrogen sulfide (H2S) is produced naturally during the reduction of sulfate and sulfur-containing organic compounds by nonspecific anaerobic bacteria [1]. Its characteristic ‘‘rotten egg’’ odor is a major nuisance in municipal, industrial and biological waste treatment processes. H2S is readily soluble in water and mobile in moist soil, aquatic and marine environments. Several microorganisms in soils, aquatic, and marine environments can oxidize hydrogen sulfide to sulfate and elemental sulfur. Hydrogen sulfide is extremely toxic to living organisms and plants. At a level of 0–5 ppm in the air, it can be detected easily. Levels greater than 10 ppm, can affect human health, while levels more than 600 ppm can cause death [2]. Methods for hydrogen sulfide removal in common use today are physicochemical processes. These processes have high operating costs and also produce chemical waste by-products that must be disposed of before discharge. Recently, the chemobiological processes are used for the gas treatment [3,4]. However, these processes still use the chemical reagent and have acidic condition for the oxidation reaction which has
* Corresponding author. Tel.: +662 2015479; fax: +662 3547166. E-mail address:
[email protected] (P. Pokethitiyook). 1359-5113/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2005.09.006
some cost of the operation. For these reasons, biological processes are more attractive because they are inexpensive and cause no environmental pollution. The major systems commonly used are biofilters, biotrickling filters and bioscrubbers [5,6]. The basic removal mechanisms are similar for all systems though differences exist in the phase of the microorganisms, which may be attached or suspended, and the phase of the liquid, which may be flowing or stationary. In bioscrubber systems, microorganisms are fixed or suspended in an aqueous phase. In a suspended growth bioscrubber, the microorganisms are dispersed freely throughout the liquid phase whereas in a fixed-film bioscrubber, the microorganisms are immobilized on packing materials such as glass, ceramics, metal, or plastic [7,8]. In addition, gas absorption and degradation occur separately. Absorption may be achieved in packed columns, spray towers, or bubble columns. After absorption occurs degradation of gas is performed. To achieve complete degradation, the liquid need to be transferred to an aeration tank. Recycling the liquid media can increase the gas removal efficiency and such a system in appropriate for water soluble odors. In addition, bioscrubbers have the advantage of better operation control over pH, nutrient content, and gas flow rate [5,9]. Fixed-film bioscrubbers work in a similar manner to biofilters but bioscrubbers can be operated with a much higher inlet gas concentration and gas flow rate than biofilters. This
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reduces space requirements and thereby construction costs [10]. The greatest advantage of bioscrubbers is their ability to deal with high odor concentrations and also severe fluctuation. The removal efficiencies of bioscrubbers are in the region of 90– 99% [7,11]. The bioscrubber system used in this experiment was a fixedfilm bioscrubber or biotrickling filter [12]. It consisted of immobilized microorganisms on plastic packing media (a gas– liquid contact column) and a recirculation tank where nutrients were added. The media provide sites for biological colonization and promote the mass transfer between gas and liquid phase on the biomass where biological oxidation occurs [13]. The important parameters for operating bioscrubber systems are types of microorganisms, recirculation rate of the liquid media, type of packing materials, and the empty bed retention time [12]. The removal efficiency can be increased by manipulating the basic parameters for this system. For these reasons, the objectives in this study were: (1) to isolate, identify and compare the efficiency of different microorganisms found and (2) to study the optimum operating parameters such as the height of packing media, inlet H2S flow rate, liquid media flow rate, and empty bed retention time (EBRT). 