The Influence of a Selected External Carbon Source on the Share of COD Fractions and the Speed of Denitrification Processes

The article presents the effectiveness of the 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 wastewater treatment processes in two SBR-type activated sludge reactors. A cycle of the SBR operation lasted 360 minutes and included the following phases: wastewater supply (2 min), anaerobic (60 min), aeration (210 min), sedimentation (60 min) and decantation (30 min). After twenty minutes of the wastewater supply, molasses was added to one of the reactors in the cycle, as a source of easily available organic compounds. The conducted tests showed that the use of molasses as an ECS during wastewater treatment resulted in higher nitrogen removal efficiency in comparison with the reactor without ECS. The 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 the 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 SBRs, 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 assessing the susceptibility of wastewater to biological treatment, additionally indicating the impurities that are difficult to decompose, which reduce the effectiveness of biological wastewater treatment. Introduction of ECS in the form of molasses to municipal wastewater caused an increase in the SS fraction by 9 mg O2/dm3, thus increasing the percentage of the 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.