Biochar marginally increases biogas production but decreases methane content of the gas in continuous-flow biodigesters charged with cattle manure

Sangkhom Inthapanya and T R Preston*

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

Abstract

Two experiments were carried out to determine the effect of biochar in continuous flow biodigesters (12 litre liquid volume) charged with cattle manure. In experiment 1, in a random block design with two treatments and 4 replicates, the biochar was added at the rate of 1% of the daily dry matter input to the biodigesters. In experiment 2, the design was a 2*2 factorial arrangement of four treatments, the factors being addition or not of biochar; and presence or not of a cloth bag located permanently inside the biodigesters.  For the biochar treatment the biochar was added to the daily manure input or placed inside the cloth bag.

The addition of biochar to the influent of the biodigesters, or by placing it inside a cloth bag, marginally (4 to 5%) increased gas production with a similar degree of reduction in the methane content of the gas. There was no advantage from putting a cloth bag inside the biodigesters as a potential support medium for biofilm formation.

Key words: biofilm, cactus, digester, greenhouse gas

Introduction

In an earlier report from our laboratory (Inthapanya et al 2012), we showed that in a batch digester charged with cattle manure, the addition of 1% biochar (DM basis) increased gas production by 31% over a 30 day incubation period. The concentration of methane in the gas was reduced by 8% but the overall effect of the biochar was a net increases in methane production of 27%. There were no advantages in increasing the biochar concentration to 3%.

The objective of the present study was to observe the effect of the biochar when added to the influent of a continuous-flow biodigester charged with cattle manure.

Materials and methods

Location and duration

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.

Experiment 1

Treatments and experimental design

The experimental design was a random block with four replications of the following treatments:

· BIO: Biochar at 3% of the DM of the influent to the digester

· NOBIO: No biochar added

Procedure

The apparatus and the general procedure of the experiment was similar to that used by Inthapanya et al (2012). In this case the digester was a plastic bottle of 15 liters fitted with inlet and outlet ports (Photo 1). The gas production was measured by water displacement. The liquid volume of the biodigester was 12 litres and the retention time was 20 days.

Manure was collected from local cattle in the farm of Souphanouvong University. They had been fed mixed grasses. The biochar was produced by combusting rice husks in a top-lit updraft (TLUD) gasifier stove; it had a particle size that passed through a 1 mm sieve and was produced at a temperature of 900-1000^oC (Olivier 2010).

On day 1, the influent to the biodigesters was composed of 3 kg fresh cattle manure and 9 liters water (with 6 g biochar for the BIO treatment). On subsequent days the quantities were 600g of the same mixture of manure and water (ie: 150g of manure and 450g of water) plus 0.3 g biochar for the BIO treatment. The incubation was carried out at ambient temperature (25-30°C) over a period of 40 days.

Data collection and measurements

The gas volume was read from the collection bottles directly every day until 40 days. The percentage of methane in the gas was measured after the incubation had proceeded for 7, 14, 21, 28, 35 and 40 days, using a Crowcon infra-red analyser (Crowcon Instruments Ltd, UK).

Statistical analysis

The data 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.

Experiment 2

Treatments and experimental design

The experimental design was a random block with three replications of the following treatments:

· BIO: Biochar at 1% of daily addition of the DM added to the biodigester

· NOBIO: No biochar added

· BIO-CB: 18 g of biochar were put in a cloth bag inside the biodigester on day 1 with no further addition of biochar on subsequent days

· CB: Cloth bag inserted in the biodigester but with no biochar

The substrate was cattle manure and water to give 5% solids as in experiment 1. The bag was made of nylon with pore size of 60 microns and measured 5x8cm (Photo 2). The general procedure and analyses were similar to those in experiment 1.

Photo 1. Experimental biodigester used in bith experimenents	Photo 2. The nylon bags (pore size 60 microns) used in experiment 2

Statistical analysis

The data 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, days, biochar, cloth bag, interaction biochar*bag and error.

