Biochar increases biogas production in a batch digester charged with cattle manure

Sangkhom Inthapanya, T R Preston* and R A Leng**

Faculty of Agriculture and Forest Resources, Souphanouvong University,
Luang Prabang, Lao PDR
inthapanyasangkhom@yahoo.com
* Finca Ecologica TOSOLY, AA48 Socorro, Colombia
** University of New England, Armidale NSW, Australia

Abstract

Two in vitro incubation experiments were conducted to test the hypothesis that  biochar  would serve as support media for biofilm development in a  biodigester and would as a result  increase the yield of biogas whether added separately or enclosed in a nylon bag  The treatments in experiment 1 were: control (no biochar), biochar added at 1% of the substrate DM in the biodigester, biochar added at 3% of the substrate DM in the biodigester. The substrate was fresh manure from cattle fed dried cassava root, fresh cassava foliage and urea. Proportions of water and manure were arranged so that the manure provided 5% of the solids in the biodigester. Gas production was measured daily over the fermentation period of 30 days; methane in the gas was measured after 21 and 28 days. In experiment 2, a 2*2 factorial arrangement with 4 replications was used to compare level of biochar: 1% of solids in the digester or none; and presence or absence of a cloth bag in the biodigester. The fermentation was followed over 21 days with daily measurement of gas production and content of methane in the gas at the end of the fermentation.

In experiment 1, incorporation of 1% (DM basis) of biochar in the biodigester increased gas production by 31% after 30 days of continuous fermentation; there were no benefits from increasing the biochar to 3% of the substrate DM. The methane content of the gas increased with the duration of the fermentation (24% higher at 28 compared with 21 days) but was not affected by the presence of biochar in the incubation medium.  In experiment 2, adding 1% of biochar (DM basis) to the substrate increased gas production by 35%, reduced methane content of the gas by 8%, increased the DM solubilized (by 2%) and increased methane production per unit substrate solubilized by 25%. Presence of the cloth bag increased gas production when it also contained biochar but decreased it when added to the biodigester without biochar. There was a similar interaction for methane produced per unit substrate solubilized.

Key words: Biofilm, greenhouse gas, in vitro incubation, methane

Introduction

Recent research with biochar in our laboratory has been focused on the concept that biochar, which has a large surface area to weight,  functions as a focal point for attachment of microbes and the formation of structured  microbial consortia enclosed in a biofilm matrix increasing the efficiency of microbial fermentation in the rumen,  both at the in vitro level  (Leng et al 2012a,b) and in vivo (Leng et al 2012c). In this paper we report on the effects of biochar on fermentation and mineralisation of organic matter when incorporated in laboratory scale batch biodigestors charged with cattle manure.

The hypotheses that were tested were: (i) that the presence of biochar in the biodigester would stimulate the development of biofilms and therefore increase rate of mineralisation of organic matter  and the production of biogas; and (ii) that it would change the potential utilization of end products, particularly through providing habitat for methanogenic and/or methanotrophic microbial consortia

We also wish to examine the possibility that biochar that had been incubated within the bulk fluid of the biodigester could become sufficiently impregnated with biofilm matrix enclosed microbes that it then could be used to increase the rate of methane production when incubated with fresh manure. To accomplish this we needed to harvest the biochar and  this was achieved by enclosing the biochar in a porous nylon cloth bag that was relatively easily  retrieved, and is the subject of on-going research.

Materials and methods

Location and duration

Two in vitro incubation experiments were conducted in the laboratory of the Faculty of Agriculture and Forest Resource, Souphanouvong University, Luang Prabang province, Lao PDR, during August to October 2012.   

Experiment 1

Treatments and experimental design 

The experimental design was a random block with 4 four replications of the following treatments:

• BC-0: Control treatment (no biochar)
• BC-1: Biochar added at 1% of the substrate in the biodigester (DM basis)
• BC-3:  Biochar added at 3% of the substrate DM in the biodigester

Procedure

The incubation apparatus (Photo 1) and the general procedure were as described by Inthapanya et al (2011).

Manure was collected from cattle in the farm of Souphanouvong University. They had been fed dried cassava root, fresh cassava foliage and urea (Leng et al 2012c). 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^°C (Olivier 2010). The ingredients in the substrate (Table 1) were put in the incubation bottles (capacity 1400ml) which were incubated at 35 ^°C in a water bath over a total period of 30 days.

