The data from regular monitoring of the concentration of nitrogen-containing compounds and nitrification indicators in the Udy river (at sites 500 m before the discharge of treated wastewater in Kharkiv city and 500 m after discharge downstream) was analyzed and in wastewater being treated at urban wastewater treatment facilities. Chemical and biological kinetic constants of nitrification in the water layer and biochemical characteristics of nitrification in the bottom deposits in these areas were determined in special laboratory experiments. It has been stated that the rate of nitrification in the water layer increases twice in the area after discharge of treated wastewater compared to the area before discharge. Biochemical and microbiological indicators of nitrification in the active sludge of wastewater treatment facilities have been established. It has been shown that the discharge of wastewater (and hence nitrifying bacteria) from wastewater treatment facilities that perform biological treatment with nitrification (without denitrification) increases the activity of nitrification processes in the water layer of a natural reservoir downstream.