Robust Nitritation Sustained by Acid-Tolerant Ammonia-Oxidizing Bacteria.

Oxidation of ammonium to nitrite rather than nitrate, , 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/(NO + NO) ratio of 95 ± 5% was stably maintained for more than 300 days, and the obtained volumetric NH removal rate (, 188 ± 14 mg N L d) was practically useful. 16S rRNA gene sequencing analyses indicated the dominance of new AOB, " Nitrosoglobus," in the nitrifying guild (, 1.90 ± 0.08% in the total community), with the disappearance of typical activated sludge nitrifying microorganisms, including , , and . This is the first identification of Nitrosoglobus as key ammonia oxidizers in a wastewater treatment system. It was found that 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 inhibition of NOB leads to accumulation of nitrite (NO), which along with protons (H) also produced in ammonium oxidation generates and sustains FNA at 3.0 ± 1.4 mg HNO-N L. 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 (NO) emission level, with the emission factors estimated at 1.57 ± 0.08 and 0.57 ± 0.03%, respectively, of influent ammonium nitrogen load.