Biogas production from water spinach combined with manure from buffaloes in an in vitro biodigester system

Sopheap Yen, T R Preston¹ and Nguyen Thi Thuy²

Svay Rieng University (SRU), Svay Rieng province, Cambodia
yensopheap2013@gmail.com
¹ Centro para la Investigación en Sistemas Sostenibles de Producción Agropecuaria (CIPAV). Carrera 25 No 6-62 Cali, Colombia
² Cantho University, Vietnam

Abstract

The objective of this study was to determine the effect on methane production of adding different proportions of fresh water spinach to buffalo manure as substrate in in vitro plug-flow biodigesters. The treatments arranged in a completely randomized design with four replicates were ratios of water spinach to buffalo manure of 0, 5, 10, 15, 20, 25, 30, 35, 40 and 45% (DM basis). 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.

Methane production was increased three times (300%) by including 15% of fresh water spinach in the substrate DM. Water spinach supported intermediate levels of methane production compared with use of household vegetable waste (the best) and fruit waste (the worst) when added to buffalo manure at replacement levels in the range of 15-25% (DM basis).

Key words: methane, pH, retention time, vegetable waste

Introduction

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. 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 (Vipul et al 2013). Another important issue concerns the site for making effective use of wastes, both from food and manure. In Europe, this is mainly done in centralized facilities (Croxatto et al 2014), which entails a considerable cost in energy for transporting the waste, while pollution results from temporary storage of the food waste at household level.

Another approach is to grow a vegetative plant as cover for the biodigester where it functions to protect the polyethylene plastic from UV damage, as a source of human food (the fruit) and of fermentable biomass supplement the manure, in this case from pigs (Photo 1).

Photo 1. Plug-flow polyethylene biodigester protected by a multi-purpose water-melon plant (Bounmay 2017)

In the present experiment, water spinach (Ipomoea aquatica) was chosen as the supplement for buffalo manure, as it could serve a dual-purpose role: as a vegetable for the family and as a supplement for the biodigestor. Previous experiments with this system compared waste from vegetables (Sopheap et al 2017a) and fruit wastes (Sopheap et al 2017b). In both cases the optimum proportion of waste to manure (DM basis) was 25:75. Higher proportions of either waste source led to a rapid fall in the rate of production of gas and in its content of methane. In the present experiment, it was decided to study a narrower range of vegetable: manure ratios, from 0:100 to 65:35 (DM basis) with 5 unit intervals.

Material and methods

Location and duration

The experiment was conducted in the experimental size of Svay Rieng University (SRU), in Svay Rieng province, Cambodia, from May to June, 2016.

Photo 2. Experimental area

Experimental design and treatments

The experiment was arranged in a completely randomized design with 10 treatments and 4 replications. The treatments were proportions (DM basis) of water spinach: 0, 5, 10, 15, 20, 25, 30, 35, 40 and 45% of the substrate with the remainder as buffalo manure.

Experimental system

The experiment was conducted in in vitro biodigesters (Photo 1), 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 liquid volume was fixed at 4 liters. The initial loading rate was 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 combinations of fresh water spinach and buffalo manure (Table 1) to provide 8 g of DM and 192 ml water.

Gas production was measured 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.

Substrates

The water spinach was planted in the experimental area at Svay Rieng University. The foliage (leaves and stems) was harvested at approximately 4-week intervals (Photo 3) and chopped by hand into small pieces (1-2 mm). Buffalo manure was collected from a farmer household near the University.

Photo 3. Water spinach being harvested

Measurements and data collection

The gas volume was read from the collection bottles directly every day over the entire experiment. Methane was determined in samples of gas which were drawn from the collection bottles every day and passed through an Infra-red analyzer (Crowcon Instruments Ltd, UK). The pH in the effluent that came out daily from the biodigester was measured with a digital meter.

Chemical analysis

Samples of water spinach and manure were analysed for DM and N using the procedures of AOAC (1990).

Statistical analysis

The data were analyzed by the GLM option in the ANOVA program of the Minitab software. Sources of variance were: replicates, source of feedstock, proportion of manure, interaction between source of feedstock*proportion of manure and error.

Results and discussion

Composition of substrate

The crude protein content in water spinach was twice that in the buffalo manure (Table 2) . In contrast, the DM content was twice as high in buffalo manure than in water spinach.

Gas production

Gas production began on day 5 and increased linearly through to 30 days (Figure 1). For comparative purposes, yields of gas and methane content were averaged over the last 5 days of measurements (days 26 to 30).

During this period, the highest gas production was recorded when the ratio of water spinach to buffalo manure was in the range15-25% (DM basis) as water spinach. With lower and higher ratios of water spinach the gas production decreased to levels of 300-400 ml/d (Figure 2).

Figure 1. Trends in gas production from mixtures of buffalo manure and water spinach

Figure 2. Mean values over days 26-30 of gas production from mixtures of buffalo manure and water spinach

Methane content

The methane content in the gas increased linearly over the period 4 to 30 days, the trends being similar for all ratios of water spinach to buffalo manure (Figure 3). The response to increasing proportions of water spinach replacing buffalo manure (days 26 to 30) was curvilinear with maximum values (40% methane) being reached with 15% (DM basis) of water spinach in the influent (Figure 4), thereafter declining to low values of only 10% when the water spinach replaced 45% of the buffalo manure. According to the data in Figure 3 the methane content was continuing to increase up to 30 days.

Figure 3. Trends in methane content of the gas from mixtures of buffalo manure (BM) and water spinach (WS)

Figure 4. Mean values of methane content in the gas from mixtures of buffalo manure and water spinach over 26- 30 days of the fermentation

Methane production

The trends in methane production were liketo those for total gas production with maximum yields of methane being recorded when water spinach replaced 15% of the buffalo manure (Figures 5 and 6).

