A conceptual model for apparent free ammonia inhibition in wastewater systems

The original hypothesis for this research program was that free hydroxylamine, and not free ammonia, is the true cause of nitrite accumulation in wastewater treatment systems. This hypothesis was developed based upon the observation that nitrite accumulation appears to be a function of both free ammonia and dissolved oxygen concentration, as well as the work of Yang and Alleman (1992) who demonstrated hydroxylamine production in batch experiments. The starting point of the research program was the development of a suitable hydroxylamine analysis method for wastewater applications. The method developed as part of this thesis combines simplicity, good yields, and a relatively low detection limit.; Measurable concentrations of hydroxylamine were not detected in completely stirred tank reactor (CSTR) or sequencing batch reactor (SBR) systems treating synthetic waste and exhibiting nitrite accumulation. In addition, even though hydroxylamine production coincided with nitrite accumulation in batch experiments, several lines of evidence suggested it was unlikely that hydroxylamine was the primary cause of observed nitrite accumulation.; Batch tests conducted using mixed liquor samples collected from reactors operated for this research program indicated free ammonia was probably not inhibitory to nitrite-oxidizers at commonly reported concentrations. Therefore, free ammonia inhibition trials were undertaken using pure cultures of Nitrospira moscoviensis, since Nitrospira spp. were the predominant nitrite-oxidizers in the systems studied for this research program. The results of these experiments support the conclusion that free ammonia is probably not inhibitory to nitrite-oxidizing organisms in wastewater systems.; Nitrous oxide emissions coincided with nitrite accumulation in every case studied. Several experiments suggested autotrophic denitrification of nitrite by ammonia-oxidizing organisms as the most likely source. FISH analyses conducted for this research program showed that ammonia and nitrite-oxidizing organisms grew in colonies in close proximity to each other. It is believed that the increase in oxygen utilization and denitrification of nitrite to nitrous oxide by ammonia-oxidizers following a free ammonia perturbation, which results in reduced substrate availability for nitrite-oxidizers, is the true cause of apparent free ammonia inhibition. A conceptual model explaining apparent free ammonia inhibition of nitrite-oxidizing organisms has been developed and is presented.