B11O-07
Isotopic Monitoring of N2O Emissions from Wastewater Treatment: Evidence for N2O Production Associated with Anammox Metabolism?

Monday, 14 December 2015: 09:30
2008 (Moscone West)
Eliza Jean Harris1, Pascal Wunderlin2, Adriano Joss2, Lukas Emmenegger1, Marco Kipf2, Benjamin Wolf3 and Joachim Mohn4, (1)Swiss Federal Institute for Materials Science and Technology, Dubendorf, Switzerland, (2)EAWAG Swiss Federal Institute of Aquatic Science and Technology, Duebendorf, Switzerland, (3)Karlsruhe Institute of Technology, IMK-IFU, Garmisch, Germany, (4)Empa, Laboratory for Air Pollution / Environmental Technology, Dübendorf, Switzerland
Abstract:
Microbial production is the major source of N2O, the strongest greenhouse gas produced within the nitrogen cycle, and the most important stratospheric ozone destructant released in the 21st century. Wastewater treatment is an important and growing source of N2O, with best estimates predicting N2O emissions from this sector will have increased by >25% by 2020. Novel treatment employing partial nitritation-anammox, rather than traditional nitrification-denitrification, has the potential to achieve a neutral carbon footprint due to increased biogas production – if N2O production accounts for <0.5-1% of total nitrogen turnover. As a further motivation for this research, microbial pathways identified from wastewater treatment can be applied to our understanding of N cycling in the natural environment.

This study presents the first online isotopic measurements of offgas N2O from a partial-nitritation anammox reactor 1. The measured N2O isotopic composition – in particular the N2O isotopic site preference (SP = δ15Nα - δ15Nβ) – was used to understand N2O production pathways in the reactor. When N2O emissions peaked due to high dissolved oxygen concentrations, low SP showed that N2O was produced primarily via nitrifier denitrification by ammonia oxidizing bacteria (AOBs). N2O production by AOBs via NH2OH oxidation, in contrast, did not appear to be important under any conditions.

Over the majority of the one-month measurement period, the measured SP was much higher than expected following our current understanding of N2O production pathways 2. SP reached 41‰ during normal operating conditions and achieved a maximum of 45‰ when nitrite was added under anoxic conditions. These results could be explained by unexpectedly strong heterotrophic N2O reduction despite low dissolved organic matter concentrations, or by an incomplete understanding of isotopic fractionation during N2O production from NH2OH oxidation by AOBs – however the explanation most consistent with all results is a previously unknown N2O production pathway associated with anammox metabolism.

  1. Harris et al. (2015) Water Res., 83: 258-270.
  2. Wunderlin et al. (2013) Env. Sci. Tech., 47: 1339-1348.
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