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7. Effect of retention time on gas production and fertilizer value of biodigester effluent

Authors: San Thy, Preston T R and Ly J 2003

Livestock Research for Rural Development (15) 7 Retrieved, from

Summary: The experiments were conducted in the experimental farm of the University of Tropical Agriculture Foundation (UTA) in Chamcar Daung, Dangkor district, Cambodia, about 10 km from Phnom Penh. Experiment 1: “ Effect of retention time in biodigesters charged daily with a constant daily amount of pig manure began on May 10, 2002 and finished on October 9, 2002 and Experiment 2 “ Effect of retention time in biodigesters charged with a constant manure concentration in the influent” began on June 14, 2002 and finished October 16, 2002. The treatments were: BRT10 retention time 10 days, 20 days and 30 days. The experimental design was a 3*3 Latin square arrangement of the three retention times with 3 periods each of 40 days, applied to 3 biodigesters. The first 30 days of each period were of adaptation to the new retention time followed by 10 days of measurements. Pig manure was collected daily in the early morning from the pig pen of Mong Cai animals. The pigs were being fed a mixed feed formulated from wheat bran, broken rice, fishmeal, salt and premix. The six plug-flow biodigesters (in each experiment there were three biodigesters) were made from tubular polyethylene film (internal diameter 0.64m) and mounted in shallow trenches lined with bricks to ensure the dimensions were exactly the same (2 m length, 0.6 m depth and 0.6m width), to provide a liquid volume in the proportion of 80% of the total biodigester capacity. This was calculated to be 510 litres. The biodigesters were installed in an area with the same microclimate condition by shading them with plastic net 3 m above the ground. At the beginning of each experimental period, the biodigesters were inoculated with 140 litres of effluent from another biodigester, followed by a mixture of 5 kg fresh pig manure and 45 litres water applied each day for 10 days. During the subsequent adaptation and data collection periods, the fresh pig manure and water were added in the proportions indicated for each loading rate treatment. 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.

Experiment 1: The amount of fresh pig manure was fixed at 5.1 kg (1.18 kg DM) per day with addition of different amounts of water in order to give the different retention times. This resulted in concentrations of dry matter in the input material of 2.3, 4.6 and 6.9 % (weight / volume basis). Experiment 2: The concentration of manure in the influent was maintained constant at 6.0% solids (DM). This required the addition daily of 10.7, 5.4 and 3.6 kg fresh manure, and 40.5, 20.2 and 13.5 litres of water. The experimental data were recorded daily during the last 10 days of each experimental period. Samples of fresh pig manure and the corresponding effluent were taken daily on days 31 to 40, immediately before (manure) and after (effluent) charging the biodigester. They were stored in a refrigerator at -12 o C. Gas production was measured daily throughout the experiment but only the data for the last 10 days were used in the statistical analysis. The samples of fresh manure were bulked and mixed every 5 days, and of effluent every 3 days, prior to taking representative samples for analysis of nitrogen and ammonia 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). Chemical oxygen demand (COD) was measured in representative samples of effluent taken two times during each collection period, and analysed immediately (Andrew et al 1996). The pH of the manure and effluent was measured daily using a glass electrode and digital meter. VFA was determined by steam distillation in three samples of effluent taken at 3 day intervals during each measurement period. Gas production was measured daily by collecting the gas in inverted light-weight containers (a plastic-covered bamboo frame) of 200 litres capacity, floating inside oil drum containers filled with water, and permanently connected to the gas outlet of each biodigester. The change in volume was recorded four times a day to determine daily gas production.

Abstract: Two experiments were carried out to study gas production and fertilizer value of the effluent in plug-flow, tubular, plastic biodigesters with hydraulic retention times of 10, 20 or 30 days (BRT10, BRT20 or BRT30). There were three biodigesters each of 510 litres liquid volume in each experiment which consisted of three consecutive periods (retention times) of 40 days arranged in a 3*3 Latin Square . In experiment 1, the quantity of fresh pig manure was 5.1 kg/day, mixed with 46, 20 or 12 litres of water to give retention times of 10, 20 or 30 days, respectively. In experiment 2, the proportions of pig manure and water were maintained constant to give a total solids content of 60 g/litre in the influent, which was added at rates of 51, 25.5 and 17 kg daily for retention times of 10, 20 or 30 days, respectively. With a fixed daily input of fresh manure, neither the rate of gas production (1.04, 1.20 and 1.12 volumes of biogas per unit liquid volume of the biodigester) nor the efficiency (493, 606 and 567 litres of biogas/kg of manure organic matter added to the biodigester), was influenced by retention time (10, 20 or 30 days, respectively). However, when the solids concentration of the influent was fixed at 60 g/kg, rates of gas production were reduced by increasing retention times (1.62, 1.19and 0.81 volumes biogas/unit liquid volume of biodigester for 10, 20 and 30 days retention); efficiency was better for 20 and 30 days retention (550 and 547 litres biogas/kg OM) than for 10 days (376 litres/kg OM). The proportion of ammonia-N in total-N increased from a range of 0.077 to 0.12 in the fresh manure to a range of 0.46 to 0.65 in the effluent and did not appear to be affected by retention time or loading rate. It is concluded that when fresh pig manure is the substrate in polyethylene plug-flow biodigesters the optimum retention time is between 10 and 20 days with a solids concentration in the influent of 60 g/litre. The retention time apparently has no effect on the degree of conversion of organic N to ammonia-N. 



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