Effect of O 2 removal in initial process on biogas production and methane content (Case: co-digestion of soybean curd whey and cow manure)

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1 Proceeding of ISBN International Conference on Sustainable Energy Engineering and Application Inna Garuda Hotel, Yogyakarta, Indonesia 6 8 November 212 Effect of O 2 removal in initial process on biogas production and methane content (Case: co-digestion of soybean curd whey and cow manure) Arini Wresta a, *, Wiratni b, Imam Prasetyo b, Aep Saepudin a, Budi Prawara a a Research Centre for Electrical Power and Mechatronics - Indonesian Institute of Sciences Kompleks LIPI, Jl. Cisitu No. 21/154D - Bandung 4135 Indonesia b Chemical Engineering Department - Faculty of Engineering, Gadjah Mada University Jl. Grafika 2 Yogyakarta Indonesia Received 9 July 212; accepted 29 August 212 Abstract In the application on biogas production process, the effort to remove O 2 in initial process will increase the operation cost. This research was conducted to understand the influence of O 2 removal in initial process on biogas production and methane content. The substrates used were a mixture of the waste water of soybean curd industry (soybean curd whey) and cow manure. Experiments were done in two batch anaerobic digesters, with same material compotition, with weight comparation of soybean curd whey, cow manure and active digester effluent of 4 : 1 : 1 in room temperature approximately of 29-3 o C. The oxygen from the air in the gas space in the first digester was removed by flushing nitrogen gas, and this effort wasn t done in the second digester. Experiment result showed that O 2 removal in initial process had no significant effects on biogas production and methane content caused by the existence of facultatively anaerobic acidogenic bacteria that immediately consumed O 2 in the slurry in the digester. By the little effect of oxygen removal on the biogas production, the methane formation will run well without oxygen removal in initial process. Key words: O 2 removal; initial process; biogas production; methane content; facultatively anaerobic acidogenic bacteria. Selection and/or peer-review under responsibility of Research Centre for Electrical Power and Mechatronics, Indonesian Institute of Sciences 1. Introduction By the increasing of the energy need and the limitation of fossil fuel crude oil and natural gas, Indonesian government has focussed the research on renewable energy. However, the high price of this energy sources cause this renewable energy are not economical reasonable for the civil community. Therefore, the effort to reduce the production cost of the biofuel, even biogas, is needed so that the price can be reasonable to substitute the fossil fuel. This research will see the opportunity to reduce the operation cost in the biogas production process by study the effect of the existence of O 2 (air) in the gas space in the biogas digester in initial process. Biogas production process is very sensitive with free molecular oxygen because methanogenic bacteria are strict anaerobs [1,2,3]. Strict anaerobs bacteria will die in the presence of free molecular oxygen. In the initial biogas production process, the gas space in the digester will be filled with the air containing oxygen from the environment. * Corresponding author: Tel: address: awresta@yahoo.com The oxygen may inhibit the methanogenic bacteria and can be removed by flushing nitrogen gas so that the air goes out. However, this effort to remove the oxygen will increase the operation cost. If the biogas production process can run well without oxygen removal, the operation cost may be reduced Biogas production process Biogas production process utilizes anaerobic bacteria metabolism activities in degrading organic compounds. This process contain of four steps, i.e. hydrolysis, acidogenesis, acetogenesis, and methanogenesis [4]. In the hydrolysis step, undissolved compounds, like cellulose, proteins, and fats are cracked into monomers, that is watersoluble fragments [4]. Insoluble organic chemicals are not readily utilizable by microorganisms, and their hydrolysis is essential for effective microbial digestion [3]. The soluble organic compounds producing in hydrolysis step will be used by fermenting bacteria for growth, producing organic acids and hydrogen as the dominant intermediate products [1]. The organic acids producing in acidogenic phase are volatile organic acids, such as acetic, butyric, formic, and propionic acids, and short-chain fatty 213 Indonesian Institute of Sciences, Research Centre for Electrical Power and Mechatronics Available online at

