The aim of this study was to determine the effectiveness of pollutant removal in sequencing batch biofilm reactors (with floating or submerged carriers) when treating nitrogen- and organic-rich real leachate generated during aerobic stabilization of the biodegradable municipal solid waste. A control reactor contained suspended activated sludge. The share of leachate in synthetic wastewater was 10%, which resulted in ratios of chemical oxygen demand and biochemical oxygen demand to total Kjeldahl nitrogen in the influent of ca. 11 and ca. 8.5, respectively. Regardless of whether the reactors contained carriers or not, the effectiveness of nitrification (84.2–84.3%) and of the removal of chemical oxygen demand (86.5–87.0%), biochemical oxygen demand (95.5–98.0%) and ammonium (88.9–89.3%) did not differ. However, the presence of carriers and their type determined in which phase of the cycle denitrification occurred. In the control reactor, denitrification took place during mixing phase with the effectiveness of ca. 43.2% (57.7% of the total nitrogen removal). During aeration, the oxygen content increased rapidly, thus reduced the possibility of simultaneous denitrification. In reactors with carriers, in the aeration phase, not only nitrification but also denitrification occurred. The increase in oxygen content in wastewater was slower, which could have caused dissolved oxygen gradients and anoxic zones in deeper layers of the biofilm and flocks. In the reactor with floating carriers, the effectiveness of denitrification and total nitrogen removal increased 1.23- and 1.10-times, respectively, as compared to the control reactor. The highest efficiencies (67.7% and 73.0%, respectively) were observed in the reactor with submerged carriers.