Results and Discussion

Experiment 1

Daily gas production and concentration of methane in the gas increased linearly with time after startup of the biodigesters,  reaching peak values after 28 days and thereafter remaining constant (Table 1; Figures 1 and 2). The addittion of biochar led to a small (4.3%) increase in gas production and a similar decrease (5%)  in the concentration of methane in the gas. T

Figure 1. Effect of biochar on daily gas production in continuous-flow biodigesters charged with cattle manure

Figure 2. Effect of biochar on methane concentration in the gas at intervals over 40 days incubation in continuous-flow biodigesters charged with cattle manure

Experiment 2

Addition of biochar to the biodigesters led to an increase in gas production and a reduction in the methane content of the gas (Tables 2 and 3; Figures 4 and 5). The relative changes (4.6% increase in gas and 6% reduction in methane) were similar to those observed in experiment 1. There was no  advantage in putting a cloth bag in the biodigesters and no interaction between biochar and the cloth bag indicating that the effect of the biochar was similar irrespective of whether it was added daily or suspended in a cloth bag. The linear increase in daily gas production and concentration of methane in the gas up to 28 days (Figures 3 and 4), and the subsequente plateau in both criteria from 28 to 40 days, were similar to what was observed in experiment 1.

Figure 3. Mean values for gas production in biodigesters charged with cattle manure and supplemented with biochar

Figure 4. Mean values for methane concentration in the gas from biodigesters charged with cattle manure and supplemented with biochar  or not supplemented	Figure 5. Relative effects on gas production of addition of biochar and suspension of a cloth bag in the biodigester (In the No Cloth Bag treatment the biochar was added daily at 1% of DM input; in the Cloth Bag treatment the biochar was put inside the bag in quantities equal to the total amount of biochar added in the No Cloth Bag treatment)

The effect of the biochar in the two experiments with continuous flow biodigesters was in marked contrast to the effect of biochar in a batch biodigester when the increase in gas production after 30 days was 31% and the reduction in methane concentration was 8% (Inthapanya et al 2012). The behavior of the biochar in a batch compared with a continuous system is likely to be quite different. In the batch system the biochar remains in contact with the substrate during the whole of the incubation. This can be expected to facilitate biofilm formation (Leng et al 2012). By contrast, in the continuous biodigester it is probable that the biochar flowed out of the biodigester at the same rate as the fermented substrate. In this situation, it is assumed that biofilm formation would not be facilitated to the same extent as in the batch digester.  It would appear that putting a cloth bag in the biodigester was also not effective in supporting biofilm formation.

Research from Mexico indicates that fibrous plant material (dried stems of the cactus Opuntia imbricata) facilitates biofilm formation in batch digesters charged with aromatic compounds (Chávez et al 2008) or waste-water from a beer factory (Martinez Amador et al 2011). The rough and porous surface of the cactus stems was thought to be a benficial feature of this material for  biofilm formation. This would appear to be a promising line of research for improving the productivity of biodigesters charged with livestock manure.

Conclusions

• Addition of biochar to the influent of continuous flow biodigesters, or by placing it inside a cloth bag, marginally (4 to 5%) increased gas production and reduced the methane content of the gas.
  
  
• There was no advantage from putting a cloth bag inside the biodigesters as a potential support medium for biofilm formation.

Acknowledgement

The authors acknowledge support for this research from the MEKARN project financed by Sida. Special thanks are given to Mr Sengsouly Phongphanith, Mr Touvieu Xayger and Mr Thonglon 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 and carry out this research.

References

Chávez A M, Balagurusamy N and Rodríguez-Martínez J 2008 Testing the Efficiency of Biofilms Formed on Natural Substratum, Coyonoxtle (Opuntia Imbricata), for the Anaerobic Degradation of Aromatic Compounds. Engineering in Life Sciences. Volume 8, Issue 4, pages 425–430,

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

Martínez Amador S Y, Garza-García, Rodríguez de la Garza J A and Rodríguez Martínez J 2011 Nitrate Reduction, Sulfate Reduction and Methanogenesis Interrelation in Fixed and Suspended Bed Batch Reactors. Current Research Journal of Biological Sciences 3(6): 591-596 http://maxwellsci.com/print/crjbs/v3-591-596.pdf

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

Olivier P 2010 The Small-Scale Production of Food, Fuel, Feed and Fertilizer; a Strategy for the Sustainable Management of Biodegradable Waste. http://www.mekarn.org/workshops/pakse/html/olivier.docx

Received 2 September 2013; Accepted 6 October 2013; Published 1 November 2013

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