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Impact of organics, aeration and flocs on N₂O emissions during granular-based partial nitritation-anammox.

ABSTRACT: 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.

SUBMITTER: Wan X

PROVIDER: S-EPMC7612980 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:For two decades now, partial nitritation anammox (PNA) systems were suggested to more efficiently remove nitrogen (N) from mainstream municipal wastewater. Yet to date, only a few pilot-scale systems and even fewer full-scale implementations of this technology have been described. Process instability continues to restrict the broad application of PNA. Especially problematic are insufficient anammox biomass retention, the growth of undesired aerobic nitrite-oxidizers, and nitrous oxide (N2O) emissions. In this study, a two-stage mainstream pilot-scale PNA system, consisting of three reactors (carbon pre-treatment, nitritation, anammox - 8 m3 each), was operated over a year, treating municipal wastewater. The aim was to test whether both, robust autotrophic N removal and high effluent quality, can be achieved throughout the year. A second aim was to better understand rate limiting processes, potentially affecting the overall performance of PNA systems. In this pilot study, excellent effluent quality, in terms of inorganic nitrogen, was accomplished (average effluent concentrations: 0.4 mgNH4-N/L, 0.1 mgNO2-N/L, 0.9 mgNO3-N/L) even at wastewater temperatures previously considered problematic (as low as 8 °C). N removal was limited by nitritation rates (84 ± 43 mgNH4-N/L/d), while surplus anammox activity was observed at all times (178 ± 43 mgN/L/d). Throughout the study, nitrite-oxidation was maintained at a low level (<2.5% of ammonium consumption rate). Unfortunately, high N2O emissions from the nitritation stage (1.2% of total nitrogen in the influent) were observed, and, based on natural isotope abundance measurements, could be attributed to heterotrophic denitrification. In situ batch experiments were conducted to identify the role of dissolved oxygen (DO) and organic substrate availability in N2O emission-mitigation. The addition of organic substrate, to promote complete denitrification, was not successful in decreasing N2O emission, but increasing the DO from 0.3 to 2.9 mgO2/L decreased N2O emissions by a factor of 3.4.

Project description:This study investigated the potential of Membrane-Aerated Biofilm Reactors (MABRs) for mainstream nitrogen removal via partial nitration/anaerobic ammonium oxidation (anammox). Four laboratory-scale MABRs were operated with real municipal wastewater characterized by low concentrations of nitrogen (varying between 31 and 120 mg-NH4-N L-1) and the presence of biodegradable organic carbon (soluble COD (sCOD) between 7 and 230 mg-O2 L-1). Two reactors were operated with different aeration strategies (intermittent vs. continuous), the other two with differences in biomass retention (recirculation or removal of detached biomass). Keeping a constant HRT caused instabilities due to difficulties with setting the optimal oxygen flux for the respective surface loadings (1.6-6 g-NH4-N m-2 d-1). Operating the MABRs with a constant surface loading (2 g-NH4-N m-2 d-1) resulted in higher and more stable total nitrogen (TN) removal independent of the aeration strategy. The intermittently aerated MABR improved from an average TN removal of 23%-69%, the continuously aerated MABR from 20% to 50% TN removal. Independent of the feeding strategy, the continuously aerated reactor removed slightly more ammonium (80-95%) compared to the intermittently aerated reactor (74-93%). Limiting the oxygen supply by intermittent aeration proofed successful to favor partial nitritation and anammox. Continuous aeration did not achieve stable suppression of nitrite oxidizing bacteria (NOB). Of the removed ammonium, approx. 26% were left in the effluent as nitrate (only 10% with intermittent aeration). Recirculation of the detached biomass resulted in reattachment onto the biofilm or membrane surface. This recirculation led to significantly higher biomass retention times and thus to better performance. Removing detached biofilm from the reactor caused a slightly lower TN removal of 33% compared to 45% with reattachment, while average ammonium removal was 58% compared to 63%, respectively. Scouring events had a significant impact on the overall operation, resulting in short term losses of TN removal capacities of 50-100%. The microbial community composition was different depending on the aeration strategy and biomass retention. The continuously aerated reactor contained significantly more AOB than the intermittently aerated MABR. The reactor with biomass retention contained less ammonium oxidizing bacteria (AOB), compared to the reactor with low biomass retention. In all MABRs, anammox bacteria established in the biofilm after an initial drop in abundance.

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Impact of organics, aeration and flocs on N₂O emissions during granular-based partial nitritation-anammox.

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Impact of organics, aeration and flocs on N2O emissions during granular-based partial nitritation-anammox.

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Impact of organics, aeration and flocs on N₂O emissions during granular-based partial nitritation-anammox.