Gruber Wenzel, Magyar Paul M, Mitrovic Ivan, Zeyer Kerstin, Vogel Michael, von Känel Luzia, Biolley Lucien, Werner Roland A, Morgenroth Eberhard, Lehmann Moritz F, Braun Daniel, Joss Adriano, Mohn Joachim
Eawag, Swiss Federal Institute for Aquatic Science and Technology, 8600 Dübendorf, Switzerland.
Department of Environmental Sciences, Aquatic and Isotope Biogeochemistry, University of Basel, Basel 4056, Switzerland.
Water Res X. 2022 Feb 28;15:100130. doi: 10.1016/j.wroa.2022.100130. eCollection 2022 May 1.
Nitrous oxide (NO) dominates greenhouse gas emissions in wastewater treatment plants (WWTPs). Formation of NO occurs during biological nitrogen removal, involves multiple microbial pathways, and is typically very dynamic. Consequently, NO 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 NO and its substrates at natural abundance have been shown to provide valuable information on formation and reduction pathways in laboratory settings, but have rarely been applied to full-scale WWTPs. Here we show that N-species isotope ratio measurements at natural abundance level, combined with long-term NO monitoring, allow identification of the NO production pathways in a full-scale plug-flow WWTP (Hofen, Switzerland). Heterotrophic denitrification appears as the main NO production pathway under all tested process conditions (0-2 mgO/l, high and low loading conditions), while nitrifier denitrification was less important, and more variable. NO production by hydroxylamine oxidation was not observed. Fractional NO elimination by reduction to dinitrogen (N) during anoxic conditions was clearly indicated by a concomitant increase in site preference, δO(NO) and δN(NO). NO reduction increased with decreasing availability of dissolved inorganic N and organic substrates, which represents the link between diurnal NO emission dynamics and organic substrate fluctuations. Consequently, dosing ammonium-rich reject water under low-organic-substrate conditions is unfavorable, as it is very likely to cause high net NO emissions. Our results demonstrate that monitoring of the NO isotopic composition holds a high potential to disentangle NO 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 modeling of NO formation or microbial assays to develop efficient NO mitigation strategies.
氧化亚氮（NO）在污水处理厂（WWTPs）的温室气体排放中占主导地位。NO的形成发生在生物脱氮过程中，涉及多种微生物途径，且通常非常动态。因此，NO减排策略需要更好地理解氮转化途径及其调控机制。对NO及其底物在自然丰度下的氮（N）和氧（O）同位素组成分析已被证明能在实验室环境中提供有关形成和还原途径的有价值信息，但很少应用于全尺寸污水处理厂。在这里，我们表明，在自然丰度水平下进行氮物种同位素比率测量，并结合长期的NO监测，可以识别全尺寸推流式污水处理厂（瑞士霍芬）中NO的产生途径。在所有测试的工艺条件（0 - 2mgO/l，高负荷和低负荷条件）下，异养反硝化似乎是主要的NO产生途径，而硝化反硝化的重要性较低且变化更大。未观察到通过羟胺氧化产生NO。缺氧条件下通过还原为氮气（N）而实现的NO部分消除，通过位置偏好、δO(NO)和δN(NO)的同时增加得到了明确指示。NO还原随着溶解无机氮和有机底物可用性的降低而增加，这代表了昼夜NO排放动态与有机底物波动之间的联系。因此，在低有机底物条件下投加富含铵的回用水是不利的，因为这很可能导致高净NO排放。我们的结果表明，监测NO同位素组成在解析工程系统（如全尺寸污水处理厂）中的NO形成机制方面具有很大潜力。我们的研究作为未来先进研究活动的起点，将污水处理厂中的同位素技术与补充方法（如NO形成的数学建模或微生物分析）相结合，以制定有效的NO减排策略。