2. Materials and methods 2.1. Microorganisms and cultivation Microorganisms were isolated from an aeration tank at Si-Phraya municipal wastewater treatment plant in Bangkok and purified by repeatedly transferring the cells to fresh medium. A thiosulfate broth (TSB) composition of KH2PO4 2.0 g l 1; K2HPO4 2.0 g l 1; NH4Cl 0.4 g l 1; MgCl26H2O 0.2 g l 1; FeSO47H2O 0.01 g l 1 and Na2S2O35H2O 8.0 g l 1 was the medium used for sulfur oxidizing bacteria culture. The final pH of the medium was adjusted to 7 by 1N NaOH and 1N HCl. The medium was autoclaved for 15 min at 15 psi and 121 8C before use. Bacto Agar (18 g l 1) was added when the medium agar was used. About 10 ml of microorganisms were inoculated into 100 ml of nutrient broth (composition: yeast extract 5.0 g l 1; bacto tryptone 10.0 g l 1; glucose 2.0 g l 1 and the final pH adjusted to 7) and incubated for 7 days at 30 8C in a rotary shaker (180 rpm). In order to screen microorganisms, 10 ml of microorganisms in nutrient broth flask were purified by repeatedly transferring the cells into 500 ml Erlenmeyer flask containing 100 ml of thiosulfate broth (TSB) and incubated at 30 8C, 180 rpm. After 3 days, microorganism isolation was done by a spread plate technique on thiosulfate agar plates. The plates were incubated at 30 8C for 3 days. After that the morphology and number of colonies were observed under a light microscope. Colonies of different morphology were isolated by picking a single colony of each type and inoculated on thiosulfate agar by a streak plate technique. The isolated microorganisms were cultivated separately in thiosulfate broth as a pure culture and then transferred 10% (v/v) at every 7 days. The growth of isolated microorganisms was studied by a plate count technique. Since pH is an essential parameter influencing hydrogen sulfide removal, the media solution was then buffered to avoid pH changes. During the growth study, pH, sulfate (SO42 ) content and maximum growth rate (at logarithmic phase) were determined by pH meter, turbidimetric method according to Standard Methods [14].
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Fig. 1. Schematic diagram of the fixed-film bioscrubber in this experiment: 1, H2S gas; 2, N2 gas; 3, flow meter; 4, regulator; 5, 3-way connector; 6, bioscrubber column; 7, peristaltic pump; 8, recirculation tank.
The weight of organism grown was the weight difference before and after the experiment.
2.3. Reactor set-up The experimental set-up is shown in Fig. 1. The bioscrubber column was made of glass of 0.03 m in diameter and 0.5 m of height. The column was packed with packing media to get the working height of 0.15 or 0.30 m. Fine control of gas flow rates was achieved by use of needle valves and a flow controller (Cole-Parmer, A03219-17, USA). Hydrogen sulfide was diluted by nitrogen gas to obtain the desirable concentrations and introduced upward into a packed bed. The H2S gas cylinder of 200 ppmv (parts per million by volume) balanced with nitrogen was manufactured by Air Liquide, USA. N2 gas cylinder of 99.9% purity was produced by Air Liquide, Thailand. The energy source for microorganism growth was derived from the hydrogen sulfide oxidation in fixed-film bioscrubber column. Therefore, Na2S2O35H2O was taken out from nutrient solution in the recirculation tank. This nutrient solution was refilled everyday at a volume of 300 ml which is equivalent to the sampling volume. This liquid medium was pumped downward and recirculated during the continuous experiment. All experiments were operated in continuous mode at room temperature.
2.4. Abiotic experiment The abiotic experiment was set-up for 360 min. The column was packed at 0.15 m without cell immobilization. The inlet H2S set at 10 ppmv was pumped upward to the bioscrubber column containing the abiotic packing media at the flow rate of 500 ml min 1 while liquid medium (TSB without Na2S2O35H2O) flowed downward from a recirculation tank at the flow rate of 13 ml min 1. The empty bed retention time (EBRT) was 12.71 s.
2.5. Short-term fixed-film bioscrubber experiments 2.2. Immobilization of bacterial cell on the packing media The packing media were polypropylene pall (PP) rings. The empty media were weighed before starting the experiments. The diameter and surface loading rate of the packing media were 0.025 m and 206.81 m2 m 3, respectively. The immobilization process was initiated by transferring the packing media into thiosulfate broth containing selected microorganisms grown at logarithmic phase. The packing media were weighed after the experiment was done.
