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Online N2O Measurement: The Next Standard for Controlling Biological Ammonia Oxidation? Pascal Wunderlin,* Hansruedi Siegrist, and Adriano Joss Eawag, Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, P.O. Box 611, 8600 Duebendorf, Switzerland suitable sensors. Toxic influents, activity loss or overaeration, all of which supply oxygen beyond the point of immediate AOB consumption, thus usually lead to microbial inhibition and NO2− accumulation. As a result, N2O emission rates have increased substantially (Figure 1). N2O off-gas analysis is consequently
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Figure 1. Correlation between N2O emissions and NO2− levels of different experimental runs. Measurements were carried out in a 400 L one-stage nitritation-anammox sequencing batch reactor fed with digester liquid from a municipal sludge digester, with continuous online N2O off-gas analysis (Rosemount Analytical X-Stream X2).
N2O EMISSION CARRIES INFORMATION ABOUT AOB ACTIVITY N2O formation during biological wastewater treatment is complex because it is produced via different pathways and influenced by many factors. At an earlier stage of the research, heterotrophic denitrifiers were considered the dominant N2O producers, a situation which changed substantially in recent years, the focus having moved to ammonia-oxidizing bacteria (AOB). The latter can generate N2O via (so far) two different pathways: (i) by reducing nitrite (NO2−), and (ii) via intermediates formed during the oxidation of hydroxylamine (NH2OH) to NO2−, known as nitrifier denitrification and NH2OH oxidation, respectively. Nitrification was recently proposed to be the dominant N2O source in full-scale biological wastewater treatment, which is mainly attributed to suboptimal process operating conditions, such as the accumulation of NO2−, the presence of low concentrations of dissolved oxygen or peak ammonia loads.1 Monitoring N2O off-gas concentrations combined with a better understanding of its dynamics and triggers is consequently helpful for improved AOB process control.
suitable for automated aeration rate control, leading to stable and more balanced operation of the nitritation-anammox process. In nitritation-anammox systems, N2O production is driven by AOB activity, either via nitrifier denitrification or NH2OH oxidation, since heterotrophic denitrification activity is negligible due to the low availability of degradable organic substrate, and anammox bacteria are considered not to produce N2O. Therefore, suitable process control strategies need to be clarified in order to (i) minimize N2O emissions, (ii) maintain low concentrations of dissolved NO2− and (iii) keep NH2OH oxidation activity low. In this context, the continuous analysis of the N2O site-specific nitrogen isotopic signature is regarded as being an excellent tool for quantitative pathway investigation. One promising approach is to feed the digester liquid continuously during aeration: at low NH4+ concentrations, N2O production via NH2OH oxidation is also low, and the N2O emissions can therefore be taken as proxy for the NO2− concentrations in the reactor (Figure 1).
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ONLINE N2O IS SUITABLE FOR CONTROLLING AOB We are currently working on the implementation of online N2O off-gas analysis for the process control of full-scale nitritation-anammox treatment. Process stability and control is still a big issue here, involving an ongoing discussion about © 2013 American Chemical Society
Received: July 9, 2013 Accepted: July 17, 2013 Published: August 14, 2013 9567
dx.doi.org/10.1021/es402971p | Environ. Sci. Technol. 2013, 47, 9567−9568
Environmental Science & Technology
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A WIN-WIN SITUATION FOR THE ENVIRONMENT AND THE PLANT OPERATORS The implementation of a financial GHG crediting system, as suggested by Wang et al.,5 is considered a powerful incentive to promote the broad application of a continuous N2O off-gas monitoring concept. We are convinced that the efforts described here have an enormous potential to reduce air and water pollution considerably and thus contribute substantially to sustainable development within the global wastewater treatment sector.
It was recently concluded that long-term continuous N2O off-gas analysis is required for the accurate recording of emission dynamics and levels of conventional wastewater treatment plants.2 As such, nitrification (AOB) process control based on N2O emissions is considered helpful, because multiple factors impact on N2O production, resulting in a complex, dynamic and plant-specific pattern of N2O emissions.6 Operating strategies minimizing overall greenhouse gas (GHG) emissions thus need to be tested individually in combination with online N2O emission analysis.
