IMPROVING THE BIOGAS APPLICATION IN THE MEKONG DELTA OF VIETNAM BY USING AGRICULTURAL WASTE AS AN ADDITIONAL INPUT MATERIAL

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1 IMPROVING THE BIOGAS APPLICATION IN THE MEKONG DELTA OF VIETNAM BY USING AGRICULTURAL WASTE AS AN ADDITIONAL INPUT MATERIAL Nguyen Vo Chau Ngan 1, 2, Klaus Fricke 2 1 Department of Environmental Engineering - College of Environment & Natural Resources Cantho University, Vietnam 2 Department of Waste Management - Leichtweiß-Institute for Hydraulic Engineering and Water Resources - Technical University of Braunschweig, Germany ABSTRACT This investigation studied the co-digestion between pig manure (PM) and spent mushroom compost (SMC) to produce biogas in lab-scale. The batch and semi-continuous anaerobic digesters were applied to co-digest PM+SMC with different mixing ratios based on organic dry matter values. The result of the 35 continuous day batch treatment showed that the more percentage of SMC contained in the mixing ratio of PM+SMC, the less biogas was produced but it was not significantly different between the treatments of 100%PM+0%SMC, of 75%PM+25%SMC, of 50%PM+50%SMC and of 25%PM + 75%SMC. The semi-continuous testing in 90 continuous days of 75%PM+25%SMC showed the relatively stable operation of the treatment. The outcomes indicated that farmers in the Mekong Delta can apply SMC as an additional feeding material to biogas digesters in case pig manure is short. KEYWORDS Batch anaerobic process, semi-continuous anaerobic process, spent mushroom compost, the Mekong Delta 1. INTRODUCTION 1.1 Background In the Mekong Delta of Vietnam, biogas plants are strongly confirmed as an optimum treatment way for livestock waste and biogas supply for household cooking, lighting, etc. The output from biogas plants can be used to feed fish in a VACB farming system. According to Tran et al. (2009) [1], more than 90% of the biogas users apply PM as the sole input material to their biogas plants. Any negative progress of the pig market will directly affect the operation of biogas plants and make the investment in installation of biogas digesters wasted. This is considered as one of primary limits to the promotion of widespread application of biogas digesters, weakening the potentials of favorable conditions of the Mekong Delta for biogas application and having an impact on the environmental pollution prevention in the area. Looking into the real conditions of the Mekong Delta, among the possible uses, SMC could be potentially used as an additional input material for biogas plants. As the residue from rice straw mushroom cultivation, SMC has already been partly degradable, helping shorten the duration of anaerobic process. However, there has been no research on the application of SMC in biogas production in the Mekong Delta. 1.2 Research objectives This study is expected to contribute locality-based evidence supporting the encouragement of the local owners of biogas digesters to take into account the usage of SMC as potentially additional input materials for their biogas digesters, especially when animal manure is in short supply. Two types of experiments were processed to give answers to the questions 22

2 of: - The influence of differential mixing ratios of SMC with PM on gas production by batch digester. - The stability of the semi-continuous anaerobic co-digestion of SMC to PM. 2. METHODOLOGY 2.1 Material preparation The input materials loaded to the digesters were prepared as follows: - PM collected from the piggeries at Hoa An Center was air-dried out at ambient temperature (25 ± 4 C) for one week before coming into use, then the dried PM was manually mashed and mixed up until it became a homogenous form. - SMC was collected from the farmers households producing straw mushroom around Hoa An ward. The SMC was manually cut into approximate cm long pieces. The chopped SMC was air-dried at ambient temperature up to unchanged weight, and then manually mixed up completely. - Inoculums: to shorten the time of gas production, the seeding material which was the effluent taken from an existing active biogas plant was used. This is a 100 m 3 activated biogas plant at Hoa An Center. 2.2 Experimental set-up For batch treatments, 17.5 L inoculum was mixed together with 665 g mixture of PM and SMC (based on organic dry matter (ODM) values). Five co-digestion batches of PM and SMC were set up to evaluate the influence of various mixing ratios on gas production. Each of the treatments was in triplicate. All of the batch treatments were allowed to be fermented for 28 days and were mixed up by shaking the digesters manually once a day so as to increase the gas production. - SMC0: 100%PM + 0%SMC - SMC1: 75%PM + 25%SMC - SMC2: 50%PM + 50%SMC - SMC3: 25%PM + 75%SMC - SMC4: 0%PM + 100%SMC A semi-continuous treatment 75%PM+ 25%SMC was set up in triplicate. The digesters were fed at the same time every day during the experiment time. In this treatment, only the action of feeding the digesters helped stir the substrate inside the digesters. The semi-continuous treatments were conducted for a 90 consecutive day period. - SMC5: 17.5 L inoculums and g ODM mixture of PM and SMC were added as starting materials. From the 2 nd day, the daily feeding rate included 875 ml inoculums and g ODM of PM plus 5.94 g ODM of SMC. There were 18 sets of airtight digestion apparatus installed of which 15 digesters for batch fermentation and 3 digesters for semi-continuous fermentation. To minimize the development of algae population that creates oxygen inside the digesters, all digesters were covered by black nylon bags throughout the testing period. Figure 1. Treatments on batch (above) and semi-continuous experiments (below) 23

