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Oxidation of ammonium to nitrite rather than nitrate, i.e., nitritation, is critical for autotrophic nitrogen removal. This study demonstrates a robust nitritation process in treating low-strength wastewater, obtained from a mixture of real mainstream sewage with sidestream anaerobic digestion liquor. This is achieved through cultivating acid-tolerant ammonia-oxidizing bacteria (AOB) in a laboratory nitrifying bioreactor at pH 4.5–5.0. It was shown that nitrite accumulation with a high NO 2 – /(NO 2 – + NO 3 – ) ratio of 95 ± 5% was stably maintained for more than 300 days, and the obtained volumetric NH 4 + removal rate ( i.e., 188 ± 14 mg N L –1 d –1 ) was practically useful. 16S rRNA gene sequencing analyses indicated the dominance of new AOB, “ Candidatus Nitrosoglobus,” in the nitrifying guild ( i.e., 1.90 ± 0.08% in the total community), with the disappearance of typical activated sludge nitrifying microorganisms, including Nitrosomonas, Nitrospira, and Nitrobacter . This is the first identification of Ca. Nitrosoglobus as key ammonia oxidizers in a wastewater treatment system. It was found that Ca. Nitrosoglobus can tolerate low pH (<5.0), and free nitrous acid (FNA) at levels that inhibit AOB and nitrite-oxidizing bacteria (NOB) commonly found in wastewater treatment processes. The in situ inhibition of NOB leads to accumulation of nitrite (NO 2 – ), which along with protons (H + ) also produced in ammonium oxidation generates and sustains FNA at 3.0 ± 1.4 mg HNO 2 -N L –1 . As such, robust PN was achieved under acidic conditions, with a complete absence of NOB. Compared to previous nitritation systems, this acidic nitritation process is featured by a higher nitric oxide (NO) but a lower nitrous oxide (N 2 O) emission level, with the emission factors estimated at 1.57 ± 0.08 and 0.57 ± 0.03%, respectively, of influent ammonium nitrogen load.
Wang et al. (Thu,) studied this question.