PhD’s thesis summary: Research on biogas production from rice straw and water hyacinth

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The quantity of rice straw and straw management survey only did in WinterSpring and Fall-Winter crops in An Giang, Dong Thap, Kien Giang province and Can Tho City. The research focused on the evaluation of simply biological pretreatment, material size and mixing rate of rice straw/hyacinth with pig manure in anaerobic batch and semi-continuous condition in vitro; trial producing biogas in polyethylene digester type (PE) in household scale in vivo

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MINISTRY OF EDUCATION AND TRAINING CAN THO UNIVERSITY PhD’s THESIS SUMMARY Major: Soil and Water Environment Major Code: 62440303 TRAN SY NAM RESEARCH ON BIOGAS PRODUCTION FROM RICE STRAW AND WATER HYACINTH Can Tho, 2016
THIS THESIS WAS COMPLETED AT CANTHO UNIVERSITY Supervisor: Assoc. Prof. Dr. Nguyen Huu Chiem Co-supervisor: Prof. Dr. Kjeld Ingvorsen The thesis was defended and approved by PhD evaluation committee Meeting at: At ……… date: ….. / ….. / ….. Reviewer 1: Reviewer 2: Reviewer 3:
PUBLICATION Trần Sỹ Nam, Nguyễn Thị Huỳnh Như, Nguyễn Hữu Chiếm, Nguyễn Võ Châu Ngân, Lê Hoàng Việt và Kjeld Ingvorsen, 2014. Ước tính lượng và các biện pháp xử lý rơm rạ ở một số tỉnh Đồng bằng sông Cửu Long. Tạp chí Khoa học Trường Đại học Cần Thơ phần A: Khoa học Tự nhiên, Công nghệ và Môi trường, số 32. 87-93. Trần Sỹ Nam, Nguyễn Phương Chi và Nguyễn Võ Châu Ngân, 2014. Khảo sát sinh trưởng của cây lục bình (Eichhornia crassipes) trên các thủy vực khác nhau. Vai trò Khoa học – Công Nghệ trong sự phát triển kinh tế - xã hội. NXB Nông nghiệp. 15-20. ISBN 978- 604-60-1858-2 Trần Sỹ Nam, Võ Thị Vịnh, Nguyễn Hữu Chiếm, Nguyễn Võ Châu Ngân, Lê Hoàng Việt và Kjeld Ingvorsen, 2014. Sử dụng rơm làm nguyên liệu bổ sung nâng cao năng suất sản xuất khí sinh học. Tạp chí Nông nghiệp và Phát triển Nông thôn - Bộ nông nghiệp và phát triển nông thôn số 15. 65-73. Trần Sỹ Nam, Nguyễn Phương Chi, Nguyễn Hữu Chiếm, Lê Hoàng Việt, Nguyễn Võ Châu Ngân và Kjeld Ingvorsen, 2015. Ảnh hưởng của các phương pháp tiền xử lý sinh học lục bình (Eichhornia crassipes) lên khả năng sinh khí biogas trong ủ yếm khí theo mẻ có phối trộn phân heo. Tạp chí Khoa học Trường Đại học Cần Thơ Số chuyên đề: Môi trường và Biến đổi khí hậu (2015)102-110. Tran Sy Nam, Huynh Cong Khanh, Huynh Van Thao, Nguyen Vo Chau Ngan, Le Hoang Viet, Nguyen Huu Chiem and Kjeld Ingvorsen, 2015. Biogas production from rice straw and water hyacinth – the effect of mixing in semi-continuous reactors. Journal of Science and Technology, Vietnam Academy of Science and Technology, Vol 53(3A): 217-222. Tran Sy Nam, Huynh Van Thao, Huynh Cong Khanh, Nguyen Vo Chau Ngan, Le Hoang Viet, Nguyen Huu Chiem and Kjeld Ingvorsen, 2015. The components of volatile fatty acids in semi- continuous anaerobic co-digestion of rice straw and water hyacinth and pig manure. Journal of Science and Technology, Vietnam Academy of Science and Technology, Vol 53 (3A): 229-234 Trần Sỹ Nam, Huỳnh Văn Thảo, Huỳnh Công Khánh, Nguyễn Võ Châu Ngân, Nguyễn Hữu Chiếm, Lê Hoàng Việt và Kjeld Ingvorsen, 2015. Đánh giá khả năng sử dụng rơm và lục bình trong ủ yếm khí bán liên tục – ứng dụng trên túi ủ biogas polyethylene với quy mô nông hộ. Tạp chí Khoa học Trường Đại học Cần Thơ phần A: Khoa học Tự nhiên, Công nghệ và Môi trường, số 36. 27-35. Nguyen Vo Chau Ngan, Tran Sy Nam, Nguyen Huu Chiem, Le Hoang Viet and Kjeld Ingvorsen, 2015. Effects of C/N rations on anaerobic co-digestion of pig manure and local biomass in the Mekong delta. Journal of Science and Technology, Vietnam Academy of Science and Technology, Vol 53(3A): 223-228. Nguyen Vo Chau Ngan, Tran Sy Nam, Nguyen Huu Chiem, Le Hoang Viet, Nguyen Thi Thuy, Kjeld Ingvorsen, 2015. Paddy straw application for energy production to reduce in situ straw burning in the Mekong delta of Vietnam. In International conference on Solid Wastes 2015: Knowledge transfer for Sustainable resource management in at Hong Kong Baptist University, Hong Kong SAR, P.R. China, ISBN 978-988-19988-9-7.