3 (1) the air-valve; (2) the gas pipe 2.3 Analytical methods The substrates before and after the experiments were taken and analyzed for ph, carbon (C), total Kjeldahl nitrogen (N), dry matter (DM), ODM, alkalinity according to methods of APHA (1995) [2]. The gas production was recorded daily by the Ritter gas-meter, and the biogas components were monitored weekly by the GA94 gas analyzer. All of the experiments and analyses were conducted at the Environmental Engineering Laboratory - College of Environment and Natural Resources, Cantho University, Vietnam. 3. RESULTS AND DISCUSSIONS 3.1 Results on feeding materials Table 1. The physical and chemical analysis of the input materials Material DM (%) ODM (%) C/N PM SMC The relationship between the amount of carbon and nitrogen available in organic materials is represented by the C/N ratio. Microorganism generally utilizes carbon and nitrogen in the ratio of 25/1 32/1 (Bouallagui et al. 2003) [4]. Because the C/N ratio in PM is low while it is high in SMC, the co-digestion PM+SMC can help adjust the C/N ratio closer to the optimum value. In this study, the C/N ratio of the input materials ranged from 9 to 25. The C/N of input materials can be significantly affected by the factors such as biomass varieties, cultivation system, soil condition, applied fertilizers, etc. 3.2 Results of produced biogas from batch treatments The daily gas production from the PM+SMC treatments was taken after two day operation and recorded for 28 continuous fermentation days. However, at the day 28 th, the material contained in the substrate was still in the original form but not much degradable. Then the treatments were kept for fermentation in 35 days. (*): ODM calculated based on DM value The mixing ratio of input materials in this study was calculated according to their ODM. Eder & Schulz (2007) [3] suggested that the optimal input should be 1 4 kg ODM day -1.m -3 for an anaerobic digester. Based on this suggestion, the input quantity was chosen to be 1.25 g ODM day -1.L -1. Table 2. Mixing ratios on dry weight basis Treatments PM SMC Total Batch treatments 100%PM+0%SMC %PM+25%SMC %PM+50%SMC %PM+75%SMC %PM+100%SMC Semi-continuous 75%PM+25%SMC Figure 2. Substrate after 28 fermented days The recorded results showed that the biogas volume of the treatments reached maximum values around the end of the 1 st week. In the 2 nd week, the peak value of biogas volume fell down and then slightly declined until the end of the testing period. At the end of the 5 th week, the daily produced biogas volume only accounted for 1.02%, 1.24%, 1.45%, 1.66% and 2.28% of the total biogas in the %PM+%SMC 24

4 treatments of 100+0, of 75+25, of 50+50, of 25+75, and of 0+100, respectively. The higher biogas production recorded in the later period of treatments with higher SMC percent was possibly caused by high lignin content remaining in SMC. Study on methane production from rice straw by Lei et al. (2010) [5] showed that the first peak of gas happened after operation days, and the second peak presented after days, but the second peak was always greater than the first peak. Because the fermentation time in this study lasted only 35 days, only PM and part of SMC were decomposed. As a result, just the first peak of produced gas occurred in this study. In respect with the various mixtures of SMC and PM, the treatments with a larger percentage of SMC generated lower biogas volume. Nevertheless, the generated biogas volumes between the testing treatments were not significantly different except the treatment of 0%PM+100%SMC. The letters a and b in Figure 3 displayed the difference in the total gas produced in the PM+SMC treatments. The results showed that SMC could apply as co-digestion with PM up to the mixing ratio of 25%PM+75%SMC without affecting the biogas production. The variation on biogas production between the treatments was caused by the difference in ph and alkalinity values resulted by the various mixing ratios of PM and SMC. In this study, the SMC substrate was the most acidic while the PM substrate was the most alkaline. It was observed that the SMC was high lignin content that exhibited a multi-stage gas production pattern. In fact, after the peak gas production in the PM+SMC treatments, there were other small peak values (on the day of 12 th, 22 nd, 27 th, 29 th and 33 rd ). It is believed there would be re-fermentation process to occur in anaerobic digestion in order to complete the gasification of SMC. Figure 4. The biogas yield of treatments Figure 3. Accumulative biogas volume Calculation on biogas yield for each treatment was based on the weekly biogas production and the fermented ODM value. The results indicated that higher biogas yield was present in the treatments with lower percentage of SMC in the mixture ratio (Figure 4). Figure 5. Control parameters of treatments Before the fermentation, the alkalinity value of the treatments decreased due to higher percentage of SMC in the mixture. 25

