Computer studies using mathematical models indicate that the step aeration process is advantageous under some conditions with the optimal design depending on the kinetic model and the parameter values within the model. Experimental studies directed toward kinetic model identification in mixed populations have been conducted. The kinetic behavior of mixed populations of activated sludge has been investigated over a wide range of operating conditions. The growth parameters change significantly and systematically with mean residence time. The results suggest that different cell populations predominated at different mean residence times with fast-growing cells predominating at low mean residence times and slow-growing cells predominating at high mean residence times. An experimental investigation of optimal conditions using evolutionary operation indicated that step aeration was desirable. (Author)