The interest in the biological alternating anoxic/oxic reactors to upgrade existing wastewater treatment plants has been recently renewed, thanks to the use of reliable automatic control systems. To discuss the treatment capabilities of the alternating systems, with particular relation to the municipal WWTPs' upgrading, this study exploits the experience gained in this field through numerous years of R&D. At first a pilot plant study and then a full scale alternating plant were used to supply experimental data that support the final schematic of methodology. The pilot experimentation was based in the real loading conditions of the italian municipal WWTPs. Real municipal sewage was used to 3 feed the plant in six steady-state periods, applying nitrogen loading rates ranging from 0.03 to 0.1 kg N m(-3) d(-1). Major problems with the nitrogen removal performances occurred in the case of overaeration of the activated sludge tank, corresponding to influent low-loaded wastewater. The experimental durations per day of the anoxic and oxic phases were in good agreement with a simplified mathematical model, which was validated by full scale data and was finally considered useful for the upgrading design. The maximum treatment capacity of the process in terms of nitrogen loading rates was estimated in the range 0.10-0.16 kg N m(-3) d(-1) according to the different rates for biological nitrification and denitrification, which influence the oxic and anoxic durations per day. Moreover, the reliability of the control device used for the experimentation was proved through the statistic analyses of the performed cycles, which were in agreement with the actual nitrogen removal performances. Finally, the schematic of methodology shows how easy and consolidated the upgrading of existing wastewater treatment plants by the alternate cycles process could be.