Data collection and measurements

The gas volume was read from the collection bottles directly every day until 30 days. The percentage of methane in the gas was measured after the incubation had proceeded for 21 and 28 days, using a Crowcon infra-red analyser (Crowcon Instruments Ltd, UK).  At the end of the experiment, the residual insoluble substrate in the incubation bottle was determined by filtering the contents through several layers of cloth that retained particle sizes to at least 0.1mm and then this was dried (100°C for 36 hours) and weighed.

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, level of biochar and error.

Experiment 2

Treatments and experimental design 

The design was a 2*2 factorial arrangement with four replications of the following treatments:

• BC: Biochar 1% of substrate DM
• BC-CB: Biochar at 1% of substrate DM enclosed in a porous nylon cloth bag
• CB: Cloth bag and no biochar
• CTL: Only incubation medium (no cloth bag and 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.

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, effect of biochar, effect of  bag, interaction biochar*bag and error.

Results

Experiment 1:

On all treatments, the rate of gas production increased during the 14 to 30 day period of the incubation compared with the startup period of 1 to 14 days (Figure 1). Incorporation of 1% (DM basis) of biochar in the biodigester increased gas production by 31% after 30 days of continuous fermentation; there were no benefits from increasing the biochar to 3% of the substrate DM (Table 2; Figures 1 and 2). Methane production increased with the duration of the fermentation (64% higher at 28 compared with 21 days) but was not affected by the presence of biochar in the incubation medium (Figure 3).  

Figure 1. Development of gas production in a batch biodigester charged with cattle manure to which was added 1 or 3% of biochar  (DM basis)	Figure 2. Effect of level of biochar on cumulative gas production over 30 days in a batch biodigester charged with cattle manure

Figure 3. Effect of biochar on methane production after 21days and 28days of fermentation in a batch biodigester (BC0 control with no biochar; BC1 biochar at 1% of substrate DM; BC3 biochar at 3% of substrate DM

Experiment 2:

On all treatments, the rate of gas production increased during the latter 12 to 21 day period of the incubation compared with the startup period of 1 to 12 days (Figure 4). Adding 1% of biochar (DM basis) to the substrate (cattle manure) in the biodigester increased gas production by 35%, reduced methane content of the gas by 8%, increased slightly the DM solubilized (by 2%) and increased methane produced per unit substrate solubilized by 25% (after 21 days of incubation) (Table 3; Figures 5 and 7). Presence of the nylon bag increased gas production when it also contained biochar but decreased it when put in  the biodigester without biochar (Figures 4 and 6; P=0.016). There was a similar interaction for methane produced per unit substrate solubilized (Figure 8; P=0.044). 

Figure 4. Development of gas production in a batch biodigester charged with cattle manure. Biochar was added at 1% of substrate DM mixed with the substrate (BC) or enclosed in a cloth bag (BCCB). Remaining treatments were the cloth bag without biochar (CB) and the control with neither biochar nor the cloth bag (CTL).

Figure 5. Effect of presence of  biochar   in a cloth bag or freely added to the substrate (no cloth bag) on biogas production after 21 days in a batch biodigester charged with cattle manure	Figure 6. Effect of cloth bag or no cloth bag  and added biochar on biogas production  after 21days  in a batch biodigester charged with cattle manure

Figure 7. Effect of presence of  biochar in a nylon cloth bag or freely added to the substrate (no cloth bag) on methane production after 21days in a batch biodigester charged with cattle manure	Figure 8. Effect of presence of  biochar in a nylon cloth bag or freely added to the substrate (no bag) on methane produced per unit substrate solubilized in a batch biodigester charged with cattle manure

Discussion

Within any anaerobic ecosystem enriched with organic matter, the complex of organic matter is degraded into methane and carbon dioxide. Fermentative bacteria initiate the catabolism producing acids and alcohols which are then readily utilised by acetogenic bacteria. Finally methanogens obtain their energy requirements from converting acetate, carbon dioxide and hydrogen to methane. These reactions require at a minimum three groups of bacteria and Archae and there is an interdependence of each group on the effective growth characteristics of each other.

