 |
 |
 |
| Photo 1. Biodigester system | Photo 2. The orange peel used as substrate | Photo 3. The liquidizer used to process the orange peel |
| Livestock Research for Rural Development 25 (11) 2013 | Guide for preparation of papers | LRRD Newsletter | Citation of this paper |
In the first of two experiments, water or effluent (from a working biodigester charged with cow manure) were used to dilute ground fresh orange peel, prior to adding it daily to eight experimental biodigesters, made from recycled water bottles. On day 24 of fermentation, cassava leaf meal or urea were added to 4 of the biodigesters (2 on the water diluent and 2 on the effluennt diluent). Charging of the biodigesters as in days 1 to 23 was continued for a further 15 days. pH and gas production were recorded daily; methane in the gas was measured on days 7, 14, 21, 28 and 35 days. In experiment 2, 12 similar biodigesters were charged continuously with orange peel as in experiment 1. Cassava leaf meal and urea, separately or together, were added to 8 of the biodigesters; 4 biodigesters received no additive. Measurements were the same as in experiment 1.
In experiment 1, the rate of gas production increased linearly up to 13 days, and then with a curvilinear increase from 13 to 23 days, and was higher when the biodigester was started with effluent (from another biodigester charged with cattle manure) than when only water was used as diluent. pH was lower when effluent was used as the startup diluent (6.43 versus 6.65). There was an immediate positive impact on rate of gas production when either urea or cassava leaf meal was added to the biodigester (on day 24) with the maximum effect of the additives (+17%) reached after a further 7 days. The methane content of the gas increased with days of incubation and was higher when effluent was used as the diluent at startup, and was increased by addition of cassava leaf meal or urea. In experiment 2, addition of either urea or cassava leaf meal, singly or in combination, increased the pH, rate of gas production and the content of methane in the gas. There were no differences in gas production and methane content of the gas between urea and cassava leaf meal added seperately but gas production was further increased when the two additives were combined.
Key word: biogas, cattle manure, effluent, fermentation, pH, recycled water bottles
We believe there is a need to develop the potential for biogas production systems at small-holder level which use food waste as the substrate and do not require manure produced by livestock. Such systems could also be incorporated in urban dwelling houses. The previous report from our laboratory, using a variety of fruit residues as substrate, showed that too rapid loading of the biodigesters led to a rapid fall in pH below 4.5, with resultant decreases in rate of gas production and the methane content of the gas, compared with use of cow manure as substrate (Sangkhom et al 2013a).
In the present study we hypothesized that using effluent from a functioning biodigester charged with cattle manure would have beneficial effects of on the fermentation, compared with use of water as the initial diluent; and that there would be advantages from supplementing the fruit waste (orange peel) with a source of fermentable nitrogen (urea) and/or a balanced source of nutients as from cassava leaf meal.
Two experiments were conducted in the farm of the Department of Animal Science of the Faculty of Agriculture and Forest Resource, Souphanouvong University, Luang Prabang province, Lao PDR, from December 2012 to March 2013.
The experiment was carried out in two steps;
In step 1 the treatments were:
Water: As diluent for the cow manure used as substrate
Effluent: As diluent for the cow manure
In step 2 the treatments were modified:
Urea (1.5 g), cassava leaf meal (15g) , or nothing, were added to the water and effluent treatments on day 23
The apparatus was similar to that used by Inthapanya et al (2012). In this case the digester was a plastic bottle of 1.5 liters fitted with inlet and outlet ports (Photo 1). The substrate was added every day, and then the inlet and outlet were covered by clay to prevent any leakage of gas. The gas production was measured by water displacement. The liquid volume of the biodigester was 1 litre and the retention time was 20 days.
|
|
|
| Photo 1. Biodigester system | Photo 2. The orange peel used as substrate | Photo 3. The liquidizer used to process the orange peel |
Fresh orange peel was collected from the market and chopped into small pieces (Phot 2) and then ground as a suspension in a kitchen liquidizer (Photo 3). The effluent was collected from a biodigester in the farm of the department that used cattle manure as substrate.
