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10. Optimization of the role of tubular plastic biodigesters in integrated farming system

Authors: San Thy, T R Preston, Khieu Borin, Pheng Buntha and Try Vanvuth, 2005.

Livestock Research for Rural Development. Volume 17, Article # 132. Retrieved November 8, 2005.

Summary: The experiment was carried out at the CelAgrid experimental farm, in Kandal province for a period of 150 days, from April 6 to August 11, 2004. The trial comprised five treatments, consisting of different proportions of pig and cattle manure; 100% of pig manure and 0% of cow manure, 75% of pig manure and 25% of cow manure, 50% of pig manure and 50% of cow manure, 25% of pig manure and 75% of cow manure and 0% of pig manure and 100% of cow manure. The experimental design was a Latin Square arrangement of five periods, 30 days each. The first 20 days of each period were for adaptation to the treatment followed by 10 days of measurement. The hydraulic retention time was fixed at 20 days. Five plug-flow biodigesters were installed using tubular polyethylene film (internal diameter 0.64m; 2m length). They were installed in an area with the same microclimate condition. Total biodigester volume was 640 litres, of which 80% corresponded to the liquid volume equal to 510 litres. Pig manure was bought from a nearby village, and stored in an enclosed plastic bag for two days prior to use. Cattle manure was collected each morning from animals at CelAgrid, in which the basal diet was untreated rice straw and ensiled cassava foliage. The loading rate was fixed at 3.27 kg manure DM per cubic metre of biodigester liquid volume. The actual amounts were determined according to the experimental treatment and the DM content of the manures. Water was added to give a solids content (DM) of 6.54% for every treatment. Thus the total daily charge to the biodigester was 25.5 litres of suspended manure and water. Samples of fresh pig and cattle manure and the corresponding effluents were taken daily on days 1 to10 of the measurement period immediately before (manure) and after (effluent) charging the biodigester. They were stored in a refrigerator at -20C until required for analysis. At this point the samples were thawed, bulked over the 10 day measurement period and analyzed for nitrogen and ammonia using a Foss-Tecator Kjeldahl apparatus and for organic matter by asking the samples in a furnace oven (AOAC 1990). DM content was determined by microwave radiation (Undersander et al 1993).Gas production was measured daily by collecting the gas in inverted plastic bags suspended in oil drums filled with water (San Thy et al 2003).

Abstract: When the proportion of pig manure was increased the rate of gas production increased by more than 50% when replacing 100% cattle manure by pig manure. The proportion of ammonia-N in total N increased as the manure was mixed with water (influent) and after being fermented in the biodigester (effluent), with higher values when the manure was from pigs rather than cattle. It is concluded that manure from pigs produced more gas that manure from cattle and effluent of biodigester laoded cow manure had lower fertilizer value. It is suggested that it may be more appropriate to recycle cattle manure through earthworms rather than using it as substrate in biodigesters.

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