Fermentation of fruit waste in an anaerobic batch digester

Sangkhom Inthapanya, Daovang Chathaokalor, Sengsuly Phongpanith, R A Leng* and T R Preston**

Faculty of Agriculture and Forest Resource, Souphanouvong University, Lao PDR
inthapanyasangkhom@gmail.com
* University of New England, Armidale NSW, Australia
** Center for Research and Technology Transfer Nong Lam University, Ho Chi Minh City, Vietnam

Abstract

The research presented here is the first step in a study aimed at developing technologiues that use food wastes for biogas production at the level of the household.  An associated objective was to develop household sufficiency in biogas using only human excreta and food waste as the raw material for biogas production. Pilot scale batch biodigesters (liqujd volume 1.5 liters) were used to measure gas production and methane concentration from fresh samples (150g) of banana skin, orange rind, papaya peel and cow manure incubated at 30 days in a water bath at 35°°C. Effuent (850g) from a plug-flow biodigester charged with cow manure was added to the biodigesters at the startup on day 1.

Gas production over the 30 day incubation period was highest with cow manure followed by orange rind, with lowest values for papaya peel and banana skins. From initial values of between 6.5 and 7, the pH of the incubation medium declined dramatically by day 4 to to 4.1-4.3 for papaya peel and banana skins, reaching 3.7 for the orange rind. The methane content of the gas, measured between 21 and 30 days of incubation, was twice as high on cow manure than on the fruit wastes. Future research will focus on the use of a buffering medium at startup, or a gradual introduction of the substrate to limit the rate of the fermentation in the first 1-2 weeks.

Key words: food waste, gas production, greenhouse gas, incubation, methane, pH, substrate

Introduction

Roughly one third of the food produced in the world for human consumption every year approximately 1.3 billion tonnes gets lost or wasted (FAO 2011). Environmentally, food waste leads to wasteful use of chemicals such as fertilizers and pesticides; more fuel used for transportation; and more rotting food, creating more methane one of the most harmful greenhouse gases that contributes to climate change (UNEP 2009). 

Per capita food waste by consumers in Europe and North-America was estimated to be 95-115 kg/year; with smaller quantities (6-11 kg/year) in Sub-Saharan Africa and South/Southeast Asia (FAO 2011). However, there are probably equivalent quantities of vegetative waste (the rinds, skins and leaves of many fruits and vegetables that are not suitable as food but which end up in land-fills with all the associated disadvantages of high transport cost and relese of methane to the armosphere.

In many industrial counntries a proportion of the food waste that normally end up in land-fills is now converted to biogas (ref to recycling in UK). However, there are still the associated high costs of transporting biomass of high moisture content to and from urban centers to recycling plants.

Small scale biogas systems are now widely used in developing countries; however, in general these require appreciable numbers of livestock (usually pigs) to generate the required amount of feedstock.

The research presented here is the first in a series aimed at developing technologiues that use food wastes for biogas production at the level of the household.  An associated objective of our study is to develop household sufficiency in biogas using only human excreta and food waste as the raw material for biogas production.

Materials and methods

Location and duration

This experiment was conducted in the laboratory of the Department of Animal Science of the Faculty of Agriculture and Forest Resource, Souphanouvong University, Luang Prabang province, Lao PDR, during July to August 2013.   

Treatments and experimental design 

The experimental design was completely randomized block arrangement with four treatments and four replications of the following treatments:

• BS: Banana skin

• OR: Orange rind

• PP: Papaya peel

• CM: Cattle manure

Procedures

All the fruit wastes (fresh) were brought from Phousi market in Luang Prabang district. The cattle manure and biodigester effluent were collected from the research farm of theDepartment of Animal Science, Souphanouvong University. The fruit wastes were chopped into small pieces 1-2 cm of length (Photos 1-3) before being added to the fermentation bottles (capacity 1500 ml) which contained fresh biodigester efluent (Table 1). The bottles were incubated at 35 ^°C in a water bath over a total period of 30 days.  The incubation system (Photo 4) was that described by Inthapanya et al (2013).

Photo 1: Papaya peel	Photo 2: Banana skin	Photo 3: Orange rind

Photo 4: The incubation system	Photo 5: Taking samples of digesta for measurement of pH

Data collection and measurements

The gas production was measured every day. The content of methane in the gas was measured after 21, 25 and 30 days. At each time interval, the gas volume was read from the collection bottles and the percentage of methane measured using a Crowcon infra-red analyser (Crowcon Instruments Ltd, UK). The gas collection bottle was filled with water after each measurement. The pH (Accumet, digital pH meter) was taken every day by using a syringe to collect the effluent from the fermentation bottle (Photo 5). 

At the end of the incubation, the DM mineralized was determined by filtering the contents of the fermentation bottle through several layers of cloth that retained particle sizes to at least 0.1mm and then this was dried (100°C for 24 hours) and weighed.

Chemical analyses

Samples of banana skin, orange rind, papaya peel, cattle manure and effluent were analysed for DM and nitrogen (N) according to methods outlined in Ly and Nguyen Van Lai (1997).