On day 1, the quantities of water (or effluent) were 750ml. Orange peel was put into each flask at the rate of 25g fresh basis on day 1. On subsequent days, the additions were 50 ml of a suspension (in water or effluent) containing 12.5 fresh orange peel (2.5g DM).
On the 23rd day, urea (1.5g) or cassava leaf meal (15 g) were added to two of the replicates (one each from the water and effluent treatments).
The gas volume was read from the collection bottles directly every day over the entire experiment that lasted for 35 days. The pH was measured in the effluent that came out daily from the biodigester. The percentage of methane in the gas was measured after the incubation had proceeded for 7, 14, 21, 28 and 35 days, using a Crowcon infra-red analyser (Crowcon Instruments Ltd, UK).
The data were analyzed by the General Linear Model (GLM) option for repeated measures in the ANOVA program of the Minitab (2000) software. Sources of variation in the model were:
Step 1: Diluent (water or effluent), days, interaction (diluent*days) replicates and error
Step 2: Additive (urea or cassava leaf meal or none), days, interaction days*additive) (the two diluents were treated as replicates as there was no interaction between diluent and additive).
The experimental design was a random block with three replications of the following treatments applied as additives to the basal suspension of orange peels :
CTL: No additive
Urea: Urea at 3% of DM of the substrate
CLM: Cassava leaf meal at 30% of the substrate DM
Urea-CLM: Urea 1.5 % and cassava leaf meal 15% of substrate DM
The apparatus and the general procedure of the experiment 2 were similar to those used in the first experiment.
On day 1, the quantities were water (or effluent) 750m and orange peel suspension 250 ml (contained 50 g of orange peel DM).
On day 2 and subsequent days, each biodigester received:
CTL: 50ml of suspension containing 2.5g DM of orange peel (ie: 12.5 fresh orange peel) suspended in water or effluent
Urea: 50ml of suspension containing 2.425 orange peel DM and 0.075 g urea
CLM: 50ml of suspension containing 0.75g cassava leaf meal DM and 1.75 g orange peel DM
Urea-CLM: 50ml of suspension containing 0.375 cassava leaf meal DM and 0.0375 g urea and 2.13 g orange peel DM
The gas volume was read from the collection bottles directly every day until 28days. The pH was measured by digital mater every day from effluent that came out of the biodigester. The percentage of methane in the gas was measured after the incubation had proceeded for 7, 14, 21 and 28 days, using a Crowcon infra-red analyser (Crowcon Instruments Ltd, UK).
The data were analyzed by the General Linear Model (GLM) option for repeated measurements in the ANOVA program of the Minitab (2000) software. Sources of variation in the model were: supplements, days, interaction supplements*days and error.
Experiment 1:
In the first part of experiment 1, the rate of gas production increased linearly up to 13 days, and then with a curvilinear increase from 13 to 23 days, and was higher when the biodigester was started with effluent (from another biodigester charged with cattle manure) than when only water was used as diluent (Figure 1). pH was lower when effluent was used as the startup diluent (Table 1; Figure 2). The methane content of the gas increased with days of incubation and was higher when effluent was used as the diluent at startup (Table 1).
