Tracing N2O formation in full-scale wastewater treatment with natural abundance isotopes

Joachim Mohn,Adriano Joss,Luzia Von Känel,Wenzel Gruber,Paul Magyar,Eberhard Morgenroth,Kerstin Zeyer,Moritz F Lehmann,Daniel Braun

doi:10.5194/egusphere-egu22-2352

Abstract

&lt;p&gt;Nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) dominates greenhouse gas emissions in wastewater treatment plants (WWTPs). Formation of N&lt;sub&gt;2&lt;/sub&gt;O occurs during biological nitrogen removal, involves multiple microbial pathways, and is typically very dynamic. Consequently, N&lt;sub&gt;2&lt;/sub&gt;O mitigation strategies require an improved understanding of nitrogen transformation pathways and their modulating controls. Analyses of the nitrogen (N) and oxygen (O) isotopic composition of N&lt;sub&gt;2&lt;/sub&gt;O and its substrates at natural abundance have been shown to provide valuable information on formation and reduction pathways in laboratory settings, but have never been applied to full-scale WWTPs.&lt;/p&gt;&lt;p&gt;Here we show that N-species isotope ratio measurements at natural abundance level, combined with long-term N&lt;sub&gt;2&lt;/sub&gt;O monitoring, allow identification of the N&lt;sub&gt;2&lt;/sub&gt;O production pathways in a full-scale plug-flow WWTP (Hofen, Switzerland). The proposed approach can also be applied to other activated sludge systems. Heterotrophic denitrification appears as the main N&lt;sub&gt;2&lt;/sub&gt;O production pathway under all tested process conditions, while nitrifier denitrification was less important, and more variable. N&lt;sub&gt;2&lt;/sub&gt;O production by hydroxylamine oxidation was not observed. Fractional N&lt;sub&gt;2&lt;/sub&gt;O elimination by reduction to dinitrogen (N&lt;sub&gt;2&lt;/sub&gt;) during anoxic conditions was clearly indicated by a concomitant increase in SP, &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O(N&lt;sub&gt;2&lt;/sub&gt;O) and &amp;#948;&lt;sup&gt;15&lt;/sup&gt;N(N&lt;sub&gt;2&lt;/sub&gt;O). The extent of N&lt;sub&gt;2&lt;/sub&gt;O reduction correlated with the availability of dissolved inorganic N and organic substrates, which explains the link between diurnal N&lt;sub&gt;2&lt;/sub&gt;O emission dynamics and organic substrate fluctuations. Consequently, dosing ammonium-rich reject water under low-organic-substrate conditions is unfavourable, as it is very likely to cause high net N&lt;sub&gt;2&lt;/sub&gt;O emissions.&lt;/p&gt;&lt;p&gt;Our results demonstrate that monitoring of the N&lt;sub&gt;2&lt;/sub&gt;O isotopic composition holds a high potential to disentangle N&lt;sub&gt;2&lt;/sub&gt;O formation mechanisms in engineered systems, such as full-scale WWTP. Our study serves as a starting point for advanced campaigns in the future combining isotopic technologies in WWTP with complementary approaches, such as mathematical modelling of N&lt;sub&gt;2&lt;/sub&gt;O formation or microbial assays to develop efficient N&lt;sub&gt;2&lt;/sub&gt;O mitigation strategies.&lt;/p&gt;

Full Text