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| Photo 1. Plug-flow configuration | Photo 2. Dome configuration |
Photo 3.
The Crowcon meter used to measure the methane in the gas |
| Livestock Research for Rural Development 28 (3) 2016 | Guide for preparation of papers | LRRD Newsletter | Citation of this paper |
The study aimed to measure gas yield, methane content of the gas and substrate pH in experimental biodigesters designed according to the "plug-flow” and “dome” configuration. The biodigesters were made from recycled "pet" plastic water bottles of 5 liter capacity. The liquid volume was set at 4 liters with a hydraulic retention time of 20 days. The influent (200 ml day-1) was fresh manure, from growing cattle fed Elephant grass (Pennisetum purpurreum) and concentrates, mixed with water to give 4% total solids. The experiment lasted 20 days.
On average the gas production was 54% greater in the "plug-flow" configuration than in the "dome" configuration. The percentage of methane in the gas increased linearly over the first 14 days of the incubation with higher values for the "plug-flow" than the "dome" configuration. After 14 days the concentration of methane stabilized at slightly over 50% with no differences between the two configurations. pH values were similar in the two configurations, falling from an initial value of 7.0 to 5.5 over the first 5 days, then stabilizing at 7.0 for the rest of the 20 day test period. Overall, mainly because of the greater daily gas production, the production of methane was 78% greater for the "plug-flow" than for the "dome" configuration.
Key words: cattle, effluent, gas production, liquid volume, manure, retention time
Several designs of biodigesters in different sizes have been developed, including floating canopy, fixed dome in bricks or concrete, plug flow digesters with different flexible materials such as low density and high-density polyethylene (HDPE). Fixed dome digesters are common in China, where digesters are built underground in the shape of a dome using bricks or concrete. However, the formation of pores from the bricks causes gas leakage as well as contamination of ground water, which is sometimes a drawback with these reactors. Recently, plug flow digesters made of polyethylene have been gaining attraction due to their cost-effectiveness and simple installation (Duong Nguyen Khang and Le Minh Tuan 2002). However, the polyethylene film is easily damaged and the working life of these biodigesters is often limited to 2 to 3 years.
There are many surveys of the benefits of biodigesters (eg: Nguyen Ngoc Thuy 2010) but there appear to be no studies on the comparative performance between fixed dome and plug flow biodigesters.
Based on the above discussion, it was considered to be important to investigate the gas yield, pH and methane content of the gas in biodigesters charged with cattle manure and designed in the two configurations of “plug-flow” and “dome”.
The experiment was carried out in the farm of the Research and Technology Transfer Center, in Nong Lam University, Ho Chi Minh City, over a 20 day period in June 2015.
The treatments in a random block design were two configuration of an in vitro biodigester system:
The apparatus and the general procedure were similar to what was used by Inthapanya et al (2012). However, in this case the experimental biodigesters were made from recycled 5 liter "pep" plastic water bottles, arranged to simulate "plug-flow" (Photo 1) or "dome" (Photo 2) configurations. Each system was fitted with an inlet port for the substrate, an outlet port for the effluent and a gas outlet leading to a 1 liter "pep" water bottle calibrated at 50ml intervals, with the bottom cut-away, and suspended in a second 5 liter water bottle with the top removed(Photo 3). This bottle was filled with water so that gas production could be measured by water displacement. Each "system" was replicated 4 times according to a completely randomized block design. The liquid volume of the bio-digester was 4 liters and the retention time was 20 days.
Manure was collected from dairy cattle which has been fed Elephant grass (Pennisetum purpurreum) and high protein concentrate supplement. On the first day, the influent to the bio-digester was composed of 1156g fresh manure and 2844ml water. On 20 subsequent days the quantities were 200g of the same mixture of manure and water (ie: 57.8g of manure and 142.2g of water). The incubation was carried out at ambient temperature (25 – 30oC).
The gas volume was read from the collection bottle directly every day over a period of 20 days. The percentage of methane in the gas was measured daily over the same using a Crowcon infra-red analyzer (Crowcon Instruments Ltd, UK).
The data were analyzed by the General Linear Model (GLM) option in the ANOVA program of the Minitab 16 software (Minitab 2015). Sources of variation in the model were: replicates, treatments and error.
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| Photo 1. Plug-flow configuration | Photo 2. Dome configuration |
Photo 3.
