Separate production of hydrogen and methane from cassava wastewater with added cassava residue under a thermophilic temperature in relation to digestibility

Abstract

In this research, the separate production of hydrogen (H₂) and methane (CH₄) from cassava wastewater with added cassava residue was investigated using a two-stage upflow anaerobic sludge blanket (UASB) system under thermophilic temperature (55 °C) in a continuous mode of operation and steady state condition. The two-stage UASB system was operated under an optimum chemical oxygen demand (COD) loading rate of 10.29 kg/m³d (based on the total volume of both bioreactors) of the cassava wastewater with different concentrations of added cassava residue. The recycle ratio of the effluent from the second bioreactor to the feed flow rate was fixed at 1:1 (v/v). In addition, the solution pH in the first bioreactor was controlled at 5.5, while that in the second bioreactor was not controlled. Under the optimum cassava residue concentration of 1200 mg/L, the produced gas from the first bioreactor contained 42.3% H₂, 55% carbon dioxide (CO₂) and 2.70% CH₄, while that from the second bioreactor contained 70.5% CH₄, 28% CO₂ and 1.5% H₂. Apart from a high H₂ and CH₄ production performance (45.2 and 150% improvement, respectively, as compared to the system without added cassava residue) under the optimum cassava residue concentration (1200 mg/L) and the controlled COD loading rate (10.29 kg/m³d) of the cassava wastewater, the degradation performance of cellulose and hemicellulose were 41% and 22%, respectively, for the first bioreactor and 23% and 11%, respectively, for the second bioreactor. The digestibility of the cassava residue at thermophilic operation was higher than that at mesophilic temperature.

Keywords: Cassava residue; Cassava wastewater; Hydrogen production; Methane production; Thermophilic temperature; Two stage upflow anaerobic sludge blanket reactor (UASB).