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International Journal of Mechanical Engineering and Technology (IJMET) Volume 10, Issue 07, July 2019, pp. 189-196, Article ID: IJMET_10_07_019 Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=7 ISSN Print: 0976-6340 and ISSN Online: 0976-6359 © IAEME Publication IMPROVING BIOGAS VOLUME PRODUCTION THROUGH CARBON TO NITROGEN RATIO MODIFICATION OF COW DUNG Hendry Sakke Tira*, Yesung Allo Padang, Salman, Ilham Ramadhan Zulkarnaen Department of Mechanical Engineering, Engineering Faculty Mataram University, Jl. Majapahit no. 62, Mataram, NTB, 83125 Indonesia *hendrytira@unram.ac.id ABSTRACT This work was performed to study the effect of C/N ratio on biogas volume production. The raw material used was cow dung while glucose and ammonium sulfate was used to modify the C/N ratio. An isolated six-liter container was used to run the anaerobic process where the temperature was kept in a range of mesophilic condition. The container pressure and temperature were investigated daily. In order to boost the biogas volume production, 10% by volume of effective microorganism-4 (EM-4) was added to the substrate (a mixture of cow dung and water with ratio of 1:1). To investigate the effect of C/N ratio of substrate on biogas volume, four different ratios were employed of 15, 22, 27, and 35 respectively. The C/N ratio of the raw material is 22.71. The work showed that the biogas volume increased when C/N ratio of the substrate was high. In the case of C/N ratio of 35, the highest biogas volume was achieved in day 16-18th then the volume decreased gradually. However, the highest volume was achieved in different day for each ratio. For instance in the second best in biogas production, C/N ratio of 27, the highest volume was achieved in day 13-15th and even it was achieved in early day for lower C/N ratios. The results give indication that increasing the C/N ratio of cow dung can improve the quantity of biogas. Key words: biogas, C/N ratio, biogas volume, anaerobic digestion Cite this Article: Hendry Sakke Tira, Yesung Allo Padang, Salman, Ilham Ramadhan Zulkarnaen, Improving Biogas Volume Production Through Carbon to Nitrogen Ratio Modification of Cow Dung. International Journal of Mechanical Engineering and Technology 10(7), 2019, pp. 189-196. http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=7 1. INTRODUCTION Uninterrupted energy supply is a major factor to keep the development pace of a country. Some obstacles faced by Indonesia government in providing sufficient energy has prompted the government to release a policy which make producer can produce renewable energy with low tariff. One of the renewable energy is biogas. Biogas can be produced by anaerobic http://www.iaeme.com/IJMET/index.asp 189 editor@iaeme.com Hendry Sakke Tira, Yesung Allo Padang, Salman, Ilham Ramadhan Zulkarnaen digestion with some different material of plants and animals [1, 2]. The utilization of manure and waste as raw material for biogas production offer positive impacts because it can reduce the environment pollution and minimizing bad odor while at the same time the byproduct can be used as organic fertilizer. Under a proper construction of the digester, higher methane emission can be eliminated [3]. Therefore, an abundant hazardous green house gasses into atmosphere can be diminished thus climate change or global warming can be well controlled. The anaerobic digestion can be affected by some factors. Those factors are digester temperature, type of raw material, content of total solid and volatile solid of input substrate, acidity level (pH) of substrate during the fermentation process, and carbon to nitrogen (C/N) ratio [4, 5]. The temperature in digester has a close correlation with the methane-producing bacteria. The bacteria which can work optimal in higher reactor temperature (43 – 55oC) is thermophilic bacteria while at lower temperature (30 – 42oC) is mesophilic bacteria. In each type of digestion process, higher temperature will lead to more efficient of biogas production. It is resulted from the bacteria is more active in higher temperature. Raw material with higher content of cellulose is better in producing biogas compared to that of lignin [6, 7]. The optimum condition for gas formation in reactor is when pH of substrate in neutral (pH of 7-8). However, during the fermentation the acidity of substrate will increase due to a formation of acid bacteria. A low pH gives an adverse effect to the gas production and even in some cases the production is stopped [2, 8]. In the other hand, the bacteria or microorganism in reactor need nutrients for microbial growth and reproduction. Some important nutrients for proper composting are phosphorus, potassium, carbon, and nitrogen. Carbon is required by microbes for energy supply and growth support. On the other hand, nitrogen is essential to support reproduction and protein supply [9, 10]. During the fermentation process, a balance composition between carbon and nitrogen is necessary to obtain optimal composting process. Some adverse effect will occur when the optimum C/N ratio is not achieved. Excess nitrogen in substrate causes an abundant release of nitrogen into the atmosphere as NH3 or N2O resulting bad odor [11]. Meanwhile, higher carbon content will slow down the rate of composting process. Therefore, a proper C/N ratio in substrate is essential to keep biogas production constant during retention time. According to the above reason, the research was investigated on how the C/N ratio influenced the biogas production form fermentation process. Different C/N ratio will be implemented to the same raw material, cow dung, in order to get better understand of its effect. 