Abstract
The performance of a mesophilic two‐stage system generating hydrogen and methane continuously from sucrose (10–30 g/L) was investigated. A hydrogen‐generating CSTR followed by an upflow anaerobic filter were both inoculated with anaerobically digested sewage sludge, and ORP, pH, gas output, %H~2~, %CH~4~ and %CO~2~ monitored. pH was controlled with NaOH, KOH or Ca(OH)~2~. Using NaOH as alkali with 10 g/L sucrose, yields of 1.62 ± 0.2 mol H~2~/mol hexose added and 323 mL CH~4~/gCOD added to the hydrogen and methane reactors respectively were achieved. The overall chemical oxygen demand (COD) reduction was 92.6% with 0.90 ± 0.1 g/L sodium and 316 ± 40 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 2.23 ± 0.2 g/L sodium, 1,885 mg/L residual acetate, a hydrogen yield of 1.47 ± 0.1 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 1.29 ± 0.3 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 522 ± 87 mg/L. Operation at 30 g/L sucrose with Ca(OH)~2~ gave poorer overall COD reduction (68%), a hydrogen yield of 1.47 ± 0.2 mol/mol hexose added, a methane yield of 138 mL/gCOD added and residual acetate 7,343 ± 715 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. Biotechnol. Bioeng. 2007;97: 759–770. © 2006 Wiley Periodicals, Inc.