Hypotheses of the ASM1 model state that the specific respiration rate of heterotrophic active biomass is a constant. It thus becomes possible to assess the active heterotrophic biomass decay rate (bH) during starvation. The object of this study is to verify if the specific respiration rate of heterotrophic biomass remains constant throughout starvation, and if it is typical of global metabolic biomass activity, as proposed by the model hypotheses. The decay rate bH was evaluated using respirometric and enzymatic measurements. Measurement of bH through respirometry on various sludges varied between 0.28 and 0.76 d−1. According to the enzyme and respirometric measurements carried out, a large portion of this value is related to enzymatic regulation. During starvation, when initial active biomass activity is high, down regulation of the respiratory chain results in a decrease in the specific heterotrophic active biomass respiration rate, thus creating an overevaluation of the decay rate as measured by respirometry. Thus a high decay rate used in the ASM1 model could eventually lead to a poor forecast of biomass production.

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