Performance Characteristics of a Two-Stage Dark Fermentative System Producing Hydrogen and Methane Continuously

ABSTRACT: The performance of a mesophilic two-stage system generating hydrogen and methane continuously from sucrose (10–30 g/L) was investigated. A hydrogengenerating CSTR followed by an upflow anaerobic filter were both inoculated with anaerobically digested sewage sludge, and ORP, pH, gas output, %H2, %CH4 and %CO2 monitored. pH was controlled with NaOH, KOH or Ca(OH)2. Using NaOH as alkali with 10 g/L sucrose, yields of 1.620.2 mol H2/mol hexose added and 323 mL CH4/ gCOD added to the hydrogen and methane reactors respectively were achieved. The overall chemical oxygen demand (COD) reduction was 92.6% with 0.900.1 g/L sodium and 31640 mg/L residual acetate in the methane reactor. Operation at 20 g/L sucrose and NaOH as alkali led to impaired volatile fatty acid (VFA) degradation in the methane reactor with 2.230.2 g/L sodium, 1,885 mg/L residual acetate, a hydrogen yield of 1.470.1 mol/mol hexose added, a methane yield of 294 mL/gCOD added and an overall COD reduction of 83%. Using Ca(OH)2 as alkali with 20 g/L sucrose gave a hydrogen yield of 1.290.3 mol/ mol hexose added, a methane yield of 337 mL/gCOD added and improved the overall COD reduction to 91% with residual acetate concentrations of 52287 mg/L. Operation at 30 g/L sucrose with Ca(OH)2 gave poorer overall COD reduction (68%), a hydrogen yield of 1.470.2 mol/mol hexose added, a methane yield of 138 mL/gCOD added and residual acetate 7,343715 mg/L. It was shown that sodium toxicity and overloading are important issues for successful anaerobic digestion of effluent from biohydrogen reactors in high rate systems.