This study shows results from four lab-scale wastewater treatment plants, which were installed behind the grit chamber of a municipal wastewater treatment plant. The lab-scale plants were fed with original wastewater, diluted with water from the final clarifier to give a constant concentration of 300 mg /l COD. The activated sludges were adapted to different sludge ages from 1.88 to 24.0 days. The steady-state behaviour was measured along with the decay rate, the yield coefficient, the oxygen demand, the C- and N- balance and the DNA of biomass XBH. The steady-state equations of Marais and Ekama were used to calculate the masses of active biomass, endogenous residue and inert VSS in the reactors. Measured parameter values were different for all sludge ages. They gave similar values for inert particulate VSS (Xi) of 7 to 9%. Parameter values of the IAWQ-Model No. 1 together with Xi-fitting could also be used to solve the equations, but in this case different Xi values for the different reactors had to be used for the same wastewater.

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