Statistical analysis

Treatment effects were analyzed by the General Linear Model (GLM) option in the ANOVA program of the Minitab (2000) Software. Sources of variation in the model were: replicates, treatments and error.

Results and Discussion

Gas production over the 30 day incubation period was highest with cow manure followed by orange rind, with lowest values for papaya peel and banana skins (Table 2). The pattern of gas production differed markedly among the substrates (Figure 1). Gas production on day 1 was much higher for the three fruit wastes than for cow manure. The orange rind continued with high gas production on days 2 and 3 while prduction form banana skins and papaya peel declined dramatically. By contrast, gas production from cow manure increased steadily from a low value on day 1 to a maximum on day 5 followed by a gradual decline over the rest of the incubation period. From day 5 to the end of the incubation, gas  production from cow manure was always higher than from the fruit wastes.

There were major differences in the methane content of the gas, measured between 21 and 30 days of incubation (Figure 3). Concentrations were twice as high on cow manure as on the fruit wastes.  The pattern of the pH changes showed major differences between the cow manure and the fruit wastes. From initial values of between 6.5 and 7, the pH of the incubation medium had declined dramatically by day 4 to 4.1-4.3 for papaya peel and banana skins, reaching 3.7 for the orange rind. The pH on the fruit residues continued to declne slightly reaching values of 3.3-3.6 with the lowest value always for the orange rind. On the cow manure the pH had fallen to 5.3 by day 10, subsequently maintaining this level to the end of the incubation. It is well established that most methanogens will not grow at pH less than 5 (see Ferry 1993) which is the logical explanation for the low methane concentration in the gas produced by the fruit wastes. Future research will require either the use of a buffering medium at startup, or a gradual introduction of the substrate to limit the rate of the fermentation in the first 1-2 weeks, a procedure well established for the management of industrial waste fermenters (Kurien et al 2012).

Figure 1. Changes in pH during 30 d incubation of fruit wastes and cow manure	Figure 2. Changes in gas production during 30 d incubation of fruit wastes and cow manure
Figure 3. Methane content ofd the gas during 30 d incubation of fruit wastes and cow manure	Figure 4. Mean values for DM digested during 30 d incubation of fruit wastes and cow manure

Conclusions

• Gas production over the 30 day incubation period was highest with cow manure followed by orange rind, with lowest values for papaya peel and banana skins.
  
  
• From initial values of between 6.5 and 7, the pH of the incubation medium declined dramatically by day 4 to to 4.1-4.3 for papaya peel and banana skins, reaching 3.7 for the orange rind.
  
  
• The methane content of the gas, measured between 21 and 30 days of incubation, was twice as high on cow manure than on the fruit wastes.
  
  
• Future research will focus on the use of a buffering medium at startup, or a gradual introduction of the substrate to limit the rate of the fermentation in the first 1-2 weeks.

Acknowledgement

The authors acknowledge support for this research from the MEKARN project financed by Sida. Special thanks are given to Miss Amphayvone Sounakean who provided valuable help in the laboratory. We also thank the Department of Animal Science, Faculty of Agriculture and Forest Resources, Souphanouvong University for providing infrastructure support and carry out this research.

References

FAO 2011 Global Food Losses and Food Waste http://www.fao.org/docrep/014/mb060e/mb060e00.pdf

Ferry J G (Editor) 1993 Methanogenesis: Ecology, Physiology, Biochemistry & Genetics. Chapman and Hall, New York, NY10119 http://books.google.com.au/books?id=sYh-W3CItsoC&pg=PA136&lpg=PA136&dq=pH+optima+methanogens&source=bl&ots=UShqpgV4L8&sig=Y7a6hzv9N1sXU4ysTbWx4rY7txs&hl=en&sa=X&ei=_bgeUsPsMKPmiAfAnoEY&redir_esc=y#v=onepage&q=pH%20optima%20methanogens&f=false

Inthapanya S, Preston T R and Leng R A 2012 Biochar increases biogas production in a batch digester charged with cattle manure. Livestock Research for Rural Development. Volume 24, Article #212. http://www.lrrd.org/lrrd24/12/sang24212.htm

Kurian R, Slade J, Holliday M, Liver S and Derjugin W 2012 Avoiding indigestion: start-up of anaerobic digesters. WEAO 2012. Technical Conference, Ottawa, Ontario.

Ly J and Nguyen Van Lai 1997 Laboratory manual. http://www.mekarn.org/labman/Default.htm

Minitab 2000 Minitab user's guide. Data analysis and quality tools. Release 13.1 for windows. Minitab Inc., Pennsylvania, USA.

UNEP 2009 The environmental food crisis, the environment’s role in Averting future food crises, a UNEP rapid response assessment. Editors: Christian Nellemann, Monika MacDevette, Ton Manders, Bas Eickhout, Birger Svihus, Anne Gerdien Prins and Bjørn P. Kaltenborn. http://www.grida.no/files/publications/FoodCrisis_lores.pdf

Received 13 August 2013; Accepted 27 August 2013; Published 4 September 2013

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