The breakdown of complex organic matter is largely carried out by microbial consortia in organised and layered biofilm communities (see Costerton 2007). In biodigesters and waste water treatment plants biofilms attach to the surfaces, either in the insoluble organic matter or on the surfaces of the solid materials suspended in the  bulk fluid fluid (see Lewandowski  and Boltz 2011). Where there are few surfaces for biofilm development, the microbial communities may form self-aggregating granules with layered populations of microbes within the consortia (Thiele et al 1988).

The need for surfaces for microbial growth is now well recognised (Costerton 2007). From previous studies with rumen digesta it appears that providing inert materials in anaerobic situations may enhance mineralisation and change the microbial ecosystem. In the present series of studies undertaken in our laboratory, we hypothesised that the biodigester process would be enhanced by inert materials with large surface areas relative to their weight by providing immediate habit for the microbial consortia involved in mineralisation of animal manure. In the present studies biochar, after  a lag period increased the rate of breakdown of organic matter and in some studies the relative net amounts of methane released. We speculate that this is due to a better habitat where microbes can establish in biofilm-associated colonies. 

The growth rate of acetogens and methanogens depends on the efficiency  of hydrogen transfer. Thus anaerobic flocs could be considered as homogeneous defined biocatalysts composed of an immobilized consortium of syntrophic acetogenic and methanogenic bacteria. If biochar provides increased habitat for close association of bacteria or they replace the development of granules or flocks of the different trophic groups, it should stimulate the overall conversion rate and growth rates in the ecosystem (Thiel et al 1988). A bacterial immobilization is often used for biofilm, floc, and granule formation in advanced anaerobic waste treatment processes (Chartrain et al 1987) with resultant  better process stability, and a lower rate of  washout of slow-growing bacteria.  A greater biomass of methanogenic  and acetogenic bacteria/Archae may also overcome  any temporary inhibitory effects of any reaction intermediate build up within  aggregates on biochar or in flocks/granules (Ozturk et al 1989).

It was recently reported that biochar when added to anaerobic digesters apparently becomes impregnated with  particular methanogens resembling methanosarcina (Anonymous 2012). However, a similar effect had already been reported in 2006 by Japanese researchers who described  immobilization and accumulation of methanogens on bamboo charcoal incubated with anaerobic sludge (Nomura et al 2006). Charcoal is in effect  biochar produced at a lower temperature and therefore with less pronounced effects as a source of biofilm, compared with biochars produced at higher temperatures. For example, as a soil amender,  biochar produced in an updraft gasifier stove supported a 53%  increase in biomass yield compared with only 24% increase when charcoal was used (Southavong et al 2012). 

The fact that biochar, exposed to anaerobic digestion of biomass waste, becomes impregnated with methanogens,  may mean that it could be be re-usable and harvested for inoculating the biodigestor at the site of entry of the substrate (manure), potentially decreasing the time lag to maximum biogas production (see Figures 1 and 4). Studies are under way to test whether  biochar that has been through a biodigester can be reused directly or after drying and whether there is an added benefit to reusing such materials.

An interesting feature of the present study was that the biochar effect was maintained when the biochar was enclosed in a nylon bag thus the effects of biochar in enhancing biogas production appear to be a stimulation of the metabolism of  soluble materials (eg: volatile fatty acids such as acetate), increasing their rate of degradation to hydrogen and carbon dioxide and actively removing the hydrogen by reduction to methane.

Conclusions

• Incorporation of 1% (DM basis) of biochar in a batch biodigester increased gas production by 31% after 30 days of continuous fermentation. There were no benefits from increasing the biochar to 3% of substrate DM. The methane content of the gas increased with the duration of the fermentation (24% higher at 28 compared with 21 days) but was not affected by the presence of biochar in the incubation medium.
  
  
• Locating a nylon cloth bag in the biodigester increased gas production after 21 days fermentation, when the bag also contained biochar, but decreased it when added to the biodigester without biochar. Presence of biochar in the substrate reduced methane content of the gas by 8%, increased the DM solubilized by 2%, and increased methane produced per unit substrate solubilized by 25%.
  
  
• It may be possible to use the ”activated’ biochar as an inoculant to further enhance biogas production.

Acknowledgement

The authors acknowledge support for this research from the MEKARN project financed by Sida. Special thanks are given to Mr Sengsouly Phongphanith, Mr Soulikham Thipsomephane and Mr Phouvanh Keophavieng, 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.  

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Received 10 October 2012; Accepted 2 November 2012; Published 2 December 2012

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