Effect of Mixing on Biogas Production Rate in a Two-Stage Digester

Abstract

This study demonstrates the effect of stirring on the biogas production rate from cassava pulp under anaerobic conditions using a two-stage digester system which is operated in the mesophilic temperature range (35 °C). Immersion heaters were installed to control the temperature in both the acidogenic and methanogenic digesters, and the initial pH was adjusted to 8. The two-stage digestion system consisted of one acidogenic digester with a working volume of 250 liters, two methanogenic digesters with working volumes of 500 liters each, and one gas storage tank, all tanks are connected in series. The system was operated under batch feeding conditions using organic substrate prepared by mixing cassava pulp with water. The hydraulic retention time was set at 12 hours, and the total solids concentration of the solution was maintained at 20% (w/v), corresponding to an organic loading rate of 0.417 g COD L^-1day^-1. The stirring frequency was varied from no stirring to intermittent stirring at frequencies of 12, 8, 4, and 2 hours per cycle, with a stirring duration of 10 min per cycle. The results showed that the average biogas production rates were 140, 160, 190, 240, and 280 L day^-1, respectively. The average electrical energy consumption for stirring was 0, 373, 1,119, 2,238, and 4,470 Wh day^-1, respectively. The average net energy outputs were 898, 654, 100, −697, and −2,672 Wh day^-1, respectively.