Abstract
To clarify the effect of sludge circulation on hydrogen fermentation, two-stage hydrogen-methane fermentation that treats food waste with different patterns of recirculation was investigated. Operation without recirculation did not perform well in terms of hydrogen yield or carbohydrate removal, perhaps owing to an increase of C/NH4+ ratio for microorganisms in the hydrogen fermenter. This indicates that the recirculation supplemented the NH4+ in the hydrogen fermenter. The operation with the circulation of heat-treated sludge performed considerably better than that with the circulation of raw sludge with respect to both hydrogen production rate and yield. In addition, hydrogen production yield decreased in proportion to the flow rate of circulation during the operation with the recirculation of raw sludge. The results of the batch experiments revealed that the sludge circulated to the hydrogen fermenter could consume hydrogen in the hydrogen fermenter at a low pH whereas the heat-treated sludge could not. These results suggest that the recirculation of active methanogenic sludge stimulates hydrogen consumption, which can be attributed to the decrease in hydrogen production yield in two-stage hydrogen-methane fermentation.
