Kinetics of denitrification reactions in single sludge systems

This paper reports denitrification studies performed using the anoxic reactor of a laboratory scale anoxic-aerobic plant as a batch reactor of variable volume. This was achieved by adding to the anoxic reactor a supplementary flow of nitrate after the shut down of the recirculation line and the interruption of the hydraulic connection to the aerobic reactor. By operating in this way, in a relatively short time, it is possible to get a number of experimental data sufficient to describe the biological process kinetics. The system is extremely flexible and gives kinetic data in short times for different experimental conditions. In fact, it is possible to operate at different COD/NO3-N ratios simply by changing the influent wastewater flowrate to the anoxic reactor. Two series of tests were performed: in the first series (use of endogenous carbon) a supplementary flow of nitrate was fed to the anoxic reactor while the wastewater influent flow was interrupted; in the second series (use of internal carbon) the influent wastewater flow was fed during the addition of nitrate. The importance of the carbonaceous substrate nature on the denitrification rate was also verified. Data analysis was performed by utilizing the integral method procedure and a zero order kinetics referring to both the substrates COD and nitrate nitrogen was considered. A satisfactory agreement between predicted and experimental data was found. Values obtained for kD range from 0.07 mg NO3-N/mg VSS•d, at which the carbon source is mostly endogenous, to 0.25 mg NO3-N/mg VSS•d, at which the carbon source consists mainly of readily biodegradable COD. Intermediate values occur when the readily biodegradable COD is limiting and denitrification takes place by utilizing the slowly biodegradable one.