The paper presents the results of full-scale research concerning the temperature, the sludge age and the COD/N ratio in wastewater effect on nitrification and results of batch tests examined the additional carbon source in wastewater and the initial nitrate concentration in anoxic zone effect on the denitrification rate, time and efficiency. It was stated that the sludge age increase above 20 days lessened the unfavorable effect of low temperatures (under 15 degrees C) and stabilized the nitrification process. The highest effectiveness of nitrification (above 95%) were achieved when the COD/N ratio was lower than 4, higher parameter values destabilized process especially with low temperature. The batch test showed that delivering an additional carbon source in the form of acetic acid solution to the biological treatment process with pre-denitrification is favorable to the denitrification process and that above particular dose (the VFA/N ratio equal to 1.67 mg CH3COOH/mg N) the further denitrification rate increase did not occur. The effect of initial nitrate concentration in the anoxic zone on the denitrification rate is less distinct. (c) 2005 Elsevier Ltd. All rights reserved.