The Crowcon meter used to measure the methane in the gas |
Gas production was stable after the 2nd day of the experiment and was 54% higher for the plug-flow configuration (Table 1; Figure 1). The methane content of the gas was 16% higher for the plug-flow configuration (Figure 2) resulting in an overall increase in production of methane of 78% in the plug-flow compared with the “ dome” configuration (Figure 3).
| Table 1. Mean values for daily gas amd methane production, and percentage of methane in the gas from plug-flow and "dome" bio-digesters | ||||
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Plug-flow |
Dome |
SEM |
p |
|
|
Gas production (ml day-1) |
718. |
465 |
4.97 |
<0.001 |
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Methane (ml day-1) |
288 |
162 |
2.2 |
<0.001 |
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Methane (%) |
39.3 |
33.9 |
0.1 |
<0.001 |
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| Figure 1. Daily gas production in the plug-flow and dome biodigester configureations |
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| Figure 2. Concentration of methane in the gas from the plug-flow and dome biodigester configurations |
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| Figure 3. Production of methane in the gas in the plug-flow and dome biodigester configurations |
The pH in the biodigesters fell from an initial value of 7.6 to 5.3 on the third day, then increased to 6.0-6.3 over the next 6 days, then stabilized at 7.0 over the remaining days of the experiment (Figure 4). There were no differences between two configurations during the 20 days incubation (Table 2; Figure 3).
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Table 2. Mean values for pH in the biodigesters during the experiment |
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Plug-flow |
Dome |
SEM |
p |
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pH |
6.4 |
6.4 |
0.006 |
>0.05 |
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| Figure 4. Trends in the pH of the biodigesters over the 20 days of the experiment. |
There is no obvious explanation for the 78% increase in methane production in the "plug-flow" as opposed to the "dome" configuration. Most manure particles appear to be concentrated in the surface layer within the biodigester. If it is assumed that the biofilms in which the microbial digestion takes place are located in this upper layer then obviously a greater surface would be available for biofilm formation in the plug-flow configuration compared with the dome configuration.
The second factor that could have influeced gas and methane production could have been the degree of disturbance caused by mixing of the incoming influent with the digesta already present in the biodigester. It can be hypothesized that the degree of disturbance inside the biodigester would have been greater in the dome compared with the plug-flow configuration. Mixing the contents of plug-flow biodigesters has been shown to reduce gas production (Kounnavongsa Bounthavone and Preston 2009). Increasing the length of plug-flow biodigesters led to increases in gas production per unit influent of manure (Bui Phan Thu Hang 2003; San Thy et al 2005). The longer the biodigester then the less would be the disturbance to the microbial communities in the biodigesters.
The results reported in this paper have to be viewed in the context of the small size of the experimental in vitro biodigesters and the influence this might have had on the microbial communities within the biodigesters. Similar studies need to be done with large scale systems as are installed in commercial biodigesters.
Bui Phan Thu Hang 2003: Effect of dimensions of plastic biodigester (width:length ratio) on gas production and composition of effluent, http://www.mekarn.org/msc2003-05/miniprojects/webpage/hangctu.htm
Duong Nguyen Khang and Le Minh Tuan 2002 Transferring the low cost plastic film biodigester technology to farmers. Proceedings Biodigester Workshop, Ho Chi Minh City, Vietnwm, March 2002 . http://www.mekarn.org/procbiod/khang2.htm
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
Karthik Rajendran, Solmaz Aslanzadeh, Fredrik Johansson and Mohammad J Taherzadeh 2013 Experimental and economical evaluation of a novel biogas digester. Energy Conversion and Management. Volume 74, October 2013, Pages 183–191 http://www.sciencedirect.com/science/article/pii/S0196890413002793
Kounnavongsa Bounthavone and Preston T R 2009: Effect of mixing on gas production in plug flow tubular plastic biodigesters. Livestock Research for Rural Development. Volume 21, Article #29. http://www.lrrd.org/lrrd21/2/boun21029.htm
Nguyen Ngoc Thuy 2010 Socio-economic aspects of biogas digesters in the Southeast region, Vietnam. International conference on live stock production, climate change and resource depletion, 9 - 11 November 2010 in Pakse, Laos. http://mekarn.org/workshops/pakse/abstracts/thuy_NLU.htm
San Thy, Buntha P, Vanvuth T, Preston T R, Duong Nguyen Khang, Soukanh K, Boualong Phouthone, Choke Mikled and Sopharoek N 2005: Effect of length: diameter ratio in polyethylene biodigesters on gas production and effluent composition. Livestock Research for Rural Development. Volume 17, Article No. 120. http://www.lrrd.org/lrrd17/11/sant17120.htm
Received 15 December 2015; Accepted 5 February 2016; Published 1 March 2016