A novel FIA configuration for the simultaneous determination of nitrate and nitrite, and its automation for monitoring an urban waste water treatment plant (WWTP) based on nitrification/denitrification criteria, are proposed. The detection range achieved is 0.25-50 mg l-1 for nitrate and 0.05-5 mg l-1 for nitrite (0.056-11.29 mg N-NO3-l-1 and 0.015-1.52 mg N-NO2 l-1). The maximum sampling frequency is 180 samples/day and the Relative Standard Deviation (RSD, n=5) was 1.1% at 5 mg l-1 nitrite and 2.3% at 50 mg l-1 nitrate plus I mg l-1 nitrite. The particle size of the reductant (Cd) and the use of Cu as a catalyst influencing the stability of the reduction column were examined and all electronic calibration system was developed in order to increase the capacity of the equipment to operate unattended with a view to its use at WWTP. The maximum unattended deployment time is four days. The proposed configuration was validated by analyzing real water samples at a pilot plant based on nitrification/denitrification criteria with optimal results.