Robust Nitritation Sustained by Acid-Tolerant Ammonia-Oxidizing Bacteria

Abstract

Oxidation of ammonium to nitrite rather than nitrate, <i>i.e.</i>, 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 NO₂^-/(NO₂^- + NO₃^-) ratio of 95 ± 5% was stably maintained for more than 300 days, and the obtained volumetric NH₄⁺ removal rate (<i>i.e.</i>, 188 ± 14 mg N L^-1 d^-1) was practically useful. 16S rRNA gene sequencing analyses indicated the dominance of new AOB, "<i>Candidatus</i> Nitrosoglobus," in the nitrifying guild (<i>i.e.</i>, 1.90 ± 0.08% in the total community), with the disappearance of typical activated sludge nitrifying microorganisms, including <i>Nitrosomonas</i>, <i>Nitrospira</i>, and <i>Nitrobacter</i>. This is the first identification of <i>Ca.</i> Nitrosoglobus as key ammonia oxidizers in a wastewater treatment system. It was found that <i>Ca.</i> 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 <i>in situ</i> inhibition of NOB leads to accumulation of nitrite (NO₂^-), which along with protons (H⁺) also produced in ammonium oxidation generates and sustains FNA at 3.0 ± 1.4 mg HNO₂-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 (N₂O) emission level, with the emission factors estimated at 1.57 ± 0.08 and 0.57 ± 0.03%, respectively, of influent ammonium nitrogen load.