Study on co-Fermentation of cow dung and giant dirt in semi-continuous anaerobic digester - Nguyen Le Phuong

Journal of Science and Technology 54 (2A) (2016) 287-292 STUDY ON CO-FERMENTATION OF COW DUNG AND GIANT DIRT IN SEMI-CONTINUOUS ANAEROBIC DIGESTER Nguyen Le Phuong 1, 2 , Thach Hai 2 , Nguyen Van Liem 2 , Duong Ngoc Tram 2 Nguyen Thi Ngoc Trang 2 , Kim Lavane 2 , Nguyen Vo Chau Ngan 2, 3, * 1 Hau Giang Industrial Zone Authority, Hau Giang province, Vietnam 2 College of Environment and Natural Resources, Can Tho University, 3/2 Street, Ninh Kieu District, Can Tho city, Vietnam 3 Department of International Relations, Can Tho University, 3/2 Street, Ninh Kieu District, Can Tho city, Vietnam * Email: nvcngan@ctu.edu.vn Received: 10 June 2016; Accepted for publication: 20 June 2016 ABSTRACT Study on co-fermentation of cow dung (CD) and giant dirt (GD - Pistia stratiotes L.) in semi-continuous anaerobic digester aims to test biogas quantity and quality by time. Three mixing rates counted based on ODM of each material were chosen: 100 %CD, 50 %CD + 50 %GD, and 100 %GD. The experiments were set up in laboratory conditions with two types of digesters: one-stage digesters and two-stage digesters (triplicate for each treatment). After 80 days, the biogas produced from the one-stage digesters was not significantly different to the two- stage digesters with similar input material. The highest production of biogas came from digesters of 100 %GD - 235.8 L for the one-stage and 240.3 L for the two-stages. The medium production came from digesters of 50 %CD + 50 %GD - 127.8 L for the one-stage and 118.4 L for the two- stages. After one month, the percentage of CH4 was high enough for burning and almost giant dirt was digested that limited of a blockage inside the digesters. The results showed that co- fermentation of GD and CD is an alternative for livestock raising households to produce biogas for energy purpose. Keywords: anaerobic semi-continuous digester, cow dung, giant dirt, one-stage digester, two- stage digester. 1. INTRODUCTION Cattle and buffaloes are two most important ruminant species in Vietnam. In 2014, there were 5.3 million beef cattle, which increase by 2.7 %; and about 253.7 thousands dairy cows [1]. In recent years, since the food requirement becomes less critical, local farmers tend to raise cattle for meat and milk production [2]. In sub-urban area, local farmers who raise dairy cows have to save a certain amount of land area for growing grass to feed the cattle because of lack of natural pasture and agricultural by-products. While in rural areas, farmers raise beef but this Nguyen Vo Chau Ngan et al. 288 activity has not been received much attention due to low price of beef compared to breeding stock. This leads to inadequate waste treatment conditions at livestock households and brings negative effects to the local people themselves as well as to the surrounding community. A survey done in some provinces of the Mekong Delta showed that most of livestock raising occurred at household scale, and farm households paid little attention on waste treatment but freely discharged to surrounding areas causing pollution to the neighborhood [3]. In addition, cattle manure is traditionally used direct as organic fertilizer source for agriculture cultivations. But due to high organic matter contain, cattle manure was applied as raw material source for biogas production through anaerobic fermentation system [4, 5]. In line to these findings, another study confirmed that anaerobic co-digestion of cow manure and corn stalk could produce biogas in specific situation of the Mekong Delta [6]. Furthermore, it is found that biogas production from batch anaerobic co-digestion of cow manure and giant dirt could give positive results due to C/N balancing by mixing these materials [7]. The study focuses on testing the semi-continuous anaerobic co-digestion of cow dung and giant dirt. This is a basic step before the technique can be transferred to farmers to help reduce the pollution from livestock activities in the region. 2. METHODOLOGY 2.1. Material preparation All experiments were processed at the Biogas laboratory of College of Environment & Natural Resources of Can Tho University. - Cow dung was collected from livestock household at Long Hoa ward of Binh Thuy district of Can Tho city. Cow dung was dried in shadow (26 ± 4 o C) in one week, grinded and mixed as union form. - Giant dirt was collected from several ponds at Long Hoa ward of Binh Thuy district of Can Tho city. - Digester effluent was collected from an active biogas plant of cow raising household at Long Hoa ward of Binh Thuy district of Can Tho city. 2.2. Experimental set-up The one-stage semi-continuous experiments were conducted in 21 L plastic bottles in which the digester effluent filled up to 17 L, while 4 L left for gas production. The two-stage semi- continuous experiments were linked by two one-stage semi-continuous digesters through a T- connector. A 15 L aluminum bag was directly connected to each digester to collect gas produced. Throughout the testing time, all bottles were covered by black nylon bags to avoid alga growth. The loading rate of each experiment was 0.8 g volatile solid (VS) per litter per day (for both one-stage and two-stage treatments) and the study time was 81 continuously days. Each experiment was conducted in triplicate. - Treatment 1: 100 %CD one-stage - Treatment 2: 100 %CD two-stage - Treatment 3: 50 %CD+50 % GD one-stage - Treatment 5: 100 %GD one-stage - Treatment 6: 100 %GD two-stage - Treatment 7: biogas effluent Study on co-fermentation of cow dung and giant dirt in semi-continuous anaerobic digester 289 - Treatment 4: 50 %CD+50 % GD two-stage Figure 1. Experimental models of one-stage (left) and two-stage (right) treatment. 