The step feed process with three stages of denitrification-nitrification reactors has been studied using simulations in order to develop criteria for the optimum selection of the design parameters and to propose efficient operational strategies. To verify the simulation results experimental studies in a pilot plant of 1100 litres were carried out. The simulation studies showed that the optimum influent flow distribution to the three anoxic reactors is in the range of 40-40-20% and 33-33-34% depending on the wastewater characteristics and effluent requirements. These two latter conditions and in turn the influent flow distribution determine the anoxic and aerobic reactor volumes. The reduction of the dissolved oxygen in the two first aerobic reactors and the use of facultative zones in the final D-N stage are proposed as operational strategy. The experimental results proved the validity of the criteria developed for design and operation. A high capacity of the IAWQ activated sludge model No. 1 to predict the performance of the step feed process was observed.