Chapter 1: INTRODUCTION 1.1. The necessity of the thesis Rice straw and water hyacinth are the two common and potential biomass sources in the Mekong Delta, but it is not used effectively. Most of the rice straw is removed from the field by burning off, burying or composting. Water hyacinths grow very fast and present with a high density in the rivers, lakes and canals; it is harm to aquatic life, preventing water transportation and aquaculture. This potential biomass sources for producing biogas and enhance decentralized energy production in rural areas where many biogas digesters inefficient operated due to a shortage of animal manure. In the period of re-generation or appearing diseases, the shortage of animal manure is one of the major drawbacks of the development of biogas in the Mekong Delta. Meanwhile, as mentioned above, straw and water hyacinth are the two common and potential biomass sources. Therefore, the thesis has been carried out in order to find a simple and applicable method in household conditions to convert rice straw and water hyacinth into biogas in the Mekong Delta. 1.2. Objectives - Determination of quantity of rice straw in the Mekong Delta that available for biogas production; - Determination of growth rate of water hyacinth and its potential for biogas production in household scale; - Finding (i) simply biological pretreatment method, (ii) suitable size, (iii) mixing rate of rice straw/hyacinth with pig manure to produce biogas from rice straw and water hyacinth in vitro; - Trial producing biogas from rice straw and water hyacinth in polyethylene biogas digester type (PE) in household scale in vivo. 1.3. Objects and scope of the study The quantity of rice straw and straw management survey only did in Winter- Spring and Fall-Winter crops in An Giang, Dong Thap, Kien Giang province and Can Tho City. The research focused on the evaluation of simply biological pretreatment, material size and mixing rate of rice straw/hyacinth with pig manure in anaerobic batch and semi-continuous condition in vitro; trial producing biogas in polyethylene digester type (PE) in household scale in vivo -1-
1.4 New findings - Pretreatment rice straw and water hyacinth by simply soaking into biogas effluent or anoxic mud can accelerate and increase biogas production in vitro; - The size of rice straw and water hyacinth (from uncut to 1cm) was not greatly effect on biogas production in vitro; - Mixing 50% - 60% rice straw with pig manure had total biogas production higher than other mixing ratios. In case of water hyacinth, the mixing ratio was 40% - 60% in vitro; - Supplementing rice straw and water hyacinth for replacing pig manure at the level of 50%VS in polyethylene biogas digester type (PE) in household scale had biogas production was not difference form the digester that loading by 100% of pig manure. 1.5 Application meaning Research result shows that rice straw and water hyacinth can be used to produce biogas in polyethylene biogas digester model (PE) in household scale in shortage period of loading materials. The research results can be applied in household’s biogas digesters to help maintain the stability of biogas model and produce clean energy sources (biogas). -2-
Chapter 2: METHODOLOGY 2.1 Research contents - Interviewing farmers about rice straw, straw treatments and sampling straw, rice for estimation the quantity of rice straw; - Surveying the growth of water hyacinth in different water bodies in the household; - Performing the experiment for determination the effect of (i) biological pretreatment and (ii) the size of rice straw and water hyacinth and (iii) mixing ratio on biogas production in batch reactors; - Performing the experiment for evaluation of the ability to producing biogas from rice straw and water hyacinth in semi-continuous reactors in vitro; - Assessing the possibility of using rice straw and water hyacinth to produce biogas in polyethylene biogas digester type (PE) in household scale in vivo. 2.2 Methodology 2.2.1. Surveying on rice straw and straw treatments in the Mekong Delta The information was collected from 400 rice farmers in An Giang, Kien Giang, Dong Thap and Can Tho city based on prepared questionnaires. Rice and straw samples were collected in 4 provinces/cities, 25 samples was collected in each location for determination ratio of straw and grain (the straw in this research is the rice biomass of rice plants from the root upwards, excluding the roots). After determining the fresh weight, the dry weight off straw and gain was determined in the laboratory for grain and straw ratio. 