2 12 Arini Wresta et al. / Proceeding of ICSEEA (212) acids [3]. According to Deublin and Steinhauser (28), in the acidogenic step it is formed short chain organic acids, alcohols, hydrogen, and carbon dioxide [4]. The products from the acidogenic phase serve as substrate for other bacteria produce acetic acid, hydrogen, and CO 2 [3]. Organic acids produced in acidogenic phase are then partially oxidized by other fermenting bacteria, which produce additional hydrogen and acetic acid [1]. In methanogenic phase, acetic acid, CO 2, and hydrogen are degraded by methanogenic bacteria to produce methane. Hydrogen and acetic acid are the main substrates used by methanogens, which convert them into methane [1]. Methane is the final product desired in biogas production because has high energy value Anaerobic microorganisms caracteristic in the presence of free molecular oxygen Biogas production is a microbiology process involving mixed culture bacteria. According to Madigan, et al. (212), with the right mixture of organisms, virtually any organic compound, even hydrocarbons, can be converted to CH 4 plus CO 2. These organic compounds (other than acetate and pyruvate) can be converted to CH 4, but only in reactions in which methanogens and other anaerobic bacteria cooperate [5]. Because there are mixed culture bacteria in biogas production process, it is necessary to understand the characteristic of the microorganisms in the presence of free molecular oxygen, so it can be analized the influence of O 2 removal on biogas production. According to Gerardi (23), based on their response to free molecular oxygen, bacteria may be divided into three groups, i.e. strict aerobes, facultative anaerobes, and anaerobes. Strict aerobes are active and degrade substrate only in the presence of free molecular oxygen, and die in an anaerobic digester in which free molecular oxygen is absent. Facultative anaerobes are active in the presence or absence of free molecular oxygen. Anaerobes are inactive in the presence of free molecular oxygen and may be divided into two subgroups: oxygen-tolerant species and oxygen-intolerant species or strict anaerobes that will die in the presence of Fig. 1. Experimental set up free molecular oxygen [2]. Microorganisms in hydrolysis and acidogenic phase are facultative and obligatorily anaerobic bacteria and the majority of these microorganisms are facultative [4]. Acidproducing organisms are a mixture of facultative anaerobes, which are called acid formers [3]. Methanogenic bacteria are strict anaerobes [1,2,3]. The oxygen inhibition of methanogenic bacteria begins at,1 mg/l O 2 [4]. Althought methanogenic bacteria are strict anaerobes, the biogas process may happen in the presence of little free molecular oxygen. According to Deublin and Steinhauser (28), under operational conditions the methanogenics always grow in the presence of facultatively anaerobic acidifying bacteria, which consume available oxygen immediately, and the anaerobic condition can be maintained in closed reactors. The small amount of air, which is injected frequently for the biotechnological desulfurization of the biogas, usually has no inhibiting effect on the methane formation [4]. 2. Experiment Soybean curd whey, the waste water from protein coagulation in soybean milk, was got from home-scale soybean curd industry in Jetis, Tirtomartani, Kalasan, Sleman, Indonesia. Analysis results showed the ph of 4, VS concentration of 7,7183 g/l and VFA concentration of 373,5871 mg acetic acid/l. Cow manure was got from Kebun Pendidikan, Penelitian, dan Pengembangan Pertanian Universitas Gadjah Mada (KP4 UGM), in Berbah, Sleman, Yogyakarta, Indonesia. Analysis results showed the ph of 7,5, VS concentration of 16,5295 g/l, and VFA concentration of 3798,135 mg acetic acid/l. Active digester effluent from cow manure biogas installation in KP4 UGM, Sleman, Yogyakarta, Indonesia, had specification: ph of 8, VS concentration of 12,2663 g/l, and VFA concentration of 249,58 mg acetic acid/l. Biogas digester was 1 ml Erlenmeyer flask, connected with gas bubbler to detect the gas formed from digester. Gas bubbler is connected with water manometer to measure the volume of gas formed. The experimental set up is presented in Figure 1. Batch experiments was done in room temperature (approximately of 29-3 o C), in the two digesters, fed with 6 gram of raw material, consisting of soybean curd whey, cow manure, and active digester