A new respirometric method for COD fractionation was applied to various wastewaters collected on French treatment plants. Great variations were observed especially in the readily biodegradable fraction (RBCOD) ranging from 1 to 16% of total COD. Variability of the results among the origin can be explained by the reactions occurring in the sewers. In one of the towns studied, the wastewater samples collected directly in the sewer show COD characteristics very different from the sewage which reaches the treatment plant. By analysing the same wastewater after aerobic and anaerobic storage, it was proved that RBCOD respectively decreased or increased significantly. Seasonal fluctuations were also quantified, showing that RBCOD fraction decreased from 75 mg/L to –15 mg/L during the summer whereas the variation of soluble fraction was less important.

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