Correspondence/Rebuttal pubs.acs.org/est
Reply to Comment on “Modeling Nitrous Oxide Production during Biological Nitrogen Removal via Nitrification and Denitrification: Extensions to the General ASM Models”
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n his comment,1 the author expresses concerns about the modeling approach presented by Ni et al.2 and suggests that the process of nitrous oxide (N2O) and nitric oxide (NO) transfer from liquid to gas phase has been ignored and that predictions of off-gas N2O and NO concentrations are not correctly made. Here, we formulate our reply to these concerns. In wastewater treatment systems, N2O and NO are produced in the liquid phase during nitrogen conversions. When N2O and NO are above saturation, they may be physically transferred into gas phase, in other words stripped from the liquid phase. Because ambient gaseous concentrations of N2O and NO are low (for N2O ca. 300 ppbv) saturation is easily attained and stripping is possible. Gaseous emissions driven by mass transfer3 are stimulated by aeration, and typically the majority of the produced N2O and NO would be emitted into the gas phase.4 The rate by which off-gas concentrations of N2O and NO increase is then determined by their volumetric mass transfer coefficient (KLa, d−1) and their liquid phase concentrations (as well as their saturation liquid phase concentrations (which, in turn are determined by the ambient gas phase concentrations and their gas/liquid equilibrium partition coefficients). In his comment,1 the author proposes that the process matrix shown in Tables S1 and S2 should include two additional processes to account for the NO and N2O release into the atmosphere, respectively. We would like to highlight that the key physical processes (i.e., the mass transfer processes of NO and N2O from liquid phase to gas phase) were included in our model applications, for example, as described for Cases 2 and 3 in Ni et al.2 (pages 7772−7773), although the process matrixes did not include nonbiological reactions. Instead, the process matrices shown in Tables S1 and S2 only present the biological reactions regarding the liquid phase NO and N2O production or/and consumption, as clearly stated in the Table captions, and in accordance with the general presentation of ASM-type models.5 In Ni et al.,2 we stated clearly that the off-gas concentrations of N2O in Cases 2 and 3 were calculated according to von Schulthess and Gujer6 (Materials and Methods section under the subtitle of Testing the Predictive Power of the Model, page 7770). Specifically, the off-gas concentration of N2O was calculated based on liquid-to-gas mass transfer, which is identical to the approach in the comment1 (see eq 1). ⎛ N2Ogas ⎞ rN2Oliquid = KLa N2O⎜N2Oliquid − ⎟ HN2O ⎠ ⎝
attention is the value of the N2O mass transfer coefficient: it is often not measured directly, but calculated from the oxygen mass transfer coefficient (KLa,O2) with corrections for their relative diffusivities. Direct estimations of KLaN2O may be warranted, especially, where very low (e.g., in nonaerated reactors) or spatially heterogeneous oxygen mass transer is expected. In his comment,1 the author then concludes that NO and N2O release into the atmosphere was not considered by Ni et al.2 and the reactants SNO and SN2O shown inTables S1 and S2 include both the dissolved and released NO and N2O. As explained above, this is an obvious misunderstanding of the method presented in Ni et al.2 In fact, SNO and SN2O shown in Tables S1 and S2 represent only the dissolved NO and N2O concentrations, as clearly stated in the model section (page 7770). There we point out that eight soluble compounds are described in Tables S1 and S2. In addition, liquid-to-gas transfer kinetics was indeed considered to predict the off-gas concentrations. In this way, the model was properly calibrated against several published N2O data derived from off-gas measurements.7,8
Bing-Jie Ni†,‡ Mael̈ Ruscalleda†,§ Carles Pellicer-Nàcher† Barth F. Smets*,† †
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AUTHOR INFORMATION
Corresponding Author
*(B.F.S.) Phone: +45 4525 2230; fax: +45 4593 2850; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Wu, J. Comment on “Modeling nitrous oxide production during biological nitrogen removal via nitrification and denitrification: extensions to the general ASM models”. Environ. Sci. Technol. 2013, 10.1027/es403417a (2) Ni, B. J.; Ruscalleda, M.; Pellicer-Nacher, C.; Smets, B. F. Modeling nitrous oxide production during biological nitrogen removal
(1)
where N2Oliquid and N2Ogas are the N2O concentrations in the bulk liquid and in the bulk gas phase, respectively, and KLaN2O is the N2O mass transfer coefficient, HN2O is the nondimensional Henry’s law coefficient. A similar approach was also applied to calculate gas-phase NO concentration. A point of © 2013 American Chemical Society
Department of Environmental Engineering, Technical University of Denmark, Miljøvej building 113, 2800 Kongens Lyngby, Denmark ‡ Advanced Water Management Centre, The University of Queensland, St. Lucia, QLD 4072, Australia § Laboratory of Chemical and Environmental Engineering (LEQUIA), Institute of the Environment, University of Girona, Campus Montilivi s/n, E-17071 Girona, Spain
Published: September 17, 2013 11910
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Environmental Science & Technology
Correspondence/Rebuttal
via nitrification and denitrification: Extensions to the general ASM models. Environ. Sci. Technol. 2011, 45, 7768−7776. (3) Foley, J.; de Haas, D.; Yuan, Z.; Lant, P. Nitrous oxide generation in full-scale biological nutrient removal wastewater treatment plants. Water Res. 2009, 44, 831−844. (4) Aboobakar, A.; Cartmell, E.; Stephenson, T.; Jones, M.; Vale, P.; Dotro, G. Nitrous oxide emissions and dissolved oxygen profiling in a full-scale nitrifying activated sludge treatment plant. Water Res. 2012, 47, 524−534. (5) Henze, M.; Gujer, W.; Matsuo, T.; van Loosdrecht, M. C. M. Activated Sludge Models ASM1, ASM2, ASM2d and ASM3, Scientific and Technical Reports; IWA Publishing: London, 2000. (6) von Schulthess, R.; Gujer, W. Release of nitrous oxide (N2O) from denitrifying activated sludge: Verification and application of a mathematical model. Water Res. 1996, 30, 521−530. (7) Butler, M. D.; Wang, Y. Y.; Cartmell, E.; Stephenson, T. Nitrous oxide emissions for early warning of biological nitrification failure in activated sludge. Water Res. 2009, 43, 1265−1272. (8) Itokawa, H.; Hanaki, K.; Matsuo, T. Nitrous oxide production in high-loading biological nitrogen removal process under low COD/N ratio condition. Water Res. 2001, 35, 657−664.
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