2. EXPERIMENTAL SETUP The study was conducted using a digester with a capacity of 6 liters. The digester is wrapped in sand with the intention to keep the temperature of the substrate constant at 27-28 oC. Therefore, the anaerobic digestion process is carried out under mesophilic condition. A number of holes are made to facilitate the release of gas from the digester which will be used to measure gas temperature and pressure in the digester as depicted in Figure 1. The composition of cow dung and water used is 1: 1 and the total volume is 3 liters. In addition, EM-4 is added by 10% to trigger the growth of bacteria so that the fermentation process can take place better. To achieve the research objectives, C/N ratio of substrate were varied, namely 15, 22, 27, and 35. Before varying the C/N ratio, the value of the C/N ratio of the raw material (cow dung without treatment (WT)) was determined first. By knowing the initial C/N ratio then the next step is to determine the amount of glucose (C6H12O6) and ammonium sulfate ((NH4)2SO4) needed to achieve the desired C/N ratio. The determination of nitrogen values was carried out using the kjeldahl method. http://www.iaeme.com/IJMET/index.asp 190 editor@iaeme.com Improving Biogas Volume Production Through Carbon to Nitrogen Ratio Modification of Cow Dung Temperature display Pressure gauge Biogas container Raw material inlet substrate insulation Figure 1 Schematic diagram of the experimental setup Observations were carried out for 30 days by measuring the pressure in the digester and the volume of biogas produced. After observing the manometer, then the gas from the digester will be inserted into the container through the prepared channel and then the gas volume obtained will be measured using the measuring instrument by water displacement method as shown in Figure 2. Figure 2 Biogas volume measuring instrument Carbon added to the substrate comes from glucose which also functions as a nutrient for the microbes that make up the methane gas. The following equation is used to find the carbon atom mass in glucose: Mass of carbon = Ar : (6 x Ar C ) Mr : (C 6 H 12 O6 ) (1)  (6  12,011) (12,011 6)  (1  12,1)  (16  6)  72,066  0,3637 180,16 http://www.iaeme.com/IJMET/index.asp 191 editor@iaeme.com Hendry Sakke Tira, Yesung Allo Padang, Salman, Ilham Ramadhan Zulkarnaen Whereas to determine nitrogen is to use the equation below: Mass of nitrogen = Ar : (2  Ar N ) Mr : (( NH 4 ) 2 SO4 ) (2)  (2  14,007) ((28,014  8,064)  32,065  64)  28,016  0,212 132,14 3. RESULTS AND DISCUSSION The experimental results show that the more glucose added (C6H12O6), the greater the carbon value (C), so that the C/N ratio increases. Conversely, the more ammonium sulfate (NH2)3SO4 added, the higher the value of nitrogen (N), causing the C/N ratio to decrease. Before the fermentation process is carried out, the acidity level or pH of the substrate is measured. At the end of the fermentation process, after 30 days, the pH of the substrate is again measured. The results obtained showed that there was a decrease in pH for each variation of C/N ratio studied. The degree of acidity (pH) has an effect on biological activity, especially in anaerobic fermentation. Changes in pH are also indicators of bacterial life found on the substrate in the digester. The growth of biomethane-producing bacteria will be optimal if the acidity level of the substrate is in an alkaline state, namely at pH of 6-7. From the result of biogas volume measurement, it was found that the C/N ratio of 35 produced the largest volume while the C/N ratio of 15 produced the least amount of biogas as showed in Figure 3. Biogas production at C/N ratio of 35 condition was achieved on days 1618 within the fermentation process. This is due to the presence of glucose on the substrate has a good impact on the bacteria growth. Glucose is a food source for bacteria producing biomethane so that the substrate decomposes rapidly and forms a relatively higher gas quantity. At low C/N ratios such as 15 and 22, the resulted biogas is lower than of the other ratios. This can be occurred by several factors. One of the factors was the use of ammonium sulfate. Ammonium sulfate is used to increase the nitrogen content of the substrate or to reduce