| A laboratory-scale, two-phase, hydrogen-producing system was inoculated with unpasteurized anaerobic digester sludge, started up, and operated for 120 days. The hydrogen reactor was operated at 35°C, pH 5.5, and a 10.2-hour SRT. The methanogenic reactor was an up-flow type operated at 28°C and an HRT of 2.7 days. The system was operated with and without methanogenic effluent recycle.; A 34--45% reduction in NaOH use for pH control was realized with recycle. Recycle also reduced the dissolved hydrogen concentration, acetic and n-butyric acid concentrations, hydrogen productivity, and hydrogen recovery. The dissolved hydrogen, methane, and carbon dioxide concentrations were supersaturated with respect to their equilibrium concentrations, irrespective of the recycle ratio. The use of recycle also reduced the methane reactor effluent pH and increased the effective organic loading rate to each reactor. Gravity settling of the recycle stream had no effect on the hydrogen reactor performance. |
