Hydrogen Sulfide Removal using Rice Husk Ash Beads as Packing Material in Biofilter

Transcription

1 Hydrogen Sulfide Removal using Rice Husk Ash Beads as Packing Material in Biofilter Nimas M.S. Sunyoto 1, 2, Warin Rukruem 3, Benjaphon Suraraksa 4, and Pawinee Chaiprasert 1 * 1 Division of Biotechnology, School of Bioresources and Technology, King Mongkut s University of Technology Thonburi, Bangkok (Thailand) 2 Agroindustrial Biotechnology, Brawijaya University, Malang (Indonesia) 3 Excellent Center of Waste Utilization Management (ECoWaste), Pilot Plant Development and Training Institute, King Mongkut s University of Technology Thonburi, Bangkok (Thailand) 4 Excellent Center of Waste Utilization Management (ECoWaste), National Center for Genetic Engineering and Biotechnology, National Science and Technology Development Agency at King Mongkut s University of Technology, Bangkok (Thailand) Abstract The presence of hydrogen sulfide as an impurity in biogas production is undesirable due to its corrosive properties to metal parts and machinery. The removal of hydrogen sulfide in biogas using porous beads from rice husk ash as packing material in biofilter was investigated. The removal efficiency and elimination capacity of the biofilter was determined. The study used a packing material namely rice husk ash beads (RHAB) in three different particle sizes of cm (RHAB 1), cm (RHAB 2) and cm (RHAB 3). The operation was carried out in 5 litres working volume biofilter under controlled DO 0.5 mg.l -1 and ph 7 and employed swine manure as the source of seed sludge. The recirculation and acclimation of sludge were done two weeks prior to its operation. The synthetic hydrogen sulfide gas at a concentration of 1,500 ppm was fed in an up flow direction at constant flow rate of 0.25 l. min -1 correspesponded to H 2 S loading rate of 6.13 g H 2 S.m -3.h -1. In the steady state performance, each of biofilter attained removal efficiencies of 92.33, and 99.53% for RHAB 1, RHAB 2 and RHAB 3, respectively within 8 hours of operation, and, the elimination capacities were 5.66, 5.90 and 6.10 g H 2 S.m -3.h -1, respectively. Keywords: Biofilter, Hydrogen sulfide removal, Rice husk ash beads 1. Introduction Hydrogen sulfide as a nuisance compound is one major problem in biogas utilization. Hydrogen sulfide (H 2 S) is produced naturally during the anaerobic digestion process as a result from reduction of sulfate content of wastewater by sulfate reducing bacteria [1]. H 2 S has a rotten egg odor, colorless, toxic, flammable and generally recognized as a hazardous gas [2]. H 2 S in biogas, which is found in the range of 50 10,000 ppm, is able to cause corrosion to the engines and pipe lines especially when used as fuel for producing electricity [3]. Moreover, H 2 S is also toxic to the The 8 th International Symposium on Biocontrol and Biotechnology 68

