Partial nitration-anammox is a resource-efficient technology for nitrogen removal from wastewater. However, the advantages of this nitrogen removal technology are challenged by the emission of N2O, a potent greenhouse gas. In this study, a granular sludge one-stage partial nitritation-anammox reactor comprising granules and flocs was run for 337 days in the presence of influent organics to investigate its effect on N removal and N2O emissions. Besides, the effect of aeration control strategies and flocs removal was investigated as well. The interpretation of the experimental results was complemented with modelling and simulation. The presence of influent organics (1 g COD g-1 N) helped to suppress NOB and significantly reduced the overall N2O emissions while having no significant effect on anammox activity. Besides, long-term monitoring of the reactor indicated that constant airflow rate control resulted in more stable effluent quality and less N2O emissions than DO control. Still, floc removal reduced N2O emissions at DO control but increased N2O emissions at constant airflow rate. Furthermore, anammox bacteria could significantly reduce N2O production during heterotrophic denitrification, likely via competition for NO with heterotrophs. Overall, this study demonstrated that the presence of influent organics together with proper aeration control strategies and floc management could significantly reduce the N2O emissions without compromising nitrogen removal efficiency during one-stage partial nitritation-anammox processes.@en

This study deals with the effect of aeration control strategies on the nitrogen removal efficiency and nitrous oxide (N2O) emissions in a partial nitritation–anammox reactor with granular sludge. More specifically, dissolved oxygen (DO) control, constant airflow and effluent ammonium (NH4+) control strategies were compared through a simulation study. Particular attention was paid to the effect of flocs, which are deliberately or unavoidable present besides granules in this type of reactor. When applying DO control, DO setpoints had to be adjusted to the amount of flocs present in the reactor to maintain high nitrogen removal and reduce N2O emissions, which is difficult to realize in practice because of variable floc fractions. Constant airflow rate control could maintain a good nitrogen removal efficiency independent of the floc fraction in the reactor, but failed in N2O mitigation. Controlling aeration based on the effluent ammonium concentration results in both high nitrogen removal and relatively low N2O emissions, also in the presence of flocs. Fluctuations in floc fractions caused significant upsets in nitrogen removal and N2O emissions under DO control but had less effect at constant airflow and effluent ammonium control. Still, rapid and sharp drops in flocs led to a peak in N2O emissions at constant airflow and effluent ammonium control. Overall, effluent ammonium control reached the highest average nitrogen removal and lowest N2O emissions and consumed the lowest aeration energy under fluctuating floc concentrations.@en