The mechanisms of nitrogen compounds transformation were investigated during four research series. The experiments were carried out at dissolved oxygen concentrations of 30% of air saturation depending on the retention time (24 h, 12 h) and substrate conditions in the reactor (SBR). Sodium acetate as carbon source and ammonium chloride as nitrate source were added to municipal waste. The activated sludge loading changed from 0,054 to 0,2 g COD/gd. Changes of nitrifying, autotrophic biomass concentration in activated sludge were observed. Addition of ammonium enhanced the increase in autotrophic biomass concentration from 10,6 to 19,3% of the whole biomass. The lowest nitrifying biomass concentration, 3,4%, was observed when the activated sludge loading was 0,2 g COD/gd under 12 h retention time.