the C/N ratio. However, the sulfur content of 24% in ammonium sulfate is believed to be an inhibiting factor for the biogas production rate. Most microorganisms can use sulfate, as a source of sulfur, to form hydrogen sulfide (H2S), a toxic compound. However, some microorganisms can assimilate H2S directly from the growth medium, but unfortunately the compound can be toxic to many organisms [12, 13]. Therefore, ammonium sulfate is an unfavorable enhancer for the development of biomethane forming bacteria in the substrate. Whereas for C/N ratio enhancing material such as glucose, it is obtained that biogas production increased compared to that of without treatment substrate. This is because glucose is an energy source for the development of microbes that form biomethane (CH4) [14]. http://www.iaeme.com/IJMET/index.asp 192 editor@iaeme.com Improving Biogas Volume Production Through Carbon to Nitrogen Ratio Modification of Cow Dung Figure 3 Biogas volume on different C/N ratios Figure 4 shows that each variation of C/N ratio has a significant impact in determining the level of total solid on the substrate. The results demonstrate that for high C/N ratio was followed by a high total solid as well. On the other hand, at a low C/N ratio the total solid was relatively low. This is due to the addition of glucose leads to the increase of solid to liquid ratio in the substrate. On the other hand, ammonium sulfate addition does not significantly increase the total solid. This is because the excess of ammonium sulfate in the substrate has been broken down by microorganisms so that it is quickly converted to ammonia (NH 3) and some are used for the nutrition of bacteria to produce biogas [15, 16]. The results obtained for volatile solid show the opposite results to the total solid. Due to the addition of ammonium sulfate, volatile solid increases. Therefore, the highest volatile solid is obtained at low C/N ratio compared to that of higher one. Figure 5 shows the result of FTIR (Fourier transform infra red) test analysis. Each variation of the C/N ratio has a different value of transmittance percentage. The low C/N ratio has higher transmittance percentage compared to that of low ratio. High transmittance percentage indicates a higher concentration of the compounds contained in the substrate. Figure 4 Total and volatile solid http://www.iaeme.com/IJMET/index.asp 193 editor@iaeme.com Hendry Sakke Tira, Yesung Allo Padang, Salman, Ilham Ramadhan Zulkarnaen Figure 5 FTIR result Figure 6 shows the results of GC-MS (gas chromatography-mass spectrometry) for without treatment and treated substrate under different C/N ratio. The results indicate that the substrate without treatment, substrate with C/N ratio of 15, and 22, have dominant compound in the form of benzene, ethyl (CAS) EB with a retention time of 5.4 minutes. Meanwhile, the substrate with C/N ratio of 27 has glyceraldehyde (C3H6O3) for its dominant compound. Substrate with C/N ratio of 35 has the dominant compound in the form of Propanol, 2,3dihydroxy- (CAS) Glycero (HOCH2CH(OH)CHO) with a retention time of 7.7 minutes. The result demonstrates that even though each variation of the C/N ratio has the same compound but with the increase in the C/N ratio the existed dominant compound will change. http://www.iaeme.com/IJMET/index.asp 194 editor@iaeme.com Improving Biogas Volume Production Through Carbon to Nitrogen Ratio Modification of Cow Dung Figure 6 Gas Chromatograph results A). without treatment B). C/N ratio 15, C). C/N ratio 22, D). C/N ratio 27, E) C/N ratio 35 4. CONCLUSIONS The C/N ratio modification of cow dung is highly recommended for better volume biogas production. The results showed that increasing the C/N ratio (ratio of cow dung is 24) to 35 will boost the produced biogas volume for almost 3 times. In addition, the peak of biogas production shifted which has resulted in longer biogas production where bacteria remain alive to produce biomethane. On the contrary, reducing the C/N ratio of cow dung will actually aggravate the biogas production. Glucose and ammonium sulfate can be good compounds to be mixed with cow dung in order to alter the C/N ratio. However, the amount of ammonium sulfate added to the raw material should not be in a large or dominant portion because can inhibit the growth of useful bacteria that produce biomethane. ACKNOWLEDGEMENTS The authors would like to thank the Department of Research, Technology and Higher Education of the Republic of Indonesia for supporting this project under 2019 PDUPT research grant. REFERENCES [1] Caporgno, M. P., Trobajo, R., Cailo, N., Ibanez, A., Fabregat, C., Bengoa, C. Biogas production from sewage sludge and microalgae co-digestion under mesophilic and thermophilic conditions. Renew. Energy, 75, 2015, pp. 254–261 [2] Tira, H. S., Padang, Y. A., Supriadi. Effect of acidity level of substrate by lime water and acetic acid addition on biogas production. AIP Conf. 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