Physico-chemical and biological parameters were monitored both throughout different oxygen cut off and starvation (OCS) times (6 h-72 h) and after the restoration of normal operational conditions. Sludge apparent viscosity and soluble extracellular polymeric substances (EPS) characteristics were measured to determine the activated sludge (AS) properties. Oxygen transfer, biological activity with specific oxygen uptake rate (SOUR) measurements during endogenous/exogenous conditions (without any external substrate/with external substrate consumption) and chemical oxygen demand (COD) removal were measured to assess the AS performances. During the different stress times, AS deflocculated as a decrease of apparent viscosity was observed and microorganisms biodegraded the released EPS to survive. After aeration return, and under endogenous conditions, size exclusion chromatographic fingerprints of soluble EPS were modified and macromolecules probably of type humic-like substances appeared in significant quantities. These new macromolecules presumably acted as biosurfactants. Consequently, the liquid surface tension, as well as the oxygen transfer rate (OTR), decreased. Under exogenous conditions, high biological activity (SOUR = 11.8 +/- 2.1 mg(O2 x g(MLVSS)(-1) x h(-1)) compensated the decrease of oxygen transfer. Finally, AS biomass maintained a constant COD degradation rate (15.7 +/- 1.9 mg(O2) x g(MLVSS)(-1) x h(-1)) before and after the disturbances for all times tested. This work demonstrates that AS microorganisms can counteract concomitant oxygen and nutrients shortage when the duration of such a condition does not exceed 72 h. Dissociation of endogenous/exogenous conditions appears to offer an ideal laboratory model to study EPS and biomass activity effects on oxygen transfer.
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