Figure 5. Trends in methane production from mixtures of buffalo manure and water spinach

Figure 6. Mean values over 26-30 days for methane production from mixtures of buffalo manure and water spinach

pH of the effluent

The pH in the effluent from the biodigesters increased slightly with the onset of fermentation, from a range of values between 5.0 and 7.0 to a range of 6.0 to 7.5 during the first 7 days of the fermentation thereafter declining to values of between 6.5 and 7.0 and remaining stable in this range (Figure 7).

Replacing buffalo manure with water spinach led to decreases in the pH from 7.0 to 6.5 as the percentage of water spinach was increased from zero to 30% in the substrate DM, thereafter remaining constant (Figure 8).

Figure 7. Trends in pH of the effluent from mixtures of buffalo manure and water spinach over the first 30 days of the fermentation

Figure 8. Mean values over 26-30 days of pH of the effluent from biodigesters charged with mixtures of buffalo manure and water spinach

Discussion

The results of this study are broadly similar to those reported by Sopheap et al (2017a,b) using vegetable and fruit wastes combined with pig and buffalo manure. However, the decision to study replacement levels at 5% intervals indicated that the optimum level of water spinach to replace buffalo manure was 15% of the influent DM. This could not be compared with the earlier findings of a 25% optimum level of replacement (Sopheap et al 2017a,b), as narrower ratios were not studied.

It is probable that the higher content of protein in the vegetable waste, and in water spinach, was responsible for the higher methane production, as compared with fruit waste as the additive. The fact that the protein in water spinach is highly soluble (Silivong et al 2015), would have contributed a superior "buffering" effect to the fermentation and hence would have maintained higher levels of pH in the system. With water spinach as the vegetable additive, the efluent pH was always above 6.0 whereas with vegetable waste it descended to values between 6.0 and 5.5 (Sopheap et al 2017a) and with fruit waste to below 5.0 (Sopheap et al 2017b) as the proportion of waste exceeded 25% of the substrate.

Unfortunately, for reasons outside our control, the experiment was halted after 30 days of fermentation, when in fact the fermentation stability indicators - total gas production and percentage methane in the gas (eg: 40% methane in the gas) - were below those reported in the first experiment with this system (70% methane; Sopheap et al 2017a). and appeared to be still increasing.

An approximation to the relative value of the three additives (Figures 9 and 10) indicates that vegetable waste was best, followed by water spinach with poorest results from fruit waste, and that these differences were probably related to the effects of the water spinach, vegetable and fruit wastes on the pH in the biodigester.

Figure 9. Gas production from biodigestors charged with optimum proportions of buffalo manure and either household vegetable waste, fruit waste of water spinach (biodigestors of 4 liters liquid volume fed with same quantities of substrate (8 g DM) and water (192ml) equivalent to 20 day retention time	Figure 10. Methane content of the gas from biodigestors charged with optimum proportions of buffalo manure and either household vegetable waste, fruit waste or water spinach (biodigestors of 4 liters liquid volume with same quantities of substrate (8 g DM) and water (192ml) equivalent to 20 day retention time

Conclusions

• The methane production from a biodigester charged with buffalo manure was increased three times (300%) by including 15% of fresh water spinach in the substrate DM.
  
  
• Water spinach supported intermediate levels of methane production compared with household vegetable waste (the best) and fruit waste (the worst) when added to buffalo manure at replacement levels in the range of 15-25% (DM basis).

Acknowledgments

This research was done by the senior author as 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, and to Svay Rieng University, for supporting this research. is acknowledged for hosting the research. The University students who enthusiastically participated in all phases of the experiment are gratefully acknowledged.

References

AOAC 1990 Official Methods of Analysis. Association of Official Analytical Chemists. 15th edition.

Croxatto Vega G A, Ten Hoeve M, Birkved M, Sommer S G and Bruun S 2014 Choosing co-substrates to supplement biogas production from animal slurry - A life cycle assessment of the environmental consequences. Choosing co-substrates to supplement biogas production from animal slurry - A life cycle assessment of the environmental consequences. Bioresource Technology, Volume 171. http://www.sciencedirect.com/science/article/pii/S0960852414012152

Minitab 2000 Minitab Software Release 16.

Thu Hien P T, Preston T R, Lam V and Khang D N 2014: Water spinach supplemented with human or animal excreta as substrate for biogas production. Livestock Research for Rural Development. Volume 26, Article #176. http://www.lrrd.org/lrrd26/10/hien26176.html

Vipul V and Shivangi G 2013 Food as Fuel: Prospects of Biogas Generation from Food Waste. International Journal of Agriculture and Food Science Technology (IJAFST) 4(2): 68-71. http://www.ripublication.com/ijafst_spl/ijafstv4n2spl_14.pdf

Silivong P and Preston T R 2015 Effect of water spinach and biochar on methane production in an in vitro system with substrate of Bauhinia acuminata or Bitter Neem (Azadirachta indica) leaves. Livestock Research for Rural Development. Volume 27, Article #57. http://www.lrrd.org/lrrd27/3/sili27057.html

Yen S, Preston T R and Thuy N T 2017a Biogas production from vegetable wastes combined with manure from pigs or buffaloes in an in vitro biodigester system. Livestock Research for Rural Development. Volume 29, Article #150. http://www.lrrd.org/lrrd29/8/soph29150.html

Yen S, Preston T R and Thuy N T 2017b Biogas production from fruit wastes combined with manure from pigs or buffaloes in an in vitro biodigester system. Livestock Research for Rural Development. Volume 29, Article #151. http://www.lrrd.org/lrrd29/8/soph29151.html

Received 3 August 2017; Accepted 7 August 2017; Published 1 September 2017

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