Short-term experiments were set as preliminary tests for studying the appropriate H2S inlet concentration, liquid flow rate, and the empty bed retention time. The optimum conditions from this experiment were used for setting the long-term fixed-film bioscrubber experiments. The height of packing media was fixed at 0.15 m for all of the short-term experiments. The short-term experiments of the fixed-film bioscrubber used in this stage are shown in Table 1. All experiments were operated only for 360 min.
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Table 1 Short-term experiments of the fixed-film bioscrubber Short-term experiments
Inlet gas concentration (ppmv)
Gas flow rate (ml min 1)
Liquid flow rate (ml min 1)
Empty bed retention time (EBRT, s)
1 2 3 4 5 6 7 8 9 10
10 10 20 20 40 40 80 80 100 100
500 500 500 500 500 500 375 375 200 200
13 35 13 35 13 35 13 35 13 35
12.71 12.71 12.71 12.71 12.71 12.71 16.94 16.94 31.77 31.77
Table 2 Long-term experiments of the fixed-film bioscrubber Long-term experiments
Inlet gas concentration (ppmv)
Gas flow rate (ml min 1)
Liquid flow rate (ml min 1)
Height of packing (m)
Empty bed retention time (s)
1 2 3
20 20 20
500 250 500
13 13 13
0.30 0.30 0.15
25.43 50.87 12.71
2.6. Long-term fixed-film bioscrubber experiments The optimum conditions obtained from the short-term experiments were used for the comparison of each isolated microorganism, and for studying the effect of gas flow rate, empty bed retention time, and height of packing. To further optimize the system, the long-term experiments of the fixed-film bioscrubber were operated for 72 h and set as shown in Table 2. The inlet gas was pumped upward to the bioscrubber column with and without immobilized cell on the packing media at the flow rates of 250 and 500 ml min 1 while liquid containing nutrient flowed downward from a recirculation tank at flow rates of 13 ml min 1. The EBRT was varied in the range of 12–50 s. The height of packing media was varied from 0.15 to 0.30 m in these experiments.
2.7. Analytical methods
Alcaligenes faecalis MU2_03. Each microbial strain was identified based on gram stain and biochemical properties using the API testing system and on the basis of their morphology described in Bergey’s manual [15]. Acinetobacter sp. and A. faecalis have never been reported as microorganisms capable for the removal of hydrogen sulfide. 3.2. Growth of microorganisms After the microorganisms were isolated, their growth was studied (Fig. 3). The highest growth rates of Acinetobacter sp. MU1_03 and A. faecalis MU2_03 and mixed culture were
The inlet and outlet H2S gas from the bioscrubber system was measured by using an H2S gas detector (OLDHAM, MX21 PLUS, France). Sulfate (SO42 ) content in the recirculation tank was determined by turbidimetric method according to Standard Method [14]. The principle of this method is the precipitation of sulfate in an acetic acid medium with barium chloride (BaCl2) so as to form barium sulfate (BaSO4) crystals of uniform size. The light absorbance of the BaSO4 suspension was measured by a spectrophotometer (Bio Aquarius, CE7200, UK) at 420 nm wavelength using deionized water as a standard. The sulfate concentration was determined by comparing to a standard curve (Fig. 2). pH was also measured by a pH meter (HANNA, HI98128, Italy) and the dissolved oxygen content was measured by an oxygen meter (YSI, 51B, USA). The dissolved oxygen content was determined only in the long-term fixed-film bioscrubber experiments.
3. Results and discussion 3.1. Microbial isolation and identification Two different dominant types of microorganisms were isolated from an aeration tank at Si-Phraya municipal wastewater treatment plant: Acinetobacter sp. MU1_03 and
Fig. 2. Standard curve of sulfate concentration.