2.2.2 Surveying the growth of water hyacinth in different water bodies The experiment was carried in four water bodies including rivers, irrigation canals, fish ponds and ditch pond in My Phung Hamlet, My Khanh village, Phong Dien district, Can Tho City. Each treatment was done in triplicate with 2m x 2m plots with ten water hyacinths plants. The fresh weight of water hyacinth is determined in each 7 days for 6 weeks. At the end of the experiment, all water hyacinth was collected for determining both dried and fresh weight. -3-
2.2.3 The effect of biological pretreatment and the size of rice straw and water hyacinth on biogas production a) The effect of biological pretreatment of rice straw and water hyacinth on biogas production Based on the actual conditions in the Mekong River delta, the researcher was chosen 04 types of water for pretreatment including tap water (1); biogas influent (2); anoxic sediment (3) and ditched pond water (4). These waters are available in the household conditions and can be used for pre-treatment rice straw and water hyacinth to accelerate the hydrolysis and microbial supplement for the anaerobic digestion process. The experiment was completely randomized design with nine treatments, each treatment was done in 5 replications with mixing rate of pig manure and rice straw/water hyacinth was 50:50 based on VS. The experiment was performed in batch anaerobic reactors with the volume of 17 L. Rice straw and water hyacinth are calculated based on volatile solids (VS), the loading rate was 1gVS.L-1.day-1. b) The effect of the size of rice straw and water hyacinth on biogas production The experiment was performed in completely randomized design in laboratory with 9 treatments; each treatment was done in 5 replications with different size of rice straw and water hyacinth (1 cm, 10 cm, 20 cm and original size). The experiment was performed in batch anaerobic reactors with the volume of 17 L. Rice straw and water hyacinth was mixed with pig manure at the rate of 50:50 (%). Daily biogas production was stored in aluminum bags to determine the total volume and CH4 concentration. 2.2.4 The effect of mixing ratio of pig manure and straw/water hyacinth on biogas production The experiment was completely randomized design with 13 treatments with different mixing rates between pig manure and rice straw/water hyacinth. The loading rate was calculated for 45 days loading, the total amount of VS was 765g/reactor and monitored continuously for 60 days. Before conducting anaerobic digestion, rice straw and water hyacinth was pretreated with biogas effluent in 5 days, well mixed every day. pH, temperature and redox potential is measured directly every day. The biogas volume and biogas composition was daily measured. 2.2.5 Biogas production from rice straw and water hyacinth in semi- continuous experiment The experiment was completely randomized design with 10 treatments pig manure mixed with rice straw (100%, 50% + 50%), pig manure mixed with water -4-
