The common consensus among researchers and practitioners alike is that high free ammonia concentrations cause nitrite oxidizer inhibition in wastewater treatment systems. The work of Anthonisen et al. (1976), who produced an operational chart providing guidance on the free ammonia concentrations
most likely to produce nitrite oxidizer inhibition, is considered by many to be the definitive work that established the inhibitory nature of free ammonia in wastewater treatment systems. Over the past three decades several investigators have reported discrepancies in the range of inhibitory
free ammonia concentrations. Anthonisen et al. (1976) reported inhibition of nitrite-oxidizers at free ammonia concentrations of 0.1 to 1 mg/L, Turk and Mavinic (1986) reported no significant nitrite-oxidizer inhibition for free ammonia concentrations below 10 mg NH3-N/L, whereas
Mauret et al. (1996) concluded that free ammonia inhibits Nitrobacter, in the range of 6.6 to 8.9 mg NH3-N/L. On the other hand, Gieseke et al. (2003) state that it is not known whether bacteria of the genus Nitrospira, now believed to be the most prevalent nitrite oxidizer in wastewater
systems, are inhibited by free ammonia. Some investigators have called into question the true cause of observed nitrite oxidizer inhibition. For example, Cecen and Ipek (1998) suggested that the dissolved oxygen to free ammonia ratio and not the free ammonia concentration itself is important,
when attempting to induce nitrite accumulation. The work of others (Stuven et al. 1992; Yang and Alleman 1992; and Hyungseok Yoo et al. 1999) has suggested that hydroxylamine – an intermediate in the ammonia oxidation process – may be the true cause of nitrite oxidizer inhibition
and therefore of nitrite accumulation. Many authors agree; however, that selective inhibition of nitrite oxidation via apparent free ammonia inhibition would allow direct oxidation of ammonia to nitrite and denitrification to dinitrogen gas with the inherent benefits of reduced aeration and
carbon requirements. The starting point to designing such a process is the development of a clearer understanding of the mechanism(s) associated with nitrite oxidizer inhibition and nitrite accumulation. This paper describes the results of exploratory work conducted to both confirm the
results of mixed culture batch tests suggesting free ammonia may not be inhibitory to nitrite oxidizers as commonly reported and elucidate the likely mechanism(s) responsible for observed phenomenon. 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. Several long-term reactor
perturbation studies were conducted with mixed microbial cultures to identify both the population shifts and gas dynamics associated with apparent free ammonia inhibition phenomenon. These studies, like others conducted for this research program, showed that nitrous oxide emissions coincide
with nitrite accumulation. 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. Data is presented to suggest that the increase in oxygen utilization and denitrification of nitrite to nitrous oxide by ammoniaoxidizers following a free ammonia perturbation, results in reduced substrate availability for nitrite-oxidizers, and 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.
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