In this research work the effects of orange azo dye over ammonia oxidizing bacteria (AOB) and anammox bacteria activities were tested. Adsorption onto the biomass was detected with both types of biomass however no biological decolourization occurred. Performed batch tests indicated that concentrations lower than 650 mgorange/L stimulated AOB activity while anammox bacteria activity was inhibited at concentrations higher than 25 mgorange/L. Long-term performance of both processes was tested in the presence of 50 mgorange/L. In the case of the partial nitritation process both the biomass concentration and the specific AOB activity increased after 50 days of orange azo dye addition. Regarding the anammox process, specific activity decreased down to 58% after 12 days of operation; however, initial values were restored 54 days after stopping the dye addition