A process for automatically controlling a water treatment process comprising subjecting the water to successive working cycles each comprising at least a carbon oxidation and nitrification stage; and a denitrification stage, by activating and deactivating, respectively, aeration means. The process comprises dynamically measuring in a working cycle at least a redox potential, ORP, of the water, and calculating a second derivative value d2ORP/dt2 of the ORP from the beginning and during the denitrification stage, the aeration means being activated to end the said denitrification stage of the working cycle and to begin a further nitrification stage of a following working cycle only once the d2ORP/dt2 of the ORP reaches at least a third zero value during the said denitrification stage. A system for putting into practice the process comprises a probe to measure the redox potential and a control unit having processor means to calculate the second derivate value of the measured redox potential and to perform a Boolean comparison of the said second derivative value with a predetermined reference value and to counter the times this second derivative value equals the predetermined reference value.
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