Air-water transfer oxygen

Reaeration in sewer networks the presence of dissolved oxygen in wastewater of sewer systems determines if, and to what extent, aerobic and anaerobic processes proceed. The air-water oxygen transfer (the reaeration) determines the potential of aerobic transformation and corresponding removal of wastewater components in many sewer... 

A number of approaches to the air-water mass transport exist. In relation to transport processes in sewer networks, the main developments have been directed toward the air-water oxygen transfer. The following are the main theoretical descriptions that are relevant in this respect ... 

As can be seen from Table 4.1, all three situations are relevant for odorous compounds. Concerning the air-water oxygen transfer (reaeration), resistance primarily exists in the liquid film. 

Empirical Models for Air-Water Oxygen Transfer in Sewer Pipes... 

Special sewer structures like junctions, manholes, bends, weirs and drops may give rise to a turbulence that is increased compared with the hydraulic conditions that exist under normal sewer pipe flow. The turbulence introduced by these structures increases the air-water oxygen transfer, and the formulas in Table 4.7 are no longer valid. These special sewer structures typically have their own site-specific characteristics, and a simple empirical description of the reaeration at sewer drops and falls that includes only the most important parameters is needed. 

Hwang, Y., T. Matsuo, K. Hanaki, and N. Suzuki (1995), Identification and quantification of sulfur and nitrogen containing odorous compounds in wastewater, Water Res., 29(2), 711-718. Jensen, N.Aa. (1994), Air-water oxygen transfer in gravity sewers, Ph.D. dissertation, Environmental Engineering Laboratory, Aalborg University, Denmark. 

The microbial transformations of the wastewater described in the concept shown in Figure 5.5 deal with the COD components defined in Section 3.2.6. The figure also depicts the major processes that include the transformations of the organic matter (the electron donors) in the two subsystems of the sewer the suspended wastewater phase and the sewer biofilm. The air-water oxygen transfer (the reaeration) provides the aerobic microbial processes with the electron acceptor (cf. Section 4.4). Sediment processes are omitted in the concept but are indirectly taken into account in terms of a biofilm at the sediment surface. Water phase/biofilm exchange of electron donors and dissolved oxygen is included in the description. 

Determination of reaeration relies on the measurement of the air-water oxygen transfer coefficient (Section 4.4.2). Measurement of this coefficient — the reaeration coefficient — in gravity sewer lines follows basically the methods that have been developed for and applied in rivers. Methods for determination... 

Jensen and Hvitved-Jacobsen (1991) developed a direct method for the determination of the air-water oxygen transfer coefficient in gravity sewers. This method is based on the use of krypton-85 for the air-water mass transfer and tritium for dispersion followed by a dual counting technique with a liquid scintillation counter (Tsivoglou et al 1965,1968 Tsivoglou andNeal, 1976). A constant ratio between the air-water mass transfer coefficients for dissolved oxygen and krypton-85 makes it possible to determine reaeration by a direct method. Sulfur hexafluoride, SF6, is another example of an inert substance that has been used as a tracer for reaeration measurements in sewers (Huisman et al., 1999). 

KLa air-water oxygen transfer coefficient, reaeration constant (s-1, h-1 or... 

One of the most common chemicals of concern in water bodies is oxygen. Without sufficient oxygen, the biota would be changed because many of the desirable organisms in the water body require oxygen to live. The rate of oxygen transfer between the atmosphere and a water body is therefore important to the health of the aquatic biota. For air-water oxygen transfer, equation (1.2) can be formulated as... 

S. Kakuno, D. B. Moog, T. Tatekawa, K. Takemura and T. Yamagishi, The effect of bubble on air-water oxygen transfer in the breaker zone, in Gas Transfer at Water Surfaces, eds. M. Donelan, W. Drennan, E. Saltzman and R. Waiminkhof (AGU, Washington, DC, 2002), pp. 265-277. 

Yu, S.L., Hamrick, J.M., 1984. Wind effects on air-water oxygen transfer in a lake. In Brutsaert, W., Jirka, G.H. (Eds.), Gas Transfer at Water Surfaces. Reidel, Boston,... 

