Livestock Research for Rural Development 21 (2) 2009 Guide for preparation of papers LRRD News

Citation of this paper

Effect of mixing on gas production in plug flow tubular plastic biodigesters

Bounthavone Kounnavongsa and T R Preston*

National Agriculture and Forestry Research Institute (NAFRI)
Livestock Research Center (LRC)
Vientiane Capital, Lao PDR
Finca Ecológica, TOSOLY, Socorro AA 48, Colombia


Four plug-flow, tubular, plastic biodigesters (2m length*0.60m diameter), charged with pig manure (hydraulic retention time of 30 days), were used to study effects of mixing frequency (every 1, 2 or 3 days or none)  on rates of gas production. Increasing the mixing frequency delayed the rime to reach stable rates of production and was associated with greater day to day variability in rate of gas production but had no effect on the final rates of gas production

It is concluded that when fresh pig manure is the substrate in polyethylene plug-flow biodigesters there are no advantages to gained by mixing the contents.

Keywords: Effluent, gas production, pig manure, polyethylene


Nowadays, developing worlds are facing a low living standard and dangers to the environment because of increasing population, exploitation of natural resources and growth of industries, and at the same time increasing demand for food and fuel. Especially the oil price has been increasing day by day and nobody knows when it will reach to the peak and then find the constant price. These problems have led to scientific and social initiatives focused on sustainable development including the use of renewable energy sources. 

There is therefore an urgent need to develop alternative energy sources. For rural areas, the use of local resources in integrated farming systems is projected to bring most benefit to small scale farmers and the environment (Leng and Preston 2005). The recycling of livestock wastes through  low cost biodigesters to produce biogas for cooking and nutrient-rich effluent as fertilizer is one of the ways to reduce dependence on fossil fuel-derived inputs in an environmentally friendly way that benefits small scale farmers (Preston 2000).

In ASEAN member countries, energy from biomass such as wood and agricultural residues represents about 40% of the total energy consumption, equivalent to some 2.5 million Tetra-joules per year. The bulk is from wood, with an estimated value of US$ 7 billions per year (Biomass energy in ASEAN member countries 1997).

Biodigesters play an important role in the recycling of organic wastes, producing methane-rich gas for cooking, with positive impacts on the environment and on human and animal health (Preston and Rodríguez 1998). The benefits from biodigesters are:  source of renewable energy or fuel for the farm household, fertilizer for crops growing on land and water, the cheapest means of minimizing pollution brought about by livestock wastes, and efficiency in the use of manure.

The appropriate use of biodigesters can also give rise to a number of related socio-economic benefits that come about through improvements to the quality of life for rural women and children due to reduce workload such as less firewood has to be collected, cleaner kitchen and cooking utensils; reduced methane emissions and less deforestation. Soeurn Than (1994) and Bui Xuan An et al (1997) showed that biodigesters made from tubular polyethylene were well accepted when introduced to households in rural areas of Cambodia and Vietnam because of the low price and simple installation.

The impact of the technology in South Vietnam is demonstrated by the more than 30,000 units that have been paid for and installed by farmers between 1996 and 2002 (Duong Nguyen Khang et al 2000). Numbers now exceed 70,000 (Duong nguyen Khang, personal communication).

Many factors influence gas production and the fertilizer value of the effluent in tubular plug-flow biodigesters. Studies have been made on the effect of retention time, temperature, types of manure and concentration of solids in the influent (Boodoo et al 1977; Bui Xuan An and Preston 1999; San Thy et al 2003).

In the Lao PDR, agricultural residues are the main sources of raw materials on which to base renewable energy technologies. Improved cooking stoves for more efficient use of fibrous fuels is one area for development. The other is the use of biodigesters. The biogas technology is more appropriate as it helps to reduce the need for wood which leads to deforestation. It also reduces Green House Gas (GHG) emissions when animal manure is exposed to the atmosphere. Besides, in rural areas, most of the households have their own animals. Thus biogas technology is a suitable technology for livestock farms in rural areas. 

In the whole of Laos, it is estimated that there are about 4 million metric tonnes of animal manure produced per year;  the gas production from  biogas technology could amount about 280 million cubic meters annually. There is thus a vast potential to adopt the biogas technology in the country.


The low cost plastic "plug-flow" biodigesters depend on the steady movement of the contents from the inlet to the outlet. It is hypothesized that mixing the whole of the contents might result in a more rapid colonization of the substrate and hence a faster rate of gas production.


The aim of this experiment was therefore to compare the gas production rate in plug-flow biodigesters when the contents were subjected to increased frequency of mixing..

Materials and methods


The experiments were conducted in the experimental farm of Kampong Cham National School of Agriculture, Kampong Cham Province which is located approximately 124 km North East of Phnom Penh City, Cambodia. 

Experimental treatments and design

Four plug-flow biodigesters made of tubular polyethylene film were used for the experiments (Photos 1-4). Each biodigester was subjected to a different frequency of mixing as shown below:

Allocation of the treatments to the biodigesters was at random (Table 1).  The experiment lasted for 30 days starting from 19 August 2008 through 17 September 2008.

Photo 1: Inlet and outlet of four biodigesters

Photo 2: Covered by bamboo frame

Photo 3: Sample collection

Photo 4: Daily measurement of gas

Mixing was achieved by inserting into each biodigester a wooden cross-piece (Photo 5) attached to a rope (Photo 6). By pulling the rope  from the inlet to the outlet and vice versa the contents could be  mixed. On each occasion, for treatments M1, M2 and M3,  the operation was conducated 4 times (4 times pulling the mixer in each direction) in the morining before adding new substrate.

Photo 5. The wooden cross-piece that
is the basis of the mixing system
Photo 6. Attaching the rope to the cross-piece.

