Physico-chemical and biological characterization methods were applied to wastewater samples originating from 7 French WWTPs. The settleable fraction (S), unsettleable-coagulable fraction (US-C) and unsettleable-uncoagulable fraction (US-UC) were separated. Special emphasis was put on the determination of hydrolysis kinetics associated with coagulable- and settleable-fractions in order to model their behaviour in activated sludge processes, as both these fractions are influenced by the SRT. The “soluble” fraction (i.e. US-UC) was composed of readily biodegradable COD (2 to 27%), readily hydrolysable COD (37 to 90%) and inerts (2 to 47%). The “colloidal” fraction (i.e. US-C) was composed of heterotrophic biomass (9 to 24%) and readily hydrolysable COD (6 to 82%), with the rest being inerts or very slowly biodegradable COD (0 to 70%). The “particulate” fraction (i.e. settleable) was composed of biomass (14 ± 6%), readily hydrolysable COD (21 ± 14%), slowly hydrolysable COD (about 45–50%) and 15 to 20% which can be considered as inert matter (XI). “Readily hydrolysable COD” was correctly modeled by a global first-order reaction. First-order constants (KH) were 9 ± 2 d−1 for raw-wastewater, 12 ± 3 d−1 for primary settled-wastewater and 16 ± 5 d−1 for coagulated-wastewater. “Slowly hydrolysable COD” was correctly modeled by a limited surface reaction. Concerning this fraction, the specific hydrolysis rate (kH) ranged from 0.25 to 1.05 d−1, and the affinity constant (KX) ranged from 0.33 to 0.95 gCOD/gCOD based on settleable solids analysis.

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