1 - digester; 2 - input pipe; 3 - output pipe; 4 - gas pipe; 5 - gas valve; 6 - gas storage bag; 7 - T-connector 2.3 Analytical methods The gas produced was collected every three days to define gas volume (by Ritter gas meter) and gas compositions (by Biogas 5000 gas analyzer). In the one-stage treatments, the gas production was collected at the gas bag; while at the two-stage treatment, the gas production was collected at total two gas bags. Experimental control parameters (pH, temperature) were monitored every three days by handheld equipment at input pipe, and output pipe. Duncan test was performed to determine significant differences among each treatment by using SPSS. 3. RESULTS AND DISCUSSIONS 3.1 Potential on biogas production 3.1.1 Biogas volume At the beginning of the experiments, the quality of the biogas produced was so low that the gas volume was first recorded after ten days. Since then, the biogas volume was recorded in the three days intervals, but the volume unstably changed during the experimental period. From the day 25 th , the biogas volume tended to increase at the treatments of GD or of CD+GD. There was no significant difference on produced biogas between the one-stage and the two-stage treatments of similar input material. The treatments with different input material produced significantly different volume of biogas. Actually the highest biogas volume recorded from 100 %GD treatments with 235.8 L for the one-stage and 240.3 L for the two-stages. The medium gas volume came from treatments of 50 %CD + 50 %GD was 127.8 L for the one-stage and was 118.4 L for the two-stage digester. The lowest biogas production came from 100 %CD treatments. Nguyen Vo Chau Ngan et al. 290 Figure 2. Gas production every 3 days (left), and the accumulate gas production (right). The gas production in this study lower than the gas production recorded in previous study from 27 to 30 % [7]. The different values on C/N ratio between the treatments with dissimilar input material could be an explanation for this difference. The C/N ratio of the 100 %CD treatment in this study was 76.26, while the C/N ratio of the 100 %CD treatment at the previous treatment only 19.63. The cow dung in this study could be store for a long time that lead to the difference on C/N ratio. In another study, there are three main advantages of two-stage anaerobic digestion compared with one-stage one, including less hydraulic retention time, higher gas production and higher CH4 concentration [8]. Other studies on treatment of wastewater sludge and on municipal solid wastes also found that two-stage digestion were proved a better efficiency than one-stage digestion [9, 10]. However, in our experiment there was no significant difference between one- stage and two-stage digesters. This could be due to the small volume of digester (21 L) and the lower amount of input materials fed into digesters (0.8 g VS/L*day -1 ) in this study. 3.1.2 Biogas compositions After 20 days, there was very limited of gas produced from treatments of 100%CD that not enough for testing by the Biogas 5000 gas analyzer. So that only gas compositions from treatments of 100 %GD and 50 %CD + 50 %GD were presented in this study. During the first 10 days, the CO2 and other gases volume produced were higher than CH4 in almost the treatments. In the next 10 days, the percentage of CH4 was increasing from 38.4 to 48.6 %, and kept stable from the next 10 days with %CH4 around 50 % (ranged from 47.4 to 54.6 %). With high content of CH4, the biogas produced could apply for energy purposes. The one-stage digester had a rapid acidification decreasing the pH and a large volatile fatty acids production, which stressed and inhibited the activity of methanogenic bacteria [8]. In this study, in comparison to one-stage and two-stage digesters of the same treatment, there was no significant difference on CH4 percentage. The explanation was given previously at section 3.1.1. Study on co-fermentation of cow dung and giant dirt in semi-continuous anaerobic digester 291 Note: A: 50%CD+50%GD one-stage B: 50%CD+50%GD two-stage C: 100%GD one-stage D: 100%GD two-stage Figure 3. Biogas compositions in treatments. 3.2 Control parameters in fermentation process The temperature in the treatments ranged from 22.8 to 29.6 o C which effected by outside temperature. The temperature inside the substrate had influence on biogas production in the treatments. It was recorded that high temperature values caused high gas production. For the pH values of substrate, it rather changed between input and output pipes. In the first week of the experiment, all of the treatments had pH from 6.8 to 8.1, then pH value decreased to 6.5 to 6.9 at the end of the study. It was noticed that more material was loaded into the digester, more acidity created that caused pH value reducing from the treatment. Table 1. The relative between pH value at input and output substrates. Treatment pH Treatment 1: 100 %CD one-stage y = 0.9601x + 0.3121 R² = 0.9035 Treatment 2: 100 %CD two-stage y = 0.9111x + 0.7019 R² = 0.8919 Treatment 3: 50 %CD + 50 %GD one-stage y = 0.8438x + 1.2062 R² = 0.7833 Treatment 4: 50 %CD + 50 %GD two-stage y = 0.9118x + 0.7828 R² = 0.8411 Treatment 5: 100 %GD one-stage y = 0.8361x + 1.2892 R² = 0.6991 Treatment 6: 100 %GD two-stage y = 0.8549x + 1.2032 R² = 0.7929 4. CONCLUSIONS This pilot scale experiment was performed to develop co-digestion process of cow dung and giant dirt in case of one-stage and two-stage processes. The reactors were fed with mixtures of CD and GD of 100 %CD, 50 %CD + 50 %GD, and 100 %GD (0.8 g VS/L*day for 17 L of substrate) and temperatures were maintained at room temperature. 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