effluent with weight rasio of 4 : 1 : 1. In the first digester, oxygen was removed by flushing nitrogen gas in the gas space in the digester, and in the second digester there was no O 2 removal from the digester. Sludge and gas sampel was taken weekly for VS and VFA analysis according to standard method [5] and CH 4 analysis using GC (Shimadzu GC 14B, Japan, with FID detector, SUS Packed Column Porapak Q, 5m x 3mm I.D., 5 o C Column Oven Temperature, and 17 kpa Inlet Pressure). The volume of gas formed was measured every day by recording the water level difference at the manometer legs. International Conference on Sustainable Energy Engineering and Application (ICSEEA) Inna Garuda Hotel, Yogyakarta, Indonesia, 6 7 November 212

3 Arini Wresta et al. / Proceeding of ICSEEA (212) Table 1 Data of Volatile Solids and Volatile Fatty Acids Concentration (Batch: P, alt : ks : ef = 4 : 1 : 1, T = 29-3 o C, total material in the digester = 6 gram) t, Without O 2 removal With O 2 removal day [VS], g/l [VFA], mg/l [VS], g/l [VFA], mg/l , , , , ,662 22, , , , ,94 3. Results and discussions 36,521 29,642 29,424 29, , , , ,72 The data got from the experiment describing the correlation between VS concentration and time, VFA concentration and time, the amount of methane formed and time, and methane content and time are presented in Figure 2. Figure 2 showed that O 2 removal in initial process had no significant effect on the digester performance, showed with the similar trends of all process parameters (VS concentration, VFA concentration, cumulative volume and methane content) and time. The trends of VFA concentration, methane cumulative volume, and methane content were almost coincides, indicated the little differences of the said process parameters in the two digesters during the process. This similar trends may happen because the digester without O 2 removal was a closed digester that no probability to air entering the digester. These trends were also caused by the little soluble oxygen in the slurry in the digester. Due to the existence of acidogenic bacteria in the cow manure [7] and active digester effluent, this little oxygen will be immediately consumed by facultatively acidogenic bacteria [4], so that the condition in the digester will be anaerobic quickly. Because the anaerobic condition was reached quickly, the presence of oxygen in initial process didn t inhibit methanogenic bacteria significantly. Althought there were no significant effect of O 2 removal on the biogas production and methane content, this paper will study the inhibition caused by the presence of O 2 in the initial process by analyse the little differences of process parameters in the two digesters. The VFA concentration in the digester with O 2 removal during the process was always higher than in the digester without O 2 removal (Figure 2b). These phenomena happened because the condition in the digester was strict anaerob since the initial process. Because free molecular oxygen wasn t exist, the degradation process occurred was an anaerobic organic compounds degradation by acidogenic bacteria produced a variety of acids [2]. In the digester without O 2 removal, little soluble oxygen in the medium is used to enzymatic activity by facultatively anaerobic bacteria to produce the major product of CO 2 and H 2 O [8]. As the result, the VFA concentration in this digester in the first week was lower than in the digester with O 2 removal. Because aerobic biodegradation is much faster than [VS], g/l [VFA], mg acetic acid/l t, day (a) 5 1 t, 15 day (b) methane cumulative volume, ml t, day (c) methane content, % volume t, day (d) Fig. 2. The correlation of process parameters and time Indonesian Institute of Sciences, Research Centre for Electrical Power and Mechatronics

4 14 Arini Wresta et al. / Proceeding of ICSEEA (212) Table 2 Data of biogas cumulative volume measured and methane content (Batch: P, alt : ks : ef = 4 : 1 : 1, T = 29-3oC, total material in the digester = 6 gram) t, days without O 2 removal cumulative methane volume, ml content, % volume 21,366 51,996 62, ,329 72, , , , , , , , , , , , , , , , , , , , , ,29 8,97 4, ,246 64,7887 with O 2 removal cumulative methane volume, ml content, % volume anaerobic biodegradation [8], organic compounds consumption in the digester without O 2 removal was faster in some initial day, caused the lower VS concentration in this digester after the degradation (compared with in the digester with O 2 removal). The organics concentration in the digester with O 2 removal after day 5 was