The plant under study is processing the wastewater of the eastern part of Montevideo city, capital of Uruguay, its population is approximately one million people, its maximum capacity is 3 m3/s. The goal of the experiment was to determine the mean residence time, MRT, of the wastewater in the decanting tanks. The plant has four of these tanks of approximately 500 m3 each. The output of this system of decanting tanks is pumped to a 2.300 m length pipe into the River Plate. At the plant input a set of mechanical filters is used and after it the material is pumped to a central 25 m length distributor, where the entrances of the decanting tanks are placed. For evaluating the whole system, it was used about 740 MBq (20 mCi) of 131I in 10 ml as an input pulse. The analysis was performed in two steps. Initially the distributor was studied, a transfer function from the main input to the entrance of each tank was determined. In the second step, the goal of the plant evaluation, the MRT of each tank was analyzed. Considering the non-impulsive shape of the input function of the decanting tanks, a numerical deconvolution software of the output-input of each tank was developed ab initio. Deconvolution results of various sequences as: main input to the distributor system-decanting tanks entrances-output were compared with meaningful conclusions.