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5. Effect of length: diameter ratio in polyethylene biodigesters on gas production and effluent composition

Authors: San Thy, P Buntha, T Vanvuth, T R Preston, Duong Nguyen Khang, Soukanh K, Boualong Phouthone, Choke Mikled and N Sopharoek, 2005.

Livestock Research for Rural Development 17 (11) 2005, Retrieved November 8, 2005.

Summary: The experiment was conducted in the four countries participating in the MEKARN project; in Cambodia (CelAgrid), Thailand ( Chiang Mai University ), Vietnam ( Nong Lam University ) and Lao (Livestock Research Centre). The activities in the different countries were initiated at different times: in Vietnam from September to November 2003, in Cambodia and Lao from February to April 2004; and Thailand from June to July 2004. The treatments arranged as a 4*2 factorial were: Length: diameter ratio and hydraulic retention times. Each location was considered as a replicate.
Length: diameter ratios

LR2: Length: diameter ratio of 2:0.63 (3:1)
LR3: Length: diameter ratio of 3:0.63 (5:1)
LR5: Length: diameter ratio of 5:0.63 (8:1)
LR8: Length: diameter ratio of 8:0.63 (13:1)

Retention time
This was 10 or 20 days.

The design was a single changeover with experimental periods of 40 days on each retention time. In each location, four experimental biodigesters were constructed according to the design developed by San Thy et al (2003). Tubular polyethylene film of 63 cm diameter was used to construct 4 biodigesters in each location (2, 3, 5 and 8m length). At the beginning, the biodigesters were inoculated with effluent from a functioning biodigester. The ratios used were 60% of digester effluent and 40 % of water. A mixture of manure and water was added to give an initial solid concentration of 4%. At the time of changeover, water was added at 50 % of biodigester liquid volume to facilitate the outflow of organic solid materials from the first period that might effect on biogas yield in the next period.

The 16 plug-flow biodigesters (in each location with 4 biodigesters) were made from tubular polyethylene film (internal diameter 0.63m), mounted in shallow trenches lined with bricks (to ensure the dimensions were exactly the same size of plastic biodigester), to provide a liquid volume in the proportion of 80% of the total biodigester capacity. The biodigesters were installed in an area with the same microclimate condition by shading them with corrugated iron roof at 3 m above the ground. During the subsequent adaptation and data collection periods, the fresh pig manure and water were added in the proportion indicated for each loading rate treatment.

Pig manure was used in each location. The loading rate was 4 kg DM per m³ of liquid volume. The manure was collected daily in the early morning from the pig pen and stored in a polyethylene sack. The pigs were fed a mixed feed formulated according to each location (Vietnam and Thailand fed by commercial feed and Lao and Cambodia fed by formulation feed, or high fiber feed such as water spinach, cassava silage,  distillery waste and brewery spentd grain mixed with commercial feed). The biodigesters were charged daily at exactly the same time and with the amounts of fresh manure and water according to the treatments and the liquid volume of the biodigester. The indicated amount of pig manure was fixed on a DM basis. The amount of water added was determined by retention time.

 The experimental data were recorded daily during the last 20 days of each experimental period. Samples of fresh pig manure and effluent were taken daily on days 21 to 40, immediately before (manure) and after (effluent) charging the biodigester.

The samples of fresh manure were bulked and mixed every 10 days, and effluent every 7 days, prior to taking representative samples for analysis of total N and ammonia-N using a Foss-Tecator Kjeldahl apparatus and for organic matter by ashing the samples in a furnace oven (AOAC 1990). DM content was determined by microwave radiation (Undersander et al 1993). Gas production was measured daily using the system of water displacement developed by San Thy et al (2003) (Photo 1). The change in volume was recorded 2 to 3 times a day to determine daily gas production.

Photo 1: The inverted containers for collecting the biogas by water displacement

Abstract: The treatments arranged as a 4*2 factorial were: Length and diameter ratio and retention time  of plastic plug-flow biodigesters.  The design was a single changeover with experimental periods of 40 days on each retention time. The length: diameter ratios wee 8:0.6, 5:0.6, 3:0.6 and 2:0.6m with hydraulic retention times of 10 or 20 days.  For each retention time, the amount of manure (DM) was kept constant by adding water according to retention times. The manure solid concentration in the influent for 10 days retention time was 4 % and for 20 days was 8 %. The proportion of ammonia-N in total N increased markedly with different forms from the raw manure of 13.2±1.83, influent slurry in the range of 27.55± 3.05 and the effluent of 48.17±1.27 as percentage .

Biogas production as litres per 100 litres liquid volume biodigester, and ammonia concentrations in effluent, did not differ between different length:diameter ratios nor between retention times.

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