# Nitrous Oxide Production and Hydroxylamine Accumulation in a Partial Nitritation Sequencing Batch Reactor: Comparison of Different Operational Strategies

## TL;DR

This study compares different operational strategies in a reactor used to treat ammonium wastewater, focusing on nitrous oxide emissions and hydroxylamine accumulation.

## Contribution

The study provides new insights into how operational strategies affect nitrous oxide production and hydroxylamine levels in partial nitritation sequencing batch reactors.

## Key findings

- Strategy II resulted in the highest nitrous oxide emission factor and net production rate.
- Hydroxylamine accumulation was highest under strategy II and lowest under strategy I.
- A strong correlation was found between ammonium oxidation rate, nitrous oxide production rate, and hydroxylamine concentration.

## Abstract

Sequencing batch reactors (SBRs) performing partial nitritation
(PN) for treating high-strength ammonium wastewater are known to exhibit
elevated levels of nitrous oxide (N2O) emissions. This
study investigated N2O production and hydroxylamine accumulation
in a PN-SBR operated using three distinct strategies. The N2O emission factor (EF) and net production rate (N2OR) were determined
under stable conditions for (i) single feeding with continuous aeration
and one microaerobic stage before settling (strategy I), yielding
EF = 4.4% and N2OR = 14 mg N g–1 VSS d–1; (ii) single feeding with multiple microaerobic stages distributed
throughout the cycle (strategy II), yielding EF = 13.5% and N2OR =
85 mg N g–1 VSS d–1; and (iii)
step feeding with one single microaerobic stage before settling (strategy
III), yielding EF = 10% and N2OR = 45 mg N g–1 VSS
d–1. The distribution of microaerobic stages throughout
the cycle (strategy II) promoted the highest hydroxylamine accumulation
(0.18 mg N L–1) during the aerated stage, whereas
strategy I showed the lowest accumulation (0.01 mg N L–1). A strong positive correlation (R
2 ≥
0.9) was observed among the specific ammonium oxidation rate (AOR),
specific N2OR, and bulk liquid hydroxylamine concentration during
the aerated stages.