Experimental site and period
The experiment was conducted in the Animal Research Farm under the Department of Surgery & Obstetrics as well as Livestock and Poultry Vaccine Research and Production Center under Department of Microbiology & Hygiene of Bangladesh Agricultural University, Mymensingh. The experiment was designed in February, 2017. Experiment set up and data were collected during April, 2017 to August, 2017. The schematic diagram of the experimental procedure is presented.
Experimental design
The experiment was designed to augment biogas production efficiency under different feeding interval. Two different feeding intervals were designed to compare the production efficiency of biogas.
Preparation of biogas plant
Two 7.5 m3 PVC digesters imported from China were set up on 2 February 2017 with gas holding capacity of 2 m3 . Plastic pipe was used as inlet and water tank was used as outlet. PVC digester is rather flexible to maintain the gas production.
Sample Collection
Fresh cow dung was collected from Animal Research Farm daily for daily feeding and after 4-day for 4-day interval feeding. The sample was carefully collected and it was free from any contamination such as straw, waste feeds, urine and any other materials.
Sample Preparation
During the set up two digesters were fed simultaneously with fresh cow dung diluted with water in a ratio of 1:1, farm digester on daily basis with 50 kg and lab x digester once in 4-day with 200kg at once, with loading rate around 1.14 – 1.64 kg ODM (vs)/m3d -1 digester volume and day, all over 5500 kg of cow dung were used for each of the digesters, with long hydrolic retention time of 55 days. The experiment was conducted over a period of 110 days. The optimum water content is likely to differ with different input materials depending up on the substrates chemical characteristics and bio-degradation rate (Valdes et al., 1996).
Experimental procedure
Two experimental set-ups were made to investigate the production of biogas from the anaerobic digestion of cow dung. Where one set-up was used to produce biogas on the basis of daily feeding and another for 4-day interval feeding. The digesters were connected with oven machine and gas meter. The gas formed in the digester passed through the pipe. Another plastic pipe was connected with a gas meter, gas suction machine and another plastic pipe was connected with the gas stove. That stove was used for burning the produced gas regularly. Digestion was done at ambient temperature. The digester temperature has also measured. During the investigation, the volume of the produced gas was measured with the help of gas meter. CO2, CH4 and gas temperature were also estimated by gas meter. The daily produced gas was burnt to reduce the environmental pollution. After burning, feeding was done in daily feeding digester and after 4-day interval feeding was done on 4-day interval basis digester. Proper mixing and dilution were done perfectly and always maintained the cow dung and water in 1:1 ratio. The pH values were monitored on 7 days intervals to determine the action of methanogens, which utilized the acids, CO2 and hydrogen produced by non-methane producing bacterial using a litmus paper.
Digester temperature and ambient temperature
The digester temperature and ambient temperature were measured by a digital thermometer, DT-2 model and this measurement was done regularly.
Gas volume
Many chemistry and physics experiments involve collecting the gas produced by a chemical reaction and measuring its volume. In the study the gas volume was measured with gas meter, BF-2000 regularly. Gas volume = Gas volume at the ending point of burning Gas volume at the starting point of burning. In the comparison of various statistical procedures, efficiency is a measure of quality of an estimator of an experimental design. The efficiency of gas volume was calculated in following way: Efficiency of digester (%) = (Average volume/ Real gas volume) * 100.
pH, CH4 and CO2
The pH was measured by litmus paper. The measurement was done weekly. CH4 were measured by gas meter and the measurement was done at the starting period of gas burning. CO2 was measured by the following formula– CO2 = (100 – CH4) %.
Statistical analysis (Two t sample test)
Data regarding gas volume (m3 ) were statistically analyzed by STATA 10.1 software package to examine the significant variation of 4-day interval feeding and regular feeding system. The data were analyzed by two t sample test.