Transfer of organic matter between wastewater and activated sludge flocs

The organic matter of wastewater was fractionated into settleable (i.e., particulate) and non-settleable (i.e., colloidal+soluble) fractions by settling followed by 0.22 μm filtration. Particulate, colloidal and soluble proportions were found to be relatively constant (45, 31 and 24% of the total COD, respectively). Transfer of soluble fraction always occurred from the wastewater to the activated sludge flocs, whereas bidirectional transfer occurred for the colloidal fraction. The transfer of soluble and colloidal matter reached a steady state after 40 min–mixing and 20 min–mixing, respectively. Desorption of a part of the colloidal organic matter pre-sorbed on the activated sludge flocs was evidenced. The biosorption capacity of activated sludge was around 40–100 mgCODg−1TSS. The biosorbable fraction of wastewater represented on average 45% of the non-settleable fraction.