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Fig. 3. Growth curves and pH values of thiosulfate media containing bacteria types MU1_03, MU2_03 and mixed culture during 7-day incubation period.
obtained on day 4 of the incubation time. The colony forming units per ml (cfu ml 1) during this time were approximately 108 cells for Acinetobacter sp. MU1_03 and 107 cells for both A. faecalis MU2_03 and the mixed culture. pH remained within 0.1 throughout the experiment and can be considered a negligible change. This drop in pH might have been caused by microorganisms that oxidized thiosulfate to sulfuric acid for their growth and respiration during that time. The growth of three bacterial types at logarithmic phase (day 4) based on a biomass concentration is shown in Table 3. The mixed culture was the mixture of MU1_03 and MU2_03 so the weight of mixed strains was more than MU1_03 or MU2_03 alone. 3.3. Abiotic experiment In the abiotic experiment, the hydrogen sulfide removal efficiency was only 10% after 300 min of operation. The sulfate concentrations were in the range of 2.5–2.8 mg l 1 and pH did not fall below 6.7 throughout the experiment. 3.4. Short-term fixed-film bioscrubber experiments The H2S removal efficiencies of each microorganism in short-term experiments are shown in Table 4. The efficiencies of Acinetobacter sp. MU1_03, A. faecalis MU2_03 and mixed microorganisms in short-term experiments 1–4 were 70% Table 3 The weight of each type of microorganisms at the logarithmic phase Microorganisms
Weight per 100 ml (mean S.D.) (g)
Acinetobacter sp. MU1_03 Alcaligenes faecalis MU2_03 Mixed culture
0.077 0.001 0.095 0.0013 0.161 0.0012
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within 1 h of the operation time while pH decreased, due to the oxidation of hydrogen sulfide to sulfate by microorganisms, as reflected by the increase in sulfate values. The without cell experiment reached only 1–5% H2S removal efficiency in all short-term experiments. The removal efficiency in experiments 5 and 6 decreased because of the increase of inlet gas concentration from 20 to 40 ppmv. This is consistent with the experiments performed earlier by Oyarzun et al. [16]. Therefore, the appropriate inlet gas concentration of 20 ppmv was chosen for setting the long-term experiments. The efficiency decreased when the inlet gas concentration increased. In experiment 6, the H2S removal efficiency of Acinetobacter sp. MU1_03 increased to 78% after 300 min of the operation time. This may be due to the increasing of liquid flow rate from 13 to 35 ml min 1 (Table 1). Increase liquid flow rate will help increase the mass transfer rate between gas and liquid [17] and hence, increase the H2S removal efficiency. When the EBRT was increased from 12.71 to 16.94 s (experiments 6 and 7), the strains MU1_03 and MU2_03 achieved more than 73% removal efficiency while the mixed culture exhibited more than 80% removal efficiency (Table 4). In addition, if the EBRT was increased to 31.77 s in experiments 9 and 10, the removal efficiency of MU1_03 and MU2_03 increased to over 90% within only 60 min, and to 98% in the mixed culture. The pH values in the recirculation flask slightly decreased in all experiments from 6.9 to 6.5 but did not fall lower than 6.5. Sulfates are a by-product of hydrogen sulfide removal and should be taken into account when considering removal efficiency. The sulfate values increased when removal efficiencies increased [11,16]. In this experiment, sulfate production increased when H2S removal increased. The sulfate concentrations of MU1_03 in experiments 1–10 were in the range 2.8– 7.7 mg l 1 at the beginning