Anaerobic Digestion of Vegetable Wastes for Biogas Production in Single Chamber and Double Chamber Reactors

The compositions of solid wastes of a rural market of Bangladesh and the results of two sets of laboratory experiments on biogas generation from the easily biodegradable wastes under daily feed condition are presented in this paper. Cow dung, cauliflower stick, papaya and potato were the major biodegradable wastes. Daily average composition of the biodegradable wastes was used in the experiments. The average Total Solids (TS) and Volatile Solids (VS) concentrations of the raw substrate were determined as 18.90% and 15.10% respectively. The experimental setups were placed in a large closed chamber containing two room heaters. The room heaters were operated alternatively at 35°C to maintain a favorable condition for anaerobic digestion of the substrate. In the first setup, a single chamber reactor and a double chamber reactor were used. In the single chamber reactor, 750 g wastes and required amount of inoculum were added initially to make the effective volume of 2 L. For the double chamber reactor, each chamber was initially fed with 350 g wastes and inoculum was added to make the effective volume of 1 L. The single chamber reactor was operated for 58 days and the double chamber reactor was run for 23 days. From the 2nd day of operation, each reactor was fed daily with a mixture of 18.75 g wastes and the required volume of tap water to make the total volume of 50 mL after taking out equal volume of slurry from the reactor. The second set of experiment was similar to the double chamber reactor of the first setup, but it was operated for 54 days including the last 16 days operation at room temperature as the heaters became out of order. In case of the first setup, the temperature varied from 31°C to 36°C and the rate of biogas generation was not affected due to this variation. The results of the experiments revealed that for the Organic Loading Rate (OLR) of 1.42 g VS/L/d, the daily stable biogas generation rate was 0.22 m3/kg of VS added for the single chamber reactor, and apparently the daily stable average rate of biogas production was 0.37 m3/kg of VS added for the double chamber reactor. During the second set of experiment, the temperature varied in between 32°C and 36°C when the chamber-heater was on and the rate of biogas generation was not affected, and the stable rate of biogas generation was 0.26 m3/kg of VS added for the OLR of 1.42 g VS/L/d. The temperature of the chamber varied from 22°C to 25°C when the heater became out of order and the sudden drop of the temperature by about 10°C affected the rate of biogas production greatly. At the ambient temperature, the stable rate of biogas generation was only 0.08 m3/kg of VS added.