The effect of digestion process on sludge treatment system was discussed in terms of thermal energy merit and energy cost merit. Thermal energy merit, expressed in F, was evaluated by the difference between the total thermal energy consumption in the sludge treatment process with digestion and that without digestion. Energy cost merit M was caluculated by summing up Mf and Me, where Mf was cost conversion of thermal energy merit and Me was cost conversion of electric power generated from the digested gas. It is indicated that adoption of digestion process reduces a total energy consumption in case of using digested gas for process fuel, and that energy cost merit due to adopting digestion process increases with sludge volatile solid contents. Though the energy cost merit M is positive at most cases, there is no case where M surpasses the deprecation cost for the digestion equipment. Thus, the digestion pro-cess equipment s, such as digestion tank, is so large in scale that the deprecation cost eats up the energy cost merit. Reduction of the construction cost by compaction of the equipment shou-id be a main object of the investigation in this field in future.

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A COD predion model was identified using a Revised GMDH with small data sets. The Revised GMDH was formed by combining the Basic GMDH Algorithm and the Filter GMDH Algorithm for taking the merits of both algorithms.
The number of layers was 3 and a simplified model was obtained. COD value of effluent was predicted using the obtained model.
The error of the prediction was estimated to be about 30% for three Sewage Treatment Plants of A, B and C.
It is considered that if point and frequency of the measlrement am investigated for the purpose of the modeling, more accurate model will be obtained.
The proposed model can be used as an output equation for COD control of the sewage treatment system.

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