of the experiments. After 60 min, the sulfate increased to of 3.0–13 mg l 1 and further to 4.5– 15 mg l 1 at the end of the experiment (360 min). In addition, the sulfate concentrations given by MU2_03 in all experiments at 0, 60 and 360 min were 5.2–9.0, 5.2–9.9 and 5.4–8.9 mg l 1, respectively. At the same time, the sulfate concentrations of mixed culture were 5.5–21.0, 5.2–21.0 and 5.9–22.0 mg l 1, respectively. The temperatures of all experiments were in the range of 28.3–33.0 8C and did not affect removal efficiency. Fig. 4 shows the relationship between H2S loading rate and the elimination capacity of each microorganism. The MU1_03, MU2_03 and mixed culture exhibited the elimination capacities between 2.70 and 5.44 g m 3 h 1 at H2S loading rates of 3.89– 7.78 g m 3 h 1 in the short-term experiments 1–4. Although the H2S loading rate increased to 23.33 g m 3 h 1 in experiments 7 and 8, the elimination capacities still increase and the mixture of the two strains reached the highest elimination capacity of 19.24 g m 3 h 1. 3.5. Long-term fixed-film bioscrubber experiments 3.5.1. Effect of gas flow rate on fixed-film bioscrubber operation Before starting the long-term fixed-film bioscrubber operation, the appropriate H2S inlet concentration and liquid flow
0 98 98 98 98 98 98 0 92 91 90 91 91 91 0 88 89 89 89 89 89 0 90 90 90 90 90 90 0 90 90 90 90 90 91 0 92 92 92 92 92 91 0 81 80 80 81 81 83 0 78 29 80 29 29 19 0 80 80 77 78 29 19 0 73 78 78 78 29 19 STE_#* stands for short-term experiments 1–10, MU1 stands for Acinetobacter sp. MU1_03, and MU2 stands for A. faecalis MU2_03.
0 73 73 73 73 73 73 0 73 73 73 73 73 73 0 70 70 70 70 70 70 0 70 70 70 70 70 70 0 73 75 75 75 78 78 0 78 65 65 68 65 68 0 65 65 65 65 65 65 0 65 65 65 65 65 65 0 70 70 70 70 70 70 0 70 70 70 70 70 70 0 70 70 70 70 70 70 0 65 70 70 70 70 70 0 70 70 70 70 70 70 0 65 70 70 70 70 70 0 70 70 70 70 70 70 0 70 70 70 70 70 70 0 70 70 70 80 80 80 0 70 70 70 70 70 70 0 70 70 70 70 70 70 0 70 70 70 70 70 70 0 60 120 180 240 300 360
STE_9* STE_8* STE_7* STE_6* STE_5* STE_4* STE_3* STE_2* STE_1*
Time (min) Removal efficiency (%)
Table 4 H2S removal efficiencies of each microorganism in short-term experiments
MU1 MU2 Mix MU1 MU2 Mix MU1 MU2 Mix MU1 MU2 Mix MU1 MU2 Mix MU1 MU2 Mix MU1 MU2 Mix MU1 MU2 Mix MU1 MU2 Mix MU1 MU2 Mix
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STE_10*
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Fig. 4. The relationship between H2S loading rate and H2S elimination capacity of each microorganism in short-term experiments 1–10.
rate should be considered because they significantly affect the H2S removal efficiency in long-term bioscrubber systems. Therefore, the optimal operating conditions used for the longterm experiments were set as shown in Table 2. The inlet gas concentration and liquid flow rate for all experiments were maintained at 20 ppmv and 13 ml min 1, respectively. When the gas flow rate was decreased, the hydrogen sulfide removal efficiency increased as shown in long-term experiments 1 and 2 (Figs. 5 and 6). When the gas flow rate was reduced from 500 to
Fig. 5. The relationship of the removal efficiency (RE) and sulfate production of the two strains, mixed culture and without cell culture in long-term experiment 1.
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Fig. 6. The relationship of the removal efficiency (RE) and sulfate production of the two strains, mixed culture and without cell culture in long-term experiment 2.
Fig. 7. The relationship of the removal efficiency (RE) and sulfate production of the two strains, mixed culture and without cell culture in long-term experiment 3.