2 humans, plants, animals and microorganisms [4]. For these reasons, the removal of H 2 S is important. There are several processes to remove H 2 S from biogas including physical, chemical and biological processes. However, recently, biological processes have been recognized as the most efficient technologies for H 2 S removal because it is inexpensive and cause no environmental pollution [5]. One of the most important biological process for H 2 S removal is biofiltration [6]. In biofilters, contaminant gases are forced through a filter bed consisting of porous packing materials that serve as supporting media for microbial attachment to metabolize volatile compounds contained in the gas phase [7]. In order to get high efficiency of biofiltration, the performance of packing material as heart of biofiltration has to be considered. Several properties should be fulfilled to be an ideal packing material including suitable particle size, void fraction, large surface area and pore size for microbial growth, high air and water retention capacity, and also high buffering capacity avoiding large ph fluctuations [8-10]. Currently, various packing materials have been used in biofilter including coconut fibre, polyurethane foam, and sugarcane baggase [11]. However, it is still necessary to search for alternative low cost packing material having good properties to remove H 2 S effectively [9-10]. Thus, this study will use the a packing material namely rice husk ash bead (RHAB), a cylindrical shape packing material made from rice husk ash with various sizes to investigate its suitability as biofilter packing material to remove H 2 S. 2. Materials and Methods 2.1 Source of Initial Seed The sludge used in this study was obtained from open pond of swine farm wastewater treatment system in the Ratchaburi Province. The total sludge used in the system was 100 g VVS.system -1. The seed sludge was recirculated and acclimated on the packing material of the reactor two weeks prior to its operation. 2.2 Packing Material Preparation Rice husk ash beads (RHAB) packing material consist of bead shaped packing material made from rice husk ash. Three types of RHAB packing material were used in this study. The types of RHAB differ based on size: type 1 ( cm), type 2 ( cm) and type 3 ( cm). The packing materials were adjusted to neutral ph of 7±0.5 by re-washing for several times. Characteristics of packing material were determined in terms of particle size, specific pore volume, water holding capacity, void fraction, pressure drop and ph. 2.3 Reactor Configuration The experiments were carried out on a laboratory scale biofilter reactor made of white polyvinyl chloride (PVC) shown in Figure 1. The reactor column s height and diameter were 0.47 and 0.14 m, respectively. The holding tank at the bottom of the column provides nutrients to the microorganisms. The biofilter column had a capacity of 7 liters and 5 liters was used as the working volume. 2.4 Reactor Operation The operation was carried out under controlled dissolved oxygen of 0.5 mg.l -1 and ph 7. The synthetic H 2 S gas in the concentration of 1,500 ppm was fed in an up flow direction at constant flow rate of 0.25 l.min -1 and gas retention time (GRT) 20 min resulting in a the steady H 2 S loading The 8 th International Symposium on Biocontrol and Biotechnology 69

3 rate at 6.13 g H 2 S.m -3.h -1. Removal efficiency and elimination capacity were determined in bioreactor containing each type of packing material Figure 1. Schematic diagram of biofilter system used in the study (1. Packing material column 2. H 2 S cylinder 3. H 2 S gas analyzer 4. H 2 S inlet port/flow meter 5. Nutrient solution tank 6. Nutrient pump 7. H 2 S outlet port) 3. Results and Discussion 3.1 Characterization of Packing Material In this study, the properties of RHAB packing material were determined, as summarized in Table 1 and the relationship of these characteristics to the performance of the packing material were also studied. Table 1. Characteristics of different types RHAB packing material Characteristics RHAB Type 1 RHAB Type 2 RHAB Type 3 Particle Size (cm) Specific Pore Volume (ml.g dry bed) Void Fraction ph Specific density (kg.l -1 ) The 8 th International Symposium on Biocontrol and Biotechnology 70

4 3.2 Pressure Drop The pressure drop is one important parameter to be considered in the operation of a biofilter. The experiment was carried out in the absence of seed sludge. The experiment used four different flow rates to study the pressure drop of each packing material. The results are shown in Figure 2. The pressure drops were increased as the flow rates increased. The highest pressure drop was provided by RHAB type 1 which has the smallest particle size. In the flow rate used in this study (0.25 l.min -1 ), the small size of the packing resulted in the highest pressure drop (12.27 mm H 2 O.m -1 ) when compared to RHAB type 2 and 3 at the same flow rate. Figure 2. The profile of pressure drop This is due to the high specific density (kg.l -1 ) of RHAB type 1. However, RHAB type 1 has the smallest value of void fraction (Table 1). This property can lead to the highest pressure drop due to the flow inhibition of the air in the packing column. This result is similar to Filho s study [11] that the pressure drop of the reactor is related to the biofilter density. The reactor having high packing density will result in high pressure drop even at the same gas velocity. Therefore, the RHAB type 1 has the highest pressure drop at all flow rates. 3.3 Buffering Capacity of RHAB during Biofilter Operation High buffering capacity avoiding large ph fluctuations is also one property of ideal packing material [9, 11]. The buffering capacity of RHAB packing material was also studied. The ph fluctuation were observed every 2 hrs during the operation and summarized in Figure 3. The ph remained stable during operation within 8 hours in the range of The ability to buffer the ph fluctuation is a beneficial property to retain and improve the biofilter performance by providing favorable conditions for microbial growth inside the packing material. It is necessary because the ph in a biofilter may change during operation. In some cases, the ph has to be regulated using buffer substances mixed with the packing material or using alkaline or acid solutions. With the ability of RHAB to buffer the ph fluctuation, it is not necessary to add other chemical substances to stabilize the ph. 3.4 Removal Efficiency The ideal properties of packing material in the biofilter application are expected to improve the performance of biofilter in terms of removal efficiency and elimination capacity. Removal efficiency is the percentage expressing the efficiency of biofilter by comparing the inlet The 8 th International Symposium on Biocontrol and Biotechnology 71

