POLICY AND SCIENCE EVALUATING THE BIOAVAILABILITY OF WWTP DISSOLVED ORGANIC NITROGEN AND PERMIT COMPLIANCE IMPLICATIONS

Total maximum daily loads (TMDLs), waste load allocations (WLAs), and National Pollution Discharge Elimination System (NPDES) permits for total nitrogen (TN) generally do not distinguish between organic and inorganic forms of nitrogen. Because many wastewater treatment plants (WWTPs) employ biological nutrient removal systems, including nitrification/denitrification for inorganic nitrogen removal, dissolved organic nitrogen (DON) can comprise a significant TN in the effluent. The Truckee Meadows Water Reclamation Facility (TMWRF) in Reno, Nevada, is facing future compliance issues with the established TN WLA in its NPDES permit. Recent studies showed that DON comprised approximately 80 percent of the TN in the TMWRF effluent that is discharged to the Truckee River. A 10-day bench scale bioavailability study was conducted to assess the fate of TMWFR effluent DON in the Truckee River system. Results showed that DON from the treatment plant was much less bioavailable than inorganic nitrogen species and that in general, initial and final DON concentrations were not statistically different, and that there was not sufficient evidence to suggest that the algal biomass was better sustained when exposed to treatment plant DON. However, additional work needs to be done to understand the observed variability in DON concentrations throughout the study duration and to further investigate changes in the algal biomass. The study provided support for future investigations of DON bioavailability.