An estimation of the mass transfer coefficients (Kq, Xl), the mass transfer area (fly), and the volume fractions of gas and liquid (ec, el) can be carried out with the correlation equations, which have been developed on the basis of hydrodynamical theories and dimension analysis. The constants incorporated into the equations have subsequently been determined on the basis of experimental data for a number of model systems (such as air-water, oxygen-water, etc.). The dependability of these correlation equations can thus be very different. Usually, the quality of the estimations falls somewhere around 10-30% of the actual values. The correlations presented in the literature should therefore be utilized with great caution, and the validity limitations should be carefully analyzed. However, these correlations are very useful, for example, when performing feasibility studies or planning one s own experimental measurements. A thorough summary of various correlation equations for gas-Kquid reactors is presented by Myllykangas [ 1 ]. Here we will only treat two common gas-Kquid reactors, namely, bubble columns and packed columns, operating in a countercurrent mode. 

Under equiUbrium or near-equiUbrium conditions, the distribution of volatile species between gas and water phases can be described in terms of Henry s law. The rate of transfer of a compound across the water-gas phase boundary can be characterized by a mass-transfer coefficient and the activity gradient at the air—water interface. In addition, these substance-specific coefficients depend on the turbulence, interfacial area, and other conditions of the aquatic systems. They may be related to the exchange constant of oxygen as a reference substance for a system-independent parameter reaeration coefficients are often known for individual rivers and lakes. 

E] Based on oxygen transfer from water to air 77 F. Liquid film resistance controls. (Dwnei- 77 F = 2.4 X 10 ). Equation is dimensional. Data was for thin-waUed polyethylene Raschig rings. Correlation also fit data for spheres. Fit 25%. See Reiss for graph. 

KLa is the volumetric oxygen mass transfer coefficient, owing to the oxygen transfer from the gas phase or air, c, the surface of the cells, cx or to the transfer of oxygen dissolved in water to the surface of the cells. 

FIG. 27 Schematic (not to scale) of the SECM-induced transfer of oxygen across a 1-octadecanol monolayer, at the air-water interface, in a Langmuir trough. 

FIG. 28 Normalized steady-state diffusion-limited current vs. UME-interface separation for the reduction of oxygen at an UME approaching an air-water interface with 1-octadecanol monolayer coverage (O)- From top to bottom, the curves correspond to an uncompressed monolayer and surface pressures of 5, 10, 20, 30, 40, and 50 mN m . The solid lines represent the theoretical behavior for reversible transfer in an aerated atmosphere, with zero-order rate constants for oxygen transfer from air to water, h / Q mol cm s of 6.7, 3.7, 3.3, 2.5, 1.8, 1.7, and 1.3. (Reprinted from Ref. 19. Copyright 1998 American Chemical Society.)... 

The results of this study demonstrated that the rate of oxygen transfer across a clean air-water interface was diffusion-controlled on the time scale of SECM measurements. The rate of this transfer process was, however, significantly reduced with increasing compression of a 1-octadecanol monolayer. Figure 28 illustrates this point, showing approach curves for O2 reduction recorded with the monolayer at different surface pressures. The transfer rate was found to depend on the accessible free area of the interface, as described by the following equation ... 

The flow in sanitary sewers may be controlled by gravity (gravity sewers) or pressure (pressure sewers). In a partially filled gravity sewer, transfer of oxygen across the air-water interface (reaeration) is possible, and aerobic heterotrophic processes may proceed. On the contrary, pressurized systems are full flowing and do not allow for reaeration. In these sewer types, anaerobic processes will, therefore, generally dominate. 

The resistance to oxygen transfer across the air-water interface mainly exists in the water film. Therefore, Equation (4.22) should only be applied to compounds that are comparable to oxygen, i.e., according to Liss and Slater (1974), those that have an HA value greater than -250 atm (mole fraction)-1. 

The DO mass balance in wastewater of sewer systems is fundamental for the microbial processes. The low solubility of oxygen in water, relatively high resistance to mass transfer across the air-water interface and potentially high removal rate of DO are maj or reasons for the fact that DO is often a limiting factor... 

K = oxygen transfer velocity (m s 1, m h 1 or m d 1) a = water-air surface area, A, to volume of water, V (m-1) dm = hydraulic mean depth of the water phase, i.e., the cross-sectional area of the water volume divided by the water surface width (m)... 

Basically, a concept for microbial transformations in sewer networks should cover soluble and particulate components and relevant processes in the water phase, in the biofilm and in the sewer sediments. In addition, mass transfer between these phases and an air-water transfer of oxygen should be taken into account (Figures 1.3 and 5.2). Although only the aerobic microbial activity will be focused on in the concept presented in this chapter, anoxic and anaerobic processes should be considered possible extensions (cf. Chapter 6). 

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