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 NO2
–/(NO2
– + NO3
–) ratio of 95 ±
5% was stably maintained for more than 300 days, and the obtained
volumetric NH4
+ 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 (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.
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