| Livestock Research for Rural Development 29 (8) 2017 | Guide for preparation of papers | LRRD Newsletter | Citation of this paper |
The objective of this study was to determine the effect on methane production of adding different proportions of fresh vegetable waste to manure from pigs or buffaloes as substrate in plug-flow biodigesters. The treatments in a completely randomized design with four replicates were ratios of fresh vegetable waste to manure (pig or buffalo) of 0, 25, 50, 75 and 100%. Measurements were made of gas production, methane content of the gas and pH of the effluent over a period of 30 days from startup of the biodigesters until the end of the experiment after 30 days.
Methane percentage in the gas and total methane production were increased threefold by including 25% of fresh vegetable waste in the substrate. Higher ratios of vegetable waste led to linear decreases in total gas and methane production. Methane production was tripled when the substrate was pig manure and vegetable waste compared with buffalo manure and vegetable waste. The ratio of vegetable waste to manure was a critical factor in determining the biogas yield and composition of methane in the gas, with the optimum being recorded at 25% of vegetable waste followed by subsequent rapid decline to zero values when the substrate was only vegetable waste.
Key words: nitrogen, methane, pH, plug-flow
Renewable energy is a major area for research and development, especially from biological waste streams (Deublein and Steinhauser 2010). Approximately 1.3 billion tonnes of food waste are unused each year; this reduces the food for human consumption, and equally serious ends up in "land-fills" with production of methane gas (FAO 2011; EPA 2012).
Anaerobic digestion in a biodigester is a way to utilize organic waste efficiently, producing not only a combustible gas (methane) but also functioning as a waste disposal system. According to: Vaid et al (2013), food waste not only makes anaerobic digestion desirable but makes it cost efficient, reduces greenhouse gas emissions at landfills, utilizes existing infrastructure for food waste diversion and meets local and state waste diversion goals. At household level, food waste; can be added to biodigesters to complement human feces and urine (Thu Thien et al 2014). This increases the availabilty of gas for cooking as well as reducing the amount of waste that has to be transported to land-fills or to centralized processing facilities (Croxatto Vega et al 2014). The pollution which results from temporary storage of the food waste at household level is also avaoided by such a strategy.
The experiment was conducted in the experimental are of Svay Rieng University (SRU), in Svay Rieng Province, Cambodia, during the dry season from January to February, 2016.
The experimental biodigesters (Photos 1-2) were made from recycled polypropylene water bottles, based on the design developed by Thu Hien et al (2014). The total volume of each biodigester was 5 liters. The biodigesters were operated with an initial loading rate set at 160g of DM in a total liquid volume of 4 liters (4% DM concentration). The retention time was fixed at 20 days thus each day were added 8 g of DM and 192 ml water. Quantities on a fresh basis are shown in Table 1. Gas was collected by water displacement using 1.5 liter bottles, with the bottoms removed, and calibrated at 50 ml intervals. These were suspended in 5 liter bottles with the tops removed and filled with water.
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| Photos 1-2. The small scale biodigesters made from recycled polypropylene water bottles |
The experiment was designed as a 2*5 factorial with 4 replications.
The factors were:
Manure:PM: Pig manure
BM: Buffalo manure
Level of vegetable waste:0, 25, 50, 75 and 100% of the substrate (DM basis)
The vegetable waste was collected from the Canteen in the Svay Rieng University. It included waste from water spinach, lettuce, bitter melon, cabbage, gourd, cauliflower, yam, bok choi, potato, pumpkin, carrot, smooth luffa, radish and amaranth. Manure from pigs and buffaloes was collected from farmer households near the University. The vegetable waste was chopped by hand into small pieces (2-3 mm length). Finally, the vegetable waste and the manure from the pigs or buffaloes were added to the biodigesters according to the quantities indicated in Table 1.
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| Photo 3. Vegetable waste (lettuce, bitter
melon, cabbage, gourd, cauliflower, yam, bok choi, potato, pumpkin, carrot |
Photo 4. Vegetable waste after chopping |
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Table 1. Quantities of pig manure and vegetable waste put in the biodigesters daily |
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Vegetable waste, % as DM |
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0 |
25 |
50 |
75 |
100 |
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Quantities, g/d fresh basis |
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Vegetable waste |
0 |
22 |
45 |
67 |
90 |
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Pig manure |
36 |
27 |
18 |
9 |
0 |
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Water, ml/d |
164 |
151 |
137 |
124 |
110 |
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The dry matter and nitrogen in the substrates were determined according to AOAC (1990) procedures. The volume of the gas was recorded daily from the calibrations on the receiving bottle. The pH of the effluent exiting the biodigester was measured every 24h using a portable digital meter (Photo 5). The percentage of methane in the gas was measured daily by passing a sample through a Crowcon Infra-red analyzer (Crowcon Instruments Ltd, UK) (Photo 6).
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| Photo 5. Measuring the pH of the effluent | Photo 6. Measuring methane content in the gas |
The data were analyzed by the GLM option in the ANOVA program of the Minitab (2000) software. Sources of variance were: replicates, source of feedstock, proportion of manure, interaction between source of feedstock*proportion of manure and error.
There were major differences in the composition of the substrates (Table 2). Manure from pigs was higher in nitrogen than manure from buffaloes.
