Correspondence Comment on “Enhancement and Inhibition of Denitrification by Fluid-Flow and Dissolved Oxygen Flux to Stream Sediments” O’Connor and Hondzo (1) are to be commended for their valuable contribution. A strong argument for the importance of the role of hydrodynamics in the denitrification process is presented. The result of interest for this discussion is the presence of a threshold leading to decreasing average bulk loss rates of nitrate (NO3-). The threshold was identified by comparing average bulk loss rates of NO3- with the shear velocity. It was observed that the average bulk loss rate increases to a point after which it decreases. The authors conclude that this threshold is dependent on both the fluid flow conditions and the sediment properties. This letter aims to show that this denitrification threshold can be explained by considering the hydraulic roughness conditions of the channel bed. Specifically, we demonstrate that the conditions near the boundary play a critical role in determining dissolved oxygen (DO) flux through the sediment-water interface, which impacts the denitrification process. This contribution, by using appropriate dimensionless parameters, has the objective of providing a more complete hydrodynamic-based explanation and, thus, the potential of providing a basis for generalizing the results. Boundary roughness classification in turbulent flow depends on the relative magnitude of two parameters: (i) the viscous sublayer thickness, δv ) 5 ν/u*, where ν is kinematic viscosity and u* is shear velocity, and (ii) the roughness height, ks ) b dc, characteristic of the material available at the channel bottom, where b is a constant and dc is a representative coarse grain size (2). While there is some ambiguity regarding the precise values associated with b and dc, it is generally accepted that when Re* ) u*ks/ν > 3.5 ∼ 5, the boundary becomes transitional (2-4), where Re* is the boundary Reynolds number. Transitionally rough boundaries introduce features of turbulence in the viscous sublayer, such as significantly augmented vertical mass and momentum transport at the fluid-solid interface due to the change from molecular dominated diffusion in the smooth boundary case to eddy dominated diffusion in the transitional and rough boundaries. The flow and average bulk loss rate data (Figure 2 in ref 1) is recast in nondimensional form, shown in Figure 1. The average bulk loss rates are made dimensionless by taking the ratio of the average bulk loss rate of DO to that of NO3-. The rate of denitrification depends on concentrations of NO3-, DO, and reducing agent, as well as temperature and the geometry of the system (5). The temperature and channel geometry were fixed for each experiment, while the microbial quantities were not measured. As denitrification is an anaerobic process, considering the ratio of the bulk loss rates emphasizes the connection between NO3- and DO concentrations. The near-bed flow parameters are represented using Re*. On the basis of the synthetic sediment description and the fact that the bed was carefully leveled, a value of 1.0 mm was selected for ks. Personal communication with one of the authors confirmed this to be a reasonable estimate (6). Figure 1 demonstrates two distinct regions. For flows with Re* e 3.52, the ratio of the average bulk loss rates remains approximately constant, with a value of about 6.5. This region corresponds to flow over a hydraulically smooth boundary. At these flow conditions, the supplied DO is consumed near the interface and NO3- is able to pass through the oxic zone. For Re* g 4.88, corresponding to flow over a transitionally 10.1021/es801841c CCC: $40.75
Published on Web 10/18/2008
2008 American Chemical Society
FIGURE 1. Ratio of average bulk loss rates for different boundary Reynolds numbers from data reported in O’Connor and Hondzo (1). rough boundary, the ratio of average bulk loss rates dramatically increases due to the decrease in denitrification and continued increase in the bulk loss rate of DO (see Figure 2 in ref 1). The introduction of near-bed turbulent eddies enhances the exchange of DO at the sediment-water interface. Past a depth within the sediments, the effects of the eddies are not felt and molecular diffusion becomes the dominant transport process. Denitrification is further inhibited by relying on molecular diffusion to transport NO3through an increased oxic zone. The region where the transitionally rough boundary begins, 3.52 < Re* < 4.88, agrees with values reported in the literature (2-4). The details of the region leading to the transitional boundary cannot be resolved because of a lack of data. The dotted line in Figure 1 represents an estimate of this transition. For the transitionally rough boundary, the interaction between the roughness elements and the channel flow significantly enhances the exchange of mass and momentum at the fluid-solid interface. When flow is over a porous bed, this interaction may affect the rate solutes and other substances are delivered to the sediments. As Re* increases, this interaction causes the vertical turbulence intensity to be further enhanced (7). Recent studies indicate that the mean turbulent flow contributes a net vertical flux of momentum into a porous bed (8) and coherent structures in turbulent flow may contribute to DO penetrating into the sediment (9). Further data for a variety of boundary and flow conditions are necessary to quantify the effects of turbulent flows on processes such as denitrification. However, this correspondence and the original paper (1) illustrate the need for nearbed velocity data to accurately classify the boundary type. Recasting the data in the form presented in Figure 1 provides VOL. 42, NO. 22, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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further support for the threshold idea proposed by the authors. It also allows for a more general framework to plot future data for validating the concept and identifying more precisely the threshold Re* value.
Acknowledgments The comments of four anonymous reviewers and an insightful discussion with John Little are gratefully acknowledged.
Literature Cited (1) O’Connor, B. L.; Hondzo, M. Enhancement and Inhibition of Denitrification by Fluid-Flow and Dissolved Oxygen Flux to Stream Sediments. Environ. Sci. Technol. 2008, 42, 119–125. (2) White, F. M. Viscous Fluid Flow, 3rd ed.; McGraw-Hill: New York, 2006. (3) Sedimentation Engineering; Vanoni, V. A., Ed.; ASCE Manuals and Report on Engineering Practice, No. 54; ASCE: New York, NY, 1975.
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(4) Schlichting, H.; Gersten, K. Boundary-Layer Theory, 8th ed.; Springer: Berlin, 2000. (5) Golterman, H. L. The Chemistry of Phosphate and Nitrogen Compounds in Sediments; Kluwer: Dordrecht, The Netherlands, 2004. (6) Personal communication with, O’Connor, B. L. June 27, 2008. (7) Nezu, I.; Nakagawa, H. Turbulence in Open-Channel Flows; IAHR Monograph; A.A. Balkema: Rotterdam, 1993. (8) Dancey, C. L.; Balakrishnan, M.; Diplas, P.; Papanicolaou, A. N. The spatial inhomogeneity of turbulence above a fully rough, packed bed in open channel flow. Exp. Fluids. 2000, 29 (4), 402–410. (9) O’Connor, B. L.; Hondzo, M. Dissolved oxygen transfer to sediments by sweep and eject motions in aquatic environments. Limnol. Oceanogr. 2008, 53 (2), 566–578.
John Petrie and Panayiotis Diplas Baker Environmental Hydraulics Laboratory, Department of Civil & Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA ES801841C