 The biodigesters were mounted in shallow trenches lined with bricks walls to ensure the dimensions were exactly the same of 2 m length, 0.60 m depth and 0.60 m width. Details of the operation of the biodigesters are in Table 1. Each biodigester was 2m long with internal diameter of 0.6. The outlet level was fixed so that approximately 80% of the total volume (560 litres) was occupied by the substrate of pig manure and water. On the first day, 30 kg fresh pig manure and 400 litres water were put in each biodigester. On subsequent days, the loading rate was 5 kg manure and 15 litres water to give a hydraulic retention time of approximately 30 days. Gas production was measured daily by water displacement (Photo 4; San Thy et al 2003).

Table 1: Details of  the biodigesters







Biodigester details 






Length, m






Diameter, m






Total volume, m3






Liquid volume, litres






Initial applied water, litres






Initial fresh pig manure, kg






Water applied daily, litres






Manure applied daily, kg






Pig manure and input

Pig manure was collected daily between 7:00 and 8:00 am from the same farm, where the pigs (crossbred) were being fed a mixed formulation of concentrate, rice bran, broken rice and mineral-vitamin premix.

Data collection and analysis

Gas production was measured daily in the early morning before adding the water and manure. The gas was collected in inverted light-weight containers (Photo 4) made of a plastic-covered bamboo frame of 200 litres capacity. These were suspended inside an oil drum  filled with water, and permanently connected to the gas outlet of each biodigester. Knowing the area (A) inside the gas container and height (H) above the water level the gas volume was calculated as "A*H".  

Samples of fresh pig manure were analyzed every 3 days for DM content by microwave radiation (Undersander et al 1993). pH of the effluent was analyzed using a digital meter every 5 days in the morning before putting pig manure and water in the biodigester.

Statistical analysis

The data were analyzed by regression using the Micrososft Excel Software.  The independent variable was the mixing rate and the dependent variable the gas production.

Results and discussion

The air temperature was measured 3 times per day (07:00, 12:00 and 17:00) and ranged from 26.8 to 35.9oC.

Gas production 

Gas production began on the 9th day after putting the initial charge of manure and water in the biodigesters and increased gradually to reach a maximum for the no-mixing treatment on day 18 (Figure 1). Gas yields with mixing were variable reaching a maximum at 23 days for mixing every 3 days which was then maintained, similar to the no mixing treatment,  until the end of the measurement period at 40days. By contrast, gas yields with mixing every day or every 2 days were very variable with stable production not being attained until 38-40 days. The much greater variability in gas production with mixing is apparent from the data for mean and SD values in the different periods (Figure 4).

Figure 1. Development of gas production in biodigesters mixed daily, every 2 days,
every 3 days or not mixed (period from 0 to 20 days from startup

Figure 2. Development of gas production in biodigesters mixed daily, every 2 days,
every 3 days or not mixed (period from 21 to 30 days from startup

Figure 3. Development of gas production in biodigesters mixed daily, every 2 days,
every 3 days or not mixed (period from 31 to 40 days from startup

 The  more frequent the mixing, the greater was the variability in daily rate of production (Figure 4)

Figure 4. Mean values for gas production (±SD)
in periods 21 to 30 and 31 to 40 days after startup

Table 2: Mean values for gas production (litres/day) in consecutive 5-day periods according to different frequencies of mixing

Period, days

No mixing

Mixing every 3 days

Mixing every 2 days

Mixing everyday































The overall tendencies in rates of gas production show that the effect of increasing the mixing frequency was to delay the time to reach stable rates of gas production (Figure 5) 

Figure 5: Gas production during experimental period, litre
pH and temperature

The air temperature was measured 3 times per day (07:00, 12:00 and 17:00) and ranged from 26.8.3 to 35.9oC. pH of the effluent ranged from 7.0 to 8.7. These parameters were not affected by the mixing frequency.



This experiment was supported by the MEKARN project, financed by the Swedish Agency for Research Cooperation with Developing Countries (SAREC).


Biomass energy in Asean member countries 1997 "Biomass: more than a traditional form of energy". FAO Regional Wood Energy Development Programme in Asia in cooperation with the ASEAN-EC Energy Management Training Center and the EC-ASEAN-COGEN programme.

Boodoo A, Delaitre C and Preston T R 1977 The effect of retention time on biogas production from slurry produced by cattle fed sugar cane. Tropical Animal Production. 4:1

Bui Xuan An, Preston T R and Dolberg  F 1997 The introduction of low-cost polyethylene tube biodigesters on small scale farms in Vietnam, Livestock Research for Rural Development, 9:2

Bui Xuan An and  Preston T R 1999 Gas production from pig manure fed at different loading rates to polyethylene tubular biodigesters. Livestock Research for Rural Development, 11:1

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Preston  T R  2000 Livestock Production from Local Resources in an Integrated Farming System; a Sustainable Alternative for the Benefit of Small Scale Farmers and the Environment. Workshop-seminar "Making better  use of local feed resources" January, 2000. SAREC-UAF (Editors: T R Preston and R B Ogle).

San Thy, Preston T R and Ly J 2003  Effect of retention time on gas production and fertilizer value of biodigester effluent. Livestock Research for Rural Development 15 (7).

Soeurn Than 1994  Low cost biodigesters in Cambodia. Proceedings National Seminar-workshop in Sustainable Livestock Prod. on local feed resources. Agricultural Publishing House, Ho Chi Minh, pp.109-112.

Undersander D, Mertens D R and Theix N 1993 Forage analysis procedures. National Forage Testing Association. Omaha pp 154.

Received 30 October 2008; Accepted 6 January 2009; Published 1 February 2009

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