The problem to be addressed:

Low cost plastic biodigesters have had a major impact in Vietnam with more than 30,000 units installed during the past ten years (Nguyen Duong Khang, personal communication). However, there is a need to continue developing the technology with the aim of increasing performance (rate of gas production) and efficiency (gas produced per unit of input substrate).  Recent work in Cambodia (San Thy et al 2003) has shown that high yields of gas can be obtained when the loading rate is 6 kg DM/m³ of digester liquid volume, in plastic plug-flow biodigesters configured with a length: diameter ratio of 3:1. This ratio is narrower than that traditionally employed for commercial use (8 or 10:1).  There is no information on the effect of biodigester configuration on the performance of plastic plug-flow biodigesters, but it can be expected that the length: diameter ratio will influence the degree of mixing and hence the rate of gas production.

To date, polyethylene tubular plastic film of 250 μm thickness has been the material used in the biodigesters.  Two and  even three layers of film are used to provide greater strength. Even so, breakages of the plastic film occur quite frequently, and the plastic has to be replaced. A recent development is the use of polypropylene as the raw material. This plastic film is stronger than that made from polyethylene.

The research proposal aims to compare different length: diameter ratios in plug-flow plastic biodigesters, and to test the use of tubular polypropylene film as an alternative to polyethylene. The research will be done in four locations: CARF, UTA (Cambodia); LRC, NAFRI, Lao;Nong Lam University, Vietnam; Chiang Mai University, Thailand

The hypotheses

Two hypotheses will be tested:

1. A length: diameter ratio of 3: 1 will result in better biodigester performance than narrower or wider ratios.

2. Tubular polypropylene film will result in simpler installation (only one layer of plastic) and longer useful life of the biodigester than tubular polyethylene.

Work plan

The study will involve two experiments repeated in each of the four locations.

Experiment 1: Compare different length: diameter ratios in plug-flow plastic biodigesters with hydraulic retention times of 10 and 20 days

Treatments and design:                           

In each location, four experimental biodigesters will be constructed according to the design developed by San Thy et al (2003). The treatments arranged as a 4*2 factorial are:

Length: diameter ratios as follows:

LR2: Length: diameter ratio of 2:1

LR3: Length: diameter ratio of 3:1

LR5: Length: diameter ratio of 5:1

LR8: Length: diameter ratio of 8:1

Hydraulic retention times:

10 or 20 days

The design is a single changeover as follows with periods of 40 days on each of the times of retention

Polyethylene tubular film of  0.64m diameter will be used. This will require digesters that are 1.28m, 2.52m, 3.80 and 5.12m long. Each location will be considered as a replicate.

Management:

Pig manure will be used in each location.  The loading rate will be 5 kg DM per m³ of liquid volume. 

Measurements
Gas production is measured daily using the system of water displacement developed by San Thy et al (2003). Ambient temperature and temperature within the biodigester are measured at 7am and 12.00am.  Total N and ammonia-N is determined in the input manure and the effluent at weekly intervals according to AOAC (1990).  

Experiment 2: Evaluation of polypropylene versus polyethylene as source material in tubular biodigesters.

Treatments and design:

In each location, at least 4 biodigesters of commercial scale will be installed in farm households using tubular polypropylene film.  Each biodigester will have a diameter of 1m and a length of 3m with retention time of 10 days and a loading rate with pig manure in the range of 2 to 5 kg DM /m³ liquid volume.

Measurements

Gas production will be measured during consecutive periods of 2 days every 14 days, using the water displacement method as described in Experiment 1.

Statistical anlaysis

References

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 Retrieved August 8, 2003, from http://www.cipav.org.co/lrrd/lrrd15/7/sant157.htm