The article presents the effectiveness of N, P, C (nitrogen, phosphorus and carbon) removal from sewage with the use of molasses as an external carbon source (ECS). The research was carried out during the sewage treatment processes in two SBR-type activated sludge reactors. A cycle of the SBR operation lasted 360 minutes and included the phases: sewage supply (2 min), anaerobic (60 min), aeration (210 min), sedimentation (60 min) and decantation (30 min). After twenty minutes of the sewage supply, molasses was added to one of the reactors in cycle, as a source of easily available organic compounds. Tests have shown that the use of molasses as an ECS during sewage treatment resulted in higher nitrogen removal efficiency in compare with reactor without ECS. Wastewater treatment in the SBR without the addition of ECS resulted in a total nitrogen removal of 80% and ammonium nitrogen of 95.9%, whereas wastewater treatment in the reactor with the addition of molasses resulted in the removal of total nitrogen at 90.3% and ammonium nitrogen at 97.3%. Despite the increase in the final nitrate concentration in both SBR, the nitrate concentration in the reactor using an external carbon source was lower by as much as 4.5 mg N/dm3. The COD fractions and their changes in wastewater were determined in order to find out their quantitative and percentage share. Determination of the COD fraction allows to assess the susceptibility of wastewater to biological treatment, additionally indicating impurities that are difficult to decompose, which reduce the effectiveness of biological wastewater treatment. Introduction of ECS in the form of molasses to municipal sewage caused an increase in the SS fraction by 9 mg O2/dm3, thus increasing the percentage of readily biodegradable dissolved organic compounds by about 10%. The increased amount of easily available carbon compounds has contributed to the increase of the denitrification rate. In the initial phase of denitrification with the addition of ECS in the form of molasses, an acceleration in the removal of nitrogen compounds by 2.48 mg N∙dm3/h compared to the control reactor, was observed.