This paper considers the problem of channel interaction in multivariable systems. As an application, nitrate removal in the activated sludge process in a wastewater treatment plant is studied. To evaluate the degree of channel interaction, two different tools are compared; the well known Relative Gain Array (RGA) and the more recently developed Hankel Interaction Index Array (HIIA). The results of the analysis are discussed from a process knowledge point of view, and are also illustrated with some control experiments. The main conclusion is that both the analysis tools provide reasonable results in this case. The HIIA, however, gives a deeper insight about the actual cross couplings in the system. This insight may also be used in order to design suitable structured multivariable controllers.
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