Physical-chemical and biological fractionation methods were used at different parts of the Evry WWTP in order to assess the influence of effluents recycled from the primary settling tanks and sludge treatment processes on the characteristics of the wastewater. The RBCOD doubled between the WWTP influent and the biological reactor inlet, thus doubling the denitrifying potential of the plant. The primary sludge gravity thickener effluent, subject to a great variability, appeared to contribute up to 38% of the RBCOD contained in the biological reactor influent, whereas only 30% seemed to come from the WWTP inlet. No notable increase of RBCOD was observed at the primary settling tanks' outlet for the longer residence time. The significant variations of RBCOD across the plant were not caused by fluctuations of the physical-chemical COD fractions.

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