 |
 |
| Figure 1. Gas production from continuous flow biodigester charged with orange peel and inoculated at startup with biodigester effluent or not inoculated (water) |
Figure 2. pH in the biodigester was lower when effluent rather than water was used as startup diluent |
|
Table 1. Mean values for pH of biodigester and methane content of the gas according to startup diluent and days from startup |
|||||
|
|
Diluent at startup |
||||
|
|
Water |
Effluent |
SEM |
p |
|
|
pH |
6.65 |
6.43 |
0.0050 |
<0.001 |
|
|
Gas, ml/d |
300 |
368 |
1.90 |
<0.001 |
|
|
Methane, % |
10.5 |
11.5 |
0.295 |
0.030 |
|
|
|
Days of incubation |
||||
|
|
7 |
10 |
21 |
SEM |
p |
|
Methane, % |
7.75a |
11.9b |
13.4c |
0.36 |
0.001 |
|
abc Means without common letter differ at p<0.05 |
|||||
There was an immediate positive impact on rate of gas production when either urea or cassava leaf meal was added to the biodigester (on day 24) with the maximum effect of the additives (+17%) reached after a further 7 days (Figures 3 and 4; Table 2). The biodigesters reached their maximum rate of gas production 31 days after startup, equivalent to 65% of the liquid volume of the biodigester and 50ml/g of DM added daily to the biodigester. The methane content of the gas was increased by 17% by adding cassava leaf meal and by 27% when urea was added. The pH in the biodigesters was higher when urea or cassava leaf meal were added to the biodigesters (Figure 5).
 |
 |
| Figure 3. Gas production from continuous flow biodigester charged with orange peel after addition (on day 24) of either urea or cassava leaf meal (CLM) |
Figure 4. Effect on methane production after adding cassava leaf meal and urea |
|
Table 2. Mean values for pH, gas production and methane content in the gas after addition of urea and cassava leaf meal to the biodigester |
|||||
|
None |
CLM |
Urea |
SEM |
p |
|
|
pH |
6.67 a |
6.74 b |
6.76 b |
0.020 |
0.004 |
|
Gas, ml |
547a |
622b |
628 b |
16 |
0.001 |
|
Methane |
27 a |
31.5 b |
34.5 c |
1.11 |
0.001 |
|
ab Means without common letter differ at p<0.05 |
|||||
 |
| Figure 5. Effect of adding ureaor cassava leaf meal on pH in the biodigesters charged with orange peel |
Experiment 2:
Addition of either urea or cassava leaf meal, singly or in combination, to biodigesters charged with orange peel increased the pH, rate of gas production and the content of methane in the gas (Table 3; Figures 6-8). There were no differences in gas production and methane content of the gas between urea and cassava leaf meal added seperately but gas production was further increased when the two additives were combined.
|
Table 3. Mean values for gas production and methane content of the gas from biodigesters charged with orange peel and supplemented with urea or cassava leaf meal singly or in combination |
||||||
|
None |
CLM |
Urea |
Urea-CLM |
SEM |
p |
|
|
pH |
6.4a |
6.56b |
6.54b |
6.55b |
0.0054 |
<0.001 |
|
Gas, ml/d |
138a |
166b |
165b |
172c |
1.24 |
<0.001 |
|
Methane, % |
11.6a |
16.0b |
16.6b |
17.4b |
0.49 |
<0.001 |
|
abc Means without common letter differ at p<0.05 |
||||||
 |
| Figure 6. Effect of adding urea or cassava leaf meal singly or together on gas production from fermentation of orange peel |
 |
 |
| Figure 7. Effect of adding urea or cassava leaf meal singly or
together on gas production from fermentation of orange peel |
Figure 8. Effect on methane production after adding cassava leaf meal and urea singly or together to a biodigester charged with orange peel |
The advantages in gas production and methane content from using effluent rather than water as the initial diluent were relatively small and appeared to decrease with increase in fermentation time. Thus use of effluent as diluyent is not a pre-requisite for efficient function of continuos flow biodigesters charged with orange skins as substrate. The same probably applies to other sources of food waste. The considerable increases in gas production and methane concentration from adding a source of fermentable nitrogen were to be expected as all the organisms involved in degradation of the original organic matter and subsequent production of methane require a source of fermentable nitrogen and orange peels are low in N content (as are most other fruit wastes). The lack of any benefit from supplying a wider range of nutrients, as provided by the cassava leaf meal, suggests that the fruit wastes contain sufficent amounts of micro-nutrients such as minerals and vitamins.
