The effect of anaerobic stabilization of returned activated sludge in a continuous system on biomass production under batch conditions at various So/Xo ratios was studied. A system without anaerobic sludge stabilization was also operated as a control unit. All experiments were carried out with Toulouse settled wastewater. Biomass growth, substrate removal, oxygen uptake and carbon dioxide production were measured using a quadrupole mass spectrometer (Inficon IQ 200) coupled to a 15-liter fermentor (Biolafitte). The results confirmed that one of the most important parameters in batch cultivation is the initial So/Xo ratio. It was found that anaerobic treatment of activated sludge changed its behaviour during batch cultivation. As a consequence, the observed biomass yield (Yobs) decreased with increasing So/Xo while it remained constant with the sludge from the control unit. In the anaerobic reactor, microorganisms are subject to a physiological shock due to lack of oxygen and food. Under the above conditions, they use ATP as a source of energy. After they are returned to aerobiosis and supplied with exogenous substrate, they rebuild energy reserves at the expense of growth.

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