hyacinth (100%, 50% + 50%) and 100% PH with and without stirring. Each treatment was done in triplications in the semi-continuous anaerobic 50L reactors. The loading rate was calculated based on 1gVS.L-1.day-1, with a retention time of 30 days. Mixing was performed once a day in 1minute. Rice straw and water hyacinth was pretreated by biogas effluent in 5 days before loaded into the reactors. pH, temperature and redox potential were daily measured. Volatile fatty acids (VFAs) were collected and measured every 5 days. The biogas volume and biogas composition was daily measured. 2.2.6 Evaluation the possibility of using rice straw and water hyacinth in semi continuous anaerobic fermentation - the application on farm scale polyethylene biogas digesters The experiment carried out in five polyethylene (PE) digesters - the common biogas digester in the Mekong Delta, with a diameter of 0.86 m and 10 m in length. The substrates was loaded to the digester at 1 kgVS.m-3.day-1, the amount of VS loading is 4.24kg.day-1. Hydraulic retention time for the system is 30 days. Rice straw and water hyacinth was pretreated by biogas effluent in 5 days before loaded into the reactors. pH of each digester was measured at the input and output every day. VFAs were collected at the inlet and outlet 2 days/times. The biogas volume and biogas composition was measured every two days. -5-
Chapter 3: RESULTS AND DISCUSSION 3.1 The estimated quantity of rice straw and straw management in Mekong Delta The survey results shows that there are 6 common methods for straw treatments such as: burning, burying, mushroom cultivation, selling, raising cattle, giving away and leaving on filed (Table 3.1). In the Winter-Spring crop, there are 4 methods including burning, mushroom cultivation, selling and giving away. Of these, 98.23 % of the surveyed households are burning straw on field after harvesting, growing mushrooms accounted for 0.99 %, 0.73% for selling and 0.06 % of households giving away. The results showed that straw burning is the most common method that used in the Winter-Spring crop. Table 3.1: The popular methods to deal with straw across season crops Winter-Spring Summer-Fall Fall-Winter Methods Area Percentage Area Percentage Area Percentage (ha) (%) (ha) (%) (ha) (%) Selling 6.24 0.73 10.92 1.27 25.03 2.92 Burning 842.31 98.23 768.98 89.67 463.89 54.1 Mushroom cultivation 8.45 0.99 10.79 1.26 69.81 8.14 Burying - - 57.01 6.65 223.78 26.1 Giving away 0.52 0.06 9.69 1.13 14.11 1.65 Raising cattle - - 0.13 0.02 3.12 0.36 Leaving on field - - - - 57.78 6.74 The estimated rice straw in the field need to treat in the Mekong Delta in 2011 was about 25.2 million tons/year, of which approximately 20.9 million tons rice straw/year is burned off. The results also showed that most of farmers tend to burning practices in the following years. Table 3.2: Rice yield and estimation of amount of straw Fall-Winter Winter-Spring 2011 Locations Yield* Straw Yield* Straw Yield* Straw Kien Giang 0.20 0.20 1.99 1.96 3.94 3.94 Dong Thap - - 1.48 1.36 3.10 2.86 Can Tho - - 0.64 0.82 1.29 1.68 An Giang 0.02 0.03 1.77 2.26 3.84 4.78 Mekong delta 1.62 1.88 10.48 11.70 23.19 26.23 Notes: unit: million tons; (*) Rice production based on GSO In additon of rice straw, water hyacinth is an abundant biomass resources. If the straw is only focused on rice harvesting time, water hyacinth is the available biomass in most of households in the Mekong delta. Therefore, water hyacinth is a -6-
potential source of biomass to produce biogas. In case of shortage of loading materials, water hyacinth can be used to supplement to the digesters in order to maintain the stability for biogas model. Therefore, it is necessary to do the research on using water hyacinth for biogas production. 3.2 The growth of water hyacinth in different water bodies The results showed that fresh weight of water hyacinth ranged from 0.41 to 0.44 kg/m2 at the beginning and reached to 1.3 to 6.9 kg/m2 after 6 weeks. Water hyacinth in irrigation canals, fish ponds and ditched pond had highest weight of dry matter at the level of 634, 804 and 603 gDM/m2, respectively. While hyacinth in the river was only 162 gDM/m2 (Figure 3.1) 1000 River a Irrigation canal Dried weight (gDM/m 2 ) 800 ab Fish pond b Ditch pond 600 400 200 c ns 0 Innitial Finish Figure 3.1: The growth rate of water hyacinth in four water bodies Note: the columns with same letters was not significant from other treatments (Duncan test, α = 0.05) The result indicated the average growth of water hyacinth increase 19.1g.m- 2 .day-1 in pond, in which VS accounted for 83.6% TS (Carina and Cecilia, 2006), VS