Application of the Activated Sludge Model for Nitrogen to Elevated Nitrogen Conditions

The Activated Sludge Model for Nitrogen (ASMN) was evaluated by conducting simulations under both steady-state and dynamic conditions using a wastewater containing high concentrations of chemical oxygen demand (COD) and nitrogen, and an inhibitor of ammonia-oxidizing bacteria. The adopted wastewater characteristics were based on data from several industrial wastewater treatment facilities. The simulations were performed at a variety of temperatures, solids retention times, dissolved oxygen concentrations, pH values, and salt concentrations. The nitrification operating window was defined, and denitrification performance was characterized. The pH and temperature were found to be the most important variables affecting nitrification performance under upset or startup conditions, with lower pH values allowing better performance at higher temperatures for the high-nitrogen wastewater used in the simulations. Emissions of nitric oxide and nitrous oxide were higher than generally thought to occur and were directly linked to depletion of the electron donor in the anoxic reactor. The findings concerning pH, temperature, and gaseous emissions were all consistent with the known growth characteristics of nitrifying and denitrifying bacteria. Parameter and process variable sensitivity studies were performed, and guidelines for improved biological nitrogen removal were developed.