The objective is the adaptation of an existing UV spectrophotometer for the on-line monitoring of nitrate, nitrite and soluble COD during cycles of SBRs. An on-line filtering system was also developed in order to provide particle-free fluids to the sensor. The modified UV-spectrophotometer sensor (called "STAC") was implemented into a pilot-scale bioreactor at INRA-Narbonne, treating a semi-synthetic dairy wastewater. After a calibration and validation steps, results obtained during five months have shown that the on-line spectrophotometer sensor was able to efficiently monitor on-line nitrate, nitrite in SBR content. Ranges of detection were between 0 to 40 mg/L for nitrate and 0 to 18 mg/L for nitrite, with an accuracy of about 10 %. The on-line estimation of soluble COD was less efficient. The major drawback was the time-consumption for the cleaning of the homemade on-line filter.