Considering these new parameters, some of the wastewater samples collected in October 2013 were again analyzed in
triplicate (n = 3) using the flow system. As can be seen in Table 4, the results obtained with the flow system improved slightly compared to those obtained in batch (see Table 3), and now, the four measured COD values overlapped with the accredited value, considering their confidence intervals (95%). It is noteworthy that the influents of the urban WWTP contain a wide variety of organic compounds, with changing load depending on the city and on the time of the day. The
spectrum of different molecular structures and functional groups makes it difficult to obtain the complete chemical
oxidation and the COD value. However, this does not seem to affect the performance of the CuO/AgO composite-derived
sensor, and results are shown to be in good agreement with those obtained by the standard dichromate method carried out in the accredited laboratory.