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.

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A computer simulation of water quality trading where COD, nitrogen and phosphorus are traded separately but simultaneously was conducted, focusing on 75 wastewater treatment plants in the Tokyo Bay basin. The result shows a reduction in the total cost of advanced treatment of about 30%. The behavior of a wastewater discharger is formulated as an optimization problem of nonlinear programming, in which the conditions for designing a cost for discharging effluent (effluent charge), which is equivalent to water quality trading, were obtained. It was confirmed by computer simulation for the Tokyo Bay basin that the behavior of each discharger in water quality trading is identical to that in an equivalently designed effluent charge. As for the equity, the effluent charge where all the charge revenue is distributed as a subsidy is superior to the equivalent water quality trading without a subsidy in that the effluent charge has more uniformity in the net cost per unit volume of effluent among dischargers.

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