The effect of pH on N₂O production in intermittently-fed nitritation reactors

The effect of pH on nitrous oxide (N₂O) production rates was quantified in an intermittently-fed lab-scale sequencing batch reactor performing high-rate nitritation. N₂O and other nitrogen (N) species (e.g. ammonium (NH₄⁺), nitrite, hydroxylamine and nitric oxide) were monitored to identify in-cycle dynamics and determine N conversion rates at controlled pH set-points (6.5, 7, 7.5, 8 and 8.5). Operational conditions and microbial compositions remained similar during long-term reactor-scale pH campaigns. The specific ammonium removal rates and nitrite accumulation rates varied little with varying pH levels (p > 0.05). The specific net N₂O production rates and net N₂O yield of NH₄⁺ removed (ΔN₂O/ΔNH₄⁺) increased up to seven-fold from pH 6.5 to 8, and decreased slightly with further pH increase to 8.5 (p < 0.05). Best-fit model simulations predicted nitrifier denitrification as the dominant N₂O production pathway (≥87% of total net N₂O production) at all examined pH. Our study highlights the effect of pH on biologically mediated N₂O emissions in nitrogen removal systems and its importance in the design of N₂O mitigation strategies.