increased in average 16.1g.m-2.day-1. Biogas yield of water hyacinth ranged from 200-300 L.kgVS-1 (O'Sullivan et al., 2010; Rajendran et al., 2012), the increasing of water hyacinth weight can provide from 3.2 to 4.8 L.m-2.day-1. One household (4 persons) needs 300-500L biogas.day-1 for cooking, so the increasing of water hyacinth from 62-156 m2 can provide adequate biogas for household use. 3.3 The effect of pre-treatment and the size of straw and water hyacinth on biogas production 3.3.1 The effects of pretreatment a. Cumulative methane The results showed that the total cumulative volume of biogas after 60 days ranged from 63.1 to 111.3 liters (Figure 3.2). The cumulative biogas from 100% pig manure treatment was the lowest volume (63.1 liters). All treatments had the -7-
cumulative methane higher than 100% pig manure treatment (p
manure generated the lowest biogas volume at 63.1 L. Water hyacinth pretreated by biogas effluent, tap water and ditch water produced 124.2, 127.6 and 114.7 L methane, respectively. At day 30, the cumulative methane from water hyacinth which pre-treated by anoxic sediment produced the highest volume and differ from other pre-treatments (p
but their methane yield higher than pre-treated by tap water (p
114.2 L, 113.3 L, 111.5 L and 114.6 L, respectively. The lowest cumulative methane came from 100% pig manure reactor (63.1L). Figure 3.5 shows the cumulative methane of all size of rice straw was not significant difference (p>0.05). Pig manure mixed with rice straw increased methane production 76.7 to 81.6 % compared to 100% pig manure. Because of the sharp of rice straw, when it was chopped into short pieces, the surface area was not significant increase, then it did not enhance decomposition process. 140 160 Original a a a a 120 Original a 1 cm 140 a a 1 cm a 10 cm a a a a Cumulative methane(L) Cumulative methane (L) 100 120 10 cm a a a a 20 cm 20 cm Pig manure 80 100 Pig manure a ab a b a a a b 80 b 60 b b b c 60 a a a a 40 b ab c 40 c bc c 20 d 20 0 0 Day 15 Day 30 Day 45 Day 60 Day 15 Day 30 Day 45 Day 60 Time Time Figure 3.5: The cumulative methane production of rice straw (left) and water hyacinth (right) Note: the columns with same letters was not significant different from other treatment (Duncan test, α = 0.05) The results showed that water hyacinth had cumulative methane ranged from 63.1 to 129.1 L, in which water hyacinth in original size produced the highest methane at 129.1 L, the lowest is 100% pig manure (63.1L), water hyacinth in 1 cm, 10 cm and 20 cm produced at 126.8 L, 124,3 L and 128 L, respectively (Figure 3.5). The cumulative methane of all size of water hyacinth was not significant difference (p>0.05). Typically, the smaller size of material the more easily of the biological decomposition process (Sanders et al., 2000). When the size was small, microorganisms can be easy contact to the surface of the material and increase biogas production (Mshandete et al., 2006). According to Mshandete et al. (2006), when the size reduced from 100mm to 2 mm, the biogas produced increased 16%. Sharma et al. (1988) reported that if the size is too small, it also gave the negatively effect on the decomposition process. The results of this study showed that the size of the straw and water hyacinth was not greatly effect on the ability produce biogas of rice straw and water hyacinth. b) Methane Yield The rresult shows that methane yield of rice straw in size of original, 1cm, 10cm and 20cm was 348; 364; 354 and 357 L CH4.kgVS-1degraded, respectively. Methane yield of water hyacinth was 361; 350; 357 and 359 L CH4.kgVS-1degraded in original size, 1 cm, 10 cm and 20 cm, respectively. Pig manure had the methane - 11 -
