Biogas production from fruit wastes combined with manure from pigs or 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 Produccion Agropecauria. Carrera 25 No 6-62 Cali, Colombia
² Cantho University, Vietnam

Abstract

The experiment was conducted to determine the effect on methane production of adding different proportions of fresh fruit waste to manure from pigs or buffaloes as substrate in plug-flow biodigesters. The treatments in a completely randomized 2*5 factorial design with with four replicates were ratios of fresh fruit waste to manure (pig or buffalo) of 0, 25, 50, 75 and 100 (% 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.

Gas production and the concentration of methane in the gas were increased by adding 25% of fruit waste to manure from buffaloes or pigs and then reduced dramatically when the fruit waste comprised 50% or more of the substrate entering the biodigester, the effect being much more dramatic when the manure component of the substrate was from buffaloes rather than pigs. The depressing effect on biodigester pH when fruit waste was the companion substrate was probably due to the high sugar content in the fruit waste.

Key words: methane, pH, plug-flow

Introduction

Residues from citrus fruit are among the most abundant organic wastes with estimated worldwide production of 15 million tonnes per year (Marin et al 2007). Almost 33% of the crop, including oranges, lemons, grapefruit and mandarins, are industrially processed for juice production, where about half of the processed citrus including peels, segment membrane and seeds end up as wastes (Wilkins et al 2007). Currently, parts of the citrus waste are deposited in landfills, constituting severe economic and environmental problems.

In a previous paper (Yen Sopheap et al 2017) we showed that adding vegetable waste to pig and buffalo manure as substrates in a biodigester tripled the methane production when the proportions added to the biodigester were 25% vegetable waste and 75% manure. Higher proportions of vegetable waste resulted in a linear decline in methane production to zero when only vegetable waste was the substrate.

The objective of the present experiment was to determine if the same pattern of gas and methane production would be observed when fruit wastes were combined with pig and buffalo manure.

Material and methods

Location and duration

The experiment was conducted in the experimental area of Svay Rieng University (SRU), in Svay Rieng province, Cambodia, during the dry season from January to February, 2016.

Experimental design

The procedure followed exactly that described in the previous paper (Yen Sopheap et al 2017). In this case the factors in the 2*5 factorial with 4 replications were:

• PM: Pig manure
• BM: Buffalo manure

0, 25, 50, 75 and 100% of the substrate (DM basis)

Experimental system

The biodigesters (Photo 1) were operated with an initial loading rate of 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 the equivalent of 192 ml water. Quantities on a fresh basis are shown in Table 1.

Substrates

The fruit waste was collected from the canteen in the Svay Rieng University and included peels from banana, pineapple, orange, melon, Jack fruit and mango (Photos 2 and 3). It was chopped into small pieces (2-3mm length) then mixed with the pig or buffalo manure according to the quantities indicated in Table 1. Manure from pigs and buffalos were collected from farmer households near the University. The substrates were kept in sealed plastic container for a maximum of 2 days before being added to the biodigesters.

Photo 1. The experimental biodigesters

Photo 2. Mixed fruit waste	Photo 3. Banana peel

Measurements and data collection

The gas volume was read from the collection bottles directly every day over the entire experiment. The pH was measured in the effluent that came out daily from the biodigester. The percentage of methane in the gas was measured every day using a Crowcon Infra-red analyzer (Crowcon Instruments Ltd, UK) (Yen Sopheap et al 2017). The pH of the effluent from the biodigesters was measured using a digital meter.

Chemical analysis

Samples of fruit waste 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 (2000) 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

There were major differences in the composition of the substrates (Table 2). Nitrogen in pig manure was higher than in buffalo manure as expected from the nature of the diets consumed by these two species. The low content of crude protein in fruit waste is in line with the characterits of the fruits producing the wastes; namely high sugar and low protein content.

Gas production

The response to adding fruit waste to the biodigesters was quite different for substrates based on pig manure and those based on buffalo manure (Table 3; Figures 1, 2 and 3). With pig manure the mixtures of 25% fruit waste and 75% pig manure gave much higher gas production that any of the other combinations (Figures 1 and 3). By contrast, with buffalo manure the levels of gas production were low and similar for all combinations of manure and fruit waste (Figures 2 and 3).

Figure 1. Trends in gas production from mixtures of pig manure and fruit waste	Figure 2. Trends in gas production from mixtures of buffalo manure and fruit waste

Figure 3. Mean values over 30 days of gas production from mixtures of manure (pig or buffalo) and fruit waste

Methane content

There was no methane in the gas when fruit waste comprised all or 75% of the substrate (Figures 4 and 5) and this was the same for both pig and buffalo manure. Reducing the fruit waste to 50% of the substrate resulted in methane concentrations of between 5 and 10%, with higher values for the combination of pig manure and fruit waste (Figures 4 and 5). For substrates with 75% or 100% pig manure the methane concentration rose to between 50 and 60% after 30 days; by contrast, with buffalo manure the methane concentration after 30 days was only in the region of 25 to 35% (Figures 5 and 6).

Figure 4. Trends in methane concentration in the gas from mixtures of pig manure and fruit waste	Figure 5. Trends in methane concentration in the gas from mixtures of buffalo manure and fruit waste

Figure 6. Mean values over 30 days of methane content in the gas from mixtures of manure (pig or buffalo) and fruit waste

Methane production

Daily production of methane was much lower for substrates based on buffalo manure compared with pig manure (Figure 9).

Figure 7. Trends in methane production from mixtures of pig manure and fruit waste	Figure 8. Trends in methane production from mixtures of buffalo manure and fruit waste

Figure 9. Mean values over 30 days of methane production from mixtures of manure (pig or buffalo) and fruit waste

pH of the effluent

The pH of the effluent from the biodigesters declined linearly, reaching values between 3 and 4 as the fruit waste replaced the manure and was always lower when the manure was from buffaloes rather than from pigs (Figures 10, 11 and 12). With 25% fruit waste the pH stabilized at 6.0 after 30 days when the manure was from pigs. By comparison, with 25% fruit waste and 75% buffalo manure the pH of the effluent had fallen to levels between 4.5 and 5.0. The depressing effect on biodigester pH when fruit waste was the companion substrate was probably due to the higher sugar content in the fruit waste compared with the vegetable waste used in a previous study. Sugars fement rapidly and were logically the reason for the lowering of pH in the present experiment

The pH in biodigesters affects directly the activities of specific acidogenic (Zhang et al 2012) and methanogenic bacteria (Ghosh et al 2000). The optimal range of pH for methane production is 6.5 – 7.5 according to several researchers (Liu et al 2008; Eckcnfelder 1989; Cheremisinoff 1994).

Figure 10. pH of effluent from mixtures of pig manure and fruit waste	Figure 11. pH of effluent from mixtures of buffalo manure and fruit waste

Figure 12. Mean values over 30 days of pH of the effluent from biodigesters charged with mixtures of manure (pigs or buffaloes) and fruit waste

Conclusions

• Gas production and the concentration of methane in the gas were reduced dramatically when fruit waste comprised more than 25% of the substrate entering the biodigester, the effect being much more dramatic when the manure component of the substrate was from buffaloes rather than pigs.
  
  
• The depressing effect on biodigester pH when fruit waste was the companion substrate was probably due to the higher sugar content in the fruit waste compared with the vegetable waste used in a previous study.

Acknowledgments

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.

References

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

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Received 13 June 2017; Accepted 30 July 2017; Published 1 August 2017

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