5 concentration with the concentration removed by biofilter. While elimination capacity is defined as the removal of a certain mass of pollutant normalised by the volume of the biofilters [11]. Figure 3. ph profiles during the operation Removal efficiency profile of each type of RHAB was shown in Figure 4. In the steady state operation, removal efficiency of biofilter was 92, 96 and 99.5% for biofilter type 1, 2 and 3, respectively. Moreover, the elimination capacity of biofilter was calculated to be 5.7, 5.9 and 6.1 g H 2 S.m -3.h -1. When the removal efficiencies of RHAB type 1, 2 and 3 were compared to their pressure drop profiles, we found that RHAB type 3 which had the lowest pressure drop also provided the highest removal efficiency. This high removal efficiency of the biofilter is due to the controlled conditions of the operation, high surface areas provided by the packing and also the ability of packing material to buffer the ph fluctuation. Figure 4. Removal efficiency profiles during cycle operation The 8 th International Symposium on Biocontrol and Biotechnology 72

6 4. Conclusions The application of RHAB as packing material to remove H 2 S under certain conditions of operation can attain removal efficiencies of 92, 96 and 99.5% for RHAB 1, RHAB 2 and RHAB 3, respectively within 8 hrs of operation. Moreover, the elimination capacity of each reactor was 5.7, 5.9 and 6.1 g H 2 S.m -3.h -1. However, we need to study the feasibility of long term utilization of using RHAB as packing material. 5. Acknowledgements The authors gratefully thank the Excellent Center of Waste Utilization Management (ECoWaste), King Mongkut s University of Technology Thonburi for the grant and facility supports. The appreciation is also extended to Beasiswa Unggulan Depdiknas and Brawijaya University for financial supportto Ms. Nimas M. S. Sunyoto for her master degree study in Thailand. References [1] Potivichayanon, S., Pokethitiyook, P. and Kruatrachue, M., Hydrogen sulfide removal by a novel fixed-film bioscrubber system. Process Biochemistry, 41, [2] Manahan, E.S., Environmental chemistry. USA, Lewis Publisher, pp [3] Pipatmanomai, S., Kaewluan, S. and Vitidsant, T., Economic assessment of biogasto-electricity generation system with H 2 S removal by activated carbon in small pig farm. Applied Energy, 86, [4] Devinny, S.J., Deshusses, M.A. and Webster, T.S., Biofiltration for Air Pollution Control, Boca Roton, CRC Press LLC, p. 93. [5] Deng, L., Chen, H., Chen, Z., Liu, Y., Pu, X. and Song, L., Process of simultaneous hydrogen sulfide removal from biogas and nitrogen removal from swine wastewater. Bioresource Technology, 100, [6] Rattanapan, C., Boonsawang, P. and Kantachote, D., Removal of H 2 S in downflow GAC biofiltration using sulfide oxidizing bacteria from concentrated Latex wastewater. Bioresources Technology, 100, [7] Boldu, F.X., Illa, J., van Groenestijn, J.W. and Flotats, X., Influence of synthetic packing materials on the gas dispersion and biodegradation kinetics in fungal air biofilters. Applied Microbiology Biotechnology, 79, [8] Hirai, M., Kamamoto, M., Yani, M. and Shoda, M., Comparison of the biological H 2 S removal characteristics among four inorganic packing material. Journal of Bioscience and Bioengineering, 91(4), [9] Dumont, E., Andres, Y., Cloirec, P. and Gaudin, F., Evaluation of a new packing material for H 2 S removed by biofiltration. Biochemical Engineering, [10] Li, Z., Sun, T., Zhu, N., Cao, X. and Jia, J., Comparative study of using different materials as bacterial carriers to threat hydrogen sulfide. Applied Microbiology Biotechnology, 81, [11] Filho, J., Sader, L.T., Damianovic, M.H., Foresti, E. and Silva, E., Performance evaluation of packing materials in the removal of hydrogen sulphide in gas-phase biofilters: polyurethane foam, sugarcane bagasse, and coconut fibre. Chemical Engineering Journal, 158, The 8 th International Symposium on Biocontrol and Biotechnology 73