The effect of free ammonia, free nitrous acid, dissolved oxygen and salinity on hypersaline nitritation stability

The prevalence of saline and hypersaline wastewaters from aquaculture and various industrial sources is increasing, often containing elevated levels of ammonia that require treatment prior to environmental discharge. In order to treat hypersaline wastewater using partial nitritation-anammox or other nitrogen removing processes involving nitritation, it is essential to develop and investigate an efficient, stable, and reliable hypersaline nitritation reactor. This study examines the stability of hypersaline (4 % salinity) nitritation by investigating the effect of free ammonia (FA), free nitrous acid (FNA), dissolved oxygen (DO) and high salinity on the inhibition of nitrite-oxidizing bacteria (NOB) in continuous and mixed fixed bed reactors. Long-term (202 days) stable nitritation was maintained under hypersaline conditions using high FA concentrations (1-10 mg N/L). At low FA levels (<1 mg N/L), the NOB Nitrospina proliferated and nitritation was lost under both high and low DO conditions. Using high FA, nitritation could be recovered even following repeated cycles of NOB proliferation. Under FNA concentrations of 0.11 mg N/L, the NOB Nitrococcus proliferated. Salinity alone above 5 % was shown to selectively inhibit NOB proliferation without significantly impacting nitritation performance. The dominant ammonia-oxidizing bacteria belonged to the species Nitrosococcus oceani and genus Nitrosomonas. Under non-saline conditions, similar reactors showed that only under high FNA (0.11 mg N/L) were NOB inhibited. These results provide valuable insights into the nitritation processes under hypersaline conditions, which have been less investigated compared to non-saline conditions.