250 ml min 1, hydrogen sulfide removal efficiency increased to more than 85%. The result was similar to that of Nishimura and Yoda [11]. Furthermore, A. faecalis MU2_03 exhibited 90% removal efficiency in only 2 h whereas with Acinetobacter sp. MU1_03 and the mixed culture removal efficiency remained above 85% throughout the 72 h operation time. In addition, the without cell fixed-film bioscrubber reached the lowest efficiency (5%) in experiment 1, but reached 10% after 2– 24 h of operation in experiment 2. Ockeloen et al. [7] suggested that the removal efficiency is more sensitive to the gas velocity than the liquid velocity because less substrate enters the bioscrubber.
addition, the without cell fixed-film bioscrubber reached only 5% of the efficiency in experiment 3. Therefore, it was suggested that the removal efficiency was able to increase when the EBRT was increased. This finding was also similar to Koe and Yang [12]. Removal efficiency exceeded 99% when the EBRT was more than 60 s [18,19]. In addition, when the EBRT was decreased to 19 s the removal efficiency decreased to 87% [18,19].
3.5.2. Effect of empty bed retention time on fixed-film bioscrubber operation The short-term results indicate that EBRT is an important parameter. The suitable ranges of EBRT were higher than 16 s even if the inlet gas concentration was increased to 100 ppmv. In the long-term experiment, the results show that Acinetobacter sp. MU1_03 had the highest removal efficiency (about 85% in only 2 h of the operation time; Fig. 5). In addition, A. faecalis MU2_03 and mixed culture achieved 80 and 70% removal efficiencies in 2 h, respectively. When EBRT was increased to 50.87 s (long-term experiment 2), the removal efficiency of A. faecalis MU2_03 reached 90% in only 2 h of the operation time whereas Acinetobacter sp. MU1_03 and mixed culture had similar removal efficiency but they took longer time to attain this (Fig. 6). However, the removal efficiency of Acinetobacter sp. MU1_03 dropped by 5% after 24 h of operation. In long-term experiment 3, the EBRT was reduced to 12.71 s so the removal efficiency of all microorganisms decreased to 75% after 72 h of the operation. The mixed culture achieved more than 80% removal efficiency at 24 h but the efficiency dropped to 75% subsequently (Fig. 7). In
3.5.3. Effect of packing media on fixed-film bioscrubber operation The fixed-film bioscrubber was operated with two different heights of packing as shown in Table 2. The inlet gas concentration was fixed at 20 ppmv while the heights of packing varied between 0.30 and 0.15 m. When the height was 0.30 m (long-term experiment 1), Acinetobacter sp. MU1_03 and A. faecalis MU2_03 exhibited higher removal efficiencies than the mixed culture (Fig. 5). Some researchers have found that removal efficiencies increased when the heights of packing were increased [7,20]. When the heights were reduced to one half in experiment 3, the removal efficiency declined (Fig. 7). Therefore, the co-culture had an advantage in the hydrogen sulfide removal with short duration EBRT. 3.5.4. The relationship of the removal efficiency, sulfate, pH and dissolved oxygen in the fixed-film bioscrubber Sulfate and pH values in the recirculation tank were studied in all experiments. Sulfates increased greatly whereas pH decreased slightly as hydrogen sulfide was oxidized to sulfate (Figs. 5–7). Oxidation reactions in the bioscrubber produces sulfate and then sulfuric acid, the most common end-product which causes an acid pH [18,21]. However, Smet and Van [22] found that 95% of the sulfates were leached as sulfates (SO42 ) salts and not as H2SO4 (sulfuric acid).
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Fig. 8. The relationship of the removal efficiency (RE) and dissolved oxygen of the two strains, mixed culture and without cell culture in long-term experiment 1.
Fig. 9. The relationship of the removal efficiency (RE) and dissolved oxygen of the two strains, mixed culture and without cell culture in long-term experiment 2.