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Table 2. The composition of the substrate |
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Vegetable waste |
Pig manure |
Buffalo manure |
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DM, % |
8.98 |
22.2 |
23.2 |
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As % of DM |
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Nitrogen |
1.22 |
1.92 |
1.04 |
Gas production increased on all treatments reaching maximum values after 12-15 days when pig manure was used (Figure 2). With buffalo manure the maximum values were reached some 3-5 days later (Figure 3). The effect of adding vegetable waste differed to a major degree according to the proportions added (Table 3; Figure 1). Gas production was tripled when 25% of the fresh weight of manure was replaced by vegetable waste. But with increasing rates of replacement gas production decreased linearly. Trends were similar for manure from buffaloes but average volumes were 50% less.
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Table 3. Mean values of biogas production over 30 days according to source of manure and proportion of vegetable waste |
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Level of vegetable waste, % |
SEM |
p |
Manure |
SEM |
p |
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0 |
25 |
50 |
75 |
100 |
Pig |
Buffalo |
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Gas, ml/d |
324 |
744 |
536 |
360 |
325 |
43.2 |
<0.01 |
561 |
354 |
27 |
<0.01 | |
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CH4, % |
24.8 |
39.5 |
20.9 |
9.91 |
2.33 |
1.82 |
<0.01 |
24.9 |
14.0 |
1.15 |
<0.01 |
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CH4, ml/d |
148 |
423 |
177 |
52.3 |
14.2 |
20.1 |
<0.01 |
242.1 |
83.7 |
12.7 |
<0.01 |
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pH |
6.9 |
6.3 |
5.8 |
5.3 |
5.1 |
0.12 |
<0.01 |
6.1 |
5.6 |
0.07 |
<0.01 |
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| Figure 1. Mean values over 30 days of gas production from mixtures of manure (from pigs or buffaloes) and vegetable waste |
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| Figure 2. Trends in gas production from mixtures of pig manure and vegetable waste |
Figure 3. Trends in gas production from mixtures of buffalo manure and vegetable waste |
The trends for methane content of the gas were similar to those recorded for gas production, with maximum values being reached after about 15-20 days (Figures 4 and 5). Similar to gas production, the highest percentages of methane in the gas were recorded when the ratio of vegetable waste to manure was 25:75 (Figure 6).
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| Figure 4. Trends in percentage methane in the gas from mixtures of pig manure and vegetable waste |
Figure 5. Trends in percentage methane in the gas from mixtures of buffalo manure and vegetable waste |
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| Figure 6.
Mean values over 30 days for methane content of the gas
from mixtures of manure (from pigs or buffaloes) and vegetable waste |
The trends in daily production of methane were similar to those for total gas production (Figures 7 and 8) with maximum yields of methane being obtained with 25% vegetable waste and 75% manure (Figure 9).
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| Figure 7. Trends of methane production from mixtures of pig manure and vegetable waste |
Figure 8. Trends of methane production from mixtures of buffalo manure and vegetable waste |
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Figure 9. Mean values over 30 days of methane production
from mixtures of manure (from pigs or buffaloes) and vegetable waste |
The pH in the biodigesters decreased over the first 7 days after which the values stabilized. (Figures 10 and 11). The pH was lower with buffalo manure than with pig manure (Figure 12) and declined linearly as the proportion of vegetable waste was increased (Figure 12).
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| Figure 10. pH of effluent from digesters with pig manure | Figure 11. pH of effluent with buffalo manure |
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| Figure 12.
Mean values over 30 days of pH of the effluent from biodigesters
charged with mixtures of manure (from pigs or buffaloes) and vegetable waste |
The results of this study are consistent with those reported by Thu Hien et al (2014) using vegetable waste combined with manure from humans, cattle or pigs. These authors showed that in similar laboratory biodigesters, the pH after 20 days had decreased from initial values of 5.5 - 6.5 to 4.5 when only vegetable waste was the substrate. They also found that the methane content of the gas was less when the concentration of vegetable waste was increased.
The greater production of gas, the higher proportion of methane in the gas, together with the higher pH in the bodigesters, when the manure was from pigs rather than buffaloes, probably reflects the nature of the feeds consumed, which were of higher digestibility - and of superior nutrient balance - in the case of the pigs versus the buffaloes. A similar finding was reported by San Thy et al (2005) that biogas yield was higher from pig than from cattle manure. There was a major interaction between the potential fermentability of the substrate (vegetable waste, manure) and the resultant effect on the pH in the biodigester which in turn played a determinant role on the gas production and the methane content of the gas, both of which declined markedly when the pH fell to values below pH 6.
From a practical standpoint the important finding from this study is that methane production from a biodigester charged with animal manure was increased threefold by including 25% of fresh vegetable waste in the substrate.
This research was done by the senior author as part contribution to the degree of Master of Science awarded by Cantho University, Vietnam. Sincere gratitude is expressed to the MEKARN II Project, financed by Sida, for supporting this research. Acknowledgment is made to Svay Rieng University for hosting the research and to the University students who enthusiastically participated in all phases of the experiment.
AOAC 1990 Official Methods of Analysis. Association of Official Analytical Chemists. 15th edition
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FAO 2011 Global food losses and food waste. www.fao.org/docrep/014/mb060e/mb060e.pdf
Minitab 2000 Minitab Software Release 16.
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Received 13 June 2017; Accepted 17 July 2017; Published 1 August 2017