yield at 234 L CH4.kgVS-1degraded. The statistical analysis showed that methane yield of rice straw and water hyacinth in all size was not significant different (p> 0.05), but their methane yield was higher than 100% pig manure. 400 a a a (a) a 300 Methane yield (L-CH4.kgVS-1 degraded) b 200 100 0 Original 1 cm 10 cm 20 cm Pig manure 400 (b) a a a a 300 b 200 100 0 Original 1 cm 10 cm 20 cm Pig manure Size Figure 3.6 : Methane yield of rice straw (a) and water hyacinth (b) in different sizes Note: the columns with same letters was not significant from other treatments (Duncan test, α = 0.05) Some studies reported the smaller size of substrate can accelerate biogas production process (Sanders et al., 2000; Sharma et al., 1998). However, in this study, rice straw and water hyacinth in difference size from whole plant to 1 cm was not difference in biogas producing. 3.4 The effect of mixing ratio of rice straw, water hyacinth and pig manure on methane production in batch experiment 3.4.1 The effect of mixing ratio of rice straw and pig manure on cumulative methane production The study showed that the highest cumulative methane was from 50% rice straw and 50% pig manure reactor after 60 days (169.4 L) and it was significant difference from other treatments such as 20% rice straw + 80% pig manure (135.1 L), 40% rice straw + 60% pig manure (149.5 L), 100% pig manure (94.1 L) and 100% rice straw (166.6 L), but it was not difference (p>0.05) from 60% rice straw + 40% pig manure and 80% rice straw + 20% pig manure (163.5) (Figure 3.7). Both of 100% pig manure and 100% rice straw had cumulative methane volume lower than rice straw mixed with pig manure (p
200 180 100%PH a a a 160 20%RO + 80%PH a ab b Cumulative methane (L) 40%RO + 60%PH b c c 140 50%RO + 50%PH d d 120 60%RO + 40%PH a ab e 80%RO + 20%PH cd bc e d 100 100%RO f e 80 f 60 a a a a a 40 b c 20 0 Day 15 Day 30 Day 45 Day 60 Time Figure 3.7: Cumulative methane from difference mixing ratio of rice straw and pig manure in 60 days Note: the columns with same letters was not significant from other treatments (Duncan test, α = 0.05) 3.4.2 The effect of mixing ratio of water hyacinth and pig manure on cumulative methane production The results showed that the cumulative methane after 60 days of 40%, 50% and 60% water hyacinth was highest at 181.4 L, 185.7 L and 190.5 L (p>0.05), respectively. The reactor that content 20%water hyacinth and 80% pig manure (151.7 L) and 80% water hyacinth and 20% pig manure had the methane (151.6 L) lower than 40%, 50% and 60% water hyacinth but higher than 100% pig manure (99 L) and 100% water hyacinth (29.8 L) reactors (p < 0.05). 250 100%PH 200 20%LB + 80%PH a a a Cumalative methane (L) 40%LB + 60%PH 50%LB + 50%PH a a a b b 150 60%LB + 40%PH b b 80%LB + 20%PH a a a 100%LB b c 100 c c d a 50 b b b d c d d 0 Day 15 Day 30 Day 45 Day 60 Time Figure 3.8: Cumulative methane from difference mixing ratio of water hyacinth and pig manure in 60 days Note: the lines with same letters was not significant different from other treatment (Duncan test, α = 0.05) The results show that when the ratio of water hyacinth increased from 40% to 60%, the cumulative methane was not significant increase (p>0.05). However, when the mixing ratio greater than 60% or less than 40% of water hyacinth, the cumulative methane will decrease (Figure 3.8). 100% water hyacinth reactor had the lowest methane production due to the low pH, ranged from 5.2 to 5.4 in the - 13 -
early incubation stages. The methanogen bacteria are inhibited in this pH range, thus the methane production of this reactor was low. 3.4.3 Methane yield The results show that methane yield of all the treatments ranged from 316 - 518 LCH4.kgVS-1degraded, in which highest yield from 50:50 rice straw and pig manure reactor (518 LCH4.kgVS-1 degraded) and the lowest yield come from 100% pig manure reactors (316 L CH4.kgVS-1degraded). Rice straw mixed with pig manure at 20:80; 40:60; 50:50; 60:40; 80:20 got methane yield higher than 100% pig manure and 100% rice straw (p