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N2O EMISSION IS ENVIRONMENTALLY RELEVANT AND MUST BE CONSIDERED IN ENERGY OPTIMIZATION SCENARIOS N2O is an environmentally harmful substance: it is a GHG and is involved in the destruction of the stratospheric ozone layer. Indeed N2O is estimated to play the biggest role in depletion of stratospheric ozone during the 21st century.3 Therefore, N2O (and other GHG) emissions from the wastewater treatment sector must be minimized. On the basis of Siegrist et al.,4 it is estimated that the average net energy consumption of Swiss wastewater treatment plants, including biogas use from sludge digestion, is in the range from 40 to 50 W hours per person and day (Wh/p/d). This value can be reduced by about 50% by introducing the nitritationanammox process for digester liquid treatment while keeping nitrogen removal at the same level. The resulting savings of 20 to 25 Wh/p/d correspond to about 150 gCO2,equivalents/p/d, assuming 700 gCO2,equivalents/kWhelectrical, which refers to emissions of about 0.5 gN2O/p/d. This accounts for approximately 3% of the daily nitrogen load of one person. Consequently, any discussion of potential future treatment schemes, must necessarily include N2O (and other GHG) emissions. Analogously, aeration energy-saving concepts for conventional treatment must include N2O emissions, which should not be increased by them: for example, emission levels in the range of 0.5−1% with respect to oxidized nitrogen are already in the same order of magnitude as GHG emissions from the production of energy for aeration.
AUTHOR INFORMATION
Corresponding Author
*Phone: +41 58 765 5037; fax: +41 58 765 5389; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was financially supported by the European Research Council within the Athene Project (grant 267897), as well as by the Swiss N2O research project. Special thanks go to Marco Kipf (Eawag) for conducting the experiments, and to the operators of the Swiss wastewater treatment plants of Bazenheid and Zürich-Werdhölzli.
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REFERENCES
(1) Ahn, J. H.; Kim, S.; Park, H.; Rahm, B.; Pagilla, K.; Chandran, K. N2O emissions from activated sludge processes, 2008−2009: Results of a national monitoring survey in the United States. Environ. Sci. Technol. 2010, 44, 4505−4511. (2) Daelman, M. R. J.; De Baets, B.; van Loosdrecht, M. C. M.; Volcke, E. I. P. Influence of sampling strategies on the estimated nitrous oxide emission from wastewater treatment plants. Water Res. 2013, 47, 3120−3130. (3) Ravishankara, A. R.; Daniel, J. S.; Portmann, R. W. Nitrous oxide (N2O): The dominant ozone-depleting substance emitted in the 21st century. Science 2009, 326, 123−125. (4) Siegrist, H.; Salzgeber, D.; Eugster, J.; Joss, A. Anammox brings WWTP closer to energy autarky due to increased biogas production and reduced aeration energy for N-removal. Water Sci. Technol. 2008, 57, 383−388. (5) Wang, J. S.; Hamburg, S. P.; Pryor, D. E.; Chandran, K.; Daigger, G. T. Emissions credits: Opportunity to promote integrated nitrogen management in the wastewater sector. Environ. Sci. Technol. 2011, 45, 6239−6246. (6) Burgess, J. E.; Stuetz, R. M.; Morton, S.; Stephenson, T. Dinitrogen oxide detection for process failure early warning systems. Water Science and Technology 2002, 45 (4−5), 247−254.
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ONLINE N2O OFF-GAS MEASUREMENT IS COST-EFFECTIVE, ROBUST AND REQUIRES LITTLE MAINTENANCE Today’s N2O off-gas analyzers are stable and robust, enabling automated calibration procedures. Monitoring off-gas concentrations avoids direct contact with activated sludge, hence considerably reduces cleaning, matrix interferences, and the overall maintenance effort compared to probes inserted in the liquid phase. Investment costs are estimated to be higher, but still in the same order of magnitude, than those for the conventional commercially available ion selective electrodes usually applied for online NH4+ and NO3− measurement in the liquid. Alternatively, monitoring dissolved NO2− would require a suitable online sensor with sufficient resolution, which needs, to the authors’ knowledge, considerably more maintenance. In addition, extending the N2O off-gas analyzer by an oxygen sensor in combination with an airflow measurement would allow the oxygen consumption to be continuously monitored and thus the aeration to be controlled via the online nitrification rate. 9568
dx.doi.org/10.1021/es402971p | Environ. Sci. Technol. 2013, 47, 9567−9568
