From a study of the performance of the autothermal thermophilic aerobic stage (45 m3) of a full-scale dual digestion plant, operated with pure oxygen, it was found that: (1) VS and/or COD removal are poor parameters for estimating biological heat generation; (2) the biological heat generation is proportional to oxygen utilized at 13.0 MJ/kgO, a value which conforms closely to that obtained from bioenergetics and thermodynamics, (3) the maximum biological heat generation rate was limited by the maximum oxygen utilization rate of the sludge (OURbio), and (4) because the aerobic reactor is oxygen limited, the temperature could be completely and instantaneously controlled with oxygen feed rate up to OURbio; (5) CO2 generation in the vent gas was 0.68 mol CO2/mol O2 utilized instead of 1.0 usually assumed.

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