incubation period. In the early stages of the digest process, organic matter was hydrolyzed into organic acids, the accumulation of these acid in the reactors caused increasing VFAs concentration. After hydrosis phrase, the methanogen play a key role and converted VFAs into methane leading the decline of VFAs concentration. 40 40 (a) (b) Concentration of VFAs (mM/L) 30 30 20 20 10 10 0 0 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Time (day) Time (day) 100%RO 50%RO+50%PHKT 100%LB 50%LB+50%PHKT 100%ROKT 100%PH 100%LB KT 100%PH 50%RO+50%PH 100%PHKT 50%LB+50%PH 100%PHKT Figure 3.10: The concentration of VFAs in semi-continuous reactors using rice straw (a) and water hyacinth (b) In this study, when the more rice straw is mixed the more VFAs accumulation and acid stage is longer, 100% RO reactor had a high concentration of VFAs until day 50 of incubation period. While VFAs concentration of all rice straw mixing pig manure reactor and 100% pig manure reactor decreased after 30 days. The suitable concentration of VFAs for the activities of methanogen was about below 4000 mg/L (equivalent to 66.7 mM/L) (Siegert et al., 2005; Ward et al., 2008). The accumulation of VFAs in the reactor was an indicator of over organic loading. In this study, VFAs concentration was suitable for methanogen activities. The results showed that the anaerobic fermented compounds were acetic acid, propionic, butyric, succinic, acrylic, fumaric, formic, malic, glucose and ethanol in which the major components of VFAs are acetic, propionic, butyric. The proportion of these compounds accounted for about 80% of VFAs concentration. Acetic acid, propionic acid and butyric acid was the main component of VFAs, especially acetic acid and propionic acid accounted for a large part of VFAs. - 15 -
25 25 (d) Acetic acid (mmol/L) 20 (a) 20 15 15 10 10 5 5 0 0 25 25 (e) (b) Popionic acid (mmol/L) 20 20 15 15 10 10 5 5 0 0 14 14 (c) Butyric acid (mmol/L) (f) 12 12 10 10 8 8 6 6 4 4 2 2 0 0 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Time (day) Time (day) 100%RO 50%RO+50%PH 100%PH 100%LB 50%LB+50%PH 100%PH 100%RO KT 50%RO+50%PHKT 100%PHKT 100%LBKT 50%LB+50%PHKT 100%PHKT Figure 3.11: The concentration of VFAs in rice straw (a,b,c) and water hyacinth (d,e,f ) semi-continuous reactors 3.5.2 The cumulative methane production from rice straw The cumulative methane of all treatments are presented in Figure 3.12. The results show that the cumulative methane after 60 days was 257, 242, 214, 215, 57.6 and 53.6 liters, respectively for 100% ROKT, 100% RO, 50%PH + 50%ROKT, 50%RO + 50%PH, 100% PH and 100% PHKT. The cumulative methane after 60 days showed mixing slightly increased the methane production. 300 a Cumulative methane production (L) 100%RO a 250 100%ROKT b b 50%RO+50%PH 200 50%RO+50%PHKT 100%PH 100%PHKT ab a ab 150 b 100 c c a a a a 50 c c b c c b b a a b 0 Day 15 Day 30 Day 45 Day 60 Time Figure 3.12: The cumulative methane from rice straw in semi-continuous reactors - 16 -
3.5.3 The volume of gas produced daily and cumulative total air volume of the treatments hyacinth Mixing just play the important role when the digesters content much substrate inside. Cumulative methane production of water hyacinth reactors was higher than rice straw reactors at 416, 322, 294, 241, 57.6 and 53.6 liters, respectively, 100% LBKT, 100% LB, 50%PH + 50% LB KT, 50%LB + 50% PH, 100%PHKT and 100%PH 500 100%LB a 400 100%LBKT Cumualative methane (L) 50%LB+50%PH b 50%LB+50%PHKT c 300 100%PH a 100%PHKT d 200 b b c a 100 b c d e e d d b a b b c e e c 0 Day 15 Day 30 Day 45 Day 60 Time Figure 3.13: The cumulative methane from water hyacinth in semi-continuous reactors 3.6 Evaluation the possibility of using rice straw and water hyacinth in semi continuous anaerobic fermentation - the experiment on farm scale polyethylene biogas digesters 3.6.1 Operation time The results showed that the operation time of 100% RO and 100% LB digesters lower than other digesters. At 1kg/VS/day load rate, 100% RO digester operated only 23 days, for 100%LB digester was 27 days (Table 3.3). While 50% RO + 50% PH, 50%LB + 50% PH and 100%PH operated continuously until day 60 without any problems. Because rice straw and water hyacinth are the materials that have high porosity and large volumes, so when it was loaded into the digesters, it is easy to make the digester full and cannot load for a long time. This is the main limitations of producing biogas from biomass, especially rice straw and water hyacinth. - 17 -