Complete hydrogen sulfide removal can occur only when the oxygen is sufficient for the oxidation reaction. Bacterial activity in the recirculation tank reduced the dissolved oxygen to as little as 0.5 mg l 1 (Figs. 8–10). The DO was as high as 6.6– 7.2 mg l 1 when the bioscrubber operated without immobilized cells. Oxygen is the key parameter that controls the level of oxidation [23]. Insufficient oxygen may cause a shift in the proportion of the end-products and prevent complete removal of H2S. Generally, sulfate is the most common end-product of H2S oxidation when the oxygen is sufficient. As dissolved oxygen is consumed by the oxidation reaction and decreases markedly, elemental sulfur may become the end-product [24]. 3.5.5. The weight of biofilm on packing materials in longterm fixed-film bioscrubber The packing materials were weighed before and after the long-term experiments 1–3. Table 5 shows the weight of each bacterial biofilm on packing materials in the long-term experiments 1–3. In the long-term experiment 1, the weight of A. faecalis biofilm was the highest (0.0026 g) and the H2S removal efficiency was 80%. In addition, the weight of MU1_03 biofilm was lower whereas the removal efficiency increased to 85%. In the long-term experiment 2, the weight of MU2_03 biofilm was the highest (0.0046 g) and the greatest
Fig. 10. The relationship of the removal efficiency (RE) and dissolved oxygen of the two strains, mixed culture and without cell culture in long-term experiment 3.
Table 5 The weight of biofilm on packing materials of each microorganism in the long-term experiments 1–3 Microorganisms
Acinetobacter sp. MU1_03 A. faecalis MU2_03 Mixed culture
Weight of biofilm on packing in long-term experiment (mean S.D.) (g) 1
2
3
0.0022 0.0003 0.0026 0.000232 0.0017 0.000348
0.0032 0.000489 0.0046 0.00008 0.0027 0.00068
0.0031 0.000319 0.0018 0.0001976 0.0016 0.0005346
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H2S removal efficiency obtained (90%). On the other hand, the mixture of MU1_03 and MU2_03 revealed the lowest weight but it exhibited the highest removal efficiency (80%; long-term experiment 3). 4. Conclusions The use of fixed-film bioscrubbers for hydrogen sulfide removal has received much attention from environmental scientists and engineers. In this experiment, the dominant strains of microorganisms found in the bioscrubber systems were Acinetobacter sp. MU1_03 and A. faecalis MU2_03. The system exhibited more than 91% of hydrogen sulfide removal efficiency. The mixture of two strains increased the removal efficiency to 98% when the EBRT and the liquid flow rate were set at 31.77 s and 13 ml min 1, respectively; although the inlet gas concentration was increased to 100 ppmv. Increased removal efficiencies yielded higher sulfate values and a decrease in dissolved oxygen because of the oxidation of hydrogen sulfide to sulfate. As a consequence, the pH of liquid media dropped but remained at or above 6.4 due to the appropriate buffering system of the liquid media used in the recirculation tank. Acknowledgements This project was supported by the Post-Graduate Education, Training and Research Program in Environmental Science, Technology and Management under the Higher Education Development Project of the Commission on Higher Education, Ministry of Education, Thailand. The authors would like to thank Mr. Phil Round for reading this article. References [1] Hesketh HE, Cross FL. Odor control including hazardous/toxic odors. Pennsylvania: Technomic Publishing, 1989. [2] Droste RL. Theory and practice of water and wastewater treatment. New York: John Wiley and Sons, 1997. [3] Park D, Lee DS, Joung JY, Park JM. Comparison of different bioreactor systems for indirect H2S removal using iron-oxidizing bacteria. Process Biochem 2005;40:1461–7. [4] Son HJ, Lee JH. H2S removal with an immobilized cell hybrid reactor. Process Biochem 2005;40:2197–203. [5] Burgess JE, Parsons SA, Stuetz RM. Development in odour control and waste gas treatment biotechnology: a review. Biotechnol Adv 2001;19: 35–63.
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