It is surprising that there is sufficient and appropriate consortia of microbes in the orange peel or water to immediately commence to grow when introduced into the closed biodigesters. The increasing rate of gas production and the rising pH with time indicate that anaerobic organisms must exist in quite large numbers even under aerobic conditions on the orange peel and can rapidly take advantage of the changed growth conditions. These organisms presumably survive on the orange peel or water and immediately begin to grow as they are subjected to biodigester conditions. As Costerton (2007) discusses perhaps these exist as ultramicrobacteria (UMB) which explode into growth when given the right conditions.
Citrus peels contain the essential oil, D-limonene which is known as an anti-microbial agent (Mixuki et al (1990). They have been demonstrated to be a potential substrate for industrial-scale biogas production following solvent extraction of the limonene (Martin et al 2010). However, attempts to improve methane yield from orange peel by prior extraction with a range of solvents gave variable results in the study reported by Huing Nguyen 2010). It appeared that residual solvent in the extracted peel could be as toxic as the limonene oil, while the extraction process often resulted in loss of volatile solids from the orange peels.
Longer-term studies are needed to confirm if the relatively low methane concentrations (<20%) observed ur studies are a permanent limitation of biogas produced from citrus fruit wastes (perhaps owing to the essential oils present in the orange peels) or if there will be further improvement in methane yields with extended times of fermentation. The other issue is that, at household level, citrus peel will not be the sole substrate, as in practice it will be accompanied by other sources of food waste as well as human excreta. A similar approach was used by Özmen and Aslanzadeht (2009) who reported that mixing citrus peels with other sources of municipal waste was a feasible solution to the limonene problem in industrial biodigesters.
Future studies in our laboratory will aim to develop response curves to levels of citrus peel so as to establish optimum levels of this resource in the overall mix of substrates for household biogas production
The authors acknowledge support for this research from the MEKARN project financed by Sida. Special thanks are given to Mr Jakkapan and Mr Souvanthon who provided valuable help in the farm. We also thank the Department of Animal Science, Faculty of Agriculture and Forest Resources, Souphanouvong University for providing infrastructure support to carry out this research.
Costerton J W 2007 The biofilm primer 1. In 'Springer Series on biofilms.' (Springer: Berlin, Heidelberg, New York)
Huing Nguyen 2010 Biogas Production from Solvent Pretreated Orange Peel. Master of Science Thesis in the Master’s Program MPISC. Chalmers University of Technology, Sweden http://publications.lib.chalmers.se/records/fulltext/157557.pdf
Inthapanya S, Chathaokalor D, Phongpanith S, Leng R A and Preston T R 2013 Fermentation of fruit waste in an anaerobic batch digester. Livestock Research for Rural Development. Volume 25, Article #155. http://www.lrrd.org/lrrd25/9/sang25155.html
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
Martin M A, Siles J A, Chica A F and Martin A 2010 Biomethanization of orange peel waste. Bioresource Technology 101(23):8993-9 http://www.ncbi.nlm.nih.gov/pubmed/20655741
Minitab 2000 Minitab user's guide. Data analysis and quality tools. Release 13.1 for windows. Minitab Inc., Pennsylvania, USA.
Mizuki E, Akao T and Saruwatari T 1990 Inhibitory Effect of Citrus unshu Peel on Anaerobic Digestion. Biological Wastes, 33, 161-168 http://bada.hb.se/bitstream/2320/5402/1/Aslanzadeh,%20%C3%96zmen.pdf
Özmen P and Aslanzadeh S 2009 Biogas production from municipal waste mixed with different portions of orange peel. University of Borĺs: Borĺs. Master thesis. University of Borĺs SE-501 90 BORĹS http://bada.hb.se/bitstream/2320/5402/1/Aslanzadeh,%20%C3%96zmen.pdf
Received 8 October 2013; Acccepted 20 October 2013; Published 1 November 2013