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. 2024 Oct 6:25:100263.
doi: 10.1016/j.wroa.2024.100263. eCollection 2024 Dec 1.

Coarse bubble mixing in anoxic zone greatly stimulates nitrous oxide emissions from biological nitrogen removal process

Haoran Duan  1   2   3 , Shane Watt  1 , Dirk Erler  4 , Huijuan Li  1 , Zhiyao Wang  1   2 , Min Zheng  3 , Shihu Hu  1 , Liu Ye  2 , Zhiguo Yuan  5   6
Affiliations

Coarse bubble mixing in anoxic zone greatly stimulates nitrous oxide emissions from biological nitrogen removal process

Haoran Duan et al. Water Res X. .

Abstract

The biological nitrogen removal process in wastewater treatment inevitably produces nitrous oxide (N2O), a potent greenhouse gas. Coarse bubble mixing is widely employed in wastewater treatment processes to mix anoxic tanks; however, its impacts on N2O emissions are rarely reported. This study investigates the effects of coarse bubble mixing on N2O emissions in a pilot-scale mainstream nitrite shunt reactor over a 50-day steady-state period. Online and offline N2O monitoring campaigns show that coarse bubble mixing in the anoxic zones significantly elevates N2O emissions, yielding an extremely high emission factor of 15.5 ± 3.5 %. Intensive sampling and isotopic analyses suggest that the elevated emissions are primarily due to the inhibition of the N2O denitrification process by oxygen in the anoxic phase introduced by coarse bubbling. Substituting coarse bubble mixing with submersible pump mixing resulted in a substantial reduction of N2O emissions, decreasing the emission factor by an order of magnitude to 1.2 ± 0.8 %. The findings reveal that a previously overlooked factor, coarse bubble mixing, can significantly stimulate N2O emissions. The use of coarse bubble mixing in anoxic tanks of biological nitrogen removal warrants caution.

Keywords: Coarse bubble mixing; Denitrification inhibition; N2O; Nitrite shunt; Nitrous Oxide.

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Conflict of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Fig. 1
Fig. 1
(A) N2O emission factors (per 6-hour cycle) in day 1–11. (B) Typical dynamics of N2O concentrations in the off-gas monitored on Day 4, showing regular peaks of N2O and NO concentrations; Intensive sampling during a typical cycle of SBR operations on day 4: (C) N2O/NO emissions and N2O liquid concentrations; (D) 15N abundances in N2O (δ15N-N2O) and site-preference (SP) of 15N in N2O; (E) Nitrogenous compounds transformation and dissolved oxygen levels during the operation. Shaded areas indicate the anoxic periods.
Fig. 2
Fig. 2
Intensive isotopic sampling on Day 12. (A) δ15N-N2O and SP values along the SBR cycle; (B) Dissolved nitrogenous compounds and DO variations along the cycle. Grey Shaded areas indicate the anoxic periods; (C) Dual isotope mapping with SP and δ15N-N2O values; (D) Dual isotope mapping with SP and δ15N-N2O values in anoxic phase 2 and anoxic phase 3. NN: Hydroxylamine oxidation pathway; ND: Nitrifier denitrification pathway; HD: Heterotrophic denitrification pathway.
Fig. 3
Fig. 3
(A) N2O emission factors with coarse bubble mixing or submersible pump mixing. Dash line indicated the switch of mixing method in the anoxic phase; (B) Typical dynamics of N2O concentrations in the off-gas monitored on Day 7 with coarse bubble mixing; (C) Typical dynamics of N2O concentrations in the off-gas monitored on Day 22 with submersible pump mixing. Shaded areas indicate the anoxic periods.
Fig. 4
Fig. 4
Intensive sampling during a typical cycle of SBR operations on day 32 and day 46 without coarse bubble mixing. (A&D) N2O/NO emissions and N2O liquid concentrations; (B&E) 15N abundances in N2O (δ15N-N2O) and site-preference (SP) of 15N in N2O; (C&G) Nitrogenous compounds transformation and dissolved oxygen levels during the operation. Shaded areas indicate the anoxic periods.
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