Fast Fourier transform analysis has been applied to decompose the variance of some temporal series from an A + B industrial wastewater treatment system, as well as to discriminate which frequencies are basically contributing to parameter variability. The basic oscillations of the effluents, evaluated by means of the total COD, are shown to be mainly dependent on the internal dynamics of the reactors, whose design and control features are the generating forces for the final effluent fluctuations. Owing to the shorter retention time in the first reactor, the output signal was more complex than the input one. Although the second reactor reduces system complexity, it does not have an important effect on the final total COD.

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