Optimization of Biological Denitrification Through Biological Activity Monitoring: System Development Terry J. Mah, Sergey Maneshin, Xin Yang, and Jaw F. Lee BioChem Technology, Inc., King of Prussia, PA 19406 Abstract This work demonstrates the principles and application of a new, in situ device designed to automatically determine the time (DNT) required to denitrify anoxic mixed liquor. By monitoring the changes in NAD(P)H-related fluroescence, the biological activity meter (BAM) can assess denitrification activity under ambient bioreactor conditions without the need for time-consuming chemical analyses. Full-scale testing demonstrated that the consisitency of the relationship between measured DNT and nitrate loading. Results also showed that when DNT exceeded actual anoxic detention time, denitrification was incomplete and indicated by high levels of nitrate in the mixed liquor leaving the anoxic stage. Furthermore, the greater the DNT exceeded the actual detention time, the higher these levels. Conversely, when actual anoxic detention time exceeded measured DNT, denitrification was complete and insignificant levels of nitrates could be detected in the mixed liquor leaving the anoxic stage. The abiltiy to conveniently assess the denitrification activity provided the basis for a denitrification optimization strategy that involves feedback control of internal nitrified mixed liquor recycle flows.