5 The alkalinity is the result of the release of amino groups (-NH 2 ) and production of ammonia (NH 3 ) as the proteinaceous wastes are degraded (Gerardi, 2003) [6]. By that fact, the treatments containing high percentage of SMC could not maintain the optimum alkalinity. After the experiment, the alkalinity of the treatments tended to increase more than that of the input substrate. The alkalinity variation was the greatest in the treatment of 0%PM+100% SMC. The output alkalinity was volatile but this tendency was incomprehensible. 3.3 Results of produced biogas from semi-continuous treatments For the semi-continuous treatments, because little gas was generated in the first week of the experiment, gas recording in the first week was skipped but only started to record the gas production from the 8 th day and continue up to day 90 th. The daily gas production of PM+SMC semicontinuous treatment is showed in Figure 6. The daily gas production was volatile and reached L. values, implying the digested materials still contained some degradable parts. In this connection, Kaparaju & Rintala (2005) [7] agreed that in order to optimize methane production in a typical co-digestion process the digestions should perform with either long hydraulic retention time or long substrate retention time. The decrease in gas production at the later phase could be explained by the lower temperature and low ph value at that time. The temperature at that time was presumably around 28 to 30 C because it was in December. In addition, the recorded ph values were only around 6.5 which was lower than the optimum value (from 6.6 to 7.6). Figure 6. Daily biogas production In the first week of fermentation, the biogas production led a sharp increase and reached the stable range from the 2 nd week onwards. The daily biogas production ranged from 4 5 L.day -1 from the 2 nd week up to the 57 th day. From the 57 th day onwards, the gas production decreased and reached an average value of 3.2 L.day -1 up to the ending experimental period. However, at the last period, the daily produced biogas displayed some peak Figure 7. Biogas compositions The methane content was expected to vary depending on feedstock. The average methane content in the biogas was 60.5% (in range of %). In fact, by daily feeding, the semi-continuous treatments supplied more nutrients to microorganism, thereby helping maintain their methanogenic activities. The ph and alkalinity values of the semi-continuous treatments were recorded for the first 7 weeks. In this study, the ph and methane content tended to decrease in the first 7 weeks but the alkalinity remained in stable range. It could be inadequate anaerobic microorganism in the substrate causing low ph and methane content. 26

6 Figure 8. Control parameters 4. CONCLUSION By mixing PM and SMC in the batch treatments, the higher percentage of SMC in the mixture ratio led to the lower biogas yield in the treatments. Compared to the treatment of 100%PM+0%SMC, the biogas yield from treatments of 75%PM+25% SMC, of 50%PM+50%SMC, of 25%PM+ 75%SMC and of 0%PM+100%SMC only got 87%, 80%, 76%, and 58%, respectively. The results showed that in case of SMC making up 75% of the mixed substrate, the gained biogas volume was not significantly different compared to the treatment fed solely with 100%PM. Besides that, the PM+SMC semicontinuous treatment remained in good operation up to the 90 th fermentation day without apply any special agitating method. The produced biogas had good quality due to the average methane content in the biogas over 60% which is significantly possible for cooking and lighting. As such, SMC is possibly an acceptable additional material for the anaerobic fermentation in case of the shortage of the pig manure. 5. ACKNOWLEDGEMENTS We would like to thank students of the KTMT33 class - Cantho University, for their help with the sampling and the lab works. We are also indebted to Mr. L. H. Viet and Mr. V. H. Nam for fruitful discussions regarding the interpretation of outcomes of the biogas experiments. 6. REFERENCES [1] Tran, V. D., Ha, V. H., & Huynh, T. L. H. Biogas user survey (pp. 94). Ha Noi, Viet Nam: SNV Vietnam [2] APHA - American Public Health Association. Standard methods for the examination of water and wastewater. Washington DC, USA [3] Eder, B., & Schulz, H. Biogas-Praxis: Grundlagen, Planung, Anlagenbau, Beispiele, Wirtschaftlichkeit: Ökobuch Magnum [4] Bouallagui, H., Ben Cheikh, R., Marouani, L., & Hamdi, M. C. Mesophilic biogas production from fruit and vegetable waste in a tubular digester. Bioresource Technology, 86(1), 5, doi: S (02) [5] Lei, Z., Chen, J., Zhang, Z., & Sugiura, N. Methane production from rice straw with acclimated anaerobic sludge: Effect of phosphate supplementation. Bioresource Technology, 101(12), 6, doi: / j.biortech [6] Gerardi, M. H. The microbiology of anaerobic digesters. United States of America: A John Wiley & Sons, Inc [7] Kaparaju, P., & Rintala, J. Anaerobic co-digestion of potato tuber and its industrial by-products with pig manure. Resources, Conservation and Recycling, 43(2), 14, doi: /j.resconrec Contact: Nguyen Vo Chau Ngan +84 (0) College of Environment & Natural Resources - Cantho University, Vietnam nvcngan@ctu.edu.vn 27

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