higher, provided the larger substrate to acidogenic bacteria. As the result, the VFA produced was large and the VFA concentration during the process was higher than in the digester without O 2 removal (Figure 2b). In some initial day (day 1 until day 7), the gas volume produced from the digester without O 2 removal was larger than in the digester with O 2 removal (Table 2). These phenomena happened because aerobic microbial oxydation is much faster than anaerobic degradation [8]. The presence of free molecular oxygen in the initial process was utilized 13, ,2662 5,211 51, , , , , , ,18 151, , , , , ,568 35, , , ,795 44, ,574 46, , , ,951 8, , , ,2524 to the faster grow by facultatively anaerobic bacteria producing CO 2 and H 2 O. The degradation was relatively fast, so that the CO 2 formed was large, and the gas volume obtained was large too. After day 1, the biogas cumulative volume and the methane content in the digester without O 2 removal were lower than in the digester with O 2 removal (Table 2). This trends happened because all of the little oxygen in the initial process had been consumed by facultative acidogenic bacteria, so that the total condition in this digester was anaerobic. This anaerobic condition caused the all next process are an aerobically, that go slower than the aerobic activity in the initial process. Because the organic compounds consumed in the initial process are larger, the substrate could be provided in the anaerobic degradation was lower. As the result, the biogas cumulative volume obtained was lower too. In the digester with O 2 removal, the higher organic compounds concentration provide the larger substrate for acidogenic bacteria so that the acidogenic process go faster, and the VFA and CO 2 produced were larger (compared with digester without O 2 removal). The great VFA amount in the digester provided more substrate to methanogenic bacteria that caused the larger methane produced (Figure 2c). There is no oxygen inhibition since the initial process in the digester, so that the methanogenic phase runs well and the methane content in the biogas obtained was high (Figure 2d). The gas produced from acidogenic and methanogenic phase was larger, and the cumulative volume of biogas obtained was increase. Totally, the performance of the two digester were not significant difference because the soluble oxygen in the slurry in the digester without O 2 removal was relatively little. 4. Conclussions Oxygen removal in the initial process had no significant effect on biogas production and methane content because there was just little soluble oxygen in the slurry in the digester without O 2 removal, immediately consumed by facultatively acidogenic bacteria. The biogas production will run well without oxygen removal in initial process. References [1] Rittmann, B. E., McCarty, P. L., Environmental Biotechnology: Principles and Application, New York, USA: McGraw-Hill Higher Education, McGraw-Hill Companies, Inc., 21, pp [2] Gerardi, M. H., The Microbiology of Anaerobic Digesters. 1st ed., New Jersey: John Wiley and Sons, Inc., 23, p. 89. [3] Shuler, M. L., Kargi, F., Bioprocess Engineering, Basic Concepts. 2st ed., New Jersey: Prentice Hall PTR, Prentice-Hall, Inc., 22, pp. 499, 5. [4] Deublin, D., Steinhauser, A., Biogas from Waste and Renewable Resources, Weinheim, Germany: WILEY-VCH Verlag GmbH and Co. KgaA, 28, pp [5] Madigan, M. T., Martinko, J. M., Stahl, D. A., Clark, D. P., Biology of Mikroorganisms. 9 th ed., USA: Pearson Education, Inc., 212. International Conference on Sustainable Energy Engineering and Application (ICSEEA) Inna Garuda Hotel, Yogyakarta, Indonesia, 6 7 November 212

5 Arini Wresta et al. / Proceeding of ICSEEA (212) [6] Eaton, A.D., Clesceri, L.S., Rice, E.W., and Greenberg, A.E. (ed.), Standard methods for the examination of water and wastewater, 21st ed., Washington DC: APHA, AWWA, and WEF, 25. [7] Damayanti, S. I., Pemanfaatan Stillage Menjadi Biogas Melalui Proses Co-digestion Stillage dankotoransapi, Program Studi Teknik Kimia, Kelompok Bidang Ilmu-Ilmu Teknik, Program Pascasarjana Universitas Gadjah Mada, Yogyakarta, Tesis, 21. [8] Maier, R. M., Pepper, I. L., Gerba, C. P., Environmental Microbiology, San Diego, California: Academic Press, A Harcourt Science and Technology Company, 2. Indonesian Institute of Sciences, Research Centre for Electrical Power and Mechatronics

6 16 Proceeding of ICSEEA (212) International Conference on Sustainable Energy Engineering and Application (ICSEEA) Inna Garuda